2-Phenyladamantane as a model for axial phenylcyclohexane. 1H NMR and molecular orbital studies of motion about the Csp2—Csp3 bond

Abstract

The 1H NMR spectra of the aromatic groups of 2-phenylcyclohexane and 2-phenyladamantane, in CS2/C6D12 solution at 300 K, are analyzed to yield the long-range coupling constants between the α and ring protons. The coupling over six bonds is related to the internal rotational potential about the Csp2—Csp3 bond in these molecules. It is confirmed that the equatorial isomer of phenylcyclohexane has the parallel conformer, that in which the aromatic plane lies in the symmetry plane bisecting the cyclohexane moiety, as the most stable. The apparent twofold barrier to rotation about the exocyclic carbon–carbon bond follows as 7.1 kJ/mol from the six-bond coupling constant. For 2-phenyladamantane, the six-bond coupling constant strongly implies that the perpendicular conformer, perhaps slightly skewed, is that of lowest energy and that the apparent twofold barrier to rotation about the Csp2—Csp3 bond is about 7.5 kJ/mol. Insofar as 2-phenyladamantane mimics axial phenylcyclohexane, these results confirm recent conclusions about the conformation of the latter and provide evidence for its internal mobility. Geometry-optimized AMI and STO-3G MO computations are reported for the internal motion in both isomers of phenylcyclohexane. The former agree best with experiment for the equatorial isomer, but both imply a significant fourfold, of opposite sign to the twofold, component of the internal rotational potential. For the axial isomer, the two sets of computations find a skewed perpendicular conformer as most stable, in rough agreement with force-field results. However, the barrier to rotation about the Csp2—Csp3 bond is computed as small and AMI has the parallel conformer as more stable than the perpendicular. Key words: 2-phenyladamantane, 1H NMR and internal rotation; phenylcyclohexane, 1H NMR and internal rotation; MO computations, 2-phenyladamantane and phenylcyclohexane.