Basicity of amines and some related compounds from energy decomposition analysis

Abstract

Acidities and basicities are normally rationalized with the help of concepts like inductive and delocalization effects. There are, however, difficulties in estimating these quantities for most of systems. In this work, energy decomposition analysis has been used to interpret the basicity of a series of nitrogen compounds. With the help of CMOEDA (LMOEDA) partitioning scheme, the increase of basicities with the increasing alkyl chain length of amines was attributed mainly to polarization (orbital rearrangement) interactions. The variation in basicites among cyclic amines, which has been interpreted as a consequence of geometry/hybridization, was also attributed mainly to polarization, with electrostatic interactions also playing an important role in some cases. The effect of the substituent (electron attractor or electron donor) on the basicity of aniline was studied as a model to aromatic compounds. In this case, electrostatic is the most important factor. Finally, some simple conjugated nitrogen compounds were investigates, namely, formamide, thioformamide and enamine. The smaller basicity of formamide compared to methylamine is attributed mainly to electrostatic interactions, as is explained in detail. Geometrical changes (from a planar to a pyramidal structure) plays a secondary role. Combination of EDA results with a solvent model allows the interpretation of pKa values.