| In this work, the structures and/or reaction mechanisms for three chemically interesting systems were investigated through ab initio and density functional theory (DFT) calculations.;I. The reaction mechanism of an analogue system of the molybdenum oxotransferases was investigated at the DFT level of theory. Kinetic measurements by Schultz and Holm suggest that the reaction MoO 2(t-BuL-NS)2 + X → MoO(t-BuL-NS) 2 + OX (t-BuL-NS = bis(4-tert-butylphenyl)-2-pyridylmethanethiolate(1-)) occurs through an associative transition state. Our results on the model reaction, MoO2(SHCH2CHNH)2 + P(CH3) 3 → MoO(SHCH2CHNH)2 + OP(CH3) 3, supports this hypothesis, and indicate that this reaction proceeds through a two-step mechanism via an associative intermediate. The DeltaH ‡ for the first (rate-determining) step was predicted to be 9.4 kcal/mol, and DeltaH‡ for the second step was predicted to be 3.3 kcal/mol, in good agreement with the experimental system (DeltaH‡ = 9.6(6) kcal/mol).;II. DFT calculations were used to investigate synthetic complexes with an diiron dioxo diamond core and intermediates in the catalytic cycle of methane monooxygenase (MMO). The synthetic complexes share an antiferromagnetically coupled diiron dioxo/hydroxo diamond core structure with the oxidized, H ox, and reduced, Hred, intermediates of MMO. DFT calculations on model complexes with ferromagnetic coupling of the synthetic species, reproduced the crystal structure data to within 0.05 A and 5° for the diamond core parameters. The B3P86 calculations strongly suggest that Hox has two bridging hydroxy ligands and the carboxylate shift, established in the crystal structure of Hred, was calculated to be a minimum at the BP86 level of theory.;III. The thermal decomposition of N,N '-diacyl-N,N'-dialkoxyhydrazines, (N(COR1)(OR2))2, to the corresponding ester and dinitrogen was investigated with ab initio and DFT calculations. The results suggest that the decomposition proceeds via a two-step 1,1-elimination. Coupled-cluster calculations on model systems (R1=R2=H; R1=CH3, R2=H; R1=R2=CH3) show that the barrier for the first elimination (rate-determining) is 24--34 kcal/mol, while the barrier for the second step is only 1--3 kcal/mol, a value that is much smaller than the exothermicity of the first step, explaining why experimentalists have been unable to trap the intermediate nitrene. An unexpected result was the failure of Hartree-Fork and Moller-Plesset second order perturbation to describe properly all aspects of this seemingly simple system. |
