Conformational studies by dynamic nuclear magnetic resonance. Part III. Rotamers arising from restricted motion in substituted benzaldehydes

Abstract

Rotational isomers due to the restricted motion of the formyl group have been unambiguously detected in ortho-substituted benzaldehydes. The conformers with the ortho-group and formyl oxygen anti have been shown, through ¹H and ¹³C n.m.r. spectroscopy, to be the more stable species, the relative abundance depending on the bulkiness of the substituents. Thermodynamic activation parameters have been also obtained by computer simulation of the spectral lines, and a rotational model is proposed to rationalize the dependence of ΔG^‡ on the steric hindrance of o-alkyl groups. Proof of the existence of cis- and trans-isomers in terephthalaldehyde has been provided by means of liquid crystal n.m.r. spectra as well as by low temperatures n.m.r. measurements. Comparison has been made between this result and that of an e.s.r. conformational investigation on the corresponding radical anion obtained by photolytic as well as by chemical reduction.