A single-crystal of ammonia, NH3, was grown in a thin-walled capillary at 178 K, and high-resolution X-ray diffraction data were obtained for this compound at 160 K in order to obtain information about electron density distribution. Conventional and multipole refinements and deformation electron density maps indicated small but significant N−H bond bending inside the NH3 tetrahedron that is in agreement with ab initio quantum-chemical calculations and the VSEPR model. Topological analysis of the experimental charge density distribution in the ammonia molecule has been performed, and the data are compared with high-level quantum-chemical calculations. Some features of the intermolecular hydrogen bonds in the crystal are discussed.

X-ray single-crystal study, molecular mechanics calculations, and calculations of the static nonlinear polarizabilities (β and γ) were performed for dicyanovinylbenzene and series of its mono- and dimethoxy-substituted derivatives. X-ray analysis has been done for dicyanovinylbenzene, its o- and p-methoxy- and 2,4-dimethoxy-substituted derivatives together with corresponding EFISH measurements of the β values for these compounds. Nonlinear polarizabilities were calculated for all series of the mono- and disubstituted methoxy derivatives of the parent compound with modified finite-field formalism that included calculation of polarization values versus static electric fields using semiempirical Hamiltonian, polynomial fit of all tensor elements of β and γ on the former data, and evaluation of the numerical instability of the calculations. All calculations were performed with optimized molecular geometries taken from X-ray data, molecular mechanics, ab initio, and semiempirical quantum chemical results. Good correlation was found between the predicted static and experimental molecular values of β. A factor of 2.0 was found to be a probable adjustment parameter to account for the solvent (1,4-dioxane) and dispersion (at 1064 nm) effects. Crystal packing analysis of the X-ray structures studied together with energetic calculations revealed the factors responsible for formation of centrosymmetric crystals. Only o-methoxydicyanovinylbenzene (also known as DIVA) forms acentric crystals (space group P21) among the experimentally studied compounds, and it was found that its molecular dipole moment orientation with respect to the polar crystal axis is close to the “optimal” for a manifestation of the high NLO responses.

The molecular and crystal structure and electron density distribution were studied for a single crystal of pentaethyl-1,5-dicarba-closo-pentaborane C 2 B 3 (Et) 5 by the high-resolution X-ray diffraction method at 120 K (5696 observed reflections, R ) 0.056). The single crystal was grown by in situ crystallization with an IR-laser beam producing a molten zone in a Lindeman capillary. In crystal the molecule of the title compound has local C 1 -symmetry with different orientations of the Et-groups with respect to the central trigonal-bipyramidal C 2 B 3 fragment, mean B-C and B-B distances are 1.571 and 1.876 Å, respectively, the axial C...C distance is 2.277 Å. An ab initio calculations of the molecular structure indicated that rotation barriers of the Et groups with respect to the central frame are very small, that may be the reason of the low molecular symmetry and diffrerent orientation of the Et groups in molecule due to the crystal packing effects. Deformation electron density (DED) maps obtained via a conventional "X-X" method, and using a multipole refinement procedure, showed charge accumulation in the B-C bonds of the central frame, and these bonds were found to be essentially bent outward of the C 2 B 3 cage. On the contrary, no charge accumulation was detected in the B-B bonds, thus indicating the absence of the direct B-B interactions. Positive and delocalized DED was also found in all B-C-B triangle faces of the central cage, that indicates the contribution of the multicenter bonding in the electronic structure of this formally electron-precise molecule. Topological analysis of the experimental charge density distribution has been performed including an analysis of the Laplacian maps, and these data confirmed all important features of the DED maps.

On the basis of high-resolution X-ray diffraction studies as well as quantum-chemical calculations of five carborane derivatives the peculiarities of electron density distribution functions have been analyzed. The data obtained permitted a deeper insight into the nature of unusual properties of the C-C bond in o-carborane and investigating intermolecular H ... H interactions in crystal. It was shown that such an approach allows estimating the values of lattice energy for the crystals of carboranes.