Theoretical study of the basicity of alkyl amines in vacuo and in different solvents: a density functional theory approach

A study is presented on the basicities of alkyl-substituted amines both in the gas phase and in solution. In the first step, calculated group hardnesses are used to clarify the origin of the inversion of the alkyl amine basicity when passing from the gas phase to solution for a series (CH3)nNH3−n with n=0, 1, 2 and 3. The basicity in the gas phase is primarily determined by the intramolecular stabilization of the positive charge in the conjugate acid whereas, in aqueous solution, the solvent accounts for this effect. Next, Sandersonʹs electronegativity equalization principle is shown to be a useful tool in describing the charge distribution in the molecules RNH2 (R=CH3, CH2CH3 and CH(CH3)3) and their conjugate acids in solution. These calculated charge distributions are then used in a study on the inversion of the alkyl amine basicities from the gas phase to aqueous solution for these molecules.