Using MIDI-1+cluster expansion method, Dunning's DZP+SDCI and 6-31 G*+MP 4, the structures and stabilities of D2h and D4h B-clusters were discussed. The obtained energy sequence of these clusters is ¹Ag(D_2h)<¹A_lg(D_4h)<³B_lu(D_2h). The optimized D_4h¹A1g-state is the transition state of the long and short axis exchange reaction of D_2h-clusters. The energy height of this transition state, however, is very low, less than 3 kcal/mol, and the stable D2h species are located on the very flat and shallow minimum region of the pote ntial surface. The potential surface of the singlet ground state of these cluster are plotted. The stabilities of these clusters are analysed and discussed by using the charge distribution of some MO's and the many-body interaction expansions.

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Deexcitation rate constants of the lowest excited argon atoms, Ar (1P₁), Ar(3P_O), Ar(3P₁), and Ar(3P₂) by N₂ and SF₆ have been obtained by means of a pulse radiolysis method combined with a time-resolved optical absorption and emission detection technique. The rate constants for SF₆ shown in Table 2 are large and strongly depe ndent on the nature of the excited states; the values for resonance states, 1P₁and 3P₁ are 84.9 and 60.5, respectively, in 10_-11cm₃s_-1 unit and are much larger than those for metastable atoms, 3P0 and 3P2, of about 31 x10^-1cms^-1
The rate constants for N₂ have also been obtained. Artifacts due to secondary thermal electrons which are evident in Ar-N₂ bicomponent gas mixture have been eliminated by the addition of small amount of SF₆ (Fig.3). The rate constants fot N₂ are small but also state dependent (Table 3). Analysis of 1 the decay of emission intensity of N₂(C³IIu, v'→B³IIg, v" ) has shown that precursors of N₂(C³IIu, v'). states are the lowest excited argon atoms. A detailed analysis has been shown that formation of N₂(C³IIu, v') is mainly contributed from Ar(¹P₁); on the other hand, formation of N₂(C³IIu, v') is dominated by excitation transfer from Ar(₃P₂). OthervibrationaVlOels (v" =1 and 2) are contributed from the three triplet argon atoms.

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A modulation technique has been developed to, determine the intra and intermode vibrational energy transfer rates in mode-selectively excited polyatomic molecules. The time evolution of the population in the i-th vibrational level of the relevant polyatomic molecule is described as
_??_
were λi is the j-th eigenvalue of the matrix A whose elements are respective vibrational energy transfer rate constants and cti is the coefficient determined by the initial condition. If the molecules are excited to the i=1 level by a laser light modulated at an angular frequency ω, the phase delay φ for the modulated part of the i-th level population is derived as
_??_
This equation has been proved to be applicable to analyze the laser induced fluorescence (LIF) data of CH3F whose v3 mode is excited by the modulated cwCO₂ laser. Observing the LIF from the 2 _v and _vil₄ levels, the inter as well as intramode vibration-to-vibration (V-V) energy transfer rate constants are determined. It has been found that the relaxation mechanism is dependent significantly on the amount of the mixed third body gas such as Ar, since the intermode V-V transfers are accelerated by increase of the CH₃F-Ar collision rate, while the intramode V-V energy transfer rate is dependent only on the self collision rate of CH₃F. The determined mechanism and rate constants are in good agreement with those obtained by the pulsed excitation method.

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It has been shown that the fluorescence of gaseous carbon disulfide (CS₂) is significantly influenced by relatively weak magnetic fields below 15 kG. Recently, one of the authors has found that the CS₂ fluorescence has a fast decay component in a picosecond region and that the lifetime of this component (τf of Eq. ( 1 )) decreases with increasing the field strength up to 15.1 kG. These facts clearly show that the magnetic field effects on the CS₂ fluorescence can be explained in terms of intramolecular relaxation processes.
In the present paper, we carry out a mechanistic study on the magnetic fields effects observed for the CS2 fluorescence, comparing the effects with those observed in the gas phase for organic molecules such as pyrazine and pyrimidene. The decoupling mechanism (see Eqs. ( 7 )--(10)) explains the effects on the singlet-triplet intersystem crossing and the direct mechanism (see Eqs. (11)--(13)) those on the singlet-singlet internal conversion.
For organic molecules, the magnetic field effects on their fluorescen ce are attributable to only the decoupling mechanism. Here, the ratio of the preexponential faCtors of the slow and fast components (A_s/A_f of Eq. ( 1 )) decrep. ses upon application of low magnetic fields below 1 kG, but rf is not affected by the fields; the magnetically induced changes are saturated at low fields. For CS₂, on the other hand, the effects on its fluorescence are attributable to a combination of the decoupling and direct mechanisms. Here, both As_s/A_f and τf decrease with increasing magnetic field without showing saturation up to 15.1 kG.

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The CH(X) and SiH(X) radicals have been detected in the dissociative excitation transfer reactions of Ar(3P_2.0) with CH₃X(X=H, Cl, Br, I), SiH₄, and Si₂H₆ by using a flowing afterglow apparatus combined with laser-induced fluoresence. The contribution of the radiative cascade process from the upper state is insiginificant except for the case of the reaction of Ar(3P_{2, 0}) with SiH₄ on the basis of the measurements of the formation rates of CH(A) and SiH(A) through observation of the emission spectra. In the reactions of Ar (³P_{2, 0})with CH₃X, the relative formation rates of CH(X) have been measured to be k(CH₄)k(CH₃Cl): k(CH₃Br): k(CH₃I )=1: 0.28: 0.15: 0.03. Upper limits of the format ion rate constants for reactions of Ar(³P_2.0) with SiH₄ and Si₂H₆ which produce SiH(X) have been estimated to be 2.0 x10^-10, and 2.2 x 10^-10 cm³⋅ molecule^-1⋅^s-1 respectively. It has been found that the fraction of the available energy converted into vibration of CH(X) or SiH(X) is very small (<5%) in the title reactions.

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The Energy redistribution which may occur upon vibrational predissociation of van der Waals complexes between rare-gas atoms and large organic molecules has attracted much interest recently because the fragmented parent molecules can be used to probe the mechanism of the energy redistribution. In this paper, the results of an investigation of the photodissociation of the rare-gas van der Waals complexes of 9-methoxyanthracene (MEOA), obtained by the use of both kinetic and spectroscopic techniques, are reported. The nonexponential fluorescence decays which are recorded for the fragmented molecules show clearly that the dissociation from the vibronic level of 12S results in relatively high yields of vibrationless MEOA. The short-lived component becomes dominant upon increasing the excess energy of excitation. A comparison of the relative fluorescence quantum yields, calculated by usin g the fluorescence decay parameters, with the relative band intensities suggests that dissociation of the Ar and Kr complexes occurs in much shorter periods of time than the radiative lifetimes of the corresponding complexes. However, an appreciable fraction of the Xe complexes decays from the 84 vibronic level via a nonradiative pathway in which dissociation does not occur. The substantial reduction in the fluorescence lifetime of the insufficiently cooled states of MEOA, together with the data obtained for the photodissociation of complexes, of 9, 10-dimethoxyanthracene with rare-gas atoms, indicates that the residual energies of the vibrational predissociation are not distributed significantly among optically inactive modes.

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The infrared multiple-photon decomposition (IRMPD) of (trifluoromethyl)benzene (TFMB)induced by a TEA CO₂ laser has been studied in order to elucidate the decomposition mechanism and the reactive species. The decomposition yield (Y_d) is proportional to the laser fluence at focus (0): Y_docg 5" (Fig.3). The value of Y_d is not dependent on the pressure of TFMB (0.3-1 Torr). The volatile products are mainly C₂F₄ and C₂H₂, while C₆1-1a₈n d C₆H₅F are formed in low yields. The product distribution is little affected b y the number of laser pulses (0-200) (Fig.5), the pressure of TFMB, and ck (39-350 Jcrn-2); 1. e., C2F4: 58--60%, C₂H₂ 34-38%, C₆H₆: 3%, and C₆H₅F: 1%. The material balance is well: 5 5-66% based on CF3, but low: 9 -12% based on C31-I5b ecause of formation of soot and polymer. The addition of Ar slightly increases C₆H₆ and C₅H₅F as products with decreasing C₂H₂ (Fig.4). Formation of C₂F₈ is not detected even at a low q5 (39 J cm^-2) a nd in the presence of 20 Torr of Ar. These results suggest that the liomolytic cleavage of the C-CF3bond occurs initially in the highly excited vibrational states of TFMB (TFMB t), which yields -CF₃ and C₆₁-₁₅(S cheme 1). The secondary IRMPD of the radicals occurs within a laser pulse to form: CF₂+F and C₂H₂, respectively. The product C₂F₄ is formed via dimerization of: CF₂: The branching ratio of the secondary IRMPD increases with increasing the internal energy of the radicals. The reactive species are trapped by Br₂ to yield CF₃Br, C₆₁-15BrC, F₂Br₂, and CF2BrCF2Br (Figs.6-9). Moreover a mixture of bromo(trifluorom ethyl)benzenes (BTMB) in the ratio of o- m-: p-BTMB =32: 37: 30 is produced via H-substitution on benzene ring of TFMB by Br (Fig.6). Even in the presence of 10 Torr of Br2, . CF3decomposes via the secondary IRMPD in a 39% and is trapped by Br₂ in a 61% yields. Therefore, the secondary IRMPD of CF₃ is particularly preferable in the IRMPD of T FMB. This is probably attributable to the effective infrared multiple-photon excitation (IRMPE) and the higher dissociation energy of C-CF₃ bond in TFMB, compared with other CF₃-containing molecules. t The IR Photochemistry of Organic Compounds.

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The detection and identification of radical intermediates produced in gas-phase photochemical reactions are very important because they determine the steady-state main products. At present, however, for the gas-phase reactions, there are few simple and convenient methods for this purpose. In this study, we applied the spin trap-ESR method which is widely used for the liquid-phase reactions, in two different ways: the reaction gases were passed through a solution containing a spin trap (liquid-phase trapping) and through silica gel adsorbing a spin trap (solid-phase trapping). In both methods, produced nitroxide radicals (radical adducts of the spin trap) produced were finally in solution (mostly in benzene) and ESR was measured at room temperatures. These methods were applied to the following photochemical reaction systems: CH₄-H₂O and ii) CH₄-NH₃-H₂O which are interesting as the methane activation processes and are known to produce 0- and N-containing compounds, respectively. In the systemi), methoxyl radical was detected both in the liquid- and solid-phase trappings. The good correlation between the radical trapping and methanol production strongly suggests the origin of methanol from this radical. Based on the product variation in the photolysis and in H-abstraction and also on the temperature dependence of CH₂O. trapping (Fig.3), we concluded that CH₃O. was mainly produced by the photolysis of methanol. In the system ii), by liquid-phase trapping, either or both of CH₂NH₂, and CH₃O. were detected depending on the relative partial pressure of ammonia to methane (P_A/P_M) (Fig.6). The solid-phase trapping yielded a similar result except that one more radical was trapped (Fig.7). Taking into account of the production of CH₃NH₂ and NH₂C₂H₄NH₂ under different conditions and the reactivity of these compounds for the photolysis and the radical attack, CH_, NH_, was concluded to be from CH₃NH₂ by the radical attacking. Favorable trapping of CH₂O. over. CH₂NH₂ under faster gas circulation (Fig.8) also suggests the different, formation mec hanisms of these radicals.

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Al, Ti, V, Cr, Mn, Fe, Co, Ni and Nb substrates were ablated in vacuum by a focused laser light beam (532 nm). An atomic monocation of each metal was generated, and it was allowed to react with ethylamine, diethylamine, or triethylamine which was injected in a pulsed molecular beam. An ion-molecular reaction occurred between the metal cation and amine. For Al ion, the product ion was a 1: 1 adduct of metal ion and amine. For Ti, V, and Nb ions there was a loss of hydrogen molecules or hydrogen molecules plus etha ne from the 1: 1 adduct ion. For Fe, Co and Ni, loss of ethylene or methane was found in addition. The resulting ions were either 1: 1 adduct ions or the ions which lost a hydrogen atom or molecule for Cr or Mn ion. These results are compared with the existing data on the metal ion-organic molecule reactions in the gas phase. Discussions are made on the possible explanations of the experimental results obtained, in terms of the promotion energies of metal ions to the d^n-2S²or d^n-2sp configurations, and bond dissociation energy D(M⁺-CH₃). The relative reactivity of ethylamine and triethylamine with Ti ⁺ion was compared using their 1: 1 (molar ratio) mixture in the molecular beam.

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Since the unstable molecules of [2], [6], [12] and [17] shown in Scheme 1 are important as intermediate molecules in organic chemical reactions, we have tried to generate these molecules by pyrolysis of 1 H-benzotriazole [1], 5-methyl-[5], 4-methyl-[11], and 5chloro-1 H-benzotriazole [16] and to observe their spectra by mass and microwave spectroscopies.
Although the spectra of the intermediate molecules could not be detected, the spectra of other pyrolysis products have been observed in gas phase.
We were able to determine the molecular structu res of 1, 3 - and 1, 4 -cyclopentadiene-1carbonitrile ([ 3] and [4 ]), 4-methyl- and 3-methyl-1, 3, -cyclopentadiene-1-carbonitrile ([7] and [8] ), 2-methyl-1, 3- and 2-methyl-1, 4 -cyclopenta diene-l-carbonitrile ( [13] and [15]), and 4-chloro-1, 3 -cyclopentadiene-1-carbonitrile0 9] generated by the pyrol ysis (600°C) of precursors of [1], [5], [11], and [16], respectively. We were also able to detect the deuterated species of[3 ], [7] and [13) generated by the pyrolysis of the deuterated derivatives (>ND) of [1], [ 5 ) and [1], respectively.
At higher temperature (800°C), the spectra of [3], [14], and benzonitrile C 10) produced by the pyrolysis of the methyl derivatives (5J and Cu])l were observed in the gas phase. In the chloro derivative C16], the spectra of[3J and [4] were detected but the specrum of CM) could not.
It was con cludedt hat i) some unstable molecules ( [4 ] [7] and [15)) were detected for the first time in the flow system as shown in Table 9; ii) the products of the pyrolysis of 4- and 5-methyl derivatives are different at low temperature (600°C); iii) the detection of benzonitrile at higher temperature (8O0°C) suggests that the carbon atom of the methyl group for methyl derivatives ( [5] and [1]) is taken into the five-memberedr ing a nd a sixmembered ring is produced; iv) the identification of the deuterated species allowed us to clarify the behavior of the hydrogen atom for NH groups of the precursors of [1], [5]and [1] shown in Scheme 1.

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Cationization and anionization of organic molecules and alkali halide salts have been studied using fast atom bombardment mass spectrometry (FAB) and tandem mass spectrometry (TMS or MS/MS). The order of the X- affinity to G, G H + A, and AX was determined by MS/MS approach, where G, A, and X represent glycerol, an alkali metal, and a halogen, respectively. This method utilizes the gas-phase fragmentation behavior of some asy mmetric cluster ions, M1 X- M2, where M1 and M2 are the species with the higher X- affinity yielding the most predominant fragment ion M1. X- in the MS/MS spectrum. The Xaffinity orders of sixteen alkali halides are divided into three classes. This classification is correlated with the ionic radii ratio (rx-fr, +). These ratios for classes (1), (2), and (3) are 2.2 89-0.923, 0.7 83, and 0.717-, --0, 6 57, respectively. The present results show for the fi rst time finding that the ionic radii ratio plays an important role in determining of the order of halide (X-) affinity to molecules and alkali halide salts in the gas phase.

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The gas-phase basicities (GB) of 2-(m, p-substituted phenyl)propenes were determined based on proton-transfer equilibrium method using a pulsed ion cyclotron 'resonance (ICR)mass spectrOmeter. The substituent effect on the GB was correlated linearly by σ⁺ to give a ρ of 13. O. The correlation whichlcovered a wide range of 35 kcal from p-NMe, to 3, 5-(CF³)² group revealed that a+ values determined in solution for most of substituents were applicable to the gas-phase substituent effects without modification accounting for solvation of substituents. This result suggested that the resonance demand (r) of LArSR Eq. ( 1 ) for the gas-phase stability of α-cumyl (1-methyl71-phenylethyl) cation was identi cal with an r of 1.00 for the solvolysis of a-cumyl chlorides.
δGB=ρ(σ°+rδδR+) ( 1 )
The identity of the r value between in the gas phase and in solution was also observed for basicities of a series of benzoyl compounds. The charge delocalization in the aryl ring within a cation will remain essentially the same in the gas phase as in solution. The solvation of cation reduces the central charge to lower the response to substituent perturbation, essentially without change in intramolecular charge delocalization. The present result leads is to the conclusion that the charge delocalization in the SN 1 transition state in the solvolysis should be quite close to that of the carbocation intermediate. The same conclusion was given by the results for a series of relevant benzylic systems.
On the basis of the substituent effect on the gas-phase stability of α-cumyl cation, the substituent effects of gas-phase basicities of benzaldehydes and chloride ion affinities of benzyl cations were analyzed to give r values of 1.00 and 1.29, respectively. The exalted r value of 1.29 for benzyl cation might be attributed to the lower stability than a-cumyl cation. The variation of the r value with the system is in complete agreement with the basic concept of varying resonance demand, supporting the validity of the LArSR Eq.

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The gas-phase basicities (GB) were determined for a series of styrenes based on protontransfer equilibrium method using a pulsed ion cyclotron resonance (ICR) mass spectrometer. The LArSR analysis of the substituent effect on the GB gave an r of 1.14 and a p of 13.6
δGB=ρ(σ°+rδδR+) ( 1 )
The r value higher than unity\ indicated clearly a greater resonance demand for 1-phenylethyl cation than that for a-cumyl cation while the p value was comparable to that of the latter. The exalted resonance demand might be attributed to the lower stability of 1-phenylethyl cation than a-cumyl cation. This trend in the r value is consistent with the observations for the relevant benzylic carbocations. Most importantly, the r value for gas-phase stability of the 1-phenylethyl cation has been found to be identical in magnitude with the r value for the solvolysis of the corresponding substrates (r=1.15). The same is generally true for a series of benzylic carbocations. The identity of th e r value between the SN 1 solvolysis and gas-phase carbocation stability suggests that the intramolecular charge delocalization in the transition state should be quite close to that of the carbocation intermediate. It is concluded that the r value of 1.15 for the 1-phenylethyl solvolysis must be intrinsic resonance demand characteristic of the parent 1-phenylethylc ation i tself. The LArSR correlationw ith r = 1.15 for the solvolysisi n aq. ethanol and aq. acetone suggests a mechanistic involvement of solvent nucleophilic assistance for substrates less reactive than m-cyano derivative.

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Excitation and ionization processes of CS radicals by electro n impact have been studied in the energy range of 10-300eV by observing UV and visible emissions from excited products. The CS(X1∑+) radicals were generated by a microwave discharge of a mixture of CS5 and Ar or He. The spectra consisted of the CS+(B2∑+-A2II) and CS(A1II-X1∑+υ=0-2) systems. The emission cross section of CS+(B-A) was smaller t han that of CS (A-X) by a factor of about 8. The CS+(B) and CS(A) vibrational populations in the CS2/He mixture were in reasonable agreement with Franck-Condon factors (FCFs) for the C S(X: υ=0)→CS+(B, υ') ionization and the CS(X, υ"=0)→ CS(A, υ') excitation. On the other hand, the CS(B) and CS(A) vibrational populations in the CS2/Ar mixture were diff erent from those predicted from the above FCFs. These deviations could be explained by taking account of a small amount of vibrational excited CS radicals (Ny=1Nυ=0=0.15) in th e CS (X) radical.

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The triplet state of 4'-methoxycarbonyl-2, 4, 6 -triis opropylbenzophenone (TIB-4'-CO-OCH₃)has been studied by pica- and nanosecond laser spectroscopy. By picoseco nd laser photolysis in benzene at room temperature, the characteristic band (λmax=500nm) of triplet-triplet (r_e-T₁) absorption of flB-4'-CO-OCH₃ shifts to blue by 320c m_-1 during-7 00 p s, This is interpreted in terms of an intramolecular rotational relaxation in the lowest triplet (T₁) state. Nanosecond laser photolysis in lrnzene at room temperature has revealed the existe nce of an intramolecular excitation energy transfer betwe9 two carbonyl groups of TIB-4'-CO-OCH₃. Based on these results, the reaction kinetics are discussed.

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Near-infrared transient absorption spectra of the charge-resonace (CR) bands of pyrene (Py)dimer cation radicals have been measured by nanosecond laser photolysis using InAs photodiode as an IR detector (Fig.2). The CR band of intermolecular Py dimer cation radical has a peak at 1450 nm in acetonitrile at 298 K (Fig.3), whereas that of intramolecular dimer cation of 1, 3-bis(1-pyrenyl)propane (PyC₃) has a peak at 1600 nm (Fig.4). The conformations of the PyC₃ cation radical are restricted to two forms: one is a fully overlapped "eclipsed"form and the other is partially overlapped “staggeKed” one (Fig.5). On the other hand, the intermolecular Py dimer cation radical may exist as the most stable arrangement of the t wo Py chromophores. Assuming that -δH of the dimer cation radical formation is half of the energy of the CR band peak position, the -41-1 values of inter- and intramolecular dimer cation radicals are determined 9.9 and 9.0 kcal/mol, respectively: the dimer cation radical formed by two intermolecular Py chromophores is more stable (0.9 kcal/mol) than that of intramolecula'r one formed in PyC₃. This suggests that the conformation of intermolecular Py dimer cation radical should not be the eclipsed one, but may be the somewhat distorted one. The stability of intramolecular PyC₃ dimer cation radical has been demonstrated by the cation radical transfer experiments (Figs 7, 8). The hole of Py cation radical is transferred to triethylamine, which is a stronger donor, by the rate constant of 6.5 x 10⁹M^-1⋅s-1whereas the transfer rate from PyC₈ cation radical to triethylamine is 2.7 x10³M^-1⋅^s-1 The decrease of the rate constants is due to the stabilization of the dimer cation radical.

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The photoamination of phenanthrene [ 1 ] with propylam ine in the presence of p-dicyanobenzene (DCNB) was investigated in solvents with relatively low polarity such as 1, 2-dimethoxyethane, tetrahydrofuran, and 1, 4-dioxane. The photoamination in the presence of tetrabutylammonium tetrafluoroborate occurred to give 9-propylamino-9, 10-dihydrophenanthrene [ 2], while photoamination in the absence of the salt did not occur at all. Since [ 1 ]-DCNB exciplex (EX) emissions were observed in these solvents and the EX emis sions were quenched efficiently with the salt, we assume the mechanism for the photoamination via salt-induced charge separation of EX into ion radicals. From kinetic analysis using p-dimethoxybenzene as cation quencher, the lifetime of ion radicals and rate constants for nucleophilic addition of n-PrNI-12 to [1]^+⋅ in these solvents were determined to be 6-7 ns and 6.3 -41.4 10⁹m ol^-1dm^3s-1 respectively. Since the lifetime of ion radicals is shorter than that of free [ 1 ]^+⋅(ca.30 ns), we proposed that the mechanisms for the photoamination proceed via the formation of salt-separated ion pairs followed by nucleophilic addition of n-PrNH₂.

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The dynamic processes of intramolecular excimer f o rmation in isotactic (mm), syndiotactic (rr), and heterotactic (mr) 6, 8, 10-triphenylpentadecanes (PS 3), which are dias tereomeric trimer model compounds of polystyrene, were investigated by picosecond pul se radiolysis. The observed multicomponent fluorescence decays were found to be explicable by the local motion of conformational change. The results suggest that the rotation of phenyl gro ups around the carbon7carbon bonds of the backbone is not always independent of each other, but rather involves a cooperative motion of backbone chain bonds. The excimer form ation times per adjacent phenyl groups were calculated to be 0.5 ns (meso diad), 7 ns (me so diad), and 25 ns (racemo diad). These were compared with relaxation times of dilute polysty rene solution observed by other measurements. An alkyl end-group effect on intra molecular excimer formation in styrene trimers was also discussed by comparing time constants of styrene trimers with methyl ends, propyl ends, and pentyl ends.

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The emission properties of a carbazole(Cz)-terephthalate(TP) exciplex formed between Cz and TP chromophores in benzene were investigated for the compounds having Cz or TP chromophore (Fig.1), and the effects of neighboring chromophores on the exciplex, in the polymer systems were discussed. The interaction between the neighboring Cz chromophores formed two kinds of exterplex (excited triple complex) one is a partially overlapped exterplex where two Cz chromophores have the same conformation as the second excimer and the other is a sandwich-type one where two Cz chromophores have the same conformation as the sandwich excimer. The partially overlapped and the sandwich-type exterplexes showed the exterplex emission around 510 nm and 560 nm, respectively (Fig.2-( b ), ( c )). The latter was found to be more stable than the former. As for PVCz-DMTP system (Fig.3), the fraction of the partially overlapped exterplex is dominant at room temperature since it is formed easily through a slight conformational change. The fraction of the more stable sandwich-type exterplex increased with increasing temperature: the increase of fraction caused longer wavelength shifts in the exterplex emission (Fig.5-( c )). As for EtCz-PVMTP and EtCz-2, 4-MTP systems (Fig.7), no emission corresponding to the exterplex was observed although the exciplex emission was shifted to a little longer wavelengths. However, the lifetime of the exciplex emission (τ_e) is shorter and the quantum efficiency (qr') of the exciplex emission is smaller than those of EtCz-DMTP system (Table 2). Hence, it is evident that the exciplex is quenched by the interaction with an adjacent TP chromophore. The results for PVMTP are considered to be due to the weak interaction between neighboring TP chromophores.

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The room temperature D₁→D₀ fluorescence spectra of various para-subsiituted benzyl radicals were induced by 248 nm laser photolysis of para-substituted benzyl halides followed by 308 nm laser pulse excitation. The fluorescence lifetimes of p-cyanobenzyl, p-fluorobenzyl, pchlorobenzyl and p-methoxybenzyl in hexane at room temperature were determined to be 58, 200, 81 and 120 ns, respectively. Such relatively long fluorescence lifetimes enabled us to examine the fluorescence quenching by electron-donating diazabicyclo[2.2.2]octane and H-donating 1, 4-cyclohexadiene (CHD). The fluorescence quenching by CHD was found to exhibit a pronounced substituent effect. The quenching rate constant (3.2 x 10⁹ M^-1. ^s-') of the p-cyanobenzyl fluorescence was about three orders of magnitude larger than that (9.4 x 10⁵ M ^-1s-1) of p-methoxybenzyl fluorescence. Completely different Dn. -D, absorpt ion spectra (λmax=295 nm and 340 nm for p-cyanobenzyl and p-methoxybenzyl) were obtained as shown in Fig.5. These results strongly indicate the different assignment (1 A₂ or 2 B₂) to the lowest excited doublet state of p-cyanobenzyl and p-methoxybenzyl. On the other hand in the 248 nm laser photolysis of p-chl orotoluene in hexane at room temperature, the triplet-doublet electronic energy transfer from p-chlorotoluene to p-chlorobenzyl radical was clearly confirmed by the detection of the long-lived D₁→D₀fluorescence due to the presence of p-chlorobenzyl (energy acceptor).

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Ionic associations between aromatic-keton anion radicals and sodium ions have been investigated by pulse radiolysis. In the electron-pulse-irradiated 2-methyltetrahydrofuran solution containing 9-fluorenone and sodium tetraphenylborate(1-), the stepwise associations of ions, free→ion→ion-pair→higher→ion aggregate, were observed. A similar ph e n omenon was also observed in solutions of benzophenone. The steady state ESR measu r e m ents on the benzophenone anion radicals produced by sodium metal reduction confirmed that th e dimerization of ion pair was forbidden in the presence of 18-crown-6. Referring the stead y state optical absorption spectra of this solution, the transient absorption spectrum with λ_max at ca.730 nm observed in the pulsed solution containing benzophenone and sodium te t r a phenylborate(1-) was assigned to monomeric benzophenone anion radical-Na+ ion-pair.1

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Employing UV nano- and millisecond flash photolysis, the dynamics of the photodephosphorylation of (4-nitrophenylmethyl)phosphonic acid [1 a], [bis(4-nitrophenyl)methyl]phosphonic acid [1b], [tris(4-nitrophenyl)methyl]phosphonic acid [1 c] in aqueous media were investigated. Transient spectrum of[1 a] above pH 6 exhibited a peak at 355 nm, whose, evolution curve was fitted with a first-order function (k=1.5 x 10^{7s -1}), but the p eak was not observed below pH 6.
The transient spectra of [1b] and [1 c ] exhibited the same broad peak of 400-550 nm at various pH's, respectively, although relative absorbance decreased with [H⁺]. These decay kinetics could be readily followed with ms-flash photolysis using MCPD. The decay curve at 355 nm in [1 a] was fitted with second-order function (half-life 60 s), while the decay curves at 400-550 nm in [1b] and [1c] were fitted with first-order functions; the rate constants of 7.0 x 10^{-3 s-1} and 7.3 x 10^{-8 s'} estimated, respectively. From photolysis of [1a], 1, 2-bis(4-nitrophenyl)ethane was obtain ed as a final product, but from [1a] and [1 c], bis(4-nitrophenyl)- and tris(4-nitrophenyl)methane were given, respectively. Primary dephosphorylation products observed in photolysis of [1a], [1b], and [1 c] could be assigned as 4-nitrophenylmethanide, bis-, and tris(4-nitrophenyl)methanides, respectively.

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In relation to the interest in photoinduced electron transfer reactions, the reactions between donor and acceptor molecules in polar solvents have been investigated by an ESR with a super pressure mercury lamp (2=290-390 nm) and a time-resolved ESR (TRESR) with a nitrogen laser (337.1 nm; 10 ns pulse duration; 16 Hz repetition) and the mechanism of the formation of the ion radicals and their electron spin polarization at the initial stage of photolysis has been discussed.
The spin polarized cation radical of the 1, 4 -diazabicyclo[2.2.2]octan (Dabco, [1]) and the xanthone (XT, C 2 D) anion radical were detected by TRESR under UV-laser photolysis in polar solvents (Fig.4). This is direct evidence of a single electron transfer from the Dabco to the phtoexcited xanthone.
The initial appear ance of total emissive spin polarization of both radicals on the flash photolysis shows that the polarization is caused by the action of the triplet mechanism. The transient xanthone of triplet state causes the initial electron transfer reaction,
Addition of electron acceptors such as benzil, p-benzoquinone and tetramethoxy-p-benzoquinone (TMQ, [3]) in the solution has produced respective anion radicals with total emissive spin polarization (Figs 5, 8 and 9). The single electron transfer has occurred from the donor to the acceptor through the photoexcited xanthone. Our experimental results have also shown that the TMQ could be a spin-trapping agent for ESR and a polarized spin trap for TRES R.
In contra st to the photolysis of the benzil (BZL) itself, emissive spin polarized anion radicals of BZL were produced when XT coexisted in the solution (Fig.8). The fact shows that the XT anion radicals are dominantly produced at first, and emissive spin polarized electrons in the XT anion radicals are transferred into the BZL as the next reaction step. The photoexcited XT acts as an electron carrier: the triplet XT is a good electron ac ceptor and the transient anion radical of XT acts as an electron donor,

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The spin polarization transfer mechanism was investigated in the triplet-doublet and doublet-doublet systems by using the time resolved ESR technique. In the triplet-double t system, phenanthrene and anthracene were used as the triplet molecules, and nitroxide radical and 2, 6-di-t-butylphenoxyl radical were used as the doublet species. An spin polarization transfer was observed only when the triplet energy was higher than the lowest excitation energy of the, radicals. Two mechanisms were discussed; one was the Dexter type energy. transfer, and the other was the spin exchange mechanism. The experimental results for the spin polarization conservation were interpreted by the electron exchange mechanism. The spin polarization conservation in the doublet-doublet system was observed in the benzill triethylamineinitroxicle -radical system. The spin polarization transfer between radica ls in the ground states was also explained by the electron exchange mechanism.

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The one-electron redox reactions of carbazole [1], 9-methylcarbazole [2 ], 9-ethylcarbazole [3], and 9-phenylcarbazole [4]have been investigated by cyclic voltamme try (CV) and ESR spectroscopy. The ESR spectra for the anion radicals [1^-⋅], [ 2^-⋅], [3^⋅], and [4-^-⋅] and the cation radicals [^1+⋅], [2⁺], and [3^+⋅] of [1], [2], [3], and [4] have been observed (Tables 2, 3, and 4). The ESR spectra of [1^+⋅] and [2^+⋅] clearly indicate that spin densities are localized mostly on the nitrogen moiety. On the other hand, the spin densities of [1-] and [2-] show the large spin distribution on the two phenyl rings (Tables 2 and 3). The hyperfine splitting constants (hfsc) obtained from their analyses are nicely supported by molecular orbital (MO) calculations of [1^-⋅], [^+⋅], [2^-⋅], [2^+⋅], [3^-⋅], [3^+⋅], and [4^-⋅].
The ion-pairs of lithium, potassium, and cesium with the anion, radial [3d^-⋅]of 9-(et hyld5)carbazole have a negative temperature dependence of the alkali metal hfsc, whereas the sodium ion-pair exhibits a positive one. These temperature dependencies (Table 4, Fig.8)are discussed in terms of the thermodynamics for the ion-pair formation.

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AllyIstannanes recently attract much interest in the field of organic synthesis and photochemistry because of their remarked reactivities toward electrophiles. In the photochemical reactions of some electron acceptable substrates with allylstannanes, photo-induced single electron transfer mechanism has been proposed mainly based on the product analyses. However, few attempts to clarify the mechanism by detecting directly the reactive intermediates have been made so far. We have applied ESR technique to the photochemical and thermal reactions of quinones with allylstannanes. During irradiation, semiquinone radical combined with trialkylstannyl moiety and, allyl radical were observed simultaneously in the case of p-quinones. No radical was detected in the dark. On the other hand, in the case of 9, 10-phenanthraquinone, intense signal of semiquinone radical coordinated with trialkyl. stannyl moiety was observed not only during irradiation but also in the dark. It is doubtless, in the latter case, that ability of the two neighboring carbonyl oxygens to coordinate stanny: moiety is the most important factor for inducing the thermal electron transier.1 he results obtained by ESR disclosed us the features of reactive paramagnetic intermediates and enabled us to make sure the mechanism of these allylations; 1) photo-induced or thermal single electron transfer occurred from allylstannane to quinone.2) Cleavage of the cation radical by assist of the quinone anion radical generated radical pair.3) Combination between semiquinone radical combined with stannyl moiety and allyl radical afforded allylated products. Evidently, regioselectivity of allylation was dependent on the spin densities of radical intermediates.

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The photochemical reactions between benzophenone and several amines, especially aromatic amines, have been studied by Ome-resolved ESR technique in polar and nonpolar solvents at room temperatrue. All the systems studied in this work show the involvement of a benzophenone ketyl radical and/or hydrogen abstracted amine radicals having a net emission pattern by polarization of triplet mechanism both in polar and nonpolar media. This leads to the conclusion that the direct hydrogen atom abstraction takes place efficiently in most of the systems studied here regardless of the polarity of solvents. Only the system of benzophenone/N, N-diethylaniline/acetonitrile suggested the existence of the ionic processes. It is also found that the position of the hydrogen atom of amines, which is abstracted by the excited triplet state of benzophenone, depends upon the kind of amines. A carbon centered radical is formed from tertiary amines and a nitrogen centered radical, is mainly formed from primary and secondary amines.

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CIDNP Spectra and its excitation profile were measured on an acidic CDC1₃ solution of tetraphenylporphine and quinones as a model system of photo-excited charge7separation reaction in the primary process of photosynthesis. Enhanced absorption of ring-H and intense eimssion of CH₃-H due to hydroquinone were observed by the addition of β-carotene. This is probably because β-carotene catalyzes the disproportionation reaction of semiquinone radicals, judging from the characteristic behavior of fast rising-up and slow decrease of quinone-H CIDNP signal whcih is very similar to that of hydroquinone-H, and its dependence on the concentration of β-carotene. It was concluded that precursor radical pair of this disproportionation reaction should not be a pair of the same species of semiquinone neutral radical, but that it may be a pair of definitely different species; semiquinone cation radical and anion radical, on the basis of CIDNP net effect and relative magnitude of enhancement factor. The latter might be produced from the former pair by proton transfer through the conjugated polyene system of 9-carotene. V

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New non-destructive methodology to study the return process in the acetolysis of aralkyl benzenesulfonate was developed based upon "C-NMR determination of oxygen-181abel scrambling within the sulfonate moiety. The splitting in the ¹³C-NMR signals, induced by isotopic oxygen substitution ¹⁶O/¹⁸O, was utilized successfully to determine quantitatively the extent of ¹⁸O scrambling within three oxygens in the sulfonate moiety.¹³C-enrichment of the target carbon permitted direct determination of the signal intensities in situ. The method was applied to the investigation of return process in the acetolysis of neophyl, 2-phenylpropyl, and 2-arylethyl tosylates. Oxygen exchange process (return process) as well as overall solvolysis process was successfully followed by 13C-NMR spectroscopy using a small amount of doubly labeled starting tosylate in a sealed NMR tube. In the acetolysis of neophyl tosylate, no ¹⁸O-scrambling at the alkoxy oxygen of the starting tosylate was detected at all for whole range of reaction at 75°C (Fig.1). There is no indication of detectable return from the intermediate, affording the first experimental evidence for the absence of internal ion-pair return. The acetolysis of 2-(p-Me0-phenyl)ethyl tosylate gives the rearranged tosylates with complete randomization of the three oxygens (Fig.7). The acetolyses of 2-phenylpropyl and 2-phenylethyl tosylates also give the rearranged tosylates of two signals, accompanying the incomplete oxygen equilibration (Figs.4 and 7). The different scrambling behaviors for substituted phenylethyl tosylates may be attributable to the nature of the ion-pairs, solvent-separated ion-pair for the p-methoxy and intimate one for the unsubstituted derivatives. In the acetolysis of 2-phenylethyl tosylate, parallel runs of (1-*C)-0Ts* and (2-*C)-0Ts* indicate the equivalence of two methylene carbons for "0-incorporation fro m sulfonyl oxygens in the return process (Fig.8). This provides strong evidence that the 180scrambling occurs only after the formation of symmetrical phenonium-ion species. This isotope shift method in NMR spectroscopy can be generally applicable to 180 equilibration kinetics of solvolysis in situ and has the advantage as non-destructive method compared to ordinary degradative method using mass spectrometric assay.

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The present paper details the method and the apparatus of the time-resolved microwave dielectric absorption technique developed for measurement of dipole moments of photochemical transients produced in solutions. The potential of the technique is illustrated by results for the lowest triplet states of some aromatic ketones (9-fluorenone, anthrone, benzil, Michler's ketone, and 4, 4'-dichlorobenzophenone). A polar molecule (ketone in the present case) dissolved in a nonpolar solvent (benzene) is photolyzed by laser pulses. A time variation of the signal which is proportional to the change of square of dipole moment for the transient relative to the ground state was detected using an X-band microwave circuit with a resonant cavity which produces reflected microwaves due to dielectric loss caused by a change of the dipole moment of the solute in the cavity. For the determination of the absolute value of dipole moment of the transient the amplitude of the detected signal was compared with that for a reference compound (diphenylcyclopropenone) for which the change of dipole moment upon photolysis is well known. The lowest triplet states of fluorenone, anthrone, and Michler's ketone have larger dipole moments than the corresponding ground states, while those for benz iland 4, 4'-dichlorobenzophenone show a marked decrease in their dipole moments, especially in benzil the value decreases down to zero. All the results except for anthrone are consitent with their characters of the lowest triplet state suggested previously (7r7r* for fluorenone, charge-transfer state for Michler's ketone, and ne for benzil and 4, 4'dichlorobenzophenone). In the case of anthrone the present result implies that the lowest triplet is 7r 7r*, while most previous results have shown that it is ne. The reason for the discrepacy is not clear. Other possible applications and limitations of this technique are suggested and discussed.

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Fragmentation of a radical cation produced upon electron ionization is accounted for on the basis of charge localization concept and of Stevenson's rule. When the radical cation produced thermally or photochemically in a solution fragments in a way similar to that occurred in mass spectrometry, we would like to call that fragmentation reaction “mass spectrometry mimetic reaction”. The radical cation of N, N-dimethyl-2, 2-diphenylethylamine [1] produced thermally or photochemically in acetonitrile were found to decompose as a typical mass spectrometry 'mimetic reaction. Treatment of [1] with tris(p-bromopheny1)amminium hexachloroantimonate gave diphenylmethylchloride, while irradiation of a solution of C I'D in acetonitrile in the presence of 9, 10-dicyanoanthracene at 70°Cgave 1, 1, 2, 2tetraphenylethane in a high yield. The formation of these products is accounted for on the basis of the charge localization concept; the unshared electron localized on the nitrogen triggers the 9-C-C bond cleavage. This fragmentation is in accord with the mass spectrum of [ 1] upon ionization, which exhibits no molecular ion and the base peak, at m/z 58(H₂C=N⁺Me₂).

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Pinacol rearrangement is one of the typical reactions via carbonium ion. The molecular structure and stability of intermediate species of pinacol rearrangement are examined with ab initio molecular orbital theory. The stability of β-hydroxy carbonium ion depends on the substituents and the migrating group, while 2-hydroxyethyl cation[11]is known to be unstable and to automatically isomerize to protonated acetaldehyde[12]. By carrying out the geometry optimization for various methyl substituted p-hydroxy carbonium ions, we have found that both methyl group and hydrogen atom spontaneously migrate to form protonated aldehyde or ketone for primary cation. The secondary carbonium ion is also found to rearrange with a very low energy barrier. Although β-hydroxy tertiary carbonium ion is possible to locate at an energy minimum on the potential energy surface, the present calculation suggests that the carbonium ion is not necessary to be formed as an intermediate of the pinacol rearrangement.

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In water containing β-cyclodextrin, guest compoun ds 2-naphthylalkanols exhibit, in addition to monomer emission, excimer emission whose emission λ_max shifts to the shorter wavelength with increasing branching in the hydroxyalkyl group of the guest molecule, i. e., 400 nm (2-naphthylmethanol), 385 nm (1-(2-naphthyl) ethanol), and 365 nm (2-(2-naphthyl)2-propanol). The depencence of monomer and excimer emission intensities on β-cyclodextrin concentration revealed the initial formation of 1: 1 inclusion compounds and subsequent dimerization to 2: 2 inclusion compounds, the latter leading to formation of excimers upon excitation. The increased steric crowding in the β-cyclodextrin cavity of the 2: 2 inclusion compound appears to prevent entrapped grest molecules from attaining stable sandwich-type excimer configuration, resulting in the observed shift of excimer emissionλ_max

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Laser flash photolysis of (CH₃)₂Sn[Mn(CO)₅]₂ [1] was studied in cyclohexane solution using 355 nm laser flash light for excitation. One transient having absorption maximum at 480 nm with a long life time (τ_1/2-0.5 s) was observed. No evidence for Sn-Mn bond photoeleavage was observed. The effect of CO on the decay kinetics of the transient as well as no inhibiting effect of CCl₄ for formation of Mn₂(CO₃)₁₀[2] showed the non-radical mechanism for formation of [2] from [1].

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The change in the electronic spectra for the reaction of 2, 4, 6-trinitroanisole with several amines (butylamine, pyrrolidine, piperidine, and diethylarnine) in dimethyl sulfoxide has been measured with the rapid scan spectroscopy. The 1, 3-σ complex M^-, which is formed at the initial stage of the reaction, is gradually conveted into the intermediate 1, 1-a complex I^-, giving the final product. As shown in Fig.1, distinct spectra of the 1, 3complex M_-and the intermediate 1, 1-complex I^- are observed. Spectral data for the a complexes observed are presented in Table 1.

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Short-lived phosphorus-containing inorganic anion radicals (PO₃^2-⋅and HP0₂^-⋅) which are produced by the one-electron oxidation of the corresponding parent ions with hydroxyl radical (OH) or CO⁴⁺ ion were trapped by water-soluble spin traps, α-(4-pyridyl)-N-tbutylnitrone N-oxide (POBN) and 3, 5-dibromo-4-nitrosobenzenesulfonate (DBNBS), as confirmed by electron spin resonance (ESR) spectroscopy. When 5, 5-dimethyl-1-pyrroline Noxide (DMPO) was used as another spin-trap, however, the oxidation product of DMPO, 2, 2-dimethyl-(5-oxo-1)-pyrrolidinyloxyl (DMPDX) was detected instead of the DMPO-phosphoruscontaining inorganic anion radical adduct, probably because of the instability of the latter adduct.

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The structure and dynamics of the intermediate in the photooxidation of isobutene by nitrogen dioxide excited at visible light (particularly 610-570 nm) were studied by lowtemperature matrix-isolation FTIR spectroscopy and discussed with those for other butenes. The reaction intermediate showed the most intense bands at 1644 cm^-1 and 754 cm^-1, which seem to be N=0 stretching and O-N stretching bands, respectively, of isobutyl nitrite radical (Fig.2; Ii). The first order rate constants, obtained from an analysis of time dependence of the infrared aborption (Fig.3) are compared with those for other butenes in Table 3.
The reaction mechanism for butenes is summarized in Fig.4. Complete retention of stereochemistry was observed upon production of the intermediate radicals because of mildness of the photooxidation by NO₂(A²B₂): trans-72-Butene gave trans-s-butyl nitrite radical, cis2-butene gave cis-s--butyl nitrite radical, and isobutene gave isobutyl nitrite radical. Each butyl nitrite radical absorbed a second visible photon, eliminated NO, and gave individual oxides: trans-s-Butyl nitrite radical gave trans-1, 2-dimethyloxirane, cis-s-butyl nitrite radical gave 2-methylptopanal caused by methyl group migration, and isobutyl nitrite radical gave 1, 1-dimethyloxirane and 2-methylpropanal caused by hydrogen atom migras tion.

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In frared spectra of the less stable conformer of isoprene have been observed in lowtemperature argon matrices (Figs 1-3). Irradiation of the matrix-isolated isoprene with a low-pressure Hg lamp increases the population of the less stable conformer. Deposition of an isoprene/argon mixture heated through a high temperature nozzle exhibits a similar effect.
The less stable conformer has been identified as the gauche form by comparing the observed spectra with the results of normal-coordinate calculations previously reported by Bock et al. Most of the observed bands in the region below 1700 cm^-1 have been reasonably assigned to the calculated modes of either the trans or the gauche form (Table 1).

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It has been found that specific species of hydrocarbon radicals can be purely trapped in a canal of thiourea by annealing the irradiated parent thiourea-hydrocarbon complexes at a certain temperature for many kinds of guests of hydrocarbons. Thiourea originating radical or trapped electrons near the sulfur atom, the object radical aimed at the study, and the other hydrocarbon radicals are produced in the complexes by 7-irradiation. Thiourea orginating radical or trapped electron decays and the other radicals are converted into the object radicals by annealing. Alkyl radicals decay through an intermolecular radical migration along the canal by annealing at higher temperatures, but ally' radicals decay at the decomposition temperatures of the host complexes.
This matrix isolation method h as been applied to an ESR study of the structure and motions of 1-methylcyclohexyl radicals. Temperature dependent ESR spectra have been observed for the 1-methylcyclohexyl radical in the 7-irradiated thiourea-methylcyclohexane canal complex at temperatures between 77 and 269 K. The radical has two pyramidal conformations, one is stable and the other is unstable in a canal at low temperatures. At temperatures from 77 to 172 K, the population ratio of the radicals increases from about O.45 to around 1. O. Between 172 and 210 K the ESR spectra have shown a selective line broadening caused by an umbrella inversion between the two pyramidal conformations, and above 210 K there has been observed another selective line broadening which indicates the occurence of a ring inversion.

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Flash Vacuum Pyrolysis(FVP) of o-(methoxycarbonyl)phenyldiazomethane yielded a mixture of 3-methylphthalide and 2-methoxy-1(2 H)-benzocyclobutenone. The formation of these cornpounds was rationalized by the formation of carbonyl ylide derived from the intramolecular interaction of the carbenic center with the carbonyl moiety. The different product ratio was observed on FVP of m- and p-(methoxycarbonyl)phenyldiazomethanes. This discrepancy was discussed on the ground of the interaction of the diazo carbon with the methoxycarbonyl function. The formation of the carbonyl ylide and its reversible isomerization to the oxirane were directly observed by IR spectroscopy in the photolysis of o-(methoxycarbony1)phenyldiazomethane in Ar matrix at 10 K. On the other hand, in the case of p-isomer the methoxycarbonyl function remained intact in the course of photoreactions at 10 K.

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Isolation of primary peroxidic intermediates has be en achieved by photochemical reaction in a cryogenic oxygen matrix. The ultraviolet photoly sis of 2, 2-dimesityl-1, 1, 1, 3, 3, 3-hexamethyltrisilane in a cryogenic oxygen matrix at 16 K has led to the first spectroscopic detection of a silylene-oxygen adduct. Isotopic labeling experiments using 1802 and 160180 provided confirmative evidence for the silanone 0-oxide structrure of the adduct. The theoretical study of products of reaction of dihydrosilylene with oxygen was also carried out. The infrared frequencies of a silanone 0-oxide intermediate and the effect on the frequencies by substituting either one or both oxygens in a silanone 0-oxide by 180 were calculated. The calculated values agree with experimental data. The UV-visible absorption spectra of diphenyl sulfide, methyl phenyl sulfide, and 3-thiatricyclo[3.2.1.02 ']octane, in an oxygen matrix at 13 K reveal a contact chargetransfer (CCT) band with the broad maximum at around 300 nm. Upon irradiation exciting CCT bands with a UV light (3b0-400 nm), the first observation of the matrix-isolated persulfoxides was achieved by FT-IR spectroscopy. The isotopic labeling experiments using '802 and 160180 priveded the confirmative evidence for the persulfoxides.

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Photolysis of sterically protected cyclopolysulfides [4], [5], and [6] in the presence of a reactive olefin resulted in a facile formation of the corresponding [4 +2]cycloadducts of 1, 2-dithione[14] and 2-selenoxo-1-thione[15].1, 2, 3-Trithiole[8] and 1, 2, 3-triselenole[9]were also photolyzed in the presence of norbornene to give the adducts of intermediary 1, 2dithione[16] and 1, 2-diselone[17]. The photolysis of 1, 2, 3-thiadiselenole[10]under similar reaction conditions gave the cycloadduct of 2-selenoxo-1-thione[25] together with the adduct of 1, 2-diselone[19]. The formation of [25] implies the competition of deselenation with desulfurization in consequence of the initial ring-opening step. Monitoring of the photol ysis of [10] in low temperature argon matrix using UV spectroscopy showed some characteristic absorption maxima which were not identical to the spectrum obrained from the photodecarbonylation of [41] under similar reaction conditions(Fig.1). These results suggest that the photolysis in the matrix has generated a primary spirothiaselenirane intermediate[28], the intermediacy of which is in good agreement with the competitive dechalcogenation in the photolysis of C[10].

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The tetraphenylporphyrins and their Zn complexes carrying one to four α-diazobenzyl groups one each at the p-position of the phenyl groups were prepared. The corresponding carbenes were generated by photolyses in MTHF at cryogenic temperatures and studied by uv/vis absorption and ESR spectroscopy. Both syn and anti regioisomers of the dicarbene s showed ESR fine structures at 270 and 400 mT characteristic of quintet species. Temperature dependences of the signal intensities in the range 10-50 K showed, however, that only the syn isomer followed the Curie law and therefore has the ground quintet state. The quintet state of the anti isomer is thermally populated and estimated from the temperature dependence to lie less than ca.54 calimol (=19 cm^-1) above the ground singlet state. It is concluded that two carbene units can interact magnetically with each other through pi conjugation of the porphyrin ring. The interaction can become ferro- or antiferromagnetic depending on the position of the carbene units in the 18 pi electron system of the porphyrin ring.

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Time-resolved ESR spectrum was observed for the triplet enol generated by intramolecular proton transfer in the excited singlet state of o-aminoacetophenone at 77 K. The phase of the polarization and the zero field splitting (ZFS) parameters suggested the formation of the corresponding enol in accordance with previous results. Polarized ESR spectrum due to the excited triplet state was also obtained under laser pulse irradiation to p-aminoacetophenone.

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Degassed dilute solutions (10^-3-10^-4 M) of dialkylbis(phenylseleno)germanes (alkyl, : ethyl, propyl and butyl) in 3-methylpentane were irradiated with low-pressure Hg lamp (253.7 nm)at 77 K and their UV spectra were observed. In each case, the UV spectrum exhibited the maximum absorption at 425 nm, which was tentatively assigned to the dialkylgermylene according to our previous report on dimethylgermylene, together with three minor bands at 330 and 490 nm with a shoulder at 380 nm.

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Triphenyl-substituted group 4 B element-centered radicals, generated from the corresponding hydrides by irradiation with a low-pressure Hg arc lamp, were examined by UV and ESR at 77 K. The group 4 B element-centered radicals showed absorption peaks at 325-330 nm and molar extinction coefficients of ca.10³-10⁴. The group 4 B element substituted cyclohexadienyl type radicals were also generated by irradiation of the hydrides.

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An ESR study has been carried out to elucidate the mechanism of radical formation from butene adsorbed on H-mordenites. The mordenites contain Fe²⁺ and Fe³⁺ as impurity ions. The Fe²⁺ ions are oxidized to give Fe³⁺ions after dehydration followed by calcination under oxygen at 670 K.2, 3-Dimethy1-2-butene molecules, generated as a by-product of Brφnsted acid-catalyzed raction of butenes, react with mainly Fe+ to afford the 2, 3-dimethy1-2-butene cation radicals. The cation radicals further react with the 2, 3-dimethy1-2-butene molecules to form the dimer cation radicals. The dimer cation radicals are slowly converted into 1, 1, 2-trimethylallyl radicals. It was concluded that the radical reaction smake no significant contribution to the catalytic reactions of alkenes on H-mordenites.

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Effects of magnetic field on the photoinduced electron-transfer reactions fro mamphiphilic zinc porphinate (ZnC₁₆PyP) located at the surface of various molecular assemblies to viologens in the aqueous phase were investigated by a laser photolysis.
In the case of positively charged DHAC bilayer, the transient absorption due to the radical pairs, as generated by electron transfer from ³ZnC₁₆PyP to zwitterionic viologen (λmax, 600nm), clearly increased in the presence of external magnetic fields (Fig.2). No magnetic field effects were detected when zwitterionic viologen was replaced by either mono- or dicationic viologens (Fig.1). Remarkably large magnetic field effects were observed with short living radical pairs(τ<10 μs) when viologens were either electronically bound to the negatively charged surface of DHP bilayer (Fig.3) or enclosed in the water pool of AOT reversed micelles (Fig.5). The decay of the short living radical pairs followed the first order reaction kinetics and the rate constants rapidly decreased with the magnetic field and reached a constant value at above 0.2 T. The magnetic field effects were reasonably explained as due to the retardation of intersystem crossing of the triplet radical pair on Zeeman splitting of sublevels at the high magnetic field (spin relaxation mechanism).
In the case of AOT/isooctane reversed m icelles, both the decay rates of radical pairs and the magnetic field effects were strongly suppressed as the water pool size increased (Fig.8). The reason was ascribed to the decrease of electron spin interaction with the increase of inter radical distance and the diffusion controlled process was found to dominate over the spin relaxation mechanism in large water pools.

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