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The structures and relative stabilities of the complexes formed by uracil and its thio- and seleno-derivatives with the Sr(2+) cation, in the gas phase, have been analyzed by means of G96LYP density functional theory (DFT) calculations. The attachment of the Sr(2+) cation to the heteroatom at position 4 is preferred systematically. Although the enolic forms of uracil and its derivatives should not be observed in the gas phase, the corresponding Sr(2+) complexes are the most stable. The enhanced stability of these tautomers is two-fold, on the one hand Sr(2+) interacts with two basic sites simultaneously, and on the other hand an aromatization of the six-membered ring takes place upon Sr(2+) association. Sr(2+) attachment also has a clear catalytic effect in the tautomerization processes involving uracil and its derivatives. This catalytic effect increases when oxygen is replaced by sulfur or selenium. The Sr(2+) binding energy with uracil and its derivatives is bigger than the tautomerization barriers connecting the dioxo forms with the corresponding enolic tautomers. Consequently, when associated with Sr(2+), all tautomers are energetically accessible and should all be observed in the gas phase.

The interaction of BeX(2) (X = H, F) with water molecules has been analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The formation of strong beryllium bonds between water molecules and the BeX(2) derivative triggers significant electron density redistribution within the whole system, resulting in significant changes in the proton donor and proton acceptor capacity of the water molecules involved. Hence, significant cooperative and anti-cooperative effects are present, explaining why there is no case in which the global minimum corresponds to a tetracoordinated beryllium atom. In fact, the most stable clusters can be viewed as the result of the attachment of BeX(2) to the water trimer and the water dimer, respectively, and not as the result of the solvation of the BeX(2) molecule. We have also shown that the decomposition of the interaction energy into atomic components is a reliable quantitative tool to describe all the closed-shell interactions present in the clusters investigated herein, namely hydrogen bonds, beryllium bonds and dihydrogen bonds. Indeed, we have shown that the changes in the atomic energy components are correlated with the changes in the strength of these interactions, and they provide a quantitative measure of cooperative effects directly in terms of energies.

The bonding interaction between non-hydrogen-bonded (NHB) hydrogen atoms and the non-nuclear attractor (NNA) observed in cavity-bound anionic water clusters has been investigated using ab initio methods. The clusters range in size from four to ten water molecules and generally consist of several water molecules hydrogen-bonded in a ring, with two of these rings stacked such that the NHB hydrogen atoms are directed towards a central cavity. Analysis of the electron density using the theory of atoms in molecules yields information about the bonding interaction that cannot be obtained with conventional computational techniques. The results indicate that this novel interaction, Ha...NNA, is a weak, closed-shell interaction. Further analysis of the atomic properties of the NHB hydrogen atoms indicate that this novel bond cannot be classified as a hydrogen bond. On the basis of the results from the current investigation, this interaction is best described as a weak dipole-ion interaction between the positively charged NHB hydrogen atoms and the negatively charged NNA.

The CNDO/BW theory is applied to triatomic and polyatomic molecules and ions to calculate their spectroscopic ground states, equilibrium geometries, bonding energies, and force constants. The calculated results are compared to the experimental results and the results of other CNDO methods.

The CNDO/BW method is used to calculate the spectroscopic states, equilibrium geometries, bonding energies, and force constants of diatomic molecules and ions. Four parameter sets are used, based on Hinze and Jaffe, and Hartree–Fock atomic parameters and on resonance integrals, β, evaluated with, and without, the inclusion of valence-state ionization potentials. All parameter sets yielded better predictions of experimental results than any other current CNDO parametrization.

Vertical ionization potentials, calculated as the difference between the total energies of the ground states of the ion and its parent molecule, are compared with estimates obtained by use of Koopmans' theorem. For some molecules the differences between the two theoretical values are quite large, while for molecules where the molecular orbitals are constrained by symmetry, the two theoretical values are identical. Differences between calculated adiabatic and vertical ionization potentials are shown to agree with the qualitative features of photoelectron spectra. The best overall fit of theoretical and experimental ionization potentials is obtained with atomic parameters evaluated from Hinze and Jaffe valence-state data and resonance integrals dependent upon valence-state ionization potentials. The theoretical predictions are generally superior to those of CNDO/2 or Hartree–Fock calculations.

The semiempirical, SCF–MO theory, using the approximation of CNDO is parameterized for the prediction of potential surfaces for both open and closed shell molecules and ions. It is designated CNDO/BW. Empirical examination of the importance of core repulsion leads to the introduction of an expression for the core repulsion energy which results in the simultaneous prediction of relatively good equilibrium geometries and bonding energies for a variety of molecules. The importance of several of the CNDO parameters is examined in this context.

The unrestricted Hartree–Fock (UHF) method of Amos and Snyder has been employed to calculate the spin density distributions in some radical anions and cations of methyl substituted condensed benzenoids. The heteroatom model has been employed for the methyl group. A least-squares-fit analysis of the data indicates a one-parameter linear relation between the methyl-proton splitting and the spin density on the attached carbon. The value of the proportionality constant is significantly higher for the cations (44 G) than for the anions (21 G).

One-centre coulomb integrals (kk/lI) evaluated from analytical Hartree–Fock valence-state energies are shown to be more accurate than those evaluated from atomic spectral valence-state energies. The simple electrostatic method of Saturno is used to calculate (kk/lI) and is found to be of limited value for estimating coulomb integrals when k≠l, since it removes the angular dependence of the integrals.