Intramolecular vibrations and electronically nonadiabatic dynamics in photodissociation reactions.

摘要

Use of the Born-Oppenheimer approximation is central to the chemist's ability to calculate adiabatic potential energy surfaces and predict the outcome of chemical reactions. Central to this approximation is the separation of electronic and nuclear degrees of freedom. Although generally true, the Born-Oppenheimer approximation can break down when nuclear motion strongly couples two adiabatic potentials. It is then inappropriate to solve the reaction dynamics on a single potential energy surface: one must include the possibility of nonadiabatic transitions to a different adiabatic surface. In this thesis, we examine two distinct approaches for describing the nonadiabatic dynamics.;The first treatment uses electronic potential energy surfaces to describe the dynamics, noting which regions are most susceptible to nonadiabatic effects. As a specific example, we compare the photodissociation of trimethylamine (N(CH3)3) at 193 nm with previous studies of the photodissociation of ammonia and methylamine. The presence of a conical intersection in the exit channel to nitrogen-methyl bond fission exerts a strong influence on the reaction, because in this region of the potential, the lowest two energy states are strongly coupled. We find that intramolecular vibrational relaxation (strongly dependent on the density of vibrational states) has a strong effect on the regions of potential energy space sampled by the nuclear dynamics, leading the dissociating trimethylamine molecule to experience more strongly the nonadiabatic effects brought about by the conical intersection.;The second treatment of nonadiabatic dynamics follows the electronic wavefunction from reactants to products. In this approach, the preferred dissociation pathways are those involving the least rearrangement of electron configuration. As a specific example, we study the photodissociation dynamics at 193 nm of N,N-dimethylformamide (HCON(CH3)2) to HCO + N(CH 3)2 products. (An additional pathway leading to HCONCH 3 + CH3 is also observed, and the relative branching to these products is quantified.) In dimethylformamide, one finds that the most electronically accessible pathway is followed along the N-CO bond fission coordinate, when the nuclear dynamics on the excited state surface are included in the analysis.