Chapter 15
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Simulating Solvent Effects on Reactivity and Interactions in Ambient and Supercritical Water Jiali Gao and Xinfu Xia Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14214
The effects of hydration on the rate acceleration of the Claisen rearrangement of allyl vinyl ether and the Menshutkin reaction of ammonia and methyl chloride were investigated by a hybrid quantum mechanical and classical Monte Carlo simulation method. In addition, the potentials of mean force for the ion pair Na Cl in ambient and supercritical water were determined. The results provided valuable insights on intermolecular interactions for these processes in solution. +
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Study of chemical transformations in solution is important because of the connection to biological processes in life. Of great challenge is to gain an atomic level understanding of structure and reactivity in aqueous solution. However, the difficult task of describing gas-phase reactions is further complicated by the need to consider the solvent effects on the reaction dynamics and potential surface (1,2). Significant progress has been made in the past decade through computer simulations and a number of methods are being developed to investigate chemical reactions in solution and enzymes (5-7). In short, the computational procedure, as summarized by Jorgensen (6), typically involves three major steps: (1) determination of the minimum energy reaction path (MERP) in the gas phase as a function of a single geometrical variable, (2) development of empirical potential functions for the reaction profile as well as for solute-solvent interactions along the entire MERP, and (3) free energy simulations to estimate the solvent effects. Valuable insights have been obtained for organic reactions in solution. Nevertheless, a major difficulty in these studies is the requirement for an accurate, analytical description, i.e., empirical potential functions, of solute-solvent interactions along the whole reaction path. The parametrization process was laborious and difficult due to a lack of experimental data, while the empirical molecular mechanics-type potentials are generally not appropriate for treatment of bond formation and
0097-6156/94/0568-0212$08.00/0 © 1994 American Chemical Society
In Structure and Reactivity in Aqueous Solution; Cramer, Christopher J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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15. GAO AND XIA
213
Simulating Solvent Effects on Reactivity
breaking processes (4), which involve electronic structure reorganizations. This has limited the application to only a few well-defined systems (6-12). The problem is further escalated by specific consideration of solvent polarization effects (15), which have been treated in an average sense in the past An alternative approach is to use a combined quantum mechanical and classical approach, in which the reacting system is treated explicitly by a quantum mechanical (QM) method, while the environmental solvent which is the most time consuming part in the computation is approximated by a standard molecular-mechanics (MM) force field (5,14-17). Since the reactant electronic structure and solute-solvent interactions are determined quantum-mechanically, the procedure is appropriate for studying chemical reactions, and importantly, there is no need to develop empirical potential functions for new systems. Furthermore, it has the advantage of taking into account the solvent polarization effects (18). Details of such a combined Q M / M M potential and contributions by other groups are available in several recent reviews (16-19). In this paper, the focus will be on results from our group on organic reactions in aqueous solution. In addition, we describe a Monte Carlo simulation of the ion pair Na Cl" in ambient and supercritical fluid water. +
Methodology The Combined QM/MM Potential. We employ a combine quantum mechanical and molecular mechanical (QM/MM) model with the semiempirical AMI and TTP3P interface to describe solute-solvent interactions in solution (16,17). The method has been reviewed previously (16-19). Thus, only a brief summary is presented here. In this approach, the condensed-phase system is partitioned into (1) a Q M region consisting of the reacting solute molecules, which are represented by electrons and nuclei and described by Hartree-Fock molecular orbital theory, and (2) an M M region containing the surrounding solvent, which is approximated by an empirical force field. Consequently, the total effective Hamiltonian for the system is t*eff
9
ffq
+ ^mm
m
+
^qm/mm
^
where H ° is the Hamiltonian for the Q M solute, ft,^ is the solvent-solvent interaction energy, and H is the solute-solvent interaction Hamiltonian. The total energy of the system is given by equation 2. q m / m m
E
tot
=
^aq\^eff\^aq
s
E
qm
+
E
mm
+
E
qm/mm
^
It should be pointed out that H depends on the partial charges and positions of the solvent interaction sites (atoms). As a result, only the oneelectron integral part in the Fock matrix needs to be modified and standard molecular orbital computation methods can be directly used. The wave function obtained through this procedure, ¥ , however, includes the solvent effects, based upon which the computed properties are further averaged in Monte Carlo q m / m m
a q
In Structure and Reactivity in Aqueous Solution; Cramer, Christopher J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
214
STRUCTURE AND REACTIVITY IN AQUEOUS SOLUTION
simulations (18). Of particular interest is the solvent polarization energy. Given the wave functions for the solute in aqueous solution, Ψ ^ , and in the gas phase, Ψ°, solvent polarization contributions to the total solvation free energy can be determined via equation 3.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 2, 2016 | http://pubs.acs.org Publication Date: September 29, 1994 | doi: 10.1021/bk-1994-0568.ch015
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ι
= -kT ln
