Barriers to internal rotation in neopentylbenzenes substituted on the benzyl group. A 13C NMR band shape study

Two series of neopentylbenzenes with one or two substituents on the benzyl group have been synthesized. In one series the substituents were H, F, Cl, Br, I, OCH3, OCOCH3, OSi(CH3)3 CH3 and CH2CH3, and in the other OH and R [R  H, CH3, CH2CH3, (CH2)3CH3, CH(CH3)2 and C(CH3)3]. Barriers to internal C C and C C rotation have been estimated by 13C NMR band shape methods. Estimated barriers were found to increase as the size of the substituent increases. The results are discussed in terms of possible initial and transition states, based on summations of results from molecular mechanics (MM) calculations, using the Allinger MMP1 program. Barriers estimated experimentally are compared with results from other systems found in the literature.