Femtosekunden Stimulierte Raman-Spektroskopie zur Photochemie von o-Nitrobenzaldehyd

Aromatic nitro-compounds are known to photochemically abstract
hydrogen atoms from adjacent hydrocarbon moieties. For
ortho-substituted nitro-aromatics these abstractions proceed
intra-molecularly and trigger secondary processes. Due to these
processes aromatic nitro-compounds are used as, e.g., photo-labile
protecting groups and as the leaving group in caged compounds.
Here, a prototypical nitro-compound, ortho-nitrobenzaldehyde
(o-NBA), which is photochemically transformed into
ortho-nitrosobenzoic acid is studied. This reaction is known for
more than 100 years but is still not totally specified. Tracing
these photochemical processes requires high temporal resolution
down to about 100 fs and a detection technique that is sensitive to
structural changes after starting the photoreaction. Femtosecond
vibrational spectroscopy fulfils these conditions in observation of
ultrafast chemical processes. So transient IR spectroscopy and the
recently developed femtosecond stimulated Raman spectroscopy (FSRS)
are used to record the ultafast structural changes in this
photoreaction. In addition the kinetics of the o-NBA photoreaction
are monitored by means of visible femtosecond absorption
spectroscopy. The novel implementation of FSRS uses a white light
continuum as the Stokes probe pulse in the stimulated Raman
process. The reaction is started by a actinic laser pulse thereby
the transient Raman spectra can be recorded in pump-probe fashion
and are compared with calculated spectra of possible intermediates
of the reaction. With the use of these techniques the photoreaction
of NBA into intermediates can be followed and a reaction mechanism
is described.