The application of low temperature 13C nuclear magnetic resonance spectroscopy to the determination of the A values of amino-, methylamino-, and dimethylamino-substituents in cyclohexane

Abstract

Determinations of nuclear Overhauser enhancements and spin–lattice relaxation times of carbon atoms in cis-2,4-dimethylpiperidine have shown that the measured integrals in a pulsed Fourier transform ¹³C spectrum recorded below 220 K are an accurate indication of the molecular proportions involved, provided (a) the comparison is between protonated carbons carrying the same number of protons, and (b) the pulse repetition time is at least 2 s. The variable temperature ¹³C spectra of cis-4-methylcyclohexylamine, cis-4,N-dimethylcyclohexylamine, and cis-4,NN-trimethylcyclohexylamine have been measured and interpreted, yielding ¹³C chemical shift parameters for conformations with equatorial and axial NH₂, NHMe, and NMe₂ groups attached to cyclohexane. The proportions of conformations obtained from the low-temperature spectra have given the following A values, in kcal mol^–1: NH₂ 1.45 (CFCl₃–CDCl₃) and 1.49 ([²H₈]toluene); NHMe 1.29 (CFCl₃–CDCl₃) and 1.11 ([²H₈]toluene); and NMe₂ 1.53 (CFCl₃–CDCl₃) and 1.31 ([²H₈]toluene). The A values are discussed with reference to previously reported values, and to the trend in the series NH₂, NHMe, and NMe₂. An A value for Me of 1.78 ± 0.12 kcal mol^–1, at 193 K, was deduced from the low temperature ¹³C spectrum of methyl cis-4-methoxycyclohexyl ether.