Extensive theoretical studies of the hydrogen-bonded complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2

Abstract

The structures and binding energies of the complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2 have been examined using much higher levels of theory than has been previously applied to these systems. These methods including large basis sets and full optimization of structures with the effects of electron correlation included, are known to give single bond energies to an accuracy of about 2 kcal mol−1 and are found in this study to give excellent agreement with the extensive experimental data available for the hydrogen fluoride and water dimers. The Cs open form of ammonia dimer remains a very shallow minimum energy structure at these levels, in agreement with previous theoretical results but seemingly in disagreement with experiment. The theoretical enthalpy of association of H5O+2 is found to be −35.0 kcal mol−1, in slight disagreement with the most recent experimental results, but in accord with earlier ones, which suggests that these experiments should be reexamined. The enthalpy of association of H2F+ is predicted to be −33.5 kcal mol−1, and that of F− with HF to be −46.4 kcal mol−1. A study of the effects of basis set expansion on the structure of the water dimer shows that the structure is much more sensitive to basis set at the Hartree–Fock level than when correlation is included. A valence triple-zeta basis plus two sets of first polarization functions and one set of diffuse functions appears to be necessary to approach the Hartree–Fock limiting structure. Counterpoise estimates of the effects of basis set deficiencies on the structure and binding energy of this complex are shown to be misleading. Examination of the complexes (HF)2, (H2O)2, (NH3)2, (H2O)2H+, (HF)2H+, and F2H− at the MP4/6-311++G(3df ,3pd)//MP2/6-311++G(2d,2p) level of theory indicates that previous studies using fourth order perturbation theory with some smaller basis sets and Hartree–Fock optimized structures are likely to be reliable, although part of the agreement reflects a cancellation of error. HF/6-31+G(d) estimates of zero-point vibrational energy contributions to association energies are found to be satisfactory for asymmetric complexes, but can both over and underestimate the contribution of this term for symmetrically bound complexes.