Long-range coupling constants for α-13C nuclei in phenyl-X-R (X = O,S,Se,Te) derivatives. Internal rotational information

Abstract

The long-range spin–spin coupling constant over six bonds between the 19F nucleus and the 13C nucleus in the side chain,6J(C,F), is reported for 4—F—C6H4—X—R, where X = O, S and R = CH3, CH2CH3, CH(CH3)2 and C(CH3)3 and 6J(C,F) depends on sin2 θ, where θ is the angle by which the side chain twists out-of-plane about the [Formula: see text] bond. Expectation values of sin2 θ are obtained from 6J(C,F), yielding magnitudes of the apparent twofold barrier to rotation about the [Formula: see text] bond. In these terms, the most stable conformation is that for θ = 0° for all compounds, with the exception of R = C(CH3)3 and of X = S, R = CH(CH3)2; there is effectively free rotation about the [Formula: see text] bond in isopropyl 4-fluorophenyl sulfide in acetone-d6 solution. Good correlations exist between 6J(C,F) and a number of other molecular properties, including certain differences of ionization potentials of the molecular orbitals in the ethers. In particular, the chemical shifts of C-4 are correlated with 6J(C,F). Because 5J(C,C), the coupling constant involving C-4, also depends on sin2 θ, it is measured for the methyl and ethyl selenides and tellurides, as are other 13C,13C couplings involving a 13C nucleus in the side chain. The literature values for the 13C nuclear magnetic resonance chemical shifts in alkyl phenyl selenides and tellurides can be related to θ preferences and also allow estimates of the extrema in 5J(C,C). The resultant values of arcsin [Formula: see text] for R = CH3 are in good agreement with estimates of θ obtained from electron diffraction patterns, photoelectron spectra, and nuclear magnetic resonance in the nematic phase.