The relation of methoxy group CH bond lengths and stretching frequencies to the oxygen lone pair orientation

Abstract

The results of ab initio gradient calculations of equilibrium molecular geometries of ROCH 3 molecules, where RCH 3, H, HCH 2, CCH, CHO, CFO, show a nearly linear dependency of theoretical CH bond lengths (both trans and gauche to oxygen lone pairs) on the (H 3)COlp angle. This trend is confirmed by experimental data. The nonbonded interactions are superimposed upon this lone pair effect and may lead to major deviations from the linear dependency in the molecules, where a fluorine atom or carbonyl oxygen are in relatively close proximity to methyl hydrogens. The experimental methyl group stretching frequencies also correlate with the (H 3)COlp angle; this correlation allows a check to be made on the assignments of the CH 3 group stretching fundamentals.