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Laimgruber, Stefan (2007): Femtosekunden Stimulierte Raman-Spektroskopie zur Photochemie von o-Nitrobenzaldehyd. Dissertation, LMU München: Faculty of Physics
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Abstract

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.