ABINITIO CALCULATIONS OF STRUCTURAL FEATURES NOT EASILY AMENABLE TO EXPERIMENT .68. STRUCTURAL TRENDS IN SOME SUBSTITUTED NITRILES AND ACETYLENES

The geometries of a number of alkynes, nitriles, and dinitriles were determined by SCF gradient optimization using the 4-21G, 4-21G∗, 5-31G∗, and 3-321G∗ basis sets. The systems investigated include: YH3-C-C-X compounds, with Y = Si and C, and X = H, F, Cl, and CN; the geminally disubstituted systems, 1,4-pentadiyne, H2C-(CCH)2, and 1,1-dicyanocarbonyl, OC(CN)2; X2Z-(CN)2 homologs with X = H, F, and Cl, and Z = Si and C; and various auxiliary structures (e.g. silylcyanide, methoxyacetylene, and S-methylacetylene) needed to establish structural trends with specific basis sets. The CC bond lengths in C C-X systems are found to decrease with increasing electronegativity of X. The Si-H bond lengths decrease in the series SiH4, SiH3-CN, SiH2-(CN)2 in contrast to C-H bond lengths in the methyl homologs. The characteristic non-linearity of the C-CC-H and Z-CN (Z = Si and C) moieties in geminally disubstituted Y2Z(CX)2 systems is considered in detail and found to depend on the electronegativity of Y. The calculated structures are in good agreement with previously published experimental geometries, some of which were derived from a limited number of experimental data.