| Laser flash photolysis (LFP) with UV-visible and infrared detection and modern theoretical calculations were used to directly observe and identify nitrenes, carbenes, triplet riboflavin tetraacetate, and intermediates derived from them, and to understand their chemical reactivity.;In the studies of arylnitrenes, singlet nitrenes such as 2,6-diethyl, 2,6-diisopropyl, 2,4,6-tri-t-butyl, 2-cyano, 2,6-dicyano, 2-phenyl, 4-phenyl, 2,4-dichloro-6-phenyl phenylnitrenes, 1-naphthylnitrene and 9-anthracenylnitrene were generated upon LFP of the corresponding azide precursors and directly observed using LFP techniques in a glassy matrix at 77 K or in condensed phase at -80°C. The crucial intermediates derived from them were directly observed using LFP methodology or time-resolved infrared (TRIR) spectroscopy. The experimental observations are supported by computational studies. The substituent effects on the reactivity of singlet phenylnitrene were discussed.;In the studies of carbene-solvent interactions, it has been demonstrated that the most stable structure of the chlorine atom-benzene complex is a pi type eta1 complex with Cl atom sitting over a carbon atom of the benzene molecule (Cs symmetry), but that this type of charge transfer complex cannot be observed experimentally or computationally for chloro-para-nitrophenylcarbene and other arylhalocarbenes with benzene. However, one cannot rule out the possibility that these complexes are present and have absolute reactivities and spectroscopic properties similar to those of free arylhalocarbenes.;In the exploratory photochemistry of riboflavin, TRIR spectra of triplet riboflavin tetraacetate, its radical anion, and neutral radical were obtained upon LFP (355 nm) of riboflavin tetraacetate (75), 75 with sodium iodide, and 75 with indole or silylated guanosine in CD3CN or CH2Cl2. The TRIR spectra are in good agreement with the calculated vibrational spectra. The adducts of riboflavin and proteins at tryptophan residues are postulated to derive from an electron-transfer, proton-transfer, and radical pair recombination mechanism, and riboflavin is concluded to photochemically react with guanosine to generate hydroflavin radical by an electron-proton transfer mechanism without the participation of sugar rings in purine nucleosides. |
