A Strategy for\nModeling Nonstatistical Reactivity Effects: Combining Chemical Activation\nEstimates with a Vibrational Relaxation Model

The kinetics of many chemical reactions can be readily\nexplained\nwith a statistical approach, for example, using a form of transition\nstate theory and comparing calculated Gibbs energies along the reaction\ncoordinate(s). However, there are cases where this approach fails,\nnotably when the vibrational relaxation of the molecule to its statistical\nequilibrium occurs on the same time scale as the reaction dynamics,\nwhether it is caused by slow relaxation, a fast reaction, or both.\nThese nonstatistical phenomena are then often explored computationally\nusing (quasi)classical ab initio molecular dynamics by calculating\na large number of trajectories while being prone to issues such as\nzero-point energy leakage. On the other side of the field, we see\nresource-intensive quantum dynamics simulations, which significantly\nlimit the size of explorable systems. We find that using a Fermi’s\ngolden rule type of model for vibrational relaxation, based on anharmonic\ncoupling constants, we can extract the same qualitative information\nwhile giving insights into how to enhance (or destroy) the bottlenecks\ncausing the phenomena. We present this model as a middle ground for\nexploring complex nonstatistical behavior, capable of treating medium-sized\norganic molecules or biologically relevant fragments. We also cover\nthe challenges involved, in particular quantifying the excess energy\nin terms of vibrational modes. Relying on readily available electronic\nstructure methods and providing results in a simple master equation\nform, this model shows promise as a screening tool for opportunities\nin mode-selective chemistry without external control.