1H nuclear magnetic resonance and molecular orbital studies of the structure and internal rotations in ethylbenzene

Abstract

Rather extensive geometry-optimized computations at the STO-3G, 4-21G, 4-31G, and 6-31G levels of abinitio molecular orbital theory suggest that the fourfold component of the barrier to internal rotation about the Csp2—Csp3 bond in ethylbenzene amounts to about 20% of the twofold component. The 1H nuclear magnetic resonance spectral parameters, extracted by complete analyses of the spectra arising from the ten protons, are reported for ethylbenzene in acetone-d6, CCl4, CS2, and perfluoromethylcyclohexane solutions. The long-range proton–proton spin–spin coupling constants demonstrate that the internal barrier is insensitive to the polarity of the solvent, in contrast to polar solute molecules such as benzyl fluoride. The coupling constants do not support a dependence of the internal barrier on the internal pressure of the solvent.