The structure and internal rotational barrier of 3-phenyl-1-propyne by molecular orbital calculations and the J method

Abstract

The 1H nuclear magnetic resonance spectral parameters are reported for 3-phenyl-1-propyne dissolved in CCl4, C6D6, and in acetone-d6. The long-range spin–spin coupling constants imply very small and perhaps vanishing barriers to internal rotation about the [Formula: see text] bond in all three solutions, in contrast to benzyl cyanide in which there exist significant solvent perturbations of the internal barrier. STO 3G MO computations, utilizing geometry optimization procedures, imply an internal rotational potential of V/kJ mol−1 = −2.8 sin2 ψ − 0.6 sin2 2ψ; the angle ψ is 90° when the C≡C bond lies in a plane perpendicular to the benzene plane. 6-31G MO energies imply V/kJ mol−1 = −0.3 sin2 ψ − 0.4 sin2 2ψ, the fourfold component being larger than the twofold. A flat minimum occurs near ψ = 50°.