Many catalytic and biomolecular reactions containing transition metals involve changes in the electronic spin state. These processes are referred to as "spin-forbidden" reactions within nonrelativistic quantum mechanics framework. To understand detailed reaction mechanisms of spin-forbidden reactions, one must characterize reaction pathways on potential energy surfaces with different spin states and then identify crossing points. Here we propose a practical computational scheme, where only the lowest mixed-spin eigenstate obtained from the diagonalization of the spin-coupled Hamiltonian matrix is used in reaction path search calculations. We applied this method to the FeO + H → Fe + H O, FeO + CH → Fe + CH OH, and Mn + OCS → MnS + CO reactions, for which crossings between the different spin states are known to play essential roles in the overall reaction kinetics. © 2018 Wiley Periodicals, Inc.

The mechanisms including spin‐inversion have been systematically studied for the M+ + OCS → MS+ + CO/MO+ + CS (M denotes a transition metal from Sc to Cu) ion‐molecule reactions using the automated reaction path search method. We used the lowest mixed‐spin potential energy surface obtained from the diagonalization of the spin‐coupled Hamiltonian matrix, whose diagonal elements are taken to be the lowest two spin states. This scheme can effectively locate approximate minimum energy crossing points between the two potential energy surfaces with different spin multiplicities. The spin‐orbit couplings at spin‐inversion points have been calculated to understand the efficiencies of nonadiabatic transitions. The obtained reaction pathways and the calculated spin‐orbit couplings are employed to interpret previous experimental studies.

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We performed reaction path search calculations for the NaCl·(H O) cluster using the global reaction route mapping (GRRM) code to understand the atomic-level mechanisms of the NaCl → Na + Cl ionic dissociation induced by water solvents. Low-lying minima, transition states connecting two local minima and corresponding intrinsic reaction coordinates on the potential energy surface are explored. We found that the NaCl distances at the transitions states for the dissociation pathways were distributed in a relatively wide range of 2.7-3.7 Å and that the NaCl distance at the transition state did not correlate with the commonly used solvation coordinates. This suggests that the definition of the transition states with specific structures as well as good reaction coordinate is very difficult for the ionic dissociation process even in a small water cluster. © 2018 Wiley Periodicals, Inc.