Basis set effects on the prediction accuracy of relative acidity constants of primary and secondary amines

Conductor-like continuum model (CPCM) has been used in conjunction with micro-solvation approach (cluster-continuum model) to study relative acidity constants of a group of amines containing methylamine, propylamine, 2-aminoethanol, 1,2-ethanediamine, 1,3-propanediamine, dimethylamine, diethylamine, piperidine, and 2,5-diazahexane in aqueous solutions. Only one water molecule was considered for the micro-solvation. Calculations were done at the level of B3LYP with 6-31G, 6-31G*, 6-31G**, 6-31+G, 6-31+G*, 6-31+G**, 6-31++G, 6-31++G*, and 6-31++G** basis sets to investigate the role of added diffuse and polarization functions on the prediction accuracies. Data showed that basis set can also affect the accuracy of the results in addition to the cavity and solvation models. The escaped charge for diffuse functions was more than the escaped charge for polarization functions. Our best results belong to the basis sets of 6-31+G* and 6-31++G** with 1:1 ratio of diffuse and polarization function.