Theoretical study on the acidities of pyrrole, indole, carbazole and their hydrocarbon analogues in DMSO

Abstract

SMD and IEF-PCM continuum solvation models have been used, in combination with three quantum chemistry methods (B3LYP, M062X, and CBS-QB3), to study the acidities of pyrrole, indole, and carbazole, as well as their hydrocarbon analogues in DMSO, following a direct thermodynamic method. Theoretical parameters such as aromaticity indices (HOMA and SA), molecular electrostatic potential (MEP), and atomic charges have been calculated using B3LYP/6-311++G(d,p) level of theory. Calculated pKa values indicate that there is generally good agreement with experimental data, with all deviations being less than the acceptable error for directly calculated pKa values. The M062X functional combined with the SMD solvation model provided the most accurate pKa values. The MEP surfaces clearly show the electron density change accompanying the deprotonation process and explain the relative stability of conjugate bases. The HOMA aromaticity indices seem to be directly related to acidity strength. The collected data have been used to elucidate the pKa trends for the series of molecules under consideration.