PREDICTING THE PROTON AFFINITIES OF H2O AND NH3

High-level ab initio molecular orbital theory has been used to predict the proton affinities of H2O and NH3 at the CCSD(T) level with con-el ation-consistent basis sets through augmented quintuple-zeta for the f ormer and augmented quadruple-zeta for the latter. Diffuse functions h ave been shown to yield faster convergence to the complete basis set l imit for the prediction of highly accurate proton affinities. For thes e two systems, core-valence correlation effects are small, 0.13 kcal/m ol, and were obtained from calculations with core-valence, correlation -consistent basis sets. The electronic component of the proton affinit ies are 171.56 kcal/mol for H2O and 211.97 kcal/mol for NH3. The zero- point vibrational corrections were taken from experimental values wher e available and from scaled theoretical values otherwise. The final pr oton affinity (PA) values at 298 K are PA(H2O) = 165.1 +/- 0.3 kcal/mo l and PA(NH3) = 204.1 +/- 0.3 kcal/mol as compared to experimental val ues of PA(H2O) = 166.5 +/- 1 kcal/mol and PA(NH3) = 204 +/- 1 kcal/mol . The calculated values together with our estimated error limits sugge st that the experimental value for H2O is too high by 1.5 kcal/mol.