Proton, carbon-13 and proton-(deuterium) INDOR n.m.r. spectra of isotopically labelled acetaldehyde dissolved in a nematic phase

Abstract

Dipolar coupling constants, obtained from the proton and carbon-13 spectra of ¹³CH₃¹³CHO dissolved in the nematic phase of p-ethoxybenzylidene-p-butylaniline, are used to obtain a structure of the molecule. Three methods of treating the internal rotation of the methyl group are examined: (a) small-amplitude vibrations neglected and the methyl group assumed to occupy only three positions, (b) averaging of methyl proton positions over internal rotation determined by solving the 3-fold Mathieu equation, but neglecting vibrational motion, and (c) same as (a) but including averaging over harmonic vibrational motion. Only method (c) gives a structure close to that found from the analysis of the rotational spectrum. Not one of the methods accounts adequately for the averaging of the dipolar coupling between methyl and aldehyde protons. The proton-deuterium INDOR spectra are shown to be consistent with the values of the deuterium quadrupole coupling constant and asymmetry parameter being close to those found for formaldehyde.