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.

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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.

Calculations have been performed on 10 structures of the cluster H+(H20)5. It is shown that the most stable ones are an open (Eigen) and a cyclic four-membered-ring structure very close in energy and possibly degenerate. This can explain that different structures were proposed by experimentalists. The easy evolution of some structures into others is likely related to the nature of the first solvation shell in larger clusters or solutions. Vibrational frequencies, useful to interpret experimental data, are computed for the two most stable structures. The Problem: Structure of H+(HzO)s and the First Solvation ShellIn 1954, Eigen et al.' proposed as a hydration model for the proton in aqueous solution and ice an oxonium ion H30+ surrounded by three water molecules in a first solvation shell. In some early theoretical work (1956) the presence of a fourth water molecule in the first solvation shell has also been suggested;* three water molecules are hydrogen bonded with the three hydrogen atoms of the ion, while the fourth water molecule is located above the oxygen atom (Figure 1, protondonor structure 1). Since then, there were many proposal^^-^^ on the structure and coordination number, and presently the issue is not settled, neither theoretically nor experimentally. Some of these studies are concemed with the cluster H30+(H20)4, others with larger systems.The first attempt by Newton et aL3 in 1971 to study such systems with ab initio quantum mechanical calculations was restricted to small hydrates involving, at best, four water molecules and the oxonium ion. Using a 4-31G basis set at the Hartree-Fock level, the authors optimized a few structures for the system &O+(H*0)3 and then added to it a fourth molecule. Within these limitations the most stable structure has three water molecules in the Eigen-like structure first shell and a fourth water molecule in the second shell, hydrogen bonded to one water molecule of the first shell with an 0. * 0 distance arbitrarily chosen (Figure 1, structure 2). The interpretation of an infrared spectrum published some time later is based on such a structure.8 However, two experimental paper^^.^ suggested that the fist solvation shell has four water molecules. In particular, from X-ray and thermal neutron studies of hydrochloric acid solutions at 20 "C, Triolo et a1.6 proposed a charge-dipole complex (Figure 1, structure 3). In a further work, ' Newton (1977) extended his studies to structures 1 and 3 in order to check this assumption. He found that neither of these two structures were stabilized with respect to H30+(H20)3 f Hz0, with a more favorable situation for the charge-dipole complex structure 3 than for the hydrogen bonded structure 1. A complete optimization of these structures with ab initio calculations was unfortunately not possible at the time, and the true minima might have been missed. Furthermore, it must be pointed out that the structure of the first shell may be different 'Abstract published in Advance ACS Abstracts, April 15, 1995.