Applications Of Quantum Chemistry

Ⅰ.Topology of Electron Charge Density for Chemical Bond from Valence Bond TheoryTo characterize the nature of bonding we derive the topological properties of electron charge density of a variety of bonds,based on ab initio valence bond methods. The electron density and its associated Laplacian are partitioned into covalent,ionic, and resonance components in the valence bond spirit.The analysis provides a density-based signature of bonding types and reveals,along the classical covalent and ionic bonds,the existence of two-electron bonds in which most of the bonding arises from the covalent-ionic resonance energy,so-called charge-shift bonds.As expected, the covalent component of the Laplacian at the bond critical point is found to be largely negative for classical covalent bonds.By contrast,in charge-shift bonds,the covalent part of the Laplacian is small or positive,in agreement with the weakly attractive or repulsive character of the covalent interaction in these bonds.On the other hand,the resonance component of the Laplacian is always negative or nearly zero,and it increases in absolute value with the charge-shift character of the bond,in agreement with the decrease of kinetic energy associated with covalent-ionic mixing. A new interpretation of the topology of the total density at the bond critical point is proposed to characterize covalent,ionic and charge-shift bonding from the density point of view.Ⅱ.DFT Study of the Monooxygenation Reactions of the Substrates Containing Sulfur Atom by Cytochrome P450One of the major and a "hot" mechanistic problems in P450 chemistry is what is actually the oxidant species that carries the oxidative processes of this enzyme and what is the root cause of its immense versatility as a primary oxidant in nature:is it the high valent iron-oxo species,so-called CompoundⅠor the precursor species,so called Compound 0? We undertook the challenge posed by the experimental data and carried out theoretical calculations,designed to addresses the CompoundⅠ—Compound 0 conundrum in sulfoxidation of thiafatty acids by P450.Our results demonstrate that CompoundⅠleads to an extremely fast process,while Compound 0 will be at least six orders of magnitude slower.We suggest that most likely,thiafatty acids promote CompoundⅠformation even in the T→A mutant of P450_BM3.In this manner we provide a clear answer to a major mechanistic puzzle and suggest some new experiments.A preliminary DFT calculations were carried out on the sulfoxidation reaction by the enzyme cytochrome P450 by taking p-methylthio-N,N-dimethylaniline (p-MT-DMA) as substrate.The results reveal that in the presence of CpdⅠ,Cpd 0 is clearly silent for sulfoxidation.While in the absence of CpdⅠ,Cpd 0 is still an inferior oxidant compared with CpdⅠ.