Proton affinities (PA) of non-hydrated, M, and hydrated forms, M(H2O)(1,2,3), of 20 organic molecules including alcohols, ethers, aldehydes, ketones and amines were calculated by the B3LY13/6-311++G(d,p) method. For homogeneous families, linear correlations were observed between PAs of the M(H2O)(1,2,3) and the PAs of the non-hydrated molecules. Also, the absolute values of the hydration enthalpies of the protonated molecules decreased linearly with the PAs. The correlation functions predicted that for an amine with PA < 1100 kJ/mol the PA(M(H2O)) is larger than the corresponding PA, while for an amine with PA > 1100 kJ/mol the PA(M(H2O)) is smaller than the PA. (C) 2016 Elsevier B.V. All rights reserved.