G. S. Koptev

Gas-phase electron diffraction study of tris(trimethylstannyl)amine, N(SnMe3)3

Abstract The geometrical parameters of tris(trimethylstannyl)amine have been determined by gas-phase electron diffraction. The a structure has been refined using mean amplitude values calculated from the force fields of a number of tin derivatives. The experimental data are consistent with a planar bond configuration at the nitrogen in N(SnMe 3 ) 3 . The final set of geometrical parameters is as follows (average bond distances, r g , in A, angles in degrees): SnC 2.166(5), SnN 2.038(3), CH 1.117(17). NSnC 108.5(1.5), SnCH 112.1(1.6). Mean amplitude values have been varied for those distances which give considerable contributions to scattering. The results obtained fill a gap in the knowledge of structures of Group IV element μ-nitrido derivatives. They confirm the conclusion that lowering of ligand MR n electro-negativity weakens the tendency to deviation from planarity in the central fragment NM 3 . This tendency may be considered as a manifestation of the second-order Jahn-Teller effect.

Kinematic models of a force field

Abstract A number of models of a force field defined by the expansion of the matrix of kinematic coefficients into multipliers is discussed. Additional relations in which sets of force constants corresponding to these models are transferable to isotopic molecules are given. It is shown that, in the case of any large molecule, e.g. 1,3-butadiene, these models cannot be used as a starting approximation for a least-squares refinement.

Gas phase electron diffraction study of basketene, C10H10

Abstract A gas phase electron diffraction study of the cage hydrocarbon, basketene, is reported. A least squares treatment of molecular intensities has been carried out in terms of a geometrically consistent rα structure. The mean amplitude values and shrinkage corrections have been calculated using the force field parameters estimated from the data on simpler molecules. Structure refinement of the C2v molecular model yields the following parameter values (bond lengths, ra, in nm; angles, rα in degrees): av. 0.1092(8); In addition to the geometric parameters listed, the mean amplitudes for all bonded and C· C nonbonded distances have been determined by the least squares method. All the other amplitudes (C· H and H· H) have been fixed at the values estimated from the spectral data. Comparison of the results obtained with the literature data on similar polycyclic molecules points to the stronger internal strain in the basketene molecule.

The conformational analysis of 2,3,4,5-tetrachloro-trans-hexa-1,3,5-triene

Abstract The potential curves for internal rotation about the single C-C bonds for conformers of the CH 2 CCl-CClCCI-CClCH 2 molecule with C i and C 2 symmetry in the gaseous phase, have been constructed by the CNDO/2 method. These curves possess minima of different depths corresponding to skew centrosymmetrical ( C i ) and skew ( C 2 ) configurations. The essential changes in the vibrational spectrum of this molecule when passing from liquid to crystal, which have been observed by us earlier, are explained by the shift of equilibrium in the direction of the skew centrosymmetrical conformer.

Concerning the physical solution of the inverse vibrational problem

Abstract The physical solution of the inverse vibrational problem suggested by Jordanov and Nikolova is discussed in detail and some errors corrected. 1