Re‐Evaluating the Transition State for Reactions in Solution

Abstract

In this microreview we revisit the early work in the development of Transition State Theory, paying particular attention to the idea of a dividing surface between reactants and products. The correct location of this surface is defined by the requirement that trajectories not recross it. When that condition is satisfied, the true transition state for the reaction has been found. It is commonly assumed for solution‐phase reactions that if the potential energy terms describing solvent‐solute interactions are small, the true transition state will occur at a geometry close to that for the solute in vacuo. However, we emphasize that when motion of solvent molecules occurs on a time scale similar or longer than that for structural changes in the reacting solute the true transition state may be at an entirely different geometry, and that there is an important inertial component to this phenomenon, which cannot be described on any potential energy surface. We review theories, particularly Grote‐Hynes theory, which have corrected the Transition State Theory rate constant for effects of this kind by computing a reduced transmission coefficient. However, we argue that searching for a true dividing surface with near unit transmission coefficient may sometimes be necessary, especially for the common situation in which the rate‐determining formation of a reactive intermediate is followed by the branching of that intermediate to several products.

Computational mechanistic study in organometallic catalysis: Why prediction is still a challenge

“…Brakestad et al demonstrate how an alternative multiwavelet approach may circumvent the basis set error, however, this option is not yet available in most available software for electronic structure calculations and the use of standard basis sets in DFT approaches means that basis set errors are present in most calculations. 2 Furthermore, the treatment of solvent and salt effects, traditionally left out of models for computational simplicity, 1 continues to be explored and improved as well, 1,[29][30][31][32][33][34] with some examples considered below. Some workflows and corrections are becoming accepted for routine cases, that is, catalytic cycles where the solvent is not directly involved, 1,17 and a growing body of knowledge is being developed about what to do for exceptions, such as solvation changing the reaction pathway 32,33 and solvent-mediated/outer-sphere pathways 35 where the use of molecular dynamics is of growing importance.…”

Dynamic effects on organic reactivity—Pathways to (and from) discomfort

“…Carpenter has discussed a related type of drag imposed by solvent. [32] The drag effect is, of course, related to the concept of enthalpy/entropy compensation. [33] The stronger the electrostatic attraction (the better the enthalpy of interaction), the less flexible is the system (the worse is the entropy).…”

“…Inspirational work in this area on small systems has been carried out again by Carpenter. [32] 2. We wonder how the time needed for solvent reorganization during a reaction is connected to the outcome of trajectories.…”

The Gibbs Energy–Potential Energy Conundrum and Chemical Reactivity: Implications for Catalysis and Enzyme Action

“…Although the said conundrum is of little concern generally, the inherent ambiguities can become manifest in certain cases, so cannot be brushed aside. These can lead to critical insights, however, so are worth pursuing in exploring the limits of transition state theory, which remain a continuing concern [8][9][10][11][12]. In particular, transition state theory is apparently focused on the transition state, hence ground state effects are generally difficult to pin down.…”