Complete basis set, Gaussian, and hybrid density functional theory evaluation of the proton affinities of water and ammonia

Using Dunning's basis set saturation approach, electronic energies for proton affinities for both ammonia and water were evaluated with hybrid density functional theory methods. It was demonstrated that the zero-point vibrational correction for both of these proton affinities computed at the B3LYP/6-311G(2d,2p) theory level is identical to the values obtained from experimental data and some other theoretical studies. Thus, computed proton affinities for ammonia and water are in excellent agreement with values obtained by G2 and CBSQ ab initio computational studies. The final proton affinity values are 201.8 kcal/mol for ammonia and 162.4 kcal/mol for water. These calculated values are 2-3 kcal/mol below the experimental values for ammonia and water, suggesting that these values might be too high. (C) 1999 Elsevier Science B.V. All rights reserved.