Rotamer Populations and Molecular Structure of 9-Isobutyl-1,4-dimethoxytriptycene: Further Evidence for the Presence of CH3···O Hydrogen Bond

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Abstract
1H NMR spectra of the title compound at low temperatures show the presence of the sc conformer as a major constituent: ΔH°=0.9±0.3 kcal mol−1, ΔS°=5±2 cal mol−1 for the sc\ightleftarrowsap equilibrium. The line shape analysis of the spectra at various temperatures afforded the following activation parameters for the internal rotation about the C9–Ci-Bu bond: ΔH\eweq=8.8±0.3 kcal mol−1, ΔS\eweq=−5.0±1.2 cal mol−1 K−1 for the ap→sc process. X-Ray crystallography of the compound revealed that the molecule exists as sc conformers in the crystal. One of the methyl groups of the isobutyl group that directs inward relative to the triptycene skeleton, is placed very closely to the 1-methoxy-oxygen to indicate the presence of the CH3···O hydrogen bond. Implications of other structural parameters are also discussed.

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