Barriers to internal rotation in 1,3,5-trineopentylbenzenes: 10—13C and 19F NMR band shape studies and force field calculations

1,3,5-Trineopentylbenzenes (TNB) with one or two benzylic substituents in each neopentyl group were synthesized. The substituents were F, Cl, Br, J, OCH₃, OCOCH₃, OSi(CH₃)₃ and CH₃ and, in cases of disubstitution, F, Cl, Br, CH₃ and Cl, CH₃ and Br and ? SCH₂CH₂S? . Barriers to internal C_sp3? C_sp2 (aryl) and C_sp3? C_sp3 rotation were estimated by ¹³C and ¹⁹F NMR band shape methods. Estimated barriers in the TNB series were found to be very close to those found for the corresponding mononeopentylbenzenes. For some of the compounds studied, molecular mechanics (MM) calculations were performed with the Allinger MMP1 program. Differences between calculated and experimental estimated barriers were found, and possible sources of these discrepancies in terms of parameters used in the MMP1 program are discussed.