Density functional study of chlorine–oxygen compounds related to the ClO self-reaction

Geometrical parameters, vibrational frequencies, relative stabilities, and dissociation energies of the three stable Cl₂O₂ isomers and the OClO and ClOO radicals were investigated by density functional theory (DFT). The present analysis shows that DFT using hybrid functionals is capable of describing these systems to at least the same degree of accuracy as ab initio methods. The average absolute bond-length deviation of ClClO₂, ClOOCl, and ClO₂ from experimental results is 0.024/0.027 Å, with a maximum deviation for the dichlorine peroxide O(SINGLE BOND)O bond equal to 0.072/0.063 Å, for the B3PW91 and B3LYP functionals, respectively. The average absolute bond-angle deviation for the hybrid functionals is 0.8°. Harmonic vibrational frequencies calculated with DFT give for all Cl(SINGLE BOND)O compounds good agreement with experiments. The dissociation energies of ClOOCl, OClO, and ClOO were found to be in good agreement with experiments, the average error being less than 1.2 kcal/mol. The two isomers chloryl chloride (ClClO₂) and dichlorine peroxide (ClOOCl) were found to be approximately 9 kcal/mol more stable than the chlorine chlorite (ClOClO) isomer. The ClOO isomer is predicted to be 3.0 kcal/mol more stable than OClO, in accordance with the experimental value of 4 kcal/mol. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 203–217, 1998     

Ketone photolysis in the presence of oxygen: A useful source of OH for flash photolysis kinetics experiments

Previous studies have shown a significant OH yield from the reaction of RCO radicals (generated from the photolysis of corresponding ketone) with oxygen below total pressures of 200 Torr. The potential of these reactions as a source of OH radicals for flash photolytic kinetic studies is investigated. The viability of the method was tested by measuring rate coefficients for the reaction of OH with ethanol using both acetone/O₂ mixtures and t‐butyl hydroperoxide photolysis. The results (with statistical errors at the 2σ level) are in excellent agreement with each other (k_EtOH(acetone) = (5.87 ± 0.34) × 10^?18 T² exp((515 ± 21)K/T) cm³ molecule^?1 s^?1 and k_EtOH (t‐butyl hydroperoxide) = (5.27 ± 0.34) × 10^?18 T² exp((557 ± 20)K/T) cm³ molecule^?1 s^?1) and with the IUPAC recommendation. The reaction of OH with methyl ethyl ketone (2‐butanone) has also been investigated using a similar technique. The results show a strong non‐Arrhenius temperature dependence, k = (3.84 ± 0.12) × 10^?24× T⁴ × exp((1038 ± 11)/t). The merits of the ketone/oxygen OH source are contrasted with other established precursors. A major advantage of the technique is the ability to cleanly generate OD without the potential for isotopic scrambling prior to photolysis. © 2008 Wiley Periodicals, Inc. 40: 504–514, 2008     

Chemically activated 1,1-dimethylcyclopropane from photolysis of isobutene–neopentane–diazomethane–oxygen mixtures

A study of the structural isomerization rate of chemically activated 1,1-dimethylcyclopropane from singlet methylene addition to the double bond of isobutene is reported. Singlet methylenes were produced from the 4358- and 3660-Å photolysis of diazomethane in the presence of added oxygen. Theoretical rates calculated via RRKM theory are in excellent agreement with experiment for calculations utilizing activated complex structures and critical energies consistent with known thermal Arrhenius parameters, and excitation energies consistent with previous determinations of ΔH(CH₂) + E*(CH₂) = 116.1 and 112.6 kcal/mole for diazomethane photolyses at 3660 and 4358 Å, respectively.     

Chemically activated 3-methyl-l-butene and 2-methyl-l-butene from photolysis of diazomethane–isobutene–neopentane–oxygen mixtures

An experimental study of the decomposition kinetics of chemically activated 2-methyl-l-butene and 3-methyl-l-butene produced from photolysis of diazomethane-isobutene-neopentane-oxygen mixtures is reported. The experimental rate constants for 3-methyl-l-butene decomposition were 1.74 ± 0.44 × 10⁸ sec^?1 and 1.01 ± 0.25 × 10⁸ sec^?1 at 3660 and 4358 Å, respectively. 2-Methyl-l-butene experimental decomposition rate constants were found to be 5.94 ± 0.59 × 10⁷ sec^?1 at 3660 Å and 3.42 ± 0.34 × 10⁷ sec^?1 at 4358 Å. Activated complex structures giving Arrhenius A-factors calculated from absolute rate theory of 10^{16.6 ± 0.5} sec^?1 for 3-methyl-l-butene and 10^{16.2 ± 0.4} sec^?1 for 2-methyl-l-butene, both calculated at 1000°K, were required to fit RRKM theory calculated rate constants to the experimental rate constants at reasonable E₀ and E* values. Corrected calculations (adjusted E₀ values) on previous results for 2-pentene decomposition gave an Arrhenius A-factor of 10^{16.45 ± 0.35} sec^?1 at 1000°K. The predicted A-factors for these three alkene decompositions giving resonance-stabilized methylully radicals are in good internal agreement. The fact that these A-factors are only slightly less than those for related alkane decompositions indicates that methylallylic resonance in the decomposition products leads to only a small amount of tightening in the corresponding activated complexes. This tightening is a significantly smaller factor than the large reduction in the critical energy due to resonance stabilization.     

A laser flash photolysis study of curcumin in dioxane-water mixtures.

Curcumin [bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was studied by means of UV-VIS absorption spectroscopy and nanosecond laser flash photolysis in 1,4-dioxane-water mixtures in a series of dioxane-water volume ratios. The transient characteristics were found to be dependent on the amount of water. In pure dioxane the triplet state of the molecule in its enolic form was detected (lambda(max) = 720 nm, tau = 3.2 micros), whereas upon water addition, the diketo form was found to prevail, because of the perturbation of intramolecular H-bonded structure. This led to hydrogen abstraction from dioxane by curcumin triplet state and the formation of the corresponding ketyl radical (lambda(max) = 490 nm, tau approximately 10 micros). Laser flash photolysis measurements, carried out in solvents of different polarity and proticity (benzene, cyclohexane and various alcohols), allowed the transient assignments to be confirmed, supporting our interpretation.     

The isothermal flash photolysis of hydrazoic acid

The flash photolysis of HN₃ was studied by coordinated time-resolved spectroscopic measurements of HN₃ NH(a¹Δ), NH(X³Σ), NH(c¹π), NH(A³π), NH₂, and N₃ following flash photolysis of mixtures of HN₃ with argon or helium. The primary photolysis is complex, but when the wavelength distribution of the flash is limited to values greater than about 200 nm, the major reactive product is NH(¹Δ), or states which quickly decay to NH(¹Δ). Disappearance of NH(¹Δ) occurs predominantly by the process The process has little, if any, energy of activation, and no detectable dependence on the pressure of inert gas below 1 atm. The rate of formation of NH₂ in its ground vibrational state depends on the inert gas pressure in a way that can be accounted for by vibrational relaxation from initial excited vibrational states. The total amount of NH₂ is roughly comparable with the amount of HN₃ decomposed by primary photolysis. The observed N₃ can be attributed to the NH(¹Δ) + HN₃ reaction, although a smaller amount could also be formed by primary photolysis. The value of k₂ is revised upward from the value given in a preliminary report on the basis of a more careful consideration of the effects of Beer's law failure in absorption measurements involving narrow spectral lines.     

Production of atomic oxygen following flash photolysis of ClONO2

The photodissociation of ClONO₂ using a broad-band ultraviolet photolysis source has been investigated using time-resolved atomic absorption spectroscopy in the vacuum ultraviolet. The predominant atomic photolysis product is O(³PJ), the quantum yield for Cl(²PJ) production being less than 4%.     

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.     

A flash photolysis study of the formation of retinal Schiff bases in a native photoreceptor cell

It is believed that the accumulation of all-trans-retinal (ATR) in retinal cells is responsible for photoinduced damage to the retina. However, ATR is formed within a photoreceptor cell disk in the process of photolysis and transferred to the cell cytoplasm, where it is converted in a high yield into the nonphototoxic compound retinol. Within the disk where ATR can be accumulated, retinol can form Schiff bases, which are also nonphototoxic compounds, with the amino groups of proteins and lipids. Thus, it is unclear in which concentrations free ATR can be accumulated inside a cell. In this study, it has been proposed to evaluate the concentration of free ATR in the cell from the yield of an excited triplet state because neither Schiff bases nor retinol can be transformed into an excited triplet state. It has been found that 70% ATR forms Schiff bases in the native cell in the equilibrium state, thereby, a considerably decreasing the probability that ATR is the main inducer of photodamage.     

Laser flash photolysis study of electron transfer processes of adsorbed anthracene on titania–silica surfaces

We have studied the photophysics of anthracene adsorbed to photocatalytic silica–titania mixed oxide systems prepared by two different methods: a sol–gel synthesis and an impregnation route. The observed photophysics depend upon the method of synthesis, with the sol–gel prepared samples resulting in enhanced radical cation formation via static excited singlet state quenching. This mechanism, whilst operative, does not give rise to long-lived charge separation in the case of the impregnated samples. These results are discussed in terms of differing sample morphologies.     

The flash photolysis of phosphorus tribromide and carbon oxysulphide

Some absorption bands attributed to the D ← X system of PBr (T_e ⋍ 46,665 cm⁻¹) are reported for the first time, following the flash photolysis of PBr₃, and their mode of formation is briefly discussed. Two further bands found when OCS is initially present possibly arise from SBr or PS.     

A flash photolysis study of the triplet state quenching of anthracene by oxygen in the presence of polystyrene

The rate constant for the quenching of triplet anthracene by molecular oxygen in the presence or absence of polystyrene was measured. The quenching ability is not affected by the presence of polymer.     

Vibrational heating effects in the flash photolysis of ozone

The vibrational temperature of ozone has been estimated during the flash photolysis of ozone under non-isothermal and strictly isothermal conditions, utilising the variation in extinction coefficient with temperature at two wavelengths. Vibrational temperatures measured in the range 300–970 K have been attributed to energy transfer from O₂

(³Σ_g^?) to ozone and the effect of this non-equilibrium vibrational excitation on the secondary reactions with non-zero activation energies is discussed.     

Laser flash photolysis study of the phase transition effect on the kinetics of triplet-triplet annihilation of tetraphenylporphine inn-decanol

The kinetics of triplet-triplet annihilation of tetraphenylporphine (TPP) inn-decanol was studied by nanosecond laser flash photolysis with monitoring of the transient absorption and concomitant delayed fluorescence in the temperature range from 100 to −100°C. The freezing of the solution results in more than tenfold acceleration of the process within the same reaction mechanism. It is supposed that the phase transition is accompanied by the concentration of the TPP molecules in defect regions of the polycrystalline structure of the frozen alcohol. These domains have a homogeneous nature and are characterized by a high molecular mobility.     

Laser flash photolysis studies of alkoxyl radical kinetics using 4-nitrobenzenesulfenate esters as radical precursors

4-Nitrobenzenesulfenate esters were used as precursors for the generation of alkoxyl radicals under laser flash photolysis conditions. The esters were efficiently cleaved using the Nd:YAG third harmonic (355 nm) to produce alkoxyl radicals and the 4-nitrobenzenethiyl radical. Rate constants for beta-scission and 1, 5-hydrogen abstraction reactions of alkoxyl radicals were measured.     

Laser flash photolysis of tert-butyl aroylperbenzoates: kinetics of the singlet and triplet states and the aroylphenyl radicals

tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 nm, pulse width approximately 350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad absorption (lambda(max) approximately 625 nm; tau approximately 20 and approximately 7.9 ps, respectively). The triplet state of each (lambda(max) approximately 530-560 nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of 3 approximately 0.74 ns). The approximately 550 nm absorption obtained from the 355 nm LFP (pulse width approximately 7 ns) of 1, 2, and 4 has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl 5 and 3-(4'-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-containing monomers have been observed to range from 7.56 x 10(7) to 1.68 x 10(9) M(-1) s(-1) in CCl(4) at room temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm absorption are discussed.     

A flash photolysis investigation of the gas phase uv absorption spectrum and self-reaction kinetics of the neopentylperoxy radical

The ultraviolet absorption spectrum of the neopentylperoxy radical, (CH₃)₃CCH₂O₂ (or C₅H₁₁O₂), and the kinetics of its self-reaction have been studied in the gas phase using a flash photolysis technique. The room temperature absorption cross-section at 250 nm was determined to be and was used to normalize the radical absorption spectrum between 210 and 300 nm. Detailed modeling of the self-reaction system was used to interpret the transient absorption kinetic decay curves over the temperature range 228–380 K, at total pressures between 25 and 100 torr. The results are discussed in relation to previous measurements of alkylperoxy radical spectra and kinetics.     

Laser flash photolysis of benzophenone in polymer films

With a nanosecond laser we studied flash photolysis of benzophenone (BP) dissolved in four different polymer films. We measured kinetics of decay of a triplet state of benzophenone (3)BP as well as kinetics of decay of benzophenone ketyl free radicals BPH(?). Polymer matrices have plenty of reactive C-H bonds, and the hydrogen abstraction by (3)BP leads to a formation of geminate pair which either recombines into molecular products or dissociates. Decay kinetics of (3)BP is well described by dispersive kinetics and in particular by the kinetic law suggested in Albery, W. J.; et al. J. Am. Chem. Soc. 1985, 107, 1854. We observed a broader distribution of rate constants in hard films. It was observed that the decay kinetics of transients radicals in the "hard" polymers is quite satisfactory described by the same law for dispersive kinetics. Kinetics of radicals decay in "soft" polymers is satisfactorily described as a diffusion-enhanced reaction. Effect of a hardness of polymer matrix on the measured kinetic parameters is discussed.     

Effect of aggregation on bacteriochlorin a triplet-state formation: a laser flash photolysis study

Bacteriochlorin a (BCA) is a potential photosensitizer for photodynamic therapy of cancer. It has been shown previously that the photoefficiency of the dye is mainly dependent on singlet oxygen (1O2) generation. Nanosecond laser flash photolysis was used to produce and to investigate the excited triplet state of the dye in methanol, phosphate buffer and dimiristoyl-L-alpha-phosphatidylcholine (DMPC) liposomes. The transients were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yield of singlet-triplet intercrossing. The lifetime of the BCA triplet state was measured at room temperature. The triplet-state quantum yield is quite high in methanol (0.7) and in DMPC (0.4) but only 0.095 in phosphate buffer. In the last case, BCA is in a monomer-dimer equilibrium, and the low value of the quantum yield observed was ascribed to the fact the triplet state is only formed by the monomers.     

The chain mechanism of ozone destruction by CFCl3 in the 214 nm photolysis of its mixtures with oxygen

The effect of CFCl₃ (0.025–0.200 mbar) addition on the formation of ozone in 214 nm photolysis of oxygen (800–2000 mbar) was investigated. Kinetic analysis of the drastic reduction in ozone formation in the presence of CFCl₃ shows that it proceeds by a chain mechanism with a chain length of 5.07 ± 0.21(2σ). This chain length is independent of CFCl₃ and O₂ pressures as well as incident light intensity and the mechanism of the chain reaction is governed by the Cl generating reactions of ClO radicals. A mechanism based only on the self reaction of these radicals: ClO + ClO → Cl₂ + O₂ (7), Cl + ClO₂ (8), and Cl + OClO (9), followed by fast decomposition of ClO₂ into Cl and O₂, predicts a chain length which is considerably lower than the observed value. Incorporation of the reaction CFCl₂O₂ + ClO → CFCl₂O + ClO₂ (11) in the mechanism satisfactorily accounts for the observed chain length. A lower limit of 3 × 10^?12 cm³ molecule^?1 s^?1 for k₁₁ is estimated.