Theoretical investigations of the electronic spectroscopy and ultrafast photochemistry of binary transition metal carbonyl complexes
Abstract
This thesis is concerned with the electronic absorption spectroscopy and photochemical
relaxation mechanisms of binary metal carbonyl complexes. These paradigm complexes
exhibit a wide range of photoinduced vibronic coupling related phenomena that are only
recently beginning to be understood with the development of modern experimental and
computational techniques. These experiments have shown that after irradiation using
ultrafast (femtosecond) laser pulses an unsaturated photoproduct is produced, and
possibly relaxes through a conical intersection at a Jahn-Teller active geometry, on the
same ultrafast timescale. However while experiment can imply the presence of conical
intersection, only theoretical methods can confirm this and accurately probe the
appropriate part of the potential energy surfaces relevant to this mechanism. The
accurate assignment of the electronic excited states of these carbonyls is also a matter of
debate with different theoretical and experimental techniques applied to these systems
over the years. The large density of excited states of different character within a small
energy range and the high computational expense of studying transition metal
complexes with highly correlated methods presents a considerable challenge to the
theoretical chemist.
The research presented in this thesis falls into two main parts, firstly the
electronic excited states of the binary transition metal carbonyl complexes Fe(CO)5,
Cr(CO)6 and Ni(CO)4 were studied with highly correlated coupled cluster methods as
well as their one-photon and two-photon absorption spectra. These results were
compared with previous experimental and theoretical results. The electronic excited
states and one-photon absorption spectra were also studied for the group 7 mixed-metal
bimetallic carbonyls (MnTc(CO)10, MnRe(CO)10 and TcRe(CO)10) for the first time
with time-dependent density functional theory (TD-DFT), the ability of TD-DFT
methods to describe charge-transfer states was also investigated here.
The second part of this thesis focussed on the relaxation pathways of the
2Mn(CO)5 and 1Fe(CO)4 initial photoproducts of the photodissociation of Mn2(CO)10
and Fe(CO)5 respectively using CASSCF. Both were found to relax to their lowest
energy state through a Jahn-Teller induced conical intersection at a Jahn-Teller active
geometry in agreement with experimental observation.