Ab Initio Studies of Intramolecular Proton Transfer and Electronic Relaxation
Coe, Joshua D.
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https://hdl.handle.net/2142/84255
Description
Title
Ab Initio Studies of Intramolecular Proton Transfer and Electronic Relaxation
Author(s)
Coe, Joshua D.
Issue Date
2007
Doctoral Committee Chair(s)
Martinez, Todd J.
Department of Study
Chemical Physics
Discipline
Chemical Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Language
eng
Abstract
"Excited state intramolecular proton transfer (ESIPT) and electronic relaxation were explored in a variety of small molecules, using a combination of quantum chemistry and ab initio molecular dynamics. The standard level of electronic structure treatment was provided by complete active space self-consistent field theory, but this was augmented by second-order perturbation corrections or multireference configuration interaction wherever greater accuracy was necessary and feasible. The simulations were performed with the ab initio multiple spawning (AIMS) method, an adaptive basis set technique for molecular dynamics with coupled Gaussians. ESIPT was examined primarily in the context of malonaldehyde and methyl salicylate, molecules possessing a labile hydrogen atom at the mouth of a five-membered ring composed of both keto and enol fragments. AIMS simulations predicted that ESIPT proceeds in 10-50 fs, and that the transfer itself is ""gated"" by the molecular backbone rearrangement necessary for interconversion. Connection to experimental probes (femtosecond depletion, transient absorption, resonance Raman) yielded satisfactory and, in some cases, quantitative agreement. S2/S1 intersection optimization in malonaldehyde yielded two extrema, a roughly planar intersection of hydrogen transfer type (HTI) and a twisted intersection of the first three singlet states (3SI). Closer examination established the connectivity of all of the known intersection extrema spanning the first three states in malonaldehyde. The impact of direct connection of S2 to S0 through the 3SI was probed with AIMS dynamics, revealing very little population transfer directly from S 2 to the ground state. It was also determined that very little population on S2 gets energetically or geometrically close to the 3SI, even though it represents the minimum for the two-state manifold connecting S 2 with S1. Both of these results were rationalized in terms of relative phase space volumes. Electronic relaxation dynamics in the neighborhood of a conical intersection was further examined for a sequence of small alpha,beta-enones differing only in the placement of a methyl group. After excitation to S 2, time-resolved photoelectron experiments predicted fast decay to S 1, followed by S1 lifetimes differing by up to a factor of four. Analysis of energetics and intersection topography failed to establish meaningful correlation with the decay rates."
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