Effects of CH3···O Hydrogen Bond on the Rotamer Populations in 9-(Alkoxymethyl)-1,4-dimethyltriptycenes

Abstract
Rotamer populations in 9-(alkoxymethyl)-1,4-dimethyltriptycenes, where the alkyl is an ethyl, a methyl, or a 2,2,2-trifluoroethyl, were examined by 1H NMR spectra. The populations of the sc isomer decreased as the alkyl group becomes electronegative. The population ratio, sc/ap, of the methyl compound was almost the same with that for 1,4-dimethyl-9-(phenoxymethyl)triptycene to indicate that the populations of the rotamers in this and related series of compounds are determined on a delicate balance of various factors such as van der Waals repulsive and charge-transfer attractive interactions.