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Research was conducted to investigate substrate oxidation in the active site of xanthine oxidase and related enzymes using a model density functional approach. Results showed the presence of a stable structure on the potential energy surface and the activation barrier of the hydride transfer to be 7.7 kcal/mol. Findings also revealed that the resulting complex of Moco with the oxidized substrate had almost the same energy as that of the parent intermediate. It was also observed that a water molecule squeezes the product with a small energy gain from the reduced Moco.

The relativistic effects of M = Ag on [M4{Fe(CO)4}4]4- clusters (M = Cu, Ag, Au) and bare clusters of M4Fe4 species was analyzed by the Gaussian-type orbitals density functional (LCTGTO-DF) method. The molecular geometry of the [M4{Fe(CO)4}4]4- anions are characterized by the presence of four mu2-bridging Fe(CO2) units that surround the M atoms. Furthermore, CO ligands stabilize the bimetallic M-Fe clusters which lead to a reduction to metal-metal bond strength of the M-Fe core.

Ground state spin-orbit effects in the molecules V[sub 2], VNb, and Nb[sub 2] have been investigated using self-consistent field configuration interaction (SCF/CI) calculations based on the intermediate neglect of differential overlap Hamiltonian parameterized for optical spectroscopy (INDO/S). Accurate results for the spin-orbit splittings of the X [sup 3]Σ[sup -] ground state of each molecule were obtained using a CI treatment designed to isolate essential correlation of the ground state and the isoconfigurational [sup 1]Σ[sup +] state causing the splittings. Energies computed for the [sup 1]Σ[sup +] states are compared to energies obtained from simple two-state perturbative models. It is observed that the consideration of only two states is a large source of error. Previous assignment of a low-lying excited state observed for V[sub 2] as the [sup 1]Σ[sub g][sup +] state is not supported. Additional results on excited states of VNb have also been obtained. A [sup 3]Σ[sup -]←X [sup 3]Σ[sup -] transition observed experimentally for VNb is assigned as a σ[sup *]←σ promotion, and the presence of a second excited [sup 3]Σ[sup -] state of VNb that has not been observed is suggested. An analysis of correlation effects in the INDO/S model is given based on a comparison of results obtained with the approximate essential correlation CI calculations and with larger restricted active space full CI calculations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An electrostatic model for physisorption at ionic solids is developed. It is based on the asymptotic density model recently introduced for the efficient calculation of electrostatic potentials. The shape of the interacting systems is generated by appropriate isodensity surfaces. An optimization procedure for orientational effects is described. The model is applied to the dimers of CO2 and H2O. Finally, the CO adsorption on NaCl(100) is simulated by NaCl clusters of varying surface size. The influence of the position and site of the adsorbed molecule, the size of the surface, and the optimization of the cluster on orientation and adsorption energy is presented and analyzed. It is found that the adsorption energy is primarily dependent on the distance between the nearest atoms of cluster and adsorbant, which is determined by the shape of the isodensity surface of the cluster and, to a lesser extent, by long range attractions. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Gradient-corrected density functional calculations were done to analyze the structure and energetics of the molybdenum site involved in the xanthine oxidase-related aldehyde oxido reductase. The results revealed that aside from the experimental structure, LMoOS(OH)2, there are two other structures, LMoOS(OH)2 and LMoO(SH)(OH) which exhibit similar energies, indicating that they can also represent the oxidized state of molybdenum cofactor.

The electronic structure and spectroscopy of the vanadium dimer has been studied with semiempirical self-consistent field-configuration interaction calculations using the intermediate neglect of differential overlap Hamiltonian parameterized for spectroscopy (INDO/S) including spin-orbit coupling effects. An approximate configuration interaction (CI) treatment is designed based on correlation effects observed in CI calculations in small active spaces, and yields good agreement with experimental observations of state energies and spin-orbit splittings. The location of a [sup 1]Σ[sub g][sup +] excited state isoconfigurational with the ground state was determined, and calls into question a previous assignment of an excited state observed near 1860 cm-1. The previously observed A [sup 3]Π[sub u]←X [sup 3]Σ[sub g][sup -] transition is assigned as a dδ[sub g]←dπ[sub u] promotion. In addition, an unassigned transition observed near 15 000 cm-1 has been assigned as B [sup 3]Σ[sub u][sup -]←X [sup 3]Σ[sub g][sup -]. Both this transition and the previously observed A[sup ′] [sup 3]Σ[sub u][sup -]←X [sup 3]Σ[sub g][sup -] transition are assigned as σ[sub u]←σ[sub g] promotions, in disagreement with previous assignments. A [sup 1]Σ[sub u][sup +] state isoconfigurational with the A[sup ′] [sup 3]Σ[sub u][sup -] state is suggested as a candidate for an unassigned transition in the range 11 250-12 500 cm-1. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]