Conformational origin of the nonequivalent C-13 NMR chemical shifts observed for the isopropyl methyl carbons in branched alkanes.

The methyl carbons belonging to an isopropyl group attached either to an alkane which possesses a chiral center, or to an achiral alkane lacking a plane of symmetry which bisects the CH(3)-CH-CH(3) angle, can exhibit nonequivalent (13)C NMR chemical shifts. As examples, in 2,4-dimethylhexane, which is chiral, and in 2,4,6-trimethylheptane, which is achiral, the isoproply methyl carbon resonances are separated by 1.1 ppm at room temperature. Using the gamma-gauche effect to describe the conformation-dependent (13)C NMR chemical shifts in branched alkanes, we are able to explain the conformational origin and magnitudes of the nonequivalent chemical shifts observed for their constituent isopropyl methyl carbons.