Conformational Preference in CH₃, CH₂F and CF₃ Compounds of 1st and 2nd Row Elements

Abstract

Energy changes and differences in geometries between the staggered and eclipsed conformations of methyl-, fluoromethyl - and trifluoromethyl - substituted compounds have been determined in a systematic ab initio study of compounds of first and second row elements. Geometry-optimized results at extended basis set level obtained on SiH₃X, SiH₂X^-, PH₃X₊, PH₂X, PHX^-, SH₂X⁺ and SHX species (X = CH₃, CH₂F, CF₃) are compared with data on the corresponding first row compounds. The structural relaxation accompanying rotation in methyl compounds of second row elements is smaller than it is in compounds of the first row but for X = CF₃ the geometrical changes are comparable. However, the relaxation contribution to energy is much smaller in the heavier compounds.