Photochemistry of tetraalkylammonium tetrachlorocuprates in low-temperature matrices

The product composition and the principles of photochemical transformations of tetrahexylammonium tetrachlorocuprate [(RH)₄N⁺]₂[Cu^IICl₄]²⁻ (RH = C₆H₁₃) in 2-chlorobutane at 77 K have been found out by ESR spectroscopy. It has been shown that the photolysis of [(RH)₄N⁺]₂ [Cu^IICl₄]²⁻ results in the formation of alkyl radicals (R^⊙), presumably, anions [Cu^ICl₃]²⁻ and organic copper(II) compounds {Cu^IIR}. A reduction in the quantum yield of primary photolysis products during the reaction, nonequivalence of the quantum yield of the buildup of paramagnetic photolysis products to that of [Cu^IICl₄]²⁻ consumption, and a decrease in the total number of paramagnetic particles in the system during the photolysis have been revealed. A photolysis mechanism involving both photochemical and thermal processes is proposed.     

The Photochemistry of [FeIIIN3(cyclam‐ac)]PF6 at 266 nm

The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [Fe^IIIN₃(cyclam‐ac)]PF₆ ([ 1 ]PF₆), where cyclam‐ac represents the 1,4,8,11‐tetraazacyclotetradecane‐1‐acetate ligand, has been shown to be wavelength‐dependent, leading either to the rare high‐valent iron(V) nitrido complex [Fe^VN(cyclam‐ac)]PF₆ ([ 3 ]PF₆) after cleavage of the azide N_α? N_β bond, or to a photoreduced Fe^II species after Fe? N_azide bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step‐scan and rapid‐scan time‐resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron‐containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe? N_azide bond, as was suspected from earlier steady‐state irradiation studies. In argon‐purged solutions of [ 1 ]PF₆, the solvent‐stabilized complex cation [Fe^II(CH₃CN)(cyclam‐ac)]⁺ ( 2 red ) together with the azide radical (N₃^.) is formed with a relative yield of 80 %, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step‐scan experiments with a higher time resolution reveal the formation of azide anions (N₃^?) during the first 500 ns after photolysis, with a yield of 20 %. These azide ions can subsequently react thermally with 2 red to form [Fe^IIN₃(cyclam‐ac)] ( 1 red ) as a secondary product of the photochemical decomposition of 1 . Molecular oxygen was further used to quench 1 red and 2 red to form what seems to be the elusive complex [Fe(O₂)(cyclam‐ac)]⁺ ( 6 ).     

Production of CO by UV irradiation of acetylacetone

Photochemistry of hexafluoroacetylacetone has been studied by photoproduct analysis using infrared spectroscopy. Hexafluoroacetylacetone was irradiated by KrF excimer laser with the oscillation wavelength of 248 nm during 100 s and the photoproducts were analyzed by transmittance measurement of FT-IR in the spectral region between 3500 and 1300 cm⁻¹. One of the photoproducts was CO. It was estimated that the formation efficiency of CO was approximately 0.6.     

Photochemistry of formic acid monomer at 222 nm

The yields of CO₂ and CO formed from the gas-phase photolysis at 222 nm of very low pressures of formic acid where the monomer predominates have been determined using FTIR spectroscopy. The observed ratio of CO₂/CO approaches unity as the formic acid pressure approaches zero. Previous studies have shown that the quantum yield for HCOOH + hv → OH + HCO (or H + CO) is 0.70 at 298 K. The present experimental results indicate that the ratio of the quantum yields of the possible molecular photolysis channels forming H₂ + CO₂ (?_1b) and H₂O + CO (?_1c) is ca. 1. Including this result in an analysis of previously reported quantum yields of CO and CO₂ determined at higher pressures of formic acid, where both monomer and dimer contribute significantly to the products, indicates that ?_1b = ?_1c = 0. © 1994 John Wiley & Sons, Inc.     

Photofragment translational spectroscopy of iodobenzene at 266 nm

The photofragmentation of C_6 H_5I at 266 nm is investigated on the nuiversal crossed molecular beam machine, and the translational spectroseopy as well as the angular distribution of I atom is measured. The results reveal that under the laser intensity of 10~8 W/cm~2 the single-phuton dissociation competes with multi-photon processes. In singlephoton dissociation the anisotropy parameter β is 0.4 and the average translational energy is only 1.04 keal/mol, which indicates that this process is a slow predissociation. In two-photon phutofragmentation the average translational energy is 51.64 kcal/mol, which accounts for about 35% of the available energy. Another photofragmentation channel is even more faster, whose peak in time-of-flight spectra corresponds to four or five photon absorptions. The branching ratio of these three channels is determined to be about 3: 3: 4.     

Fluorescence response of a 4-trifluoroacetylaminophthalimide to iodide ions upon 254 nm irradiation in MeCN

The title trifluoroacetylaminophthalimide derivative produced a violet fluorescence (lambdaFLmax 392 nm) in MeCN, and it displayed a green emission (lambdaFLmax 506 nm) after irradiation at 254 nm in the presence of iodide ions. The corresponding amidate ion of the trifluoroacetamide was identified as the green fluorescence emitter. The deprotonation reaction may be caused by proton-abstracting solvated electrons generated by a photochemical charge--transfer-to-solvent process from I(-) to MeCN.     

Photodissociation Dynamics of Nitromethane and Nitroethane at 266 nm

Measurements of the nascent OH product from photodissociation of gaseous nitromethane and nitroethane at 266 nm were performed using the single-photon laser induced °uorescence technique. The OH fragment is found to be vibrationally cold for both systems. The rotational state distribution of nitromethane are Boltzmann, with rotational temperature of Trot=2045§150 and 1923§150 K for both 2|3=2 and 2|1=2 states, respectively. For nitroethane, the rotational state distribution shows none Boltzmann and cannot be well characterized by a rotational temperature, which indicates the di?erent mechanisms in producing OH radicals from photodissociation of nitromethane and nitroethane. The rotational energy is calculated as 14.36§0.8 and 4.98§0.8 kJ/mol for nitromethane and nitroethane, respectively. A preferential population of the low spin-orbit component (2|3=2) is observed for both nitromethane and nitroethane. The dominant population of |+ state in two ¤-doublet states is also observed for both nitromethane and nitroethane,which indicates that the unpaired ? lobe of the OH fragment is parallel to the plane of rotation.     

Two-photon Dissociation Study of Carbon Disulfide in Acetonitrile at 266 nm

Using laser flash photolysis/transient absorption technique for the study of two photon photodissociation of carbon disulfide in acetonitrile solution at 266 nm, the transient UV-Vis absorption spectrum of Rydberg state CS2 （6sσg） within 240-370 nm and subsequent dissociation product CS （α^3П） with the maximum absorption at 260 nm were directly observed. The lifetime of CS （α^3П） in the nitrogen and oxygen saturated solution is also studied in our experiment.     

Photochemistry of the vitamin D series. I. Kinetics and quantum yields in ergosterin irradiation by lambda = 253,7 nm

Irradiation of Δ^5,7-9α, 10β-steroids in solution with λ = 253,7 nm yielded no Δ^5,7-9β, 10α-steroids and vice versa. No back reaction from precalciferol₂ to ergosterol could be detected by kinetic measurements. This leads to the conclusion that with the mercury resonance line only ring opening and 6,7-cis/trans isomerization but no ring closure occurs.     

Crystal field splitting of rovibrational transitions of water monomers isolated in solid parahydrogen

We report polarized infrared absorption spectra of water isotopologues isolated in solid parahydrogen (pH2) which reveal the crystal field induced splittings of the 1 01<--0 00 R(0) lines in the nu1 HDO, nu3 D2O, nu3 HDO, and nu3 H2O fundamental bands. For annealed pH2 solids, these spectra also reveal a strong alignment of the hexagonal-close-packed crystallites' c axes with the deposition substrate surface normal. This alignment effect explains our failure to detect the parallel-polarized components of these R(0) lines in spectra of pH2 solids produced on a transparent deposition substrate [M. E. Fajardo et al., J. Mol. Struct. 695, 111 (2004)]. This lesson applies more generally to comparison of solid pH2 spectra obtained in different laboratories. The spectra are consistent with water monomers existing in solid pH2 as very slightly hindered rotors. The individual components of the R(0) absorption lines show a Lorentzian lineshape, with vibrational depopulation the most important source of line broadening.     

Dimerization of acetylacetone upon the action of 3,6-di-tert-butyl-ortho-benzoqutnone and tetrafluorosilane

1,3,5,7-Tetramethyl- 2,4,6,8-tetraoxaadamantane was obtained by the dimerization of acetylacetone in the presence of 3,6-di-tert-butyl-orthobenzoquinone and tetrafluorosilane. An x-ray diffraction structural analysis was carried out for the tetraoxaadamantane obtained.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2375–2377, October, 1989.     

Photochemistry of uridine on high intensity laser UV irradiation

The method of high performance liquid chromatography was applied to investigate the products of single-quantum and two-quantum photo-reactions which occur when an aqueous solution of uridine is irradiated with high intensity nanosecond and picosecond laser UV pulses. A comparison of the experimental results obtained with theoretical models enables a number of photophysical parameters of the highly excited electronic states of the uridine molecule to be determined.     

Photochemistry of cyclohexene oxide radical cations in freonic matrices at 77 K

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Photodissociation of Dibromobenzenes at 266 nm by the Velocity Imaging Technique

A velocity imaging technique combined with (2+1) resonance‐enhanced multiphoton ionization (REMPI) is used to detect the primary Br(²P_3/2) fragment in the photodissociation of o‐, m‐, and p‐dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o‐ and m‐dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (π,π*) state followed by predissociation along a repulsive triplet (n,σ*) state localized on the C? Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53–0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p‐dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet–triplet crossing energy. The fractions of translational energy release are determined to be 6–8 and 29–40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.     

Detection of OH Radical in the Photodissociation of p-Aminobenzoic Acid at 266 nm

Photodissociation of p-aminobenzoic acid at 266 nm was investigated by probing the nascent OH photoproduct employing the laser-induced fluorescence technique. It was found that the nascent OH radical was vibrationally cold and its rotational state distribution conformed to be a Boltzmann behavior, characterized by a rotational temperature of 1040±110 K. The rotational energy of OH was determined to be 8.78±0.84 kJ/mol. Between the two spinorbit states of OH, ^2Ⅱ3/2 and ^2Ⅱ1/2, the former was found to be preferentially populated. The distribution of the II（A＇） state for the A-doublet was dominant. Finally, a probable mechanism for the formation of OH produced from the photodissociation of p-aminobenzoic acid is discussed.     

Picosecond laser study of the collisionless photodissociation of dimethylnitramine at 266 nm

We report a picosecond pump-probe study of the UV photolysis of gaseous dimethylnitramine. After photolysis by picosecond laser pulses at 266 nm, efficient monophotonic collision-free photodissociation occurs within 6 ps. NO₂ fragments are formed in the ground electronic state and in a fluorescent excited state; the quantum yields for both of these channels are estimated.     

Angular distribution of photo electrons from above-threshold-ionization (ATI) of xenon by 532 nm, 355 nm and 266 nm radiation

A time-of-flight electron energy spectrometer has been used to measure the angular distributions of photoelectrons emitted after the absorption of up to four excess photons above the ionization threshold of Xenon at 532 nm. For shorter wavelength less efficient ATI is observed. The shape of the angular distributions and the branching ratios for the two ionic fine structure states Xe⁺(² P _3/2) and Xe⁺(² P _1/2) may be plausibly attributed to the influence of excited states of the atom.     

Ultraviolet photodissociation at 266 nm of phosphorylated peptide cations

Ultraviolet (UV) photodissociation (PD) experiments using 266 nm light were performed for a series of phosphopeptide cations in a Fourier transform mass spectrometer. The objective of the experiments was to determine whether 266 nm UV irradiation on the phosphopeptide cations would induce unique peptide backbone dissociation. In addition, the general behavior of the phosphate loss (?80 or ?98 Da) was monitored, particularly for those phosphopeptides with a phosphotyrosine residue that itself is a UV chromophore. For phosphopeptides with a UV chromophore, their photodissociation behavior was very similar to that of low‐energy sustained off‐resonance irradiation collisionally activated dissociation (SORI‐CAD), with a few exceptions. For example, b‐ and y‐type peptide backbone fragments were prevalent, and their dephosphorylation behavior was consistent with that of the SORI‐CAD results. For phosphoserine peptides, the loss of a phosphate group was always observed. On the other hand, for phosphotyrosine peptides, the phosphate loss was found to be dependent on the presence of a basic amino group in the sequence and the charge state of the precursor ions, in agreement with the CAD results in the literature. However, hydrogen atom loss or aromatic side chain loss, which is known to be the excited state specific fragmentation pathway, was rarely observed in our 266 nm UV PD experiments, in contrast to the previous UV PD literature (particularly at 220 nm). The mechanism for these observations is described in terms of dominant internal conversion followed by intramolecular vibrational energy redistribution (IVR). Copyright © 2009 John Wiley & Sons, Ltd.     

Photoionization of DNA and RNA bases,nucleosides and nucleotides through a combination of one- and two-photon pathways upon 266 nm nanosecond laser excitation

The 266 nm nanosecond laser photolysis of various purine and pyrimidine derivatives results in their photoionization (PI) as one of the primary photochemical pathways. Electron photoejection occurs through a combination of one- and two-photon mechanisms. The PI values depend on the substituents attached to the chromophore of the base. The net PI of the purine bases at 266 nm are of the same order of magnitude (10(-2)) as those of the pyrimidine bases under similar experimental conditions. The monophotonic component is approximately one-third of the net PI yield of the bases. A nonrelaxed singlet excited state intermediate is tentatively proposed for this pathway. It is proposed that this state is significantly stabilized by water solvation, transforming it into a charge transfer to solvent state from which the hydrated electron evolves.