Author: Arkajit Mandal

Publisher:

Published: 2021

Total Pages: 263

ISBN-13:

"Direct quantum dynamics simulation is often an essential tool for investigating complex chemical reactivities that involve the quantum mechanical interplay of electrons, protons, phonons, and photons. Quantum dynamics simulations can provide crucial mechanistic insights which can reveal the basic principles of new chemical reactivities, lead to new strategies for controlling or enabling chemical reactivities and help resolve mysteries in emerging fields such as polariton chemistry. One of the challenges for performing an on-the-fly quantum dynamics simulation is that it requires combining quantum dynamics methods with electronic structure approaches which are usually formulated under two different representations. While many quantum dynamics methods are developed in the diabatic representation, most of the electronic structure approaches provide outputs in the adiabatic representation. In this thesis, we have resolved this incompatibility challenge by developing the quasi-diabatic (QD) propagation scheme that allows a seamless interface between any adiabatic electronic structure method with a diabatic quantum dynamics approach. This is the first key finding in this thesis. With this new theoretical tool, we investigated proton-coupled electron transfer (PCET) reactions. We combined the instantaneous adiabatic electron-proton vibronic states, with path-integral quantum dynamics approaches using the QD propagation scheme. We found that this approach is accurate in obtaining population dynamics and provides reliable mechanistic insights of thermal as well as photoinduced PCET reactions. This is the second key finding in this thesis. With the success in simulating quantum dynamics molecular systems, we decided to investigate new chemical reactivities in light-matter hybrid systems. In particular, we investigated polariton chemistry, where new chemical reactivities are enabled by coupling molecular systems to quantized radiation in an optical cavity. We demonstrated that an isomerization reaction can be tuned by coupling molecules to radiation modes in a cavity. Using direct quantum dynamics simulations and analytical rate theories, we also demonstrated that the kinetics of photoinduced electron-transfer reaction can be suppressed or enhanced by coupling molecular system to quantized radiation in an optical cavity. This is the third key finding in this thesis. We found that the existing theoretical models for describing light-matter interactions between atoms and photons is inadequate for describing light-molecule hybrid systems. We developed the polarized-Fock state (PFS) representation for describing molecule-cavity interactions. The PFS representation provides an intuitive understanding of new phenomena and at the same time provides numerical convenience. We also discovered that the light-matter Hamiltonian in the PFS representation resolves the gauge ambiguity as it reduces to a coulomb gauge Hamiltonian that provides consistent result compared to the dipole gauge Hamiltonian under finite electronic truncation. This is the fourth key finding in this thesis. Finally, we explored vibrational polaritonic chemistry where ground-state chemical kinetics is modified when the vibrational degrees of freedom (DOF) of molecular systems are coupled to radiation modes inside an optical cavity. Such chemical kinetics modification has been demonstrated in recent experiments. However, a clear theoretical understanding of such an effect remains elusive. We found that the radiation mode can dynamically cage a solvent coordinate near the dividing surface suppressing the rate of a chemical reaction. We developed a non-Markovian rate theory for vibrational polaritonic chemistry. We extended this theory to show the same effect arises when collectively coupling radiation mode to the vibrational DOF of solvent molecules that strongly couples to a reaction coordinate. This is the final key finding in this thesis"--Pages xiii-xiv.