Potential energy surface Proton-transfer

Figure 2.9 Model potential energy surface for combined electron and proton transfer. is the solvent coordinate for electron transfer and Q2 that for proton transfer. (See color insert.)...

Figure 2.10 Potential energy surface for combined electron and proton transfer.

However, it is not known into which part of the potential energy surface these species couple. The reactions of H2CO+ with OH and CH2OH+ with O-atoms would also access the surface although these are not experimentally very tractable. The surface is also accessed to a limited extent by the gas kinetic proton transfer from HCO+ to H20 yielding H30+. 

The reaction potentials displayed in Scheme 2.6 are those appropriate for the symmetric transfer of a proton in a vacuum, AH+ A <-> A HA+. However, when the system is placed in a polar solvent, the effect of the polar solvent upon the stability of the reactant and product state must be taken into account. The reactant and the proton state will have different solvent structures (Scheme 2.7). The effect of having different solvent structures associated with the reactant and product state is to break the symmetry of the potential energy surface associated with the proton-transfer coordinate. 

The effect of the solvent upon the breaking of the symmetry of the potential energy surface for proton transfer has a profound consequence for the reaction dynamics for proton transfer. The tunneling of the proton out of the reactant state... 

This model has obvious shortcomings. For example, the interaction with the solvent in the initial state is straightforward since the proton is in the ionic form, whereas in the final state, the proton is the nonionic adsorbed H atom and its interaction with the solvent should be negligible. No consideration of this fact was made in the potential of the final state Uf m Eq. (43). However, this treatment incorporates the basic feature of the proton transfer reaction interaction with the solvent, tunneling as well as classical transition of the proton, and the effect of the electric field on the potential energy surfaces of the system. 

The empirical valence bond (EVB) approach introduced by Warshel and co-workers is an effective way to incorporate environmental effects on breaking and making of chemical bonds in solution. It is based on parame-terizations of empirical interactions between reactant states, product states, and, where appropriate, a number of intermediate states. The interaction parameters, corresponding to off-diagonal matrix elements of the classical Hamiltonian, are calibrated by ab initio potential energy surfaces in solu-fion and relevant experimental data. This procedure significantly reduces the computational expenses of molecular level calculations in comparison to direct ab initio calculations. The EVB approach thus provides a powerful avenue for studying chemical reactions and proton transfer events in complex media, with a multitude of applications in catalysis, biochemistry, and PEMs. 

One such feature is the potential energy surface for proton transfer in the contracted hydrogen bonds. The time-averaged potential surfaces are almost symmetrical (especially for the Zundel ion) without significant barriers, i.e., the proton is located near the center of the bond. Whether its location is off-center at any time instance is mainly dependent on the surrounding hydrogen-bond pattern, and it is the... 

Figure 8. Qualitative potential energy surface proposed to explain the clustering of methoxide onto methanol and the "slow" rate of symmetric proton transfer. Energies in parentheses have units of kcal mol . ...

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

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