On the use of density functional theory based descriptors for the interpretation of the influence of alkyl substitution on the basicity of amines

Abstract

The influence of alkyl substitution on the gas phase basicity of amines was studied using density functional based reactivity descriptors, together with two more traditional quantum chemical properties. Softness related local properties such as the Fukui function f − k and local softness s − k are found to be inadequate for predicting the reactivity order of the alkyl amines studied in this work. As a first approximation to local hardness, the Mulliken population on the nitrogen atom q N and the minimum of the molecular electrostatic potential (MEP) near the nitrogen were calculated. Both were found to correlate inversely with the amine basicity order, a result consistent with the literature. As the polarizability of the alkyl substituent is often cited as a factor determining alkyl amine basicity, the corresponding group property (group softness) and the group electronegativity were combined in a dual correlation, providing a successful interpretation of the basicity order. The group softness or polarizability was found to be the more important factor in determining the basicity of the alkyl amines.