Molecular orbitals from group orbitals. IV. Quantitative perturbational molecular orbital analysis of the methyl rotational barriers in (CH3)2X molecules. Effect of the fragmentation mode upon the results of the analysis

Abstract

A quantitative perturbational molecular orbital (PMO) analysis has been performed on ab initio SCF-MO wavefunctions associated with the rotation of the methyl groups in a series of (CH3)2X molecules (X = CH2, O, S, C=O, C=CH2). Two fragmentation modes have been investigated: Method a, in which the system is dissected into X and (CH3)2; and Method b, in which the system is dissected into CH3X and CH3. Both fragmentation modes reproduce the principal property of these molecules, viz., that the more crowded SS conformation is preferred. However, whether this conformational preference is controlled by two-electron stabilizing effects or four-electron destabilizing effects is found to depend upon both the mode of fragmentation and the nature of the substituent X. The quantitative results are supplemented by a detailed qualitative description of the nature of the group orbitals associated with the two fragmentation modes and the various types of orbital interactions. It is shown that orbital energy differences control the qualitative discussion of Method a, and overlap effects control that of Method b. Although the final result, i.e., the preference for the SS conformation, and the behaviour of individual orbital interactions are anticipated correctly by the qualitative arguments, these are unable to assess the relative contributions of the stabilizing and destabilizing interactions.