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Approach for Predicting the Standard Free Energy Solvation of H+ and Acidity Constant in Nonaqueous Organic Solvents

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journal contribution
posted on 13.11.2014, 00:00 by Hossein Farrokhpour, Mohammad Manassir
In the present work, the free energy solvation of H+, ΔGsolH+, is estimated in various important nonaqueous solvents including alcohols and polar aprotic solvents, semiempirically. The selected solvents are methanol, ethanol, 2-propanol, 2-methyl-2-propanol, dimethyl sulfoxide, dimethylformamide, and acetonitrile. The value of ΔGsolH+ in 2-propanol and 2-methyl-2-propanol were reported for the first time. The estimated values of ΔGsolH+were used to predict the acidity constants (pKa) of different compounds in the selected solvents. These pKa values had better agreement with the experimental values than the corresponding values obtained using the other ΔGsolH+ reported in literature. This confirmed that the values of ΔGsolH+, predicted in this work, are reasonable. The calculated pKa values of selected compounds in nonaqueous solvents were further corrected by accounting for the error of the applied computational method for predicting the pKa values of the compounds in water. New correlation equations were proposed and used to estimate the pKa values of carboxylic acids (aliphatic and aromatic derivatives), phenol derivatives, a series of active pharmaceutical ingredients, and some anti-inflammatory agents in the considered solvents when their experimental pKa values are known in water. The calculations presented in this work were performed at four different levels of theory including B3LYP, M062X, MP2, and CCSD using 6-311++G­(d,p) basis set, separately.