An Ab Initio Molecular Orbital−Valence Bond (MOVB) Method for Simulating Chemical Reactions in Solution

Abstract

A mixed molecular orbital and valence bond (MOVB) method for describing the potential energy surface of reactive systems has been developed and applied to a model proton transfer reaction in aqueous solution. The MOVB method is based on a block-localized wave function (BLW) approach for defining the diabatic electronic states. Then, a configuration interaction Hamiltonian is constructed using these diabatic states as the basis function. It was found that the electronic coupling energy is large with a value of about 30 kcal/mol for the H 3 N-H-NH 3 + system, whereas the predicted activation barrier is only 1.2 kcal/mol using the 3-21G basis set. The MOVB results are found to be in good accord with the corresponding ab initio Hartree-Fock calculations for the proton transfer process. We have also incorporated solvent effects into the MOVB Hamiltonian in the spirit of combined QM/MM calculations, and have modeled the proton transfer between ammonium ion and ammonia in water using Monte Carlo simulations. The potential of mean force was computed via free energy perturbation coupled with umbrella sampling techniques using (1) an energy gap mapping approach, and (2) a geometrical mapping procedure. Solvent effects increase the barrier height by about 2.2 kcal/mol from the MOVB and HF ground state potential energy surface. The present study demonstrated the feasibility of ab initio MOVB method for studying chemical reactions by incorporating explicit solvent effects in the description of the reaction coordinate in combined QM/MM simulations.

A survey of recent developments in ab initio valence bond theory

“…As a typical example, the ODP has been applied to calculate the resonance energy of the allyl cation. 101 As an extension of the BLW method, the Molecular-OrbitalValence Bond method 102 (MOVB, not to be confused with the MO-VB method of Raimondi and coworkers, [85][86][87][88][89][90] described above, which carries the same acronym) allows to calculate the electronic coupling energy resulting from the mixing of two, or more, diabatic states. The diabatic states are first calculated by the BLW method, then a non-orthogonal configuration interaction Hamiltonian is constructed using these diabatic states as the basis functions.…”

“…103,104 Importantly, solvent effects can be incorporated in the MOVB method. Thus, the MOVB method has been used to model the proton transfer between ammonium ion and ammonia in water, 102 as well as a solvated S N 2 reaction, 105 using Monte Carlo simulations.…”

The Diabatic Picture of Electron Transfer, Reaction Barriers, and Molecular Dynamics

“…Allyl Radical has two structures -|CH 2 = CH −ĊH 2 and |ĊH 2 − CH = CH 2 -that place the double bond in a fixed location irrespective of which C-C bond is actually shorter. The VB-diabatic connection has been used to describe S N 1 reactions [47], proton-coupled electron transfer [48], and is the basis for the molecular orbital VB [49] and empirical VB [16,17] …”

Multidimensional Tunneling, Recrossing, and the Transmission Coefficient for Enzymatic Reactions

“…161,400 Various types of reaction coordinates can be used in this stage, for example, a geometry-based reaction coordinate, such as a distinguished reaction coordinate (DRC) described by the difference of the breaking and forming bond distances, 30,97,144,170,171,404 or a collective reaction coordinate, 27,120,131,161,168,265,295,299,[405][406][407] defined in terms of the energy gap 406 between the valence bond states corresponding to the reactant and product states. [512][513][514] Alternatively, for H transfer, when the donor and acceptor experience changes of the hybridization states, recent studies emphasized the usefulness of employing the change of one or both hybridization states as the DRC. 408 Any combination of bond distances, bond angles, or torsion angles is called a valence coordinate, whereas quantities based on diabatic potentials or electrostatic fields that depend on solvent or bath coordinates are often called collective solvent coordinates.…”

“…The fluctuations of the collective energy gap coordinate do not "drive" the dynamics; rather the solvent coordinate can be used to define a transition state dividing surface through which the equilibrium one-way local flux provides a good approximation to the net global reactive flux, just as the reaction coordinate of eq 9 can be used for this purpose. [512][513][514] Even when a valence coordinate is a good reaction coordinate, solvent molecules may participate in the reaction and respond to the change of electronic properties of the system. 458 The variational transition state, which is the dividing surface with the smallest calculated rate constant, depends on the choice of reaction coordinate z since it is defined by a hypersurface of constant z.…”