Predicting Kinetics of Spin-Forbidden Unimolecular Reactions with Nonadiabatic Transition State Theory
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The central theme of this work is computational modeling of spin-forbidden kinetics of unimolecular reactions in the gas phase. We address in detail the nonadiabatic transition state theory (NA-TST), discuss its main advantages and shortcomings, and describe all critical steps to be taken to calculate the rate constants of spin-forbidden unimolecular reactions and the lifetimes of excited states decaying through intersystem crossing. We also discuss the recent implementation and use of the Zhu-Nakamura formulas coupled with NA-TST. These formulas provide complete set of solutions for the probability of transition between two electronic states for all coupling regimes and, in addition, they account for the curvature of the crossing potentials. We demonstrate that the 3B1→1A1 intersystem crossing rates in GeH2 predicted by NA-TST are in good agreement with the rates obtained with the nonadiabatic ab initio multiple spawning molecular dynamics. We also investigate the T1→S0 intersystem crossing in Cl2CS using single- and multireference electronic structure methods and compare the predicted microcanonical rate constants with previously reported theoretical and experimental values. We propose a zero-point energy correction scheme that can improve the intersystem crossing rate constants at the low energies. In the last part of this work, we study the fluorescence of a newly synthesized M2BiQ fluorophore, which is a promising candidate for chemical sensors and light-emitting devices. We show that fluorescence of this compound can be tuned to specific wavelengths throughout the visible spectrum by insertion of metal cations and by protonation. We show that coordination of M2BiQ to the metal cation is followed by stabilization of the excited emitting state with respect to the ground state. This stabilization results in reduction of the electronic gap and therefore in the large red shift observed for M2BiQ complexes.