ABSTRACT
A computational investigation based on the density functional theory (DFT) has been performed for predicting the pKa value of alkylamines. Amines are versatile and widely studied class of organic compounds. Their importance in biochemistry, natural product chemistry, industrial 4purposes, etc., make them highly frequent subject in physical as well as in (bio) chemical studies. The pKa studies are important in explaining the acid-base properties of amines. In the study of macroligands with a large number of ionisable sites such as dendrimers, we must calculate the pKa values of their chemical building blocks, i.e., amines and for that we have to get validate and accurate method. Another important and developing property of amines is that they can serve as the novel candidates for CO 2 capture and sequestration processes. Here also the values of pKa are important. The pKa values are very important in explaining the mechanism (not included in this work) of CO 2 capture process and their capturing capacity. Reasonable prediction of pKa values would therefore be of great value in screening for new candidates for CO 2 capture process. In the present work, we have employed a DFT/B3LYP level of theory with 6 - 311 + + G(d, p) basis set for the amines and the computational works were done through Gaussian 03 version. We have computed the gas phase basicity, gas phase proton affinity, free energy of solvation, electronic and non‐electronic contributions to free energy and pKa values of alkylamines. The free energy of solvation was computed using IEF‐PCM models with water as the solvent. The gas phase basicity, gas phase proton affinity and the electronic and non‐electronic contributions to free energy were found to increase with the number of carbon atoms. The pKa values were calculated using different types of basis sets; 6 - 311 + + G(d, p) has been found to be the best. The pKa values of all the amines studied were computed using the basis set 6 - 311 + + G(d, p) and are optimized both in gas and solution phase (water). The alkylamines studied include primary, secondary and tertiary amines. It has been found that the computed values are in good agreement with the experimental results which shows that the DFT (B3LYP) based analysis with 6 - 311 + + G(d, p) basis set is a reliable and easy method for the computation of pKa values.
