Collision-Induced Dissociation Study of Strong Hydrogen-Bonded Cluster Ions Y − (HF) n (Y = F, O 2 ) Using Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry Combined with a HF Generator

Hydrogen fluoride (HF) was produced by a homemade HF generator in order to investigate the properties of strong hydrogen-bonded clusters such as (HF) n . The HF molecules were ionized in the form of complex ions associated with the negative core ions Y − produced by atmospheric pressure corona discharge ionization (APCDI). The use of APCDI in combination with the homemade HF generator led to the formation of negative-ion HF clusters Y − (HF) n (Y = F, O 2 ), where larger clusters with n ≥ 4 were not detected. The mecha-nisms for the formation of the HF, F − (HF) n , and O 2 − (HF) n species were discussed from the standpoints of the HF generator and APCDI MS. By performing energy-resolved collision-induced dissociation (CID) experiments on the cluster ions F − (HF) n ( n = 1–3), the energies for the loss of HF from F − (HF) 3 , F − (HF) 2 , and F − (HF) were evaluated to be 1 eV or lower, 1 eV or higher, and 2 eV, respectively, on the basis of their center-of-mass energy ( E CM ). These E CM values were consistent with the values of 0.995, 1.308, and 2.048 eV, respectively, obtained by ab initio calculations. The stability of [O 2 (HF) n ] − ( n = 1–4) was discussed on the basis of the bond lengths of O 2 H–F − (HF) n and O 2 − H–F(HF) n obtained by ab initio calculations. The calculations indicated that [O 2 (HF) 4 ] − separated into O 2 H and F − (HF) 3 . and an open the of Attribution which use, distribution, and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.


INTRODUCTION
Mass spectrometry (MS) provides data on the hydrogen bond dissociation characteristics of complex gaseous ions such as hydrated cluster ions H 3 O + (H 2 O) n and Y − (H 2 O) n (Y=OH, O 2 , CO 2 ), [1][2][3] and it also gives experimental evidence of the thermodynamic stabilities of small clusters. 4,5) Although it is di cult to apply the collision-induced dissociation (CID) method to weakly bound hydrogen-bonded systems, it is possible to use it to examine strong hydrogenbonded systems such as the uoride ion complex F − (HF). 6) A strong-hydrogen bonding (SHB) can be distinguished from a weak-hydrogen bonding (WHB) by means of the bond energy (WHB <30 kJ/mol, SHB >50 or 100 kJ/mol). 7) SHB is often indicated as A-H-B or A⋯H⋯B, while normal or WHB is expressed by A-H⋯B. Another method for representing WHB and SHB is based on the use of the potential pro le, i.e., WHB and SHB are denoted by a symmetric or asym-metric double-minimum potential with a high energy barrier and a symmetric double-or single-minimum potential with a low energy barrier, respectively. 7) Hydrogen uoride (HF), its clusters (HF) n , and related chemicals have attracted both theoretical and experimental interest as hydrogenbonded systems owing to their strong binding energies, i.e., 150-240 and 160-230 kJ/mol from the thermochemical and theoretical data, respectively, of F − -(HF) X (X=N + Me 4 and alkali-metal cations), 7) in contrast to the WHB energy of the neutral cluster HF-HF that is 25.09 kJ/mol. 8) e HF cluster negative ions have been studied from the standpoints of the electron and hydrogen transfer 9) as well as complex formation with heterocyclic organic cations in solution. 10) Wenthold and Squires have estimated the hydrogenbonding energy of a hydrogen uoride anion F − (HF) by means of the center-of-mass energy (E CM ) that was determined by low-energy CID experiments. 6) Here, in order to study the properties of strong hydrogen-bonded systems, we focused on the CID analysis of hydrogen uoride cluster anions of the type F − (HF) n . e energy for the loss of HF from F − (HF) n (n=1-3) was estimated on the basis of the E CM energy. In order to produce HF clusters, a generator of hydrogen atoms H · and HF molecules was assembled and the resulting HF molecules were ionized as complexes associated with the negative core ions Y − (Y=F, O 2 ) produced by atmospheric pressure corona discharge ionization (APCDI). e dissociation energy of F − (HF) n estimated from the CID experiments was compared with the energy calculated by ab initio methods combined with density functional theory (DFT). e stability of other complex ions such as O 2 − (HF) n (n=1-4) was discussed on the basis of the results of ab initio calculations.

Generation of hydrogen uoride
Gaseous HF molecules were produced by using a homemade generator for H · /HF species. e H · /HF generator consisted of an ultraviolet (UV) light source incorporating a deuterium lamp D200F (Heraeus, Tokyo, Japan), a reaction tube made of polycarbonate (PC tube), and a polytetra uoroethylene (PTFE) tape, as shown in Fig. 1. e deuterium lamp generated UV photons over a wavelength range of 160-400 nm (3.8-7.7 eV). Upon supplying hydrogen gas (H 2 ) to the reaction tube, the H 2 molecules dissociated into hydrogen atoms H · owing to the lower dissociation energy of the H-H bond (4.5 eV) compared to the UV photon energy (7.7 eV, reaction 1). e UV photons may produce uoride atoms F · from the PTFE tape (reaction 2). Although it is expected that the HF molecules are mainly formed according to reaction 3, another minor reaction 4 may occur following the uorine atom abstraction with H · from the PTFE tape in view of the dissociation energies of C-F (5.07 eV) and H-F (5.89 eV).
n n (4) e resulting HF molecules were transferred using N 2 as carrier gas to the dri region adjusted to a gap length of 3 mm between the tip of the corona needle and the ori ce of the mass spectrometer (Fig. 1).
Atmospheric pressure corona discharge ionization e detailed schematic illustration of the APCDI apparatus used in this study has been described elsewhere. 11) e corona discharge experiments for generating negative atmospheric core ions Y − (Y=O 2 , HO x , NO x , CO x ) 11,12) were performed under ambient laboratory conditions in the presence of nitrogen (N 2 ), oxygen (O 2 ), water vapors (H 2 O), and minor amounts of other species, such as CO 2 and Ar. e laboratory temperature and relative humidity were 298 K and 40-70%, respectively. e corona needle used as point electrode with a tip radius of ca. 1 µm was a headless insect pin (Shiga, Tokyo, Japan), made of stainless steel with a diameter and length of 200 µm and 20 mm, respectively. e discharge gap between the needle tip and the ori ce plate of the mass spectrometer was adjusted to 3 mm with a π/2 rad needle angle with respect to the ori ce plate axis (Fig. 1). A DC voltage of −2.0 kV was applied to the needle relative to the ori ce plate.
Mass spectrometry e mass spectra were obtained with a TSQ7000 triplequadrupole mass spectrometer ( ermo Fisher Scienti c, San Jose, CA, USA). e resulting ions were introduced into an ori ce hole with a diameter and length of 320 µm and 114 mm, respectively. e ori ce was heated at 70°C to prevent the generation of large water cluster ions. e ions introduced into the ori ce hole were focused onto a skimmer opening by the tube lens, and they were then transported to the ion guide. e voltages applied to the skimmer and tube lens were 0 and 67.8 V, respectively. e applied rf voltage on the ion guide was 3 V. e transported ions were accelerated to 20 V at the focusing lens electrode. e assignment of the negative ion species generated by the corona discharge and the ion/molecule reactions in the atmospheric pressure dri region (3 mm) between the needle tip and the ori ce plate was performed by using CID experiments. us, the precursor ion selected by the rst quadrupole (Q1) was injected into the rf-only second quadrupole (Q2) collision cell, and the product ions were mass-analyzed by the third quadrupole (Q3). e target collision gas and laboratory frame collision energy (E lab ) used were argon at 2.2×10 −3 Torr and 1-25 eV, respectively. For the energy-resolved CID experiments, the laboratory frame axial collision energy (E lab ) was set by the Q2-rod o set voltage, and then the E CM was determined according to the equation where m t and m p represent the masses of the target gas and precursor ion, respectively.

Ab initio calculations
All calculations were performed using ab initio methods combined with the DFT unrestricted B3LYP 13,14) level of theory and 6-311++G(2d,p) basis set in the Gaussian 09 15) suite of programs. e initial structures of HF, F − (HF) n , and O 2 − (HF) n were generated using CS Chem3D Ultra (Cambridge So , Cambridge, MA, USA). e input le was minimized under a semi-empirical MO (PM7) run through a Winmoster interface using MOPAC2012.

RESULTS AND DISCUSSION
Formation of hydrogen uoride anion clusters F − (HF) n using APCDI MS e negative-ion APCDI mass spectra of the background in the absence of HF supply and HF molecules associated with the preformed core ions are shown in Fig. 2. e background spectrum (Fig. 2a) exhibited peaks corresponding to the negative atmospheric core ions Y − (Y=O 2 , CO 3 , CO 4 , HCO 3 , and HCO 4 ) and those of the water clusters Y − (H 2 O) n , 11) although the source of the carbon atoms of CO x and HCO x ions has not been yet clari ed. e spectrum obtained in the presence of the HF supply (Fig. 2b) displayed peaks corresponding to the HF clusters F − (HF) n (n=2, 3) at m/z 59 and 79, respectively, while clusters larger than n=3 could not be observed. A weak peak corresponding to the hydrogen-bonded ion F − (HF) at m/z 39 was also observed.
Other HF cluster ions were observed, such as O 2 − (HF) n (n=1-3), O 2 − [(HF) n +HCOOH] (n=1, 2), and HCO 2 − (HF) n (n=1, 2). From the viewpoint of the stability of cluster ions, it is worth mentioning that F − (HF) n and O 2 − (HF) n with n≥4 were not detected, as shown in Fig. 2b. e HF clusters associated with ions Y − and/or neutral byproducts B (such as H 2 CO 3 , HCO 2 , HO 2 , and HNO 3 ) 12) can be formed according to the gas-phase reactions (5) and (6) in the dri regions and/or adiabatic expansion cooling in the ori ce vacuum region. 12) Considering the hydrogen-bonding energies of 0.141 and 0.211 eV calculated for the neutral clusters HF-HF and HF-(HF) 2 , respectively, it is reasonable to assume that the HF molecules form hydrogen-bonded cluster (HF) n .
Regarding the formation of uoride anion clusters F − (HF) n , it may be possibly due to the proton abstraction by the negative ions Y − from the HF molecules. On the basis of the strong proton a nity of superoxide O 2 − (PA=15.3 eV), 16) higher stability of the cluster ion F − (HF) n than (HF) n+1 and (HF) n − , 9) and thermochemical data (16.12 eV for HF→F − +H + , 17) 1.99 eV for F − (HF)→F − +HF 6) ), the overall process (7a and 7b) may occur as an exothermic reaction to

Estimation of the energy for the loss of HF from the anion clusters F − (HF) n
e CID spectra of the cluster ions F − (HF) n (n=1-3) obtained with laboratory frame collision energies (E lab ) of 5 and 15 eV, are shown in Fig. 3. All the spectra showed the peaks relative to the product ions F − at m/z 19, F − (HF) at m/z 39, and/or F − (HF) 2 at m/z 59 originated from the loss of neutral HF from the precursor ions F − (HF) n (n=1-3).
e CID spectra of the cluster ions F − (HF) and F − (HF) 2 obtained with a 15 eV/E lab exhibited a peak corresponding to the uoride anion F − , while those with 5 eV/E lab did not show such peak (Figs. 3a and 3b). e peak corresponding to the uoride ion F − was also absent in the CID spectrum of F − (HF) 3 (Fig. 3c), suggesting an insu cient internal energy of the precursor ion F − (HF) 3 . With regard to this, the internal energy deposited in the precursor ions can be roughly estimated from the E CM .
In order to estimate the dissociation energy from the E CM of the F − (HF) n ions (n=1-3), breakdown diagrams were extrapolated from the CID data, as shown in Fig. 4. e breakdown diagram for the hydrogen-bonded cluster F − (HF) indicated that the appearance energy for the F − ion was about 2 eV/E CM or higher (Fig. 4a). e estimated energy was consistent with the E CM value of 1.99 eV reported by Wenthold and Squires. 6) On the other hand, the E CM data for F − (HF) 2 and F − (HF) 3 showed that the rst loss of HF from these precursor ions occurred at 1 eV/E CM or lower to form the product ions F − (HF) and F − (HF) 2 , respectively (Figs. 4b and 4c). Furthermore, it can be seen in Fig. 4c that the second loss of HF from F − (HF) 2 at m/z 59 to form the product ion F − (HF) at m/z 39 occurred at about 1 eV/E CM or higher. e results discussed above indicate that the energy for the loss of HF from F − (HF) n (n=1-3) is dependent on the rst, second, and third loss, as summarized in Table 1. e E CM order for the loss of HF was F − (HF) 3 <F − (HF) 2 <F − (HF).

Ab initio calculations for F − (HF) n (n=1-3)
e structure of the cluster ions F − (HF) n has been examined by low-temperature solid-state infrared (IR) spectroscopy, 18) nuclear magnetic resonance (NMR) in the liquid state, 19) and ab initio calculations under vacuum. 9) e IR solid-state study suggested that the F − (HF) 3 species possesses the highest clustering number n=3. 18) e NMR liquid-state study indicated the presence of a central uoride ion involved in multiple hydrogen bonds of the HF molecules to form F − (HF) n (n=2-4), although the F − (HF) 4 was only tentatively assigned. 19) On the other hand, the ab initio calculations indicated that limited zig-zag chains such as FH⋯F − ⋯HF and FH⋯F − ⋯HF⋯HF are more stable than (HF) 3 − and (HF) 4 − , respectively. 9) With regard to the structure of the cluster F − (HF) 3 , Groenewold et al. reported that the average binding energy of the rst, second, and third step addition of HF to the central F − ion was 1.3 eV, while the binding energy for the third step addition of HF to F − (HF) 2 was 0.82-1.0 eV on the basis of ab initio calculations. 10) e authors emphasized that the further addition of HF to F − (HF) 3 could not be observed in the gas-phase owing to a weak binding energy. Actually, the cluster ion F − (HF) 4 as well as larger clusters could not be detected in the present study. e E CM estimated here (Table 1) were well consistent to the reports of Wenthold and Squires 6) and Groenewold et al., 10) although the E CM just only gives maximum energy contents or rough values.
Herein, we calculated the binding energies of F − -HF, F − (HF)-HF, and F − (HF) 2 -HF by using an ab initio method, as summarized in Table 1. e E CM values estimated from the CID experiments were qualitatively in agreement with the calculated values (Table 1). e H-F and F − -HF bond lengths for the cluster ions F − (HF), F − (HF) 2 , and F − (HF) 3 were calculated for the D ∞h and D 3h symmetries of F − (HF) 2 and F − (HF) 3 , respectively (Scheme 1), as summarized in Table 2. e length of the covalent bond of the neutral H-F decreased with increases of the size of the clusters F − (HF) n , while the length of the hydrogen bond between F − and HF increased with increasing cluster sizes. e results of the binding energy and bond length calculated in this study support the E CM values and the order of the energy for the loss of HF from the cluster ions F − (HF) n , i.e., F − (HF)> F − (HF) 2 >F − (HF) 3 .

Stability of O 2 − (HF) n (n=1-4) based on ab initio calculations
As described above, the APCDI mass spectrum of the HF cluster associated with F − and O 2 − ions did not exhibit any peak corresponding to cluster ions of the Y − (HF) n (Y=F, O 2 ) type larger than n=3 (Fig. 2a), although Groenewold et al. have suggested that F − (HF) 4 could not be observed owing to the weak binding energy of HF to F − (HF) 3 . 10) In order to examine the stability of the superoxide/HF cluster ions O 2 − (HF) n (n=1-4), we calculated the bond length of O 2 − -(HF) n and O 2 − H-F using DFT calculations. e most stable structures of each cluster for n=2 and 3 were zig-zag forms; thus, the bond lengths were calculated on the basis of the zig-zag structures (Scheme 2), and they are summarized in Table 3 Fig. 2b may be due to a less stable structure of the ion, which may be separated into O 2 H and F − (HF) 3 under the present experimental conditions.

CONCLUSION
In order to study the properties of strong hydrogenbonded systems, a homemade generator of HF was assembled and its use was combined with atmospheric pressure corona discharge ionization mass spectrometry. e negative-ion APCDI MS combined with the H · /HF generator allowed the formation of complex cluster ions such as  and F − (HF) 2 at m/z 59, depending on the collision energy and the size of the HF clusters. e E CM values for the loss of HF from F − (HF) 3 , F − (HF) 2 , and F − (HF) ions were 1 eV or lower, 1 eV or higher, and 2 eV, respectively. e E CM energies estimated above were in good agreement with the energy and bond length data obtained by ab initio calculations. e ab initio calculations of the cluster ions O 2 − (HF) n (n=1-4) indicated that the ion [O 2 (HF) 4 ] − possessed the structure O 2 H⋯F − ⋯(HF) 3 , while the absence of the F − (HF) 4 ion peak in Fig. 2b was due to the weak bonding energy of HF to F − (HF) 3 . 10)