Nuclear magnetic resonance study of the hydrogen bonding of chloroform with aliphatic tertiary amines and ethers

Abstract

The parameters for the hydrogen bonding of chloroform with 16 aliphatic tertiary amines and 10 aliphatic ethers in cyclohexane medium have been determined by n.m.r. spectrometry. The hydrogen-bond chemical shift Δ_AB correlates extremely well with the change in enthalpy ΔH° in each series of the closely related bases. The ethers have a more uniform negative temperature coefficient of Δ_AB, than do the tertiary amines. In both the oxygen and the nitrogen bases, the alkyl substituents having the β-methyl group, such as n-propyl and isobutyl, have greater steric hindrance effect in the hydrogen bonding than bulkier substituents having the α-methyl or the γ-methyl group, such as t-butyl or n-butyl, respectively. There is a very good correlation between Δ_AB and the sum of the polar substituent constants (Taft σ* values) in the ethers, while in the tertiary amines this correlation is fairly good except for t-butyldimethyl- and cyclohexyldimethyl-amines, in which the Δ_AB(and ΔH) values are relatively large. It is suggested that a necessary condition for an uniform temperature coefficient of Δ_AB and for a good correlation between Δ_AB and Σσ* is that the hydrogen bonds should be formed in the minimum potential energy configuration. Severe steric hindrance causes the hydrogen bonds to be formed in higher energy configurations than the minimum.