Theoretical and experimental data on the internal rotational potential in isopropylbenzene

Abstract

Geometry-optimized STO-3G MO computations of the internal rotational potential about the [Formula: see text] bond in isopropylbenzene yield V (kJ/mol) as 14.1(3) sin2 ψ − 2.8(3) sin2 2ψ + 1.4(3) sin2 3ψ; ψ is the angle by which the α C—H bond deviates from the benzene plane. This result provides no support for the very low internal barrier once hypothesized from microwave data for isopropylbenzene in the vapor phase, nor for the local minimum at ψ = 90° inferred from hyperfine coupling constants in π electron radicals of isopropylbenzene derivatives. An alternative explanation in the literature for such microwave spectra is therefore a more likely one. Similar MO computations with the 6-31 G basis at three values of ψ can be fit by 13.40 sin2 ψ − 2.61 sin2 ψ for the energies in kJ/mol. In CS2 and acetone-d6 solutions, the long-range proton–proton coupling constants imply the independence of the rotational potential of solvent polarity and also that the barrier is rather lower than that computed for the free molecule. An apparent twofold barrier of 6.5 kJ/mol is consistent with the coupling constants but so is a combination of twofold and fourfold barrier components amounting to 9.7, −3.5 and 7.2, −1.5 kJ/mol. The coupling constants do not allow discrimination among these possibilities. The internal rotational potential is not altered significantly in the presence of a 4-hydroxyl substituent in isopropylbenzene.