On the electronic origins of barriers to methyl rotation: CNDO/2 calculations on (CH 3) 2XH n (X = C, Si, N, P, O, S) molecules

Abstract

CNDO calculations are performed for the homogeneous series of (CH 3) 2XH n compounds (X = C, Si, N, P, O, S) in order to determine the molecular equilibrium configuration. In agreement with available experimental data, for all investigated molecules, the theoretical energy minimum is found for the (θ = 60°, ψ = 60°) conformation in which one hydrogen of each methyl group is in the X heavy atoms plane but pointing outside the CXC angle. A partitioning of total energy shows that the variations of this quantity are completely reflected by the variations of the interaction energy between non-bonded terms. A more detailed analysis reveals, only for the third-row compounds, the essential role of the interactions between central atom and methyl hydrogens.