The geometries, interaction energies, and harmonic vibrational frequencies of water clusters (with up to 8 molecules) have been studied using density functional theory (DFT) at the gradient corrected level. The water monomer and water dimer calculations have been used as benchmarks to investigate different choices for basis sets and density functionals. Our results for larger clusters agree with both available high-level ab initio calculations and experimental information. The calculations of the vibrational frequencies and IR absorption intensities for the larger clusters, for which no other reliable quantum-chemical calculation is available, are presented to facilitate the frequency assignment of experimental spectra.

The structure and the relative energies of all possible tautomeric forms of the uracil and cytosine molecules have been determined using both local and gradient-corrected density functional methods. The calculations have been performed with double-zeta plus polarization basis sets and the geometries optimized with analytic gradient techniques. The vibrational frequencies and the contribution of the zero-point energies have also been computed. In the uracil case, the dioxo form is predicted to be the most stable. In the cytosine case, three tautomers are found to be very close in energy with the oxo-amino form slightly more stable. The infrared absorption intensities and frequencies for the uracil and the two more stable tautomeric forms of the cytosine molecules are reported and compared with experimental spectra. The agreement with experiment and correlated ab-initio methods is good for geometries, energetics, and vibrational frequencies.

The influence of the solvent on the structure and IR spectrum of the [Fe(CN)(5)NO](2)(-) ion is investigated by using gradient corrected density functional theory. IR spectra are also measured on different solvents and the results obtained are compared with the predicted ones. We have treated the solvent effects with a continuum model, based on the Onsager's reaction field approach; in order to mimic strong specific interactions, calculations were also performed on the complex protonated at the cyanide trans to the nitrosyl group. The reaction field calculations predict only qualitatively the most important observed trends, e.g., the shifts in the nitrosyl stretching wavenumber, but fail in accounting quantitatively for the differences between the spectra in water and acetonitrile. The possible role of specific interactions is consistently accounted for by interpreting the experimental shifts of the NO stretching wavenumber nu(NO), as well as the visible absorption energies, when changing the Lewis acidity of the solvent, as measured by the Gutmann's acceptor number. Ligand population analysis was performed to relate the solvent effects with the sigma donor and pi acceptor behavior of cyanide and nitrosyl ligands. The significance of nu(NO) shifts as a result of changes in the medium is discussed in view of the physiological relevance of transition-metal nitrosyl chemistry.

mThe effects of the solvent on the tautomeric equilibria of cytosine and uracil are studied using Onsager's reaction field model in the framework of density functional theory. In this model, the solute molecule is placed in a spherical cavity of radius a, immersed in a continuous medium with a dielectric constant E , treating the solute-solvent electrostatic interactions at the dipole level. The cavity radius is evaluated by determining the molecular volume with a quantum mechanical approach. The solvent effect brings significant changes in the geometrical parameters of some cytosine tautomers, but only very small changes to those of the uracil tautomers. Our results are in good agreement with available experimental results and confirm that the polarization of the solute by the continuum has important effects on the absolute and relative solvation energies. Frequency shifts and intensity variations in the infrared spectra due to the presence of the solvent are also presented. 0 1995 John Wiley & Sons, Inc.tomers may have important biological implications, such as mutagenesis. In addition, the effect of the environment on tautomeric equilibria may be dramatic, changing equilibrium constants several orders of magnitude. Recently, we reported gas-phase calculations on the tautomerism of uracil and cytosine molecules [9] using density functional theory (DFT); the results are in reasonable agreement with the avail-

Theoretical calculations of the potential energy surface (PES) for the [NH, + HCl] system are presented using several standard ab initio methods such as Hartree-Fock (HF), second-order Mdller-Plesset perturbation theory (MP2), coupled cluster (CC), complete active space self-consistent-field (CASSCF), density functional theory (DFT), and less traditional ab initio approaches such as Dirac-Fock four-components and the use of effective Hamiltonian techniques, such as the recently proposed K functional. All calculations predict a single minimum for the complex, corresponding to a hydrogenbonded structure, confirming early studies. The dynamical and nondynamical contributions to the correlation energy are discussed for different cuts of the PES, involving different N-Cl distances. The complex has also been characterized by performing a full geometry optimization within the HF and DFT schemes; with the latter we have performed also the vibrational analysis. The predicted binding energies and infrared (IR) spectrum are compared with other theoretical and experimental results. For the gas phase, we propose a binding energy of -5.3 f 0.5 kcal/mol, thus revising the experimental value of -8.0 k 2.8 kcal/mol; for the minimum, the predicted N-H and * is done with approximate inclusion of solvent effects (Onsager reaction field), the minimum is shifted and it corresponds to the ion pair NH:. C1-structure, similar to Mulliken's outer complex. Since the first ab initio computation for the NH,Cl complex is the pioneer work in 1967 by E. Clementi, the present work provides us with an opportunity to comment on some aspects of the evolution in computational chemistry, particularly for energy determinations. We have concluded our comments with the invitation to use four-components Fock-Dirac for molecules both with high and low Z atoms, rather than the traditional Hartree-Fock and related methods. In other words, we are of the opinion that the time is ready in quantum chemistry to switch from the Schrodinger to the Dirac representation, due to new developments in computer hardware and software. In addition, the use of effective Hamiltonians, like the recently proposed " K functional," seems to deserve attention, because of their computational simplicity and physical reliability in predicting correlation corrections. 0 1996 John Wiley & Sons, Inc.