Electronic aspects of the hydride transfer mechanism. A b i n i t i o analytical gradient studies of the cyclopropenyl-cation/lithium hydride model reactant system

Abstract

The electronic mechanisms of a model hydride transfer reaction are theoretically studied with ab inito RHF and UHF SCF MO procedures at the 4-31G basis set level and analytical gradient methods. The model system describes the reduction of cyclopropenyl cation to cyclopropene by the oxidation of lithium hydride to lithium cation. The molecular fragments corresponding to the asymptotic reactive channels characterizing the stepwise mechanisms currently discussed in the literature have been characterized. The binding energy between the fragments is estimated within a simple electrostatic approximate scheme. The results show that a hydride-ion mechanism is a likely pathway for this particular system. The system is thereafter thoroughly studied from the supermolecule approach. Reaction paths for the ground and first triplet electronic states have been calculated. The hypersurface is explored from a geometrical disposition of the reactants that mimics the one found in several dehydrogenases (perpendicular configuration). A hydride ion is found to be the particle transferred on the unconstrained as well as the constrained reaction pathways in the ground electronic state. In the triplet state (perpendicular configuration) the mechanism is stepwise: electron transfer followed by a hydrogen atom transfer. It has been noticed that the perpendicular geometrical disposition of the reactants plays an important role by polarizing the susceptible cyclopropene C–H bond in the sense of increasing the electronic density at the hydrogen nucleus. This provides a clue to rationalize several dehydrogenase’s active site structure and mechanism. The reactant molecular complex found in the inverted potential energy curves, namely the LiH---Cp+ association has an electronic distribution which can be described as a hydride ion cementing two electron deficient centers corresponding to the cyclopropenyl and the lithium cations. Direct CI calculations confirm the overall picture obtained above.

On Transition Structures for Hydride Transfer Step in Enzyme Catalysis. A Comparative Study on Models of Glutathione Reductase Derived from Semiempirical, HF, and DFT Methods

“…Our previous results 16,19,20,23,73 on models of the dehydrogenase enzyme family show that two mechanisms are possible: a hydride ion mechanism and an electron transfer accompanied by a hydrogen atom migration. Within the diabatic approach used in our work, 16a, the electron-hydrogen atom mechanism appears to be a plausible path. On the other hand, Bertrán et al.…”

Hydride transfer reaction dynamics of OD++C3H6

“…OH + has two unpaired electrons and no empty p orbital. Following the discussion of Tapia et al, 17 we expect the hydride transfer mechanism to first involve electron transfer, which is then followed by hydrogen atom transfer. In the present system, ground state OD + +C 3 H 6 on the triplet surface may first undergo rapid charge transfer.…”

On the Molecular Mechanism of Liver Alcohol Dehydrogenase: Monte Carlo Simulations and X-Ray Studies of Water in the Substrate Channel

“…Quantum chemical studies [23,24] have shown a hydride ion as the most likely structure for this set of transferred particles. In this case the environment to the exchang ing centers has to be hydrophobic, otherwise, the hydride ion could react with a water mol ecule to produce molecular hydrogen, with particularly nasty consequences for the bio system.…”