All-electron generalized-gradient density functional (DF) calculations have been carried out on both the nonrelativistic and relativistic levels to address the mechanism of N2 cleavage by L3Mo(III) complexes. The reaction features computed for the model reactants with L = NH2, NMe2, and Me are compared to the recently published values for the (H2N)3Mo derivative obtained with the help of the effective core potential (ECP) DF method and to the experimental data reported for the reaction of N2 with (RArN)3Mo (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2). Despite a qualitative agreement between the reaction parameters of the present relativistic and the previous ECP DF calculations, some important quantitative differences are found, like a lower activation barrier by more than 10 kJ mol-1 and a higher overall reaction exothermicity by about 80 kJ mol-1. The influence of the electronic and steric properties of the ligands L on the parameters of the cleavage reaction has been analyzed. The Mo(III) trialkyl complexes, at least those with small ligands, are predicted to scissor N2 molecules less efficiently than the triamido derivatives. The relativistic effects, which are unusually large for 4d metal compounds, have been calculated and are attributed to the exceptionally strong Mo⋮N bonds.

The universal force field approach is used to investigate the steric demand in nitrogen molecule cleavage by three-coordinate molybdenum complexes MoL3 of different ligand types L (L = NH2, NMe2, N(mesityl)(tert-butyl), O(tert-butyl), Me, tert-butyl, neopentyl). Calculated geometries of the intermediates L3Mo−N2−MoL3, of the products L3MoN, and of the undersirable side product dimers L3MoMoL3 are presented. The primary role of ligand sterics appears to be the prevention of dimerization of MoL3 monomers.

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Molecular dynamics simulations have been carried out to study structural aspects of the photo repair mechanism of DNA photolyase. In particular, we investigated the docking and binding of bare and dressed model pyrimidine dimers, U〈〉T and dU〈p〉dT, respectively, in the enzyme pocket. These dimers, which split after photoinduced electron transfer, are essentially inflexible in the gas phase, in water, and inside the enzyme pocket. Details of the dimer docking and the binding inside the pocket are presented and the influence of the desoxyribose and phosphate link on the dimer docking are discussed. The minimum van der Waals distances for the electron transfer between the electron donor FADHand the accepting dimers are found to be about 5 Å for U〈〉T and about 9 Å for dU〈p〉dT. Analysis of the structure of the dimer models and their orientation in the enzyme pocket as well as the orientation of the FADHdonor suggests that indirect electron transfer to the dimer may prevail.

A simple method for incorporating bond-length constraints in Monte Carlo simulations of cyclic and linear molecules is described. As an example, the conformational behavior of five even-numbered cyclic alkanes is studied using Monte Carlo simulation and the MM2 force field. 0 1994