The study of reaction mechanisms involves systematic investigations of the
correlation between structure, reactivity, and time. The challenge is to be able to
observe the chemical changes undergone by reactants as they change into products via
one or several intermediates such as electronic excited states (singlet and
triplet), radicals, radical ions, carbocations, carbanions, carbenes, nitrenes,
nitrinium ions, etc. The vast array of intermediates and timescales means there is
no single "do-it-all" technique. The simultaneous advances in contemporary
time-resolved Raman spectroscopic techniques and computational methods have done
much towards visualizing molecular fingerprint snapshots of the reactive
intermediates in the microsecond to femtosecond time domain. Raman spectroscopy and
its sensitive counterpart resonance Raman spectroscopy have been well proven as
means for determining molecular structure, chemical bonding, reactivity, and
dynamics of short-lived intermediates in solution phase and are advantageous in
comparison to commonly used time-resolved absorption and emission spectroscopy.
Today time-resolved Raman spectroscopy is a mature technique; its development owes
much to the advent of pulsed tunable lasers, highly efficient spectrometers, and
high speed, highly sensitive multichannel detectors able to collect a complete
spectrum. This review article will provide a brief chronological development of the
experimental setup and demonstrate how experimentalists have conquered numerous
challenges to obtain background-free (removing fluorescence), intense, and highly
spectrally resolved Raman spectra in the nanosecond to microsecond (ns–μs) and
picosecond (ps) time domains and, perhaps surprisingly, laid the foundations for new
techniques such as spatially offset Raman spectroscopy.
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