Photochemistry Of Benzene Research Articles

Photochemistry of benzene (C6H6) hydrogen cyanide (HCN) co-condensed ices part 1: A source of solid-state production of volatile nitrile compounds in Titan's stratosphere

Since 2015, during northern spring, a massive “noxious” stratospheric cloud has been detected at 250 km of altitude at the south pole of Titan, which persisted until the last targeted Cassini's flybys in July 2016. Its chemical composition seems to be the result of a co-condensation process of benzene and hydrogen cyanide in a 4:1 mixing ratio. Since, its altitude of detection allows to undergo long-UV solar radiations known to trigger photochemical aging processes, we experimentally simulated its evolution under stratospheric-like radiation conditions. The ice photo-processing (λ > 200 nm) led to the detection of nitriles by infrared spectroscopy such as acetonitrile (CH3CN), benzonitrile (C6H5CN) as well as a partial identification of propionitrile (CH3CH2CN) and acrylonitrile (C2H3CN). Gas chromatography coupled to mass spectrometry (GC–MS) analyses of the volatile fraction released in the gas phase during the warming of the photo-processed ice have been confirmed by previous assignments and provided the detection of several other hydrocarbons ranging from C3 to C8. These experimental results are of prime interest in the context of the future Dragonfly mission. They provide a list of nitrile derivatives that can be produced from this stratospheric cloud. Indeed, they may contribute at the end to the organic layer that recovers Titan's surface and would be analyzed by the mass spectrometer (Dra-MS) of this space mission.

Quantum dynamics study of the competing ultrafast intersystem crossing and internal conversion in the “channel 3” region of benzene

Time-resolved photoelectron spectroscopy can obtain detailed information about the dynamics of a chemical process on the femtosecond timescale. The resulting signal from such detailed experiments is often difficult to analyze and therefore theoretical calculations are important in providing support. In this paper we continue our work on the competing pathways in the photophysics and photochemistry of benzene after excitation into the "channel 3" region [R. S. Minns, D. S. N. Parker, T. J. Penfold, G. A. Worth, and H. H. Fielding, Phys. Chem. Chem. Phys. 12, 15607 (2010)] with details of the calculations shown previously, building on a vibronic coupling Hamiltonian [T. J. Penfold and G. A. Worth, J. Chem. Phys. 131, 064303 (2009)] to include the triplet manifold. New experimental data are also presented suggesting that an oscillatory signal is due to a hot band excitation. The experiments show that signals are obtained from three regions of the potential surfaces, three open channels, which are assigned with the help of simulations showing that following excitation into vibrationally excited-states of S(1) the wavepacket not only crosses through the prefulvenoid conical intersection back to the singlet ground state, but also undergoes ultrafast intersystem crossing to low lying triplet states. The model is, however, not detailed enough to capture the full details of the oscillatory signal due to the hot band.

Photochemistry of benzene and quinoxaline fused Δ 2-1,2,3-triazolines and their trapping products

The benzene and quinoxaline fused Δ 2-1,2,3-triazolines 1a and 1b were synthesized in good yields using Knoevenagel condensation and intramolecular 1,3-dipolar cycloaddition as two of the key reactions. Photolysis (254 nm) of Δ 2-1,2,3-triazoline 1a or 1b in acetonitrile led to the homolytic cleavage of nitrogen that generated diethyl diazomalonate 7, highly reactive intermediates aziridines 8a, b, and isoindoles B. The latter two species subsequently underwent rearrangement to give the nitrogen extrusion products 9a, b, and polymers. Furthermore, the reactive intermediates were trapped by dienophiles to give the corresponding cycloadducts. Subsequent rearrangement of the N-bridged cycloadducts gave N-substituted pyrrolo[3,4- b]quinoxalines 12b and 15b in 6% and 9% yields, respectively. Irradiation of 1a in the presence of fumaronitrile led to the isolation of cycloadduct 16a with retention of stereochemistry. Thermal reaction of 1b gave more nitrogen extruded product 9b (58–63% yield) than that by photolysis (5–23% yield), which implied that zwitterionic intermediate might be involved in the former.

Automatic generation of active coordinates for quantum dynamics calculations: Application to the dynamics of benzene photochemistry

A new practical method to generate a subspace of active coordinates for quantum dynamics calculations is presented. These reduced coordinates are obtained as the normal modes of an analytical quadratic representation of the energy difference between excited and ground states within the complete active space self-consistent field method. At the Franck-Condon point, the largest negative eigenvalues of this Hessian correspond to the photoactive modes: those that reduce the energy difference and lead to the conical intersection; eigenvalues close to 0 correspond to bath modes, while modes with large positive eigenvalues are photoinactive vibrations, which increase the energy difference. The efficacy of quantum dynamics run in the subspace of the photoactive modes is illustrated with the photochemistry of benzene, where theoretical simulations are designed to assist optimal control experiments.

FTIR study of adsorption, thermal reactions and photochemistry of benzene on powdered TiO2

The adsorption, thermal reactions, and photochemistry of benzene on powdered TiO2 have been investigated using Fourier-transform infrared spectroscopy. Benzene is adsorbed with its π electrons interacting with the surface Ti4+ ions, and is stable even when the TiO2 temperature is increased to 400°C. However, it is subject to thermal decomposition to H2O(g), CO(g) and CO2(g) in the presence of O2 and the dissociation occurs below ∽225°C. With the aid of an adsorption study of phenol and iodobenzene, it is found that phenoxy groups are generated on the surface in the photooxidation of benzene on TiO2 and the amount greatly increases in the presence of H2O or O2. Other surface products containing carbonyl or carboxylate groups are produced as well for benzene photoreaction in O2. The present in situ study shows that the products extracted using solvents from TiO2 catalysts after photooxidation of benzene in previous investigations may not reveal the real products formed on the surface during photoillumination processes.

‘Classical wavepacket’ dynamics through a conical intersection. Application to the S 1/S 0 photochemistry of benzene

Excited state dynamics calculations (using semiclassical surface-hopping trajectories first described by Tully and Preston) have been carried out in order to explain the low quantum yield for the photochemical production of benzvalene from benzene. Trajectories are propagated using a series of local quadratic approximations to the potential energy surface. Initial conditions are determined by random sampling of each excited state normal mode within an energy threshold which generates an ensemble of trajectories - a ‘classical wavepacket’. Points on the potential energy surface are computed as required using a parametrised quantum-mechanical/force-field method (molecular mechanics-valence bond, MMVB) which has been designed to simulate the CASSCF potential for ground and covalent excited states. The low benzvalene quantum yield is shown to result from the fact that, after decay via an S 1/S 0 conical intersection, most trajectories lead back to S 0 benzene and do not reach the region of the prefulvene intermediate.

Photochemistry of benzene and oxygen in supersonic cluster beams

A reaction is seen to occur between benzene and oxygen in a supersonic beam when mixed clusters are irradiated by 193nm. This reaction has been investigated by multiphoton ionization mass spectrometry, with primary reaction products including the introduction of up to four oxygen atoms onto the benzene, as well as species resulting from hydrogen abstraction. A two laser experiment indicates that the initial step in the photochemical reaction results in a neutral species which lives for longer than 100 ..mu..s.

Photochemistry of benzene in aerated aqueous solutions in the range of 214 to 265 nm

Photochemistry of benzene in aerated aqueous solutions in the range of 214 to 265 nm

Fluorescence and photochemistry of benzene in solution: Heavy atom effect of Xe

Solutions of benzene in O2 free cyclohexane gave on illumination at 254 nm fluorescence yields which were lower significantly in the presence of Xe, than in its absence. The photochemical yield of benzvalene was unaffected. The results support the assumption that this photochemistry originates in non-relaxed states before the fluorescent level is reached.

Intermolecular vibronic coupling and the mechanism of formation of hexatrienes from benzene in viscous media

The photochemistry of benzene and some of its alkyl derivatives has been studied in deuterated alcohol at 77°K. Intermolecular vibronic coupling of triplet benzene occurs with C-H but not O-H vibrational modes in the solvent. The coupling is sensitive to the relative orientation of the neighbouring molecules and in favourable cases, leads to the formation of a hexatriene in a concerted process.

Photochemistry of benzene in oxygenated aqueous solution in the B2u first excited singlet state

Photochemistry of benzene in oxygenated aqueous solution in the B2u first excited singlet state

Spectroscopy and photochemistry at low temperature. Part 2.—Hydrogen bonding and photochemical addition to the benzene ring

Perturbations in the near u.-v. absorption spectra of benzene and its derivatives are induced by CHCl3 and other halogenated protic solutes in hydrocarbon solution, at temperatures below 200°K; they have been ascribed to H-bonding with the aromatic ring. Two stages of association have been distinguished, and ascribed to specifically oriented complexes involving one, and two molecules of the non-aromatic solute. Thermodynamic parameters for the benzene + CHCl3 association at 77°K, have been evaluated, and the structure of the complex compared with that of pyridine and CHCl3 at 77°K. Association modified the low-temperature photochemistry of benzene and its alkyl derivatives, and substituted polyene adducts are produced with perchloro-alkanes. With substituted ethylenes, the photochemical reaction producing octatetraenes in fluid solution, is suppressed in a rigid solvent medium or by association. The two types of reaction are compared, and possible mechanisms discussed.

Cationic intermediates in the photochemistry of benzene

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