Implications of Unusual Population Ratios in Rotational Isomers of 9-(4-Substituted Benzyl)-8,13-dichloro-1,4-dimethyltriptycenes and 4-Substituted 9-Benzyl-8,13-dichloro-1-methyltriptycenes: CH3···π Hydrogen Bond

Population ratios of rotational isomers in substituted 9-benzyl-8,13-dichloro-1-methyltriptycenes were examined by equilibration in solution. The sc isomers increase in their populations as the electron density on the 9-benzyl group increases and the acidity of the 1-methyl group increases. The sc/ap ratios were larger than 2, the statistical value, in all the compounds examined, except the 4-nitrobenzyl compound. By contrast, MM2 calculations show that the sc isomers are less stable than the ap by ca. 1 kcal mol−1. This discrepancy together with the substitutent effects on populations of the rotamers are attributed to the presence of CH3···π interactions in the sc forms that stabilize the system mainly by charge-transfer interactions but are not included in the MM2 calculations.