Molecular orbitals from group orbitals. II Conformational analysis of Systems with three heavy atoms. The XCH2CH2 system and the anomeric effect

A procedure for the conformational analysis of Systems of the type AHmBHnDHk is proposed, in terms of interactions between AHmBHn— and —DHk fragments. The nodal properties and relative level orderings of the group orbitals of these fragments are obtained from the molecular orbitals of the molecules AHmBHn+1 and DHk+1. Attention is focussed upon two-orbital two-electron stabilizing and two-orbital four-electron destabilizing interactions between these fragments. A consistent interpretation of the conformational behaviour of n-propyl and 2-fluoroethyl carbonium ions and carbanions is attempted within the framework of this group molecular orbital procedure. The conformational behaviour of FCH2OH, a model for the anomeric effect, has also been examined. It is shown that the group molecular orbital approach accommodates two earlier interpretations of the anomeric effect, one of which focusses upon bond–bond interactions; the second focusses upon effects of the oxygen lone pairs. Some limitations of the model are noted, in particular the problem associated with explicit calculation of the magnitudes of different stabilizing and destabilizing interactions.