ABSTRACT
The photo tautomerisations of ortho-nitrotoluene (oNT) and its methylated derivative ortho-ethylnitrobenzene (oENB) have been studied by means of femtosecond spectroscopy and (TD)-DFT computations. In UV/Vis transient absorption spectroscopy a band peaking at 400 nm is seen to rise in a bi-modal manner with time constants of 1-10 ps and 1500 ps. Femtosecond stimulated Raman experiments clearly identify aci-nitro forms as the spectroscopic carriers of the 400 nm band. The assignment of the Raman spectra is based on TD-DFT computations. The quantum yields of the aci-nitro forms after 3 ns are 0.08 (oNT) and 0.3 (oENB). The aci-nitro forms are formed via a singlet channel (1-10 ps) and a triplet channel (1500 ps). There are indications that the triplet channel involves a bi-radical intermediate. In between 3 ns and 1 ms the spectrum of the aci-nitro form shifts from 400 to 390 nm. This could indicate a tautomerisation from Z-aci-nitro to an E form.
Subject(s)
Photochemical Processes , Spectrum Analysis/methods , Toluene/analogs & derivatives , Absorption , Benzaldehydes/chemistry , Furans/chemistry , Models, Chemical , Models, Molecular , Molecular Structure , Nitrobenzenes/chemistry , Nitrobenzenes/radiation effects , Quantum Theory , Spectrophotometry, Ultraviolet , Spectrum Analysis, Raman , Toluene/chemistry , Toluene/radiation effectsABSTRACT
The first intermediate of the photochemical transformation of ortho-nitrobenzaldehyde to ortho-nitrosobenzoic acid in acetonitrile solvent has been characterized by femtosecond spectroscopy and time-dependent density functional theory (TDDFT) calculations. Femtosecond stimulated Raman spectroscopy (FSRS) indicates that this intermediate adopts a ketene structure. This assignment is supported by the TDDFT results. A kinetic analysis of FSRS and transient absorption data points to two channels for the formation of the ketene. For the predominating first channel the formation takes 0.4 ps. For the second channel it is much slower and takes 220 ps. We assign the first channel to a reaction via an excited singlet state. The second one might involve a triplet state.
