Calculation of pKa Values of Alkanolamines – A DFT-B3LYP Computational Analysis

Abstract

Greenhouse effect, an important environmental concern today, is particularly due to the emission of carbon dioxide. Removal of CO2 and its storage called carbon dioxide capture and storage (CCS) is thus a noble area in research. Many experimental works are done in this area to screen a suitable solvent for CCS. But the experimental measurements on pKa values, free energy values, enthalpy values, etc. are very difficult and have less accuracy. The present work employs a computational analysis to predict the pKa values of alkanolamines, a suitable candidate for CCS. The poly-functionality makes the alkanolamines as an industrially important molecule. A DFT-B3LYP level of theory was employed on different alkanolamines with 6-31+G (d, p) as basis set. Since most of the reactions starts with protonation or deprotonation reaction, the study of acid dissociation constants, i.e., the pKa values is very useful. The pKa values of alkanolamines are of importance that they play a major role in explaining the mechanism of carbon dioxide capture by them. Different alkanolamine molecules including primary, secondary, and tertiary are studied. The present method uses the free energy calculation to get the pKa values of alkanolmines. The linear relationship of the pKa value with the free energy of protonation is verified. Also computed the gas phase basicity and gas phase proton affinity of different alkanolamines. The structure of alkanolamin molecule has an effect on their pKa value and this determines the trend in their pKa values. They have pKa values less than that of corresponding alkylamines. The hydrogen bonding, sterric effect, etc. have major role in the pKa values of different alkanolamines. All the values are computed except the Gibb’s free energy (in gas phase and in solution phase) and the enthalpy values of proton (in gas phase), which is taken from the literature. The gas phase basicity and gas phase proton affinity are also varied with the structure of different alkanolamine molecules. The method employed is simple and the results are in good agreement with the experimental results. The study can further extended to the temperature dependence of pKa values, as the postcombustion of CCS has temperature dependence and to screen suitable candidates for carbon dioxide removal.

Keywords: alkanolamines, density functional theory, gas phase basicity, pKa values, proton
affinity