The acetyl radical studied by flash photolysis and kinetic spectroscopy

The acetyl radical absorption spectrum is a broad band with maximum decadic extinction coefficient of (1.0 ± 0.1) × 10⁴ ? mole^?1 cm^?1 at 215 nm and an oscillator strength of 0.23 ± 0.03. Absolute rate constants were estimated as 4.5 × 10¹⁰ ? mole^?1 s^?1 for the mutual interaction of acetyl radicals, and as 7.5 × 10¹⁰ ? mole^?1 s^?1 for the cross interaction of acetyl and methyl radicals.     

Mutual interactions of the methyl and methylperoxy radicals studied by flash photolysis and kinetic spectroscopy

The decadic extinction coefficient of the methyl radical at 216.4 nm and the rate constant for mutual combination were redetermined as: . The application of the Beer–Lambert law to these measurements was justified experimentally. The absorption spectrum of the methylperoxy radical was characterized as a weak, broad, structureless band, having a maximum at 240 nm with ?(240) = 1.55 × 10³ l./mol cm. The mutual interaction of methylperoxy radicals leads to the generation of methoxy and hydroperoxy radicals as a consequence of the nonterminating interaction . Each derivative radical may consume a significant fraction of the methylperoxy radicals, and either of these cross interactions may be made predominant by a suitable choice of oxygen pressure. The mutual interaction was studied under both conditions. The overall mechanism was analyzed by a precise computational method, and the rate constant of the total mutual interaction was estimated as .     

Bimolecular self-reactions of 2-arylindandione- 1,3-yl radicals studied by flash photolysis

Kinetic and thermodynamic data for reaction (1) of certain C-centered aromatic radicals (referred to in this paper by the numbers I to X) in chlorobenzene: have been obtained. The k₁ values of radicals varied between (1.1 ± 0.2) × 10⁶M^?1·sec^?1 (radical VIII) and (3.6 ± 0.7) × 10⁹M^?1 sec^?1 (radical VI) at 20°C. An investigation of the relationship between the recombination rates of radicals I–VIII and X and the solvent viscosity (mixture of toluene and dibutylphthalate, 0.6 < η < 18.4 cP) has shown that the recombination reactions involving radicals I–IV are limited by diffusion in solvents having a viscosity η> 10 cP and are activation reactions in solvents having a viscosity η < 10 cP. The recombination of radicals VIII and IX is an activation reaction, while that of radicals V–VII is diffusion-controlled in the entire viscosity range. The recombination of radical X is limited, in the viscosity range of 18.4 to 2 cP, by intrusion into the first coordination sphere of the partner, the effect of viscosity on the radical X recombination rate in the specified range being the same as its effect on diffusion-controlled reactions. The possible reasons of the discrepancies between the experimental fast recombination rate constants and the theoretical values calculated by the Debye–Smoluchowski theory are discussed. The equilibrium constant depends strongly on the nature of the substituent in the phenyl fragment: the substituents which increase unpaired electron delocalization in the radical intensify the dissociation of the respective dimer. Long-wave absorption bands have been recorded for radicals I–X and their extinction coefficients obtained. Dimers I–V are thermo- and photochromic compounds.     

The flash photolysis of azomethane. Minor products from the photolysis of methyl radicals and a rate constant for CH3 + NO

The flash photolysis of azomethane in a quartz reaction vessel produces mainly ethane (>75%) plus smaller quantities of methane, ethylene, and acetylene. The minor products are interpreted quantitatively in terms of methyl radical photolysis at 216 nm to give CH₂ and H. This interpretation is substantiated by the dependence of the minor products on flash intensity. The reduction of the ethane yield on adding NO is employed to obtain a rate constant for CH₃ + NO as a function of total pressure, based on a value for methyl radical recombination of 4.2 × 10^?11 cm³/molec · sec. An RRKM analysis is used to extrapolate the data to give a limiting high-pressure rate constant for CH₃ + NO of (1.2 ± 0.1) × 10^?11 cm³/molec · sec at 298°K.     

Triplet excitation dynamics of β-carotene studied in three solvents by ns flash photolysis spectroscopy

Upon anthracene-sensitizing, triplet excitation dynamics of β-carotene(β-Car) were studied in nhexane, in methanol, and in acetonitrile, respectively, by ns flash photolysis spectroscopy. In n-hexane,only the bleaching of the ground state absorption(GSB) and the excitation triplet(~3Car*) absorption were observed, and there were no cationic species detected. In both methanol and acetonitrile, similar excitation dynamics were observed, i.e.,~3Car* having a similar lifetime to that in n-hexane, and the immediate generation of the cation dehydrodimer(~#[Car]2~+) upon excitation following transformation into the radical cation Car*~+, since Car*~+ has much longer lifetime in acetonitrile than in methanol. The results prove that both solvent and carotenoid structure determine the triplet excitation mechanism.     

Kinetics of reactions between methyl and acetyl radicals in gas phase produced by flash photolysis of acetic anhydride

The reaction products of photolysis of acetic anhydride in gas phase at 25°C, where He or CO₂ was present as buffer gas, were analyzed by gas chromatography. The extent of photodissociation was 52% ± 5% and the extent of intramolecular hydrogen transfer reaction producing acetic acid and ketene was 48% ± 5%. The rate constants of the hydrogen exchange and radical combination reactions between methyl and acetyl radicals were calculated from the amounts of products. The value of the ratio of the rate constants of hydrogen exchange and radical combination reactions between methyl and acetyl radicals, ??₇/??₆ = 0.15, indicates that acetyl radical is a relatively poor hydrogen donor. The corresponding ratio of rate constants for the reactions between two acetyl radicals, ??₉/??_s = 0.42, indicates that acetyl radical is a better hydrogen acceptor than methyl radical.     

The ethylperoxy radical spectrum and rate constant for mutual interaction measured by flash photolysis and kinetic spectroscopy

Intrinsic spectral and kinetic parameters have been measured for the ethylperoxy radical, which was formed in the gas phase by the flash photolysis of azoethane in the presence of an adequate excess of oxygen. Absolute values of the extinction coefficient ?(λ) were derived from complementary measurements of the yield of nitrogen and the absorbance of an equivalent concentration of ethylperoxy radicals. The absorption spectrum is broad, structureless and comparatively weak; ?(236) = 1.02 × 10³ liter mole^?1 cm^?1 at the maximum, and the oscillator strength is 3.4 × 10^?2. This spectrum resembles the spectrum of the methylperoxy radical closely in form, but it is less intense; the ratio of the values of oscillator strength is 0.5. The bimolecular reactions of mutual interaction of ethylperoxy radicals are not exclusively terminating, and ethoxy and hydroperoxy radicals are formed in kinetically significant quantities. A computer program was designed to simulate the rise and fall of the concentration of each radical species, and to perform the related kinetic analysis. This program predicted that a second-order plot of the decline of the absorbance of the ethylperoxy radical during the dark period would not show a significant departure from linearity, a conclusion which was confirmed by experiment. Accordingly, the gradient of each such plot yielded a value of k'/?(λ), where k' is the apparent value of the rate constant for the collective reactions of mutual interaction. This rate constant was evaluated from the product of corresponding values of k'/?(λ) and ?(λ); individual values are independent of the wavelength of measurement, and the mean value is k' = (6.6 ± 0.5) × 10⁷ liter mole^?1 sec^?1. Further kinetic analysis yielded the corresponding absolute value: k = (6.0 ± 0.6) × 10⁷ liter mole^?1 sec^?1. This value fits the pattern of a relationship between rate constant and structure shown by the methylperoxy, isopropylperoxy, and tert-butylperoxy radicals. Adequate sensitivity for the characterization of the spectrum of the ethylperoxy radical was achieved by the use of a pulsed xenon arc as the monitoring light source in conjunction with a dual beam detection system with twin cells and balanced photomultipliers, and the apparatus is described in detail.     

Thermal addition reactions to benzocyclobutenones studied by flash photolysis

Ortho-quinoid vinylketenes 2 have been generated through flash photolysis of benzocyclobutenones 1. A kinetic study of intermolecular addition reactions of 2 competing with the recyclization 2 → 1 reveals strikingly different substituent effects for the addition of methanol and of dienophiles.     

A quantitative study of alkyl radical reactions by kinetic spectroscopy. IV. The flash photolysis of azopropanes

The flash photolysis of azo?n?propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in theregion of 220–260 nm have been assigned to the n-propyl and isopropyl radicals. For the n-propyl radical, ?_max = 744 ± 39 l/mol cm at 245 nm and the rate constants for the mutual reactions were measured to be k_c = (1.0 ± 0.1) × 10¹⁰ l/mol sec (combination) and k_d = (1.9 ± 0.2) × 10⁹ l/mol sec (disproportionation). For the isopropyl radical, ?_max = 1280 ± 110 l/mol cm at 238 nm, with k_c = (7.7 ± 1.6) × 10⁹ l/mol sec and k_d = (5.0 ± 1.2) × 10⁹ l/mol sec The rate constant for the dissociation of the vibrationally excited triplet state of the azopropanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo-n-propane k = (6.6 ± 1.3) × 10⁷ sec^?1, and for azoisopropane k = (1.6 ± 0.4) × 10⁸ sec^?1. Collisional deactivation of the vibrationally excited singlet and triplet states was found to occur on every collision for n-pentane; but nitrogen and argon were inefficient with a rate constant of 1.1 × 10¹⁰ l/mol sec. Spectra observed in the region of 220–260 and 370–400 nm areattributed to the cis isomers of the parent trans-azopropanes. These are formed, as permanent products, in increasing amounts as the pressure is increased.     

The umbrella motion of core-excited CH3 and CD3 methyl radicals

An accurate experimental and theoretical study of the lowest core excitation of CH(3) and CD(3) methyl radicals is presented. The complex vibrational structure of the lowest band of the x-ray absorption spectrum (XAS) is due to the large variation of the molecular geometry, which is planar in the ground state and pyramidal in the core-excited state. The XAS spectra of the two radicals were recorded at high resolution and assigned by theoretical simulations of the spectra, taking into account the coupling of symmetrical stretching and symmetrical bending (umbrellalike) deformations of the radicals. An excellent agreement between experimental and theoretical spectral profiles allowed us to accurately characterize the vibrational structure of the electronic transition. The similarities, as well as the differences, of the peculiar vibrational progression observed for the two radicals are explained by the strong anharmonicity along the umbrella coordinate and by the isotopic variation, leading to a different probing of the double-well potential energy surface of the core excited state during the nuclear motion.     

Formation and characterization of 1-substituted 1,2-dithiolanyl radicals by laser flash photolysis

Photolyses of 1-alkylthio-3-propanethiols, 1,3-propanedtthiol and 1-phenylthio-3-propanethiol in acetonitrtle, 2-propanol or aqueous mixtures of these solvents lead to five membered cyclic sulfuranyl radicals of the type \(\left[ {R - \overline {S - S - CH_2 - CH_2 - C} H_2 } \right]\) which exhibit characteristic uv/vis absorption spectra. These 1-substituted 1,2-dithiolanyl radicals are formed by intramolecular cyclization of the primarily formed thiyl radicals generaeed by S-H- bond cleavage in the excited starting material. The wavelength of the absorption maximum observed for these transients depends on the natuee of the substituntt R on the sulfur atom (R=H, alkyl, aryl).     

Reactivities of sulfur-centered radical toward polyisoprenes and polybutadienes studied by flash photolysis method

The kinetic behavior of the thiyl radical in the solution containing polyisoprenes and polybutadienes has been studied by the flash photolysis method. For benzothiazole-2-thiyl radical, the addition rate constants toward these polymers and the model compounds of the polymers were evaluated. The relative reverse rate constants and equilibrium constants were also estimated. The addition rate constants decrease with an increase in the degree of polymerization; the ratio of the addition rate constant for polyisoprene (3.1 × 10⁴ M^?1 s^?1 (in monomer unit); M_v = 674,000) to that for 2-methyl-2-butene (1.5 × 10⁵ M^?1 s^?1) is about 1/5. This indicates that the polymer chain effect appears in the free-radical addition reaction. The relative reverse rate constants for the polymers are also smaller than those for 2-methyl-2-butene, suggesting a kind of polymer effects; i.e., it can be presumed that the bonded-thiyl radicals migrate very rapidly to the neighboring double bonds in the polymer. Significant differences in the rate parameters were observed between polyisoprene and polybutadiene, between cis- and trans-polyisoprenes, and between 1,4- and 1,2-polybutadienes.     

Oxygen binding to tyrosinase from streptomyces antibioticus studied by laser flash photolysis

Tyrosinases catalyze the o-hydroxylation of monophenols (monophenolase activity) and the oxidation of o-diphenols to o-quinones (diphenolase activity) and possess a dinuclear copper active site. The O2 binding kinetics of oxytyrosinase is studied by flash-photolysis measurements, and the O2 binding rate constant (kO2) is obtained as kO2 = 13 +/- 3 muM-1 s-1. Small molecules, such as carbon monoxide and p-nitrophenol (a substrate-analogue inhibitor), are demonstrated to affect O2 binding kinetics. The activation enthalpy of the rate-limiting step of O2 binding is calculated by the temperature dependence of kO2 to be 12.8 +/- 2.6 kcal/mol.     

Infrared absorption of CH3OSO and CD3OSO radicals produced upon photolysis of CH3OS(O)Cl and CD3OS(O)Cl in p-H2 matrices

Irradiation at 239 ± 20 nm of a p-H(2) matrix containing methoxysulfinyl chloride, CH(3)OS(O)Cl, at 3.2 K with filtered light from a medium-pressure mercury lamp produced infrared (IR) absorption lines at 3028.4 (attributable to ν(1), CH(2) antisymmetric stretching), 2999.5 (ν(2), CH(3) antisymmetric stretching), 2950.4 (ν(3), CH(3) symmetric stretching), 1465.2 (ν(4), CH(2) scissoring), 1452.0 (ν(5), CH(3) deformation), 1417.8 (ν(6), CH(3) umbrella), 1165.2 (ν(7), CH(3) wagging), 1152.1 (ν(8), S=O stretching mixed with CH(3) rocking), 1147.8 (ν(9), S=O stretching mixed with CH(3) wagging), 989.7 (ν(10), C-O stretching), and 714.5 cm(-1) (ν(11), S-O stretching) modes of syn-CH(3)OSO. When CD(3)OS(O)Cl in a p-H(2) matrix was used, lines at 2275.9 (ν(1)), 2251.9 (ν(2)), 2083.3 (ν(3)), 1070.3 (ν(4)), 1056.0 (ν(5)), 1085.5 (ν(6)), 1159.7 (ν(7)), 920.1 (ν(8)), 889.0 (ν(9)), 976.9 (ν(10)), and 688.9 (ν(11)) cm(-1) appeared and are assigned to syn-CD(3)OSO; the mode numbers correspond to those used for syn-CH(3)OSO. The assignments are based on the photolytic behavior and a comparison of observed vibrational wavenumbers, infrared intensities, and deuterium isotopic shifts with those predicted with the B3P86∕aug-cc-pVTZ method. Our results extend the previously reported four transient IR absorption bands of gaseous syn-CH(3)OSO near 2991, 2956, 1152, and 994 cm(-1) to 11 lines, including those associated with C-O, O-S, and S=O stretching modes. Vibrational wavenumbers of syn-CD(3)OSO are new. These results demonstrate the advantage of a diminished cage effect of solid p-H(2) such that the Cl atom, produced via UV photodissociation of CH(3)OS(O)Cl in situ, might escape from the original cage to yield isolated CH(3)OSO radicals.     

Photoreactions of 1,4-naphthoquinone with lysozyme studied by laser flash photolysis and steady-state analysis

Photoprocesses of 1,4-naphthoquinone (NQ) and its photoreactions with lysozyme in acetonitrile/water (3:1, v/v) solution were studied using 355nm laser flash photolysis technique combined with electrophoresis and turbidimetric assay. The transient spectra of NQ were observed and the transient species were assigned. The electron transfer process from N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) to NQ triplet state ((3)NQ( *)) was investigated and the rate constant was determined to be k(t1)=2.0x10(10)M(-1)s(-1). It has been found that (3)NQ( *) can abstract hydrogen atom from lysozyme with a rate constant of k(t2)=2.4x10(10)M(-1)s(-1). Furthermore, the results of steady-state analysis suggested that lysozyme can be damaged by NQ irradiated with UVA light influenced by the concentration of NQ and the gas saturated in the solution. The mechanisms of photosensitized damage of lysozyme were discussed.     

A study of isomeric retinal triplets by low temperature spectroscopic and kinetic flash photolysis

Spectroscopic and kinetic data of triplets of all-trans retinal and several of the cis isomers obtained with frozen samples at liquid nitrogen temperature are reported. They are believed due to unisomerized triplets of the corresponding ground state isomers. Interestingly, decay of transients from 7-cis-retinal was found to be wavelength dependent.     

Triplet states of humic acids studied by laser flash photolysis using different excitation wavelengths

The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (T_HFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of T_HFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of T_HFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of T_HFA quenching by molecular oxygen are equal to (7—8)·10⁸ L mol^–1 s^–1.     

Determination of rate constants for some reactions of CBr radicals by VUV laser flash photolysis

The CBr radical has been produced by VUV laser flash photolysis of CHBr₃ and absolute rate constants for reactions with O₂, CO₂ and N₂ have been measured. The possible mechanisms of these reactions have been discussed.     

Investigation of perfluoroperoxy radicals self-coupling reactions by kinetic EPR spectroscopy

The rate constants and activation parameters for the self-coupling of perfluoroperoxy radicals of structure A and B: C₇F₁₅OO (A) and R_FOCF₂OO (B) have been determined in perfluorohexane solution in the temperature range 228-258 K. The magnitude of the rate constants obtained ranks between 6.6×10⁸ and 2.5×10⁹ l mol⁻¹ s⁻¹ and are therefore, among the largest rate values so far reported in the literature for primary peroxy radicals couplings. The activation energy is positive and lower for the peroxy radicals (A) with respect to the peroxy radicals (B) (10.5 and 23.0 kJ mol⁻¹, respectively).Analysis by kinetic modeling has shown that the peroxy radicals decay curves are compatible with the participation of peroxy radicals↔tetroxide equilibria to the reaction mechanism.Upper limit values of k_bs<10 and <20 s⁻¹ were inferred for the β-scission reactions of the perfluoroalkylperoxy radicals at 228 and 258 K, respectively.