Recently there appeared a lot of new findings in researches on molecular crystals. For example, superconductivity and ferromagnetism, which have been thought to be a characteristic nature of metals, have been observed for organic crystals. Moreover, new solid-state reactions caused by only grinding reactant crystals in a mortar have been observed. The crystalline-state reactions, which proceeds retaining the single crystal form, have been found. In order to elucidate the mechanism of these new findings, it is inevitable to make clear dynamic nature of molecules in a crystal. This special issue makes a survey of the recent development in this field.

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Recently we have developed a method, based on a lattice-variable constant-pressure molecular dynamics, to predict the structure of a molecular crystal without any assumptions on the lattice constants and spatial symmetry. This method, coded in the MDCP program, has been applied successfully to CO₂, benzene, pyrimidine, 1, 2-dimethoxyethane, and several molecules showing nonlinear optical properties. In addition, our method with the potential functions derived by an ab initio molecular orbital method predicted correctly the packing structure of CO₂ both at 1 atm and 20 GPa, and reproduced the phase transition observed at around 12 GPa, breaking new ground in predicting crystal structure of a molecule from its chemical composition.

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Application of nuclear magnetic resonance (NMR) spectroscopy for the study of molecular dynamics in solids is reviewed. Effects of molecular motion on the NMR parameters such as motional averaging of the spectra, nuclear spin-lattice relaxation, two-dimensional exchange spectra, and pulsed-field-gradient spin-echo decay are discussed. A case of molecular rotation of C₆₀ and K₃C₆₀ are treated as typical examples.

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Vibrational spectra of molecular crystals offer valuable information on a crystal structure and intermolecular interactions. Several typical examples including our recent results of the humidity-induced phase transition of nucleotides and guanosine are reviewed.

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Recent studies are reviewed of the molecular motion and phase transition in simple molecular solids, putting great emphasis on the thermodynamic properties. Interesting phenomena such as structural phase transition and glass transition occur in molecular crystals concerning not only orientational but also internal twisting degrees of freedom.

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The role of the researches on the X-ray electron densities in the studies of chemical reactions in the crystalline state is discussed. And the application of the X-ray AO analysis to PtP₂ and NiTiO₃ crystals were described in conjunction with the future possible application of the method to the studies of chemical reactions in the crystalline state. In both measurements the effect of multiple diffraction on structure factors was significant. Structure factors were measured avoiding the effect with the Ψ-scan technique in PtP₂, while in the case of NiTiO₃, the structure factors measured in the past were examined and those apparently affected by the effect were discarded. 5d- and 3d-orbital functions of Pt and Ni were deter-mined by the X-ray AO analysis. However the residual density maps implies that better model which explains the electron density in the whole unit cell is necessary. Hence the basic problems inherent in the X-ray MO analysis, which analyze the molecular orbitals from the measured X-ray structure factors, is also discussed.

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Recently we designed and made a new no-screen Weissenberg camera type diffractometer for rapid data collection to determine the structures of reaction intermediates. The new features of the diffractometer are a κ-type goniometer, two imaging plates for recording and reading the intensity data, and spiral motion reading mechanism. The crystal mounted on a goniometer is aligned automatically and the three-dimensional intensity data were collected within two hours. Using this new diffractometer, we determined the structure of a syn-tricyclooctane derivative. The structure has not been analyzed with the four-circle diffratometer because the molecule easily reacts under X-ray irradiation .

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Solid state reaction is strictly controlled by the crystal structure of the reactive molecules. As the reactions occur between the neighboring two or three molecules, their local arrangement is most important. In the polymerization, however, an additional factor is required to complete the reaction; that is, repeated relative arrangement of the neighboring molecules in some direction to form a backbone of the polymer molecule. Since the required condition is very difficult to desigh in the crystal lattice, new development in this field of chemistry is strongly expected.

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The generation of asymmetry in a chiral crystal environment is very interesting in view point of the origin of life as well as organic syntheses. Two subjects are discussed; one is the asymmetric photoisomerization in chiral crystalline field and the other is a racemic-to-chiral transformation in a racemic crystal on exposure to visible light. Since the reactions proceed with retention of the single crystal form, the processes can be easily observed by X-ray analysis and the mechanism is explained on the basis of the reaction cavity which represents the void space around the reactive group in the crystal structure.

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Nitrite ion is a well-known ligand which shows linkage isomerism. The photo-isomerization in the solid state is governed by at least two factors: (a) electronic effect by coexisting ligands, and (b) reaction cavity of the nitro group. Recent X-ray study has revealed that the nitro-nitrito photo isomerization in a crystal is severely governed by intermolecular steric restrictions.

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Studies of phase transitions of organic crystals are briefly introduced with examples taken from our recent works utilizing structure analysis of single crystals, temperature dependent powder diffraction and calorimetry. The first example is the crystals of N, N'-dialkyl-1, 4-diazoniabicyclo [2.2.2] octane dibromides which exhibit the first-order phase transition and showed the halide anion conductivity in the high temperature phase. The second examples are the phase transformations between the polymorphic crystals of the N-picrylaniline derivatives. The dynamic behavior of molecules during the phase transformations is discussed.

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The series of halogen-bridged N^II-X-M^IV mixed valence compounds (M=Pt, Pd; X=Cl, Br, I) has attracted much interest from solid state physicists and chemists as one dimensional compounds having strong electron-lattice interactions. Their structures are well described as Peierls distorted linear chains with repeating unit…M^II…X-M^IV-X…. They show characteristic physical properties such as strong charge transfer absorption and luminescence with large Stokes shift that can be interpreted by using an extended Peierls-Hubbard model. The relation between the Peierls energy gap and lattice distortions for the compounds has been established by a number of structural and optical studies. The remarkable feature that the physical and structural parameters of the compounds can be controlled by changing chemical parameters such as central metals, bridging halogens, and counter ions has been also elucidated. From an expectation based on the relation, the halogen-bridged Ni^III-X-Ni^III compounds with an extreme limit of the M^II-X-M^IV compounds were synthesized. The novel structure having no Peierls distortion has been determined by detailed X-ray diffraction and physical studies. Further studies on the interesting physical properties such as an extremely strong antiferromagnetic coupling between electronic spins localized on the Ni^III atoms are now in progress.

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Molecular crystals had been long cosidered to be electrically unattractive materials. But this old image was renewed dramatically by the appearance of the recently developed π-molecular metals and superconductors. The Fermi surfaces calculated by the simple extended-Hü ckel tight-binding band calculations have been confirmed by the oscillatory magnetoresistances and magnetic susceptibilities in some organic metals and molecular conductors based on transition metal complex molecules. Several typical molecular conducting systems, which played crucial roles in the development of the new frontier of molecular conducting systems, are described with putting a special stress on the structure chemical aspects. New types of molecular metals with π-metal electons interacting with magnetic ions are also mentioned briefly.

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Magnetic interactions and ordered states in magnets composed of molecular species, especially organic radicals, are summarized. Specific molecular structures that may yield intermolecular ferromagnetic (FM) interaction are discussed. Magneto-structural correlations are presented for the molecular magnetic materials exhibiting intermolecular FM interactions. Magnetic structures of the ordered states are demonstrated on the basis of the neutron diffraction and muon spin rotation (relaxation) techniques. Finally, spin dynamics such as spin solitons, magnetic domains, spin waves and spin resonances in the molecular magnetic materials are briefly shown.

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