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Photochemistry of Aromatic Nitro Compounds Probed by
Femtosecond Stimulated Raman Scattering
S. Laimgruber, W. Schreier, P. Gilch
Department für Physik, Ludwig-Maximilians-Universität München, Oettingenstr. 67, D80538 München, Germany. Phone: +49-89-2180-9243, Fax: +49-89-2180-9243, e-Mail:
[email protected] , Internet: www.bmo.physik.uni-muenchen.de
Photo-excitation of aromatic nitro compounds can induce hydrogen abstraction from
“nearby” hydrocarbon residues. Provided that the residues are in ortho position to the nitro
group this abstraction can proceed intramolecularly. The abstraction often induces secondary
processes like rearrangements and bond fissions which are the basis for several applications in
practical photochemistry. These applications include photoliable protecting groups, and caged
compounds. A prominent example for the photochemistry of aromatic nitro compounds is the
photo induced transformation of o-nitrobenzaldehyde NBA to o-nitrosobenzoic acid (reaction
scheme in Fig.). Picosecond absorption experiment1 have revealed that the kinetics of this
process is very sensitive to the solvent properties but not its quantum yield which is ~ 0.5. We
here look in detail at this reaction by means of femtosecond absorption spectroscopy and
femtosecond stimulated Raman scattering. The latter experiments being the first example of
an application of this technique in reactions other than the isomerisation of polyenes2,3.
Left: Femtosecond absorption spectra of NBA in ethanol following 258 nm excitation. Right: Raman difference
spectrum recorded 10 ps after excitation in comparison with a B3LYP 6-311G** calculation. Discrepancies
between the two spectra are most likely due to shortcomings of the DFT method to reproduce Raman intensities.
Femtosecond absorption spectroscopy (left) reveals three kinetic processes. The excited state
of NBA decays within 0.5 ps, an intermediate I has a lifetime of 70 ps and an intermediate II
vanishes in ~ 2 ns. Addition of water reduces the lifetime of the intermediate I but does not
affect the other processes. This effect makes it seem likely that intermediate I has a ketene
structure (right). Time resolved Raman spectroscopy substantiate this assignment. The Raman
difference spectrum recorded after 10 ps strongly resembles predictions for this spectrum
based on DFT calculations (right). The ketene intermediate then presumably reacts by adding
a solvent molecule and that adduct then transform into the nitroso product. Further
spectroscopic support for this mechanism particularly relying on time resolved IR
spectroscopy will be given.
R.W. Yip, and D.K. Sharma, “The Reactive State in the Photo-Rearrangement of o-nitrobenzaldehyde”,
Research on Chemical Intermediates, 1989, 11, 109-116
2
M. Yoshizawa, H. Aoki, M. Ue, H. Hashimoto, “Ultrafast Relaxation Kinetics of Excited States in a Series of
Mini- and Macro--carotenes”, Physical Review B 2003, 67, 174302-1-7
3
P. Kukura, D.W. McCamant, and R. Mathies, “Femtosecond Time-Resolved Stimulated Raman Spectroscopy
of the S2 (1BU+) Excited State of -carotenes ”, Journal of Physical Chemistry A 2004 108 5921-5925
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