Modeling Quantum Tunneling in Nonadiabatic Statistical Theory
Authors
Dergachev, Ilya D.
Issue Date
2024
Type
Dissertation
Language
Keywords
Alternative Title
Abstract
The central topic of this work is predicting the kinetics of spin-dependent processes. In the first part of this dissertation, we provide an overview of the nonadiabatic statistical theory (NAST) and our NAST program package for predicting the rate constants of spin-dependent processes. We also compare the capabilities of NAST with those of the nonadiabatic molecular dynamics (NAMD), a complementary approach for studying spin-dependent processes. This comparison is supplemented with several examples to highlight the advantages and limitations of both approaches. In the second part, we focus on the application of NAST to study the kinetics of spin-dependent processes in the quantum tunneling regime. Comparison of the semiclassical Landau-Zener and quantum mechanical weak coupling rate constants allows us to estimate the role of quantum tunneling in reaction kinetics at low temperatures. We use the model of linear crossing potentials to investigate how molecular properties, such as energy gradients, spin-orbit coupling, and mass affect the kinetics of a spin-dependent reaction in the tunneling regime. We extend this work to study the kinetics of spin-forbidden isomerization reactions in three complexes of Ni(II) and two spin-crossover reactions in nitrenes at cryogenic temperatures. We also use these molecules to test the performance of a new Landau-Zener-Eckart method for describing quantum tunneling in spin-dependent reactions. The third part of this dissertation is motivated by the discussion of the ground state multiplicity of iron(II)-porphyrin, the prosthetic group of hemeproteins. The ground state of iron(II)-porphyrin remains under debate due to the close-lying electronic states of different spin multiplicities arising from the partially filled 3d shell of iron. We propose the existence of a nonadiabatic equilibrium between the lowest energy triplet and quintet spin states. The last part of this dissertation is motivated by the remarkable ability of vulpinic acid, a natural sunscreen pigment found in lichens, to protect the organism against harmful UV radiation. The photostability of vulpinic acid drastically increases when it is embedded in a polysaccharide matrix. We propose that this enhanced photostability originates from the formation of a H-bonding network between the pigment and the matrix. We perform computational studies on the ground and excited electronic states of vulpinic acid that are proposed to participate in the energy dissipation mechanism behind the photoprotective activity of vulpinic acid.