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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.

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A series of equation-of-motion coupled-cluster ͑EOM-CC͒ calculations of the vertical excitation energies of benzene, pyridine, pyrazine, pyrimidine, pyridazine, symmetric triazine, and symmetric tetrazine have been performed. Single and double excitations have been included fully, and a noniterative approximation has been used to estimate triple excitation effects ͓the EOM-CCSD(T) method͔. The basis set contains polarization functions and has reasonable diffuseness. Comparison is made with experimental data and second-order perturbation theory complete active space ͑CASPT2͒ theoretical data. The average EOM-CCSD(T) error for →* transitions is 0.11 eV and the error for n→* transitions is 0.15 eV. Based on these small errors, several uncertain assignments for pyrazine and pyrimidine are substantiated.

Anharmonicity and matrix effects play important roles in determining the proton-stretching frequencies in hydrogen-bonded complexes of HCl and HBr with NH 3 and N(CH 3 ) 3 . These effects have been investigated through ab initio calculations carried out at MP2/aug′-cc-pVDZ for complexes with HCl and at MP2/6-31+G-(d,p) for complexes with HBr. The potential surfaces of these complexes are very anharmonic, since the region surrounding the global minimum may be very broad and relatively flat, or a second region of the surface, displaced from the global minimum, can be accessed in either the ground (V ) 0) or the first excited (V ) 1) state of the proton-stretching mode. As a result, two-dimensional anharmonic frequencies, particularly for the proton-stretching vibration, can be dramatically different from the corresponding harmonic frequencies. Moreover, the zero-point energy contribution to binding enthalpies based on harmonic vibrational frequencies can be significantly overestimated in some complexes. To model the effects of matrices on the structures and spectra of these complexes, potential surfaces have been generated in the presence of external electric fields applied along the hydrogen-bonding X-H-N direction. These fields preferentially stabilize more polar hydrogen-bonded structures. The changes in anharmonic frequencies computed from these surfaces depend on the strength of the field and the nature of the equilibrium structure at zero field. Comparisons between computed frequencies for these complexes and experimental frequencies obtained in Ar and N 2 matrices provide insight into the dependence of proton-stretching frequencies on the environment. It is now possible to understand the apparently disparate effects of Ar and N 2 matrices on the spectra of closely related complexes.