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Theoretical and experimental data on the internal rotational potential in isopropylbenzene

1988; NRC Research Press; Volume: 66; Issue: 6 Linguagem: Inglês

10.1139/v88-241

1480-3291

Ted Schaefer, Rudy Sebastian, Glenn H. Penner,

Photochemistry and Electron Transfer Studies

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) sin 2 ψ − 2.8(3) sin 2 2ψ + 1.4(3) sin 2 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 sin 2 ψ − 2.61 sin 2 ψ for the energies in kJ/mol. In CS 2 and acetone-d 6 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.