Exploring the Role of Solvents on the Chemical Dynamics of Reactive Intermediates using Ultrafast Transient Absorption Spectroscopy

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The research reported in this thesis explores how different types of solvent-solute
interactions influence photochemical outcomes. The research combines ultrafast transient
absorption spectroscopy (TAS) measurements with quantum chemical calculations to unravel
competing reaction pathways for three chemical systems.
The ultraviolet (UV) photoexcitation of α-diazocarbonyl compounds produces ketenes
by both concerted and stepwise Wolff rearrangements. The stepwise mechanism proceeds
through singlet carbene intermediates which can also participate in bimolecular reactions.
Transient absorption spectra provide direct evidence for competitive production of singlet -
carbonyl carbene and ketene intermediates, with ylide formation by carbene capture in
aprotic nucleophilic solvents. A new enol-mediated pathway is identified in reactions of
singlet -carbonyl carbenes with alcohols, and a computed two-dimensional cut of the
potential energy surface for the reaction with methanol shows that the enol forms without a
barrier. This pathway is promoted by an intermolecular hydrogen bond from methanol to the
carbonyl oxygen atom of the singlet -carbonyl carbene.
Because of the zwitterionic character of ylides, they interact strongly with polar
solvent molecules. Ultrafast TAS reveals the specific solvation dynamics of a tetrahydrofuranderived ylide with various hydrogen-bond donor solvents. The specific interaction of ethanol
with the ylide isfurther characterised using quantum chemical calculations and atom-centred
density matrix propagation trajectories which show preferential hydrogen-bonding to an αcarbonyl group.
The final system studied by ultrafast TAS is the 255-nm UV photoexcitation of 2-
aminothiazole, revealing competing N-H cleavage and heterocyclic ring-opening pathways. In
non-polar solvents, the TAS data indicate that N-H cleavage outcompetes the ring-opening
pathways. In contrast, polar solvents are shown to inhibit N-H fission, and ring-opening is the
preferred pathway to photoproducts. Initial quantum chemical calculations suggest the
height of the barrier to access the dissociative * S1-state responsible for N-H bond scission
is highly sensitive to the solvent and controls the product branching.
Date of Award24 Jun 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMichael N R Ashfold (Supervisor) & Andrew J Orr-Ewing (Supervisor)

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