Potential surfaces for proton transfer reactions

VB Potential Surface for Proton Transfer Reactions in Solutions... 

FIGURE 9.1. The potential surface for proton transfer reaction and the effect of constrainir the tiA B distance. The figure demonstrates that the barrier for proton transfer increasi drastically if the A — B distance is kept at a distance larger than 3.5 A. However, in solutic and good enzymes the transfer occurs through pathway a where the A - B distance is arour 2.7 A. 

VB potential surface for proton transfer reactions in solutions. Let us consider a proton transfer reaction in solution, which can be written as... 

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. 

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. 

Very different proton transfer reaction efficiencies from these two potential wells have been measured, and only the clusters in which the internal energy is sufficient to cross over the barrier will lead to the reaction. The empirical potential surface and the potential barrier between the two wells have been estimated (Steadman and Syage 1991). For ammonia clusters, upper limits for the barriers were estimated to be about 1.5 eV (n = 1) to 1.0 eV (n = 4). 

It had been anticipated that the reaction pathway for a proton transfer reaction should be characterized by a double minima in the potential surface.100 The failure of Qementi s calculation to predict a barrier along the reaction pathway for the transfer of H + from HQ to NH3 to form NHJC1-, must be accepted with some caution in view of the lack of polarizing functions on N and Cl in the basis set. This work, however, broke new ground and dementi99 includes in his paper a detailed discussion of the merits and faults to be expected for the Hartree-Fock method in the different regions of the potential surface. 

Fig. 30. Potential surfaces (energy versus reaction coordinate) for typical gas-phase, ion-molecule reactions (a) proton-transfer reactions, (b) electrophilic displacement reactions.

The hyperspherical and related coordinates which have been considered in this work have served for the visualization of critical features of potential energy surfaces [91,92], crucial for the understanding of reactivity (role of the ridge [93] and the kinetic paths [94]). In [95], the PES for the O + H2 reaction was studied. A discrete hyperspherical harmonics representation is presented in [96] for proton transfer in malonaldehyde. 

Figure 10 Potential surface for interconversion of uncorrelated protons in a centrosymmetric hydrogen bonded dimer. Coordinates x 1 and x-2 correspond to the stretching modes. The long dash lines are schematic representations of the reaction paths for uncorrelated proton transfers. The potential function is V(a i, 2) = Vo(a i) + V0(x2)+a0(x1 +x2), where Vo (a ) is the symmetric part of the double minimum potential represented in Fig. 7.

As it follows from the quantum chemical calculations, two minima on the potential curves for the interaction of organic molecules with the silica surface OH group are due to formation of linear hydrogen bonded and following cyclic donor-acceptor complexes prior to the transition state for proton transfer in these reactions. In case of large molecules interaction, the second minimum often missing from the potential curve and the donor-acceptor complex is consistent with the transition state of proton transfer reaction. 

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