First-Principles Calculation of pK_aValues for Organic Acids inNonaqueous Solution

摘要

Electronic structure theory, mainly the density functional theory (DFT), is applied to calculate the pKavalues for a variety of organic acids in several nonaqueous solvents: namely DMSO, MeCN, and THF.Following the supermolecule approach the solute molecule, together with a few solvent molecules inclose proximity, is treated explicitly by the electronic structure theory, and the remaining solventenvironment is approximated by using a standard dielectric continuum model. It is found that in mostcases including only one explicit solvent molecule gives satisfactory results for pKa estimations. Next,the equilibrium position free energy difference is calculated between a reference acid—base pair whosepKa is known experimentally and the acid—base pair whose pKa is to be determined theoretically. Thisbypasses the step of treating the solvated proton that most of the current theories have difficulty withand, to a large extent, induces favorable error cancelations in the final theoretical results. Accuratetheoretical predictions of pKa values are thus obtained at a moderate level of theory (MP2 single pointon B3LYP/6-31+G(d) optimized geometry) for a series of organic acids spanning a wide range of aciditiesin DMSO, MeCN, and THF. Furthermore, the correlation between the pKa values of these acids in differentsolutions is investigated theoretically, and excellent agreement is found with the experimental results.