Identification of flurbiprofen and its photoproducts in methanol by gas chromatography-mass spectrometry

A sample of 10 mM flurbiprofen in methanol (or ethanol) was photoirradiated with sixteen 8 W low-pressure quartz mercury lamps irradiated at 306 nm in a Panchum PR-2000 photochemical reactor. In total, four major photoproducts derived from each sample were observed from the HPLC chromatogram. The photoproducts were separated and their structures elucidated by various spectroscopic methods. Alternatively, using GC-MS, 11 major photoproducts were observed. A reaction scheme of flurbiprofen in methanol is proposed: the photochemical reaction routes occur mainly via esterification and decarboxylation, followed by oxidation with singlet oxygen to produce a ketone, alcohols and other derivatives.     

Cobalt(III) complexes of unsaturated carboxylic acids: synthesis, characterization, and photochemical studies in aqueous medium

Cobalt(III) ammine complexes coordinated to carboxylic acids are classic examples showing photoredox reactions originating from the ligand to metal charge transfer excited states. Cobalt(III) pentaammine complexes coordinated to unsaturated carboxylic acids as ligands were synthesized and characterized by spectroscopic methods. The photolysis of these complexes produces a carboxylate free radical. This free radical undergoes further transformation to form organic photoproducts. The organic photoproducts were characterized by spectroscopic methods. The nature of the decomposition products from the organic photoproduct was examined in detail. The quantum yields were also determined. The photochemical reactions have the potential to produce novel compounds from the decarboxylation of unsaturated acids, which shows interesting reaction pathways.     

Time-resolved events on the reaction pathway of transcript initiation by a single-subunit RNA polymerase: Raman crystallographic evidence

The nucleotidyl transfer reaction leading to formation of the first phosphodiester bond has been followed in real time by Raman microscopy, as it proceeds in single crystals of the N4 phage virion RNA polymerase (RNAP). The reaction is initiated by soaking nucleoside triphosphate (NTP) substrates and divalent cations into the RNAP and promoter DNA complex crystal, where the phosphodiester bond formation is completed in about 40 min. This slow reaction allowed us to monitor the changes of the RNAP and DNA conformations as well as bindings of substrate and metal through Raman spectra taken every 5 min. Recently published snapshot X-ray crystal structures along the same reaction pathway assisted the spectroscopic assignments of changes in the enzyme and DNA, while isotopically labeled NTP substrates allowed differentiation of the Raman spectra of bases in substrates and DNA. We observed that substrates are bound at 2-7 min after soaking is commenced, the O-helix completes its conformational change, and binding of both divalent metals required for catalysis in the active site changes the conformation of the ribose triphosphate at position +1. These are followed by a slower decrease of NTP triphosphate groups due to phosphodiester bond formation that reaches completion at about 15 min and even slower complete release of the divalent metals at about 40 min. We have also shown that the O-helix movement can be driven by substrate binding only. The kinetics of the in crystallo nucleotidyl transfer reaction revealed in this study suggest that soaking the substrate and metal into the RNAP-DNA complex crystal for a few minutes generates novel and uncharacterized intermediates for future X-ray and spectroscopic analysis.     

Photolysis of NSAIDs. IV. Photoproducts of zomepirac determined by LC-ESI-MS

A sample of 10 mm zomepirac in methanol was photo-irradiated with a Hanovia 200 W high-pressure quartz Hg lamp for 14 days. In total, four photoproducts were observed from the HPLC chromatogram. The preparative HPLC included an YMC-Pack Pro C18 column (250 x 20 mm i.d.), a mobile phase of CH3CN-CH3OH-1%HOAc (10:60:30, v/v/v), and UV detection at 254 nm. The most probable structures of the four photoproducts were determined by LC-MS. Two major photoproducts were separated, and their structures were further confirmed by the spectroscopic methods. A reaction scheme of zomepirac was proposed that the photochemical reaction routes occur mainly via bond fission between carbonyl-pyrrolyl groups (alpha-cleavage of a ketone), and decarboxylation followed by oxidation with singlet oxygen to produce an aldehyde.     

Separation and structure determination of nicardipine photoproducts by LC-ESI-MS

A sample of 0.104 m nicardipine in methanol was photoirradiated with a Philips 400 W UV lamp for 3 h in a photochemical chamber. A total of four major photoproducts were found from the HPLC chromatogram. The same sample was used for taking LC-MS, while eight major photoproducts were observed and the structures elucidated by analyzing the CID patterns of their respective mass spectra. A reaction scheme of nicardipine is proposed that the photochemical reactions occur mainly via oxidation of 1,4-dihydropyridine moiety, following the stepwise photo-reduction of the m-nitro group and demethylation of the ester group at 5-position of the pyridine ring. Copyright (c) 2008 John Wiley & Sons, Ltd.     

A study of photochemistry of flavins in pyridine and with a donor

Abstract— Both anaerobic and aerobic photolysis of riboflavin in pyridine yielded several photoproducts, analogous to the photochemical reaction in aqueous solution. Lumiflavin was also photoreduced in pyridine (an electron donor) without decomposition of the isoall-oxazine ring Differences in the reactivity of excited singlet and triplet states with respect to formation of photoproducts have been confirmed in pyridine. In the photoreduction of riboflavin by N, N'-dimethyl-N-benzylethylene diamine, an initial rate with a higher relative quantum yield than that of the reaction at a later stage was observed both in water and pyridine. Photolysis in D₂O with the donor showed no significant solvent isotope effect. These results strongly suggest that the photolysis of riboflavin in water does not involve the water molecule in the primary photochemical act. A detailed mechanism of flavin photoreaction in water to account for solvent oxygen incorporation into a photoproduct (benzaldehyde) from N, N'-dimethyl-N-benzylethylene diamine has been proposed without involving photochemical splitting of water based on our results and molecular orbital computations. Preliminary results on the kinetics of free radical decay were obtained using an ESR spectrometer and the significance of the results are discussed.     

Time-resolved resonance Raman observation of the dimerization of didehydroazepines in solution

Time-resolved resonance Raman (TR3) studies of the photochemistry of phenyl azide, 3-hyroxyphenyl azide, 3-methoxyphenyl azide and 3-nitrophenyl azide in acetonitrile:water solutions is reported. After photolysis of these four aryl azides in room temperature solutions, only one species was observed in the TR3 spectra for each azide, respectively at the probe wavelengths employed in the TR3 experiments. The species observed after photolysis of 3-nitrophenyl azide was assigned to 3,3'-dinitroazobenzene, an azo compound formed from the dimerization reaction of triplet 3-nitrophenylnitrene. In contrast, the species observed after photolysis of phenyl azide, 3-hydroxyphenyl azide and 3-methoxyphenyl azide were tentatively assigned to intermediates formed from the dimerization of didehydroazepines that are produced from the ring expansion reaction of the respective singlet arylnitrene. To our knowledge, this is the first time-resolved vibrational spectroscopic observation of the dimerization reaction of didehydroazepines in solution. In addition, these are the first resonance Raman spectra reported for dimers formed from didehydroazepines. We briefly discuss the structures, properties and chemical reactivity of the dimer species observed in the TR3 spectra and possible implications for the photochemistry of aryl azides.     

Time-resolved resonance Raman identification and structural characterization of a light absorbing transient intermediate in the photoinduced reaction of benzophenone in 2-propanol

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic study of the triplet state benzophenone reaction with the 2-propanol hydrogen-donor solvent and subsequent reactions is presented. The TR3 spectra show that the benzophenone triplet state (npi*) hydrogen-abstraction reaction with 2-propanol is very fast (about 10 to 20 ns) and forms a diphenylketyl radical and an associated 2-propanol radical partner. The temporal evolution of the TR3 spectra also indicates that recombination of these two radical species occurs with a time constant of about 1170 ns to produce a LAT (light absorbing transient) intermediate that is identified as the 2-[4-(hydroxylphenylmethylene)cyclohexa-2,5-dienyl]propan-2-ol (p-LAT) species. Comparison of the TR3 spectra with results obtained from density functional theory calculations for the species of interest was used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the TR3 spectra. The structures and properties of the reaction intermediates observed (triplet benzophenone, diphenyl ketyl radical, and p-LAT) are briefly discussed.     

Flash photolysis studies of charge-transfer photochemistry of nickel(II) and cobalt(III) complexes

The photochemical behavior of cobalt(III) and nickel(II) complexes on excitation in the charge-transfer bands is reviewed in this article with particular reference to the study of intermediates. Investigations on the photoredox reactions of cobalt(III) and nickel(II) complexes using flash kinetic spectroscopic methods reveal details on the characteristics of the intermediates produced from the charge-transfer excited states of these metal complexes. The reactive species produced on photolysis of cobalt(III)-amine complexes activate molecular oxygen, producing mononuclear and dinuclear dioxygen species coordinated as superoxo and peroxo forms. Cobalt(III)-amino-acid complexes on photolysis lead to the formation of cobalt(III)-alkyl complexes which are identified as transients. The spectra and the decay kinetics are described with the view to elucidate mechanistic details. Nickel(II) macrocyclic complexes on excitation in the charge-transfer bands lead to oxidation of the metal centre. Scavenging experiments using dioxygen, alcohols and acids were carried out to understand the mechanistic details.     

Photoproducts of indomethacin

In a photochemical chamber, a sample of 27.9 mM indomethacin (IN) in methanol was exposed to a Philips 400-W UV lamp for 5 days in normal atmosphere. Four photoproducts were separated using high-performance liquid chromatography, and their structures were elucidated by various spectroscopic methods, including liquid chromatography–electrospray ionization–mass spectrometry. A reaction scheme of IN in methanol is proposed: The photochemical reaction routes occur mainly via esterification and decarboxylation, followed by oxidation with singlet oxygen to produce an aldehyde. A β-hydroxy-γ-lactone was also formed through a photochemical [2 + 2] cycloaddition, and its structure was confirmed by single-crystal X-ray diffraction.     

Ring-closing photoisomerization of some 2,6-diarylstyrenes

The structure, spectroscopy and photochemical reactions of three symmetrical 2,6-diarylstyrenes (aryl = phenyl, 2-furyl and 2-thiophenyl) have been investigated. The ground-state structures are highly nonplanar, having large aryl-phenyl and vinyl-phenyl dihedral angles. All three diarylstyrenes have broad UV absorption bands attributed to allowed, delocalized pi,pi* (highest occupied molecular orbit-lowest unoccupied molecular orbit) transitions. The 2-furylstyrene is weakly fluorescent, with a large Stokes shift attributed to a change in geometry from the nonplanar ground state to a more planar singlet state. Irradiation in fluid solution results in efficient conversion of the diarylstyrenes to cyclized 9,10-dihydrophenanthrene and 4,5-dihydronaphthofuran or thiophene products, thus extending the scope of the 2-vinylbiphenyl photocyclization reaction to heterocyclic analogs. Irradiation at low temperatures in glassy media permits observation of the UV absorption spectra of the unstable primary photoproducts. Upon warming of the glass, these intermediates undergo rapid hydrogen migration to form the stable dihydroarene products.     

Photolysis of NSAIDs. Part 3: Structural elucidation of photoproducts of tolmetin in methanol

A sample of 10 mM tolmetin in methanol was photo-irradiated with a Hanovia 200 W high-pressure quartz Hg lamp for four days. In total, eight photoproducts were observed from the HPLC chromatogram. Three major photoproducts were separated, and their structures were elucidated by spectroscopic methods. The structures of all photoproducts were further determined by LC-ESI-MS. A reaction scheme of tolmetin was proposed.     

Time-resolved resonance Raman and density functional study of an azirine intermediate in the 2-fluorenylnitrene ring-expansion reaction to form a dehydroazepine product

We report time-resolved resonance Raman spectra for the azirine intermediate produced in the 2-fluorenylnitrene ring-expansion reaction to form a dehydroazepine product. The Raman bands obtained with a 252.7 nm probe wavelength and 500 ns delay time exhibit reasonable agreement with predicted vibrational frequencies from density functional calculations for two isomers of azirine intermediates that may be formed from a 2-fluorenylnitrene precursor. The Raman bands observed for delay times of 15 ns and 10 micros were consistent with predicted vibrational frequencies from density functional calculations for the 2-fluorenylnitrene and dehydroazepine product species as well as previously reported 416 nm time-resolved Raman spectra obtained on the ns and micros time scales. Our results demonstrate that the 2-fluorenylnitrene ring-expansion reaction to produce dehydroazepine products proceeds via relatively long-lived 2-fluorenylnitrene and azirine intermediates. Substitution of a phenyl ring para to the nitrene group of phenylnitrene appears to lead to significant changes in the ring-expansion reaction so that longer lived arylnitrene and azirine intermediates can be observed. This should enable the chemical reactivity of azirine intermediates formed from arylnitrenes to be examined more readily.     

Structural and photochemical properties of organosilver reactive intermediates MeAg2(+) and PhAg2(+)

Although there is growing interest in silver promoted carbon-carbon bond formation, a key challenge in developing robust and reliable organosilver reagents is that thermal and photochemical decomposition reactions can compete with the desired coupling reaction. These undesirable reactions have been poorly understood due to complications arising from factors such as solvent effects and aggregation. Here the unimolecular decomposition reactions of organosilver cations, RAg(2)(+), where R = methyl (Me) and phenyl (Ph), are examined in the gas phase using a combination of mass spectrometry based experiments and theoretical calculations to explore differences between thermal and photochemical decompositions. Under collision-induced dissociation conditions, which mimic thermal decomposition, both PhAg(2)(+) and MeAg(2)(+) fragment via formation of Ag(+). The new ionic products, RAg(+?) and Ag(2)(+?), which arise via bond homolysis, are observed when RAg(2)(+) is subject to photolysis using a UV-vis tunable laser OPO. Furthermore, comparisons between the theoretical and experimental UV-vis spectra allow us to unambiguously determine the most stable structures of PhAg(2)(+) and MeAg(2)(+) and to identify the central role of the silver part in the optical absorption of these species. The new photoproducts result from fragmentation in electronic excited states. In particular, potential energy surface calculations together with the fragment charges highlight the role of triplet states in these new fragmentation schemes.     

The Studies of the Resonance Raman Scattering and the Chemical Reactions Involved in the Gaseous Iodine Bromide Under Argon Ion Laser Light

The resonance Raman spectra of gaseous iodine bromide IBr have been studied with the excitation of various argon ion laser lines from 5017 to 4579Å. The fine structures of the fundamental and few overtones of IBr are also studied by various power of 4880Å laser line. The resonance Raman scattering is found to be strong as that of Br₂ and ICI. A new term “apparent spectroscopic temperature” is suggusted for the case of the resonance Raman scattering. The apparent spectroscopic temperatures measured in this cell show that the system is not in thermal equilibrium. Br₂ is the hottest and I₂ is the coldest. IBr is in the middle. Unfortunately, no chemical reaction enhanced phenomenon is found although there should be some chemical reactions occurring under the laser light. The initiating reaction is the photodissociation of the main component IBr which also has large absorptivity. Because of the non-crossing between the B³Π and the ¹Π states, the primary products of the photodissociation should be I and Br. The chemical reactions of I and Br with IBr follow. The reactions of I and IBr is endothermic but the reaction of Br with IBr is exothermic. Therefore vibrational hot Br₂ is produced and its apparent spectroscopic temperature should be higher. On the other hand, the apparent spectroscopic temperature of I₂ is lower.     

2-氨基苯并噻唑的结构及激发态质子转移动力学

通过傅里叶变换红外光谱（FTIR）、傅里叶变换拉曼（FT-Raman）和488 nm拉曼光谱,结合密度泛函理论（DFT）计算研究了2-氨基苯并噻唑（ABT）在晶态和溶剂中的二聚体结构,并解释了质子性溶剂中ABT二聚体与溶剂分子间的氢键作用.电子光谱实验揭示了ABT二聚体的光物理和光化学反应;紫外吸收和荧光发射光谱结果表明,溶剂、激发波长和pH值对ABT二聚件激发态衰变具有调控作用;含时密度泛函理论（TD-DFT）解释了ABT二聚体双荧光现象,提出了高激发态的质子转移机理.     

Spectroscopic studies of the photochromism of N-salicylideneaniline mixed crystals and glasses

The reaction mechanism of the photochromism of N-salicylideneaniline (SA) in crystalline and glass matrices has been investigated at low temperatures. The structures of the initial E and final QC states as well as those of the intermediates QA and QB have been analyzed by optical absorption and emission spectroscopy. The potential barriers involved in the photochromic activity have been deduced from the temperature-dependent kinetics of the absorption spectra. An energy-level scheme of the forward and back reactions is presented.     

Combined experimental-theoretical study of the lower excited singlet states of paravinyl phenol, an analog of the paracoumaric acid chromophore

The low-lying excited singlet states of paravinyl phenol (pVP) are investigated experimentally and theoretically paying attention to their similarity to excited states of paracoumaric acid, the chromophore of the photoactive yellow protein (PYP). Resonance enhanced multiphoton ionization and laser induced fluorescence spectroscopic techniques are employed to obtain supersonically cooled, vibrationally resolved excitation and emission spectra related to the lowest (1)A'(V') excited state of pVP. Comprehensive analyses of the spectral structures are carried out by means of the equation-of-motion coupled cluster singles and doubles and time dependent density functional theory methods in combination with the linear vibronic coupling model and Franck-Condon calculations. The assignments of the spectral patterns are given, mostly in terms of excitations of totally symmetric modes. Weak activity of the non-totally-symmetric modes indicates low probability of photochemical processes in the Franck-Condon region of the (1)A'(V') state. The second (1)A'(V) and third (1)A" (Ryd) excited states of pVP are characterized with regard to their electronic structure, properties, and effects of geometry relaxations. The lengthening of the double bond relevant to the trans-cis isomerization of the PYP chromophore is found for the (1)A'(V) state. A possibility of photochemical processes and strong vibronic interactions in this state can be expected. The theoretical results for the (1)A"(Ryd) state predict that dissociation with respect to the O-H bond is possible.     

Applications of time-resolved step-scan Fourier-transform infrared spectroscopy in revealing the light-initiated reactions in condensed phases

Step-scan Fourier-transform infrared spectroscopy (ssFTIR) simultaneously provides the spectroscopic and kinetic information of a given reaction. ssFTIR has been extensively employed to acquire the transient absorption and emission spectra in gas phase for identifying unstable species, for example, various Criegee intermediates, and elucidating the dynamics and kinetics of the reaction, such as the molecular elimination dynamics of haloalkenes and the bimolecular reactions involving chlorine atoms and singlet oxygen atoms. In addition to gaseous studies, ssFTIR has been also utilized to record the time-resolved difference spectra of the photochemical reactions in condensed phases, such as the photolysis of metal–ligand complexes, photocycles of the retinal proteins, coordination capability of solvents to unstable transient species, chemical reactions of atmosphere-related molecules in aqua, and the exciplex dynamics of organic light emitting materials. Moreover, my group has pioneered the recording of the transient thermal infrared emission of gold nanostructures upon photoexcitation. The experimental setups and the working principles for probing the time-resolved infrared absorption and emission in condensed phases will be revealed and a number of studies on chemical, biological, and materials systems will be described. These reported results demonstrate that ssFTIR is a versatile tool for exploring the properties of novel materials and photoreactions in condensed phases.