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The singlet and triplet states of AlCu and AlAg below about 32 000 cm-1 are studied using the internally contracted multireference configuration-interaction method. A more elaborate study of the X 1Σ+ ground state of AlCu is undertaken using extended Gaussian basis sets, including the effect of inner-shell correlation and including a perturbational estimate of relativistic effects. Our best estimate of the spectroscopic constants (r0, ΔG1/2, and D0) for the X 1Σ+ state with the experimental values in parentheses are: 4.416(4.420) a0, 295(294) cm-1, and 2.318(2.315) eV. The calculations definitively assign the upper state in the observed transition at 14 892 cm-1 to the lowest 1Π state. The calculated spectroscopic constants and radiative lifetime for the 1Π state are in good agreement with experiment. The calculations support the tentative assignments of Behm et al. for three band systems observed in the visible region between 25 000 and 28 000 cm-1. However, the computed spectroscopic constants are in very poor agreement with those deduced from an analysis of the spectra. Analogous theoretical results for AlAg suggest that the (2)3Π, (3)3Π, and (3)1Σ+ states account for the bands observed, but not assigned, by Duncan and co-workers. [ABSTRACT FROM AUTHOR]

The lowest 5Π and 5Σ+ states of CO are characterized using very large one-particle basis sets and the internally contracted multireference configuration–interaction treatment of electron correlation. It is found that the (1)5Σ+ state is weakly bound (De=572 cm-1) with an re of 4.76 a0, in good agreement with earlier theoretical calculations by Rosenkrantz et al. [Theor. Chim. Acta 82, 153 (1992)]. The (1)5Π state (hereafter designated a‘ 5Π) has a weakly bound outer well (De=18 cm-1, re=5.80 a0) and a significantly bound inner well (De=6043 cm-1, re=2.76 a0). It is shown that the emission spectrum between about 360 and 520 nm observed by Bahrdt et al. [Chem. Phys. 144, 273 (1990)] can be assigned to the a‘ 5Π-a’ 3Σ+ and a‘ 5Π-d 3Δ transitions. Franck–Condon factors are presented to aid in the experimental identification of the a‘ 5Π state by absorption from the low-lying triplet states of CO. [ABSTRACT FROM AUTHOR]

Ab initio all-electron quantum mechanical methods were applied to the tungsten hexahydride (WH6) molecule using very large basis sets. Seven distinct structures were investigated with full geometry optimizations at the self-consistent field (SCF) level of theory. The effects of electron correlation were estimated with second-order Mo\ller–Plesset perturbation theory (MP2), modified coupled pair functional (MCPF) method, and the coupled cluster with single and double excitations (CCSD) method, and relativistic effects estimated using the Cowan–Griffin (mass–velocity and Darwin) corrections. Our results suggest that the ground state of the tungsten hexahydride (WH6) molecule is a closed-shell triangular prism belonging to the C3v point group, with a set of three hydrogens stacked on top of another set of three hydrogens. From the perspective of inorganic chemistry, it is truly remarkable that the octahedral structure lies 130 kcal/mol above the C3v ground state. Furthermore, these results imply major qualitative differences between WH6 and the well-known W(CH3)6 molecule. Another C3v structure, with one set of three hydrogens staggered with respect to the other set, is energetically nearby. With MCPF and relativistic corrections at the SCF optimized geometry, a third low-lying structure belonging to the C5v point group is virtually degenerate with the triangular prism C3v structure. [ABSTRACT FROM AUTHOR]

Potential energy curves have been calculated for the X 3Σ- and 5Σ- states of NH using a complete-active-space self-consistent-field/multireference configuration-interaction procedure and extensive Gaussian basis sets. An accurate dipole moment function is computed for the X 3Σ- state. Transport cross sections have been calculated for the collisions of hydrogen and nitrogen atoms in their ground states. The mean transport cross sections are tabulated for collision energies in the range 0.0001–1 Eh; the mean collision integrals are listed for temperatures between 100 and 100 000 K. The ab initio energies for the 5Σ- state of NH are found to be consistent with the results of correlation studies and agree well with the prediction from combining relations using the energies of the highest spin states of H2 and N2. [ABSTRACT FROM AUTHOR]

The N + N[SUB2] exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground [SUP4]A″ state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N + N[SUB2] (J = 0). The J-K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasiclassical calculation using a London-Eyring-Polanyi-Sato PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate three-dimensional quantal dynamics study for the N + N[SUB2] reaction system and the ab initio PES reported here is the first such surface for N[SUB3]. [ABSTRACT FROM AUTHOR]

Matrix infrared spectra of Co[SUB2]CO were obtained from cobalt vapors condensed with different isotopically substituted mixtures of carbon monoxide diluted in argon at 10 K. New bands at 1953.3, 488.7, 357.9, and 270.9 cm[SUP-1] are assigned to the v[SUBC-O], v[SUBM-CO], v[SUBM-C-O], and v[SUBM-M] fundamentals, respectively. Computations performed using density functional theory with the generalized gradient approximation (DFT-GGA) for the exchange-correlation potential predict the lowest energy state of Co[SUB2]CO to be [SUP5] A″ with the angle Co-Co-C of 116°. Our DFT-GGA vibrational frequencies of this state agree with the experimental frequencies extremely well--within 15 cm[SUP-1]. Our estimate for the Co[SUB2]-CO bond strength is 1.60 eV. Computed vibrational frequencies for Co[SUB2]CO[SUP+] and Co[SUB2] CO[SUP-] are included for comparison. [ABSTRACT FROM AUTHOR]