An 1H NMR conformational analysis of 3-phenylpentane in solution

Abstract

Some 30 proton chemical shifts and proton–proton coupling constants are reported for a 4.7 mol% solution of 3-phenylpentane in a CS2/C6D12/TMS solvent mixture at 300 K. The long-range coupling constant over six formal bonds between the methine and para protons is used to deduce an apparent twofold barrier of 15.0 ± 0.3 kJ/mol to rotation about the Csp2—Csp3 bond, at least twice as large as that for isopropylbenzene in solution. AM1 computations agree with experiment in finding the conformation of lowest energy as that in which the methine C—H bond is situated in the phenyl plane, but predict a barrier height of only 13.9 kJ/mol. The vicinal coupling constants are consistent with a fractional population, 0.38(2), of the TT conformer, that in which all the carbon atoms of the alkyl chain lie in a plane. A doubly degenerate conformer, TG+(G−T), in which one methyl group is twisted away from the phenyl substituent, then has a fractional population of 0.62(2). The assumption that only these three conformers are present is tested with the signs and magnitudes of the four different coupling constants over four bonds. These coupling constants are consistent with the absence of significant proportions of the other six all-staggered conformers. These six are characterized by a close approach of the methyl groups (1,5 interactions) or by proximity of the methyl and phenyl moieties.