Showing papers on "Flash photolysis published in 1973"
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01 Jan 1973
TL;DR: Hammett et al. as discussed by the authors proposed a triplet-state diffusion-controlled triplet state diffusion-controlled rate for photolysis of organic molecules in solution, where triplets are derived from triplets of the triplets.
Abstract: . Photophysics of Organic Molecules in Solution Triplet-State Energies: Ordered Flash Photolysis: Designing Experiments Low Temperature Photophysics of Organic Molecules Ground-State Absorption Spectra ESR and ODMR Parameters of the Triplet State Diffusion-Controlled Rate Constants Rate Constants of Singlet-State Quenching Rate Constants of Triplet-State Quenching Ionization Energies, Electron Affinities, and Redox Potentials of Organic Compounds Bond Dissociation Energies Solvent Properties Chemical Actinometry Transmission Characteristics of Light Filters and Glasses Spectral Distribution of Photochemical Sources Spin-Orbit Coupling Fundamental Constants and Conversion Factors Hammett (T Constants Bibliography and Indexes References Compound Name Index Molecular Formula Index
2,513 citations
TL;DR: In this paper, the rate constants for oxygen quenching of aromatic hydrocarbon triplets have been measured by the laser flash photolysis technique, and the importance of restrictive Franck-Condon factors which are determined by the hydrocarbon is discussed.
Abstract: The rate constants for oxygen quenching of aromatic hydrocarbon triplets have been measured by the laser flash photolysis technique. The quenching of high triplet energy compounds is characterised by rates which are inversely proportional to triplet energy. The reaction probabilities, which are as small as 10–2 in hexane, have been found to increase in polar or viscous solvents. Molecules with low triplet energies are quenched at one-ninth the measured diffusion controlled encounter rate.These data are analysed in terms of the non-radiative transitions of a collision complex of the aromatic triplet and ground state oxygen. The importance of restrictive Franck–Condon factors which are determined by the hydrocarbon is discussed. The data are shown to be consistent with electronic matrix elements for the energy transfer quenching processes that are dependent on orbital symmetry matching and charge transfer interactions.
221 citations
121 citations
116 citations
TL;DR: The polyenes include several which contain carbonyl groups whose triplet states, unlike retinal, could not be detected unsensitized by flash photolysis, and appear to be a function of the number of conjugated double bonds.
Abstract: — The triplet-triplet absorption spectra of six polyenes have been characterised using flash photolysis, in the presence of anthracene as sensitizer, and pulse radiolysis, in the absence of a sensitizer. The polyenes include several which contain carbonyl groups whose triplet states, unlike retinal, could not be detected unsensitized by flash photolysis. The triplet lifetimes appear to be a function of the number of conjugated double bonds, and vary between 7 and 14 μ sec. In general, the longer the polyene, the shorter the lifetime. An empirical linear relation was found between the frequencies of the polyene triplet-triplet absorption maxima, and the frequencies of the corresponding ground singlet-singlet maxima. The rate constants for quenching by oxygen of nine polyene triplet states were determined to lie in the range 2–7 × 109M-1 sec-1. The possible mechanisms for oxygen quenching of triplet states are discussed and analogies between the results for oxygen quenching of polyenes and of polyacenes are drawn. The rate constant for oxygen quenching of all-trans-β-carotene triplet was the same in benzene and hexane.
101 citations
TL;DR: Computer fits of the time course of the CO combination reaction could not be achieved with a simple two-state model of hemoglobin, but rather required intra-dimer interactions.
100 citations
75 citations
TL;DR: In this paper, the mechanism of the photoreduction of 9,10-anthraquinone (AQ) in alcohol and hexane has been studied by flash photolysis.
Abstract: — The mechanism of the photoreduction of 9,10-anthraquinone (AQ) in alcohol and hexane has been studied by flash photolysis. The fluorescence spectrum of the photoproduct, 9,10-dihydroxy anthracene shows a large shift between hexane and ethanol. The quantum yields of photoreduction for AQ are solvent-dependent, the reaction between the solvent radical and AQ determining the quantum yield.
The absorption spectrum of the 9,10-anthrasemiquinone (AQH.) has a long-wavelength absorption band with peaks at 631 and 678 nm. The second-order decay constants for AQH. were estimated to be 1.3 × 109, 6.7 × 108 and 2.0 × 108M-1 sec-1 in ethanol, 2-propanol and ethylene glycol, respectively.
A long-wavelength absorption band was observed for 9,10-anthrasemiquinone radical anion, having peaks at 776 and 860 nm; epsi;max= 1900 at 776 nm. This spectrum is compared with the spectra of 9,10-dihydroxy anthracene mono- and di-anions. The 9,10-anthrasemiquinone radical anion was found to photoreduce quantitatively to 9,10-dihydroxy anthracene mono-anion with a quantum yield of 0.1.
61 citations
TL;DR: In this article, an inner-sphere substitution-controlled path with a rate constant of (3·6 ± 0·4)×106 l mol-1 s−1 at 25 °C was presented.
Abstract: The radical anions Br2– and Cl2–, generated by flash photolysis of the corresponding halide solutions or of the iron(III) complexes FeBr2+ and FeCl2+, oxidize iron(II) to iron(III). The product of the reaction between the ions Br2– and Fe2+ is the FeBr2+ complex. The reaction proceeds by an inner-sphere substitution-controlled path with a rate constant of (3·6 ± 0·4)×106 l mol–1 s–1 at 25 °C (ΔH‡= 25·2 ± 2 kJ mol–1; ΔS‡=–42 ± 12 J K–1 mol–1). The reaction between the ions Cl2– and Fe2+ proceeds by two paths at 25 °C, an inner-sphere substitution-controlled path with a rate constant of (4·0 ± 0·6)× 106 l mol–1 s–1(ΔH‡= 31·5 ± 4 kJ mol–1; ΔS‡=–21 ± 15 J K–1 mol–1), and an outer-sphere path with a rate constant of (1·0 ± 0·2)× 107 l mol–1s–1(ΔH‡= 22·7 ± 4 kJ mol–1; ΔS‡=–42 ± 15 J K–1 mol–1).
58 citations
TL;DR: In this article, the rate constants for quenching of aromatic hydrocarbon triplets by nitric oxide and by the free radical di-t-butyl nitroxide have been measured by the laser flash photolysis technique.
Abstract: The rate constants for quenching of aromatic hydrocarbon triplets by nitric oxide and by the free radical di-t-butyl nitroxide have been measured by the laser flash photolysis technique. For low energy triplets the efficiency of quenching is inversely proportional to triplet energy. At high triplet energy the rates are proportional to triplet energy and there is a large effect of solvent polarity.These data are analysed in terms of an enhanced intersystem crossing process catalysed by the doublet state nitric oxide molecule. The data are shown to be consistent with important contributions from charge-transfer interactions for high triplet energy compounds in non-polar solvents.
56 citations
TL;DR: In this article, the absolute second-order rate constants describing the removal of the excited atom on collision with the molecules H2, D2, CH4, NO, NO2, N2O, and C3O2 were reported.
Abstract: Time-resolved atomic absorption spectroscopy in the vacuum ultraviolet has been employed to monitor electronically excited oxygen atoms, O(21D2), following their genera-tion by the flash photolysis of ozone in the Hartley band region. We report the first values for the absolute second-order rate constants describing the removal of the excited atom on collision with the molecules H2, D2, CH4, NO, NO2, N2O, and C3O2. Where possible, these data are considered within the context of restrictions arising from spin and orbital symmetry and are further discussed in tems of previously reported relative rate data derived from indirect measurements. Consideration is given to the importance of these rate con-stants in discussing processes taking place in the earth's atmosphere and in systems giving rise to chemical laser action.
TL;DR: Flash photolysis studies in a number of labora tor ie~ have shown that 3Chl a in fluid solutions is efficiently quenched by quinones, probably via electron transfer to the quencher, and the triplet state of the chlorophyll is strongly implicated as the electron donor.
Abstract: The existence of triplet state chlorophylls in uitro has been well e~tablished,'-~ but the role of such intermediates in photosynthesis is ~ n c e r t a i n . ~ For example, it is thought that one of the functions of carotenoids in the chloroplast is to protect the system from damage by photooxidation.6 This effect is apparently due to oxidation by singlet oxygen ('A#), which is formed by energy transfer' from the lowest triplet state of chlorophyll a (Chl a) to molecular oxygen. Another possibility-perhaps one within the mainstream of biochemical events-is electron transfer from the triplet state of chlorophyll ( T h l ) to form a wcation radicaLs Oxidized forms of the respective chlorophylls are evidently involved in both green plant9 and bacterial'O photosynthesis, and there is evidence that in uitro photooxidation of Chl a can proceed via the triplet state. Flash photolysis studies in a number of labora tor ie~ ' .~~ ' ' '~ have shown that 3Chl a in fluid solutions is efficiently quenched by quinones, probably via electron transfer to the quencher. In frozen solutions, both quinones15 and aromatic nitrocompounds'6 can photooxidize Chl a, the efficiencies being proportional to the redox potentials of the oxidant. However, involvement of T h l a in these reactions can only be inferred, since the experiments were carried out under steady illumination. Seely\" employed a series of substituted nitrobenzenes as oxidants (Ox) and measured the rates of pyrochlorophylland Chl a-sensitized photoreduction of the nitro-compounds by hydrazobenzene in 7:3 (v/v) ethanol-pyridine at room temperature. He found a good correlation between the quantum yields and the polarographic quarter-wave reduction potentials of the nitro-compounds. Since fluorescence quenching is negligible under the conditions employed, the triplet state of the chlorophyll is strongly implicated as the electron donor. Thus, Seely's results show that the reaction:
TL;DR: In this paper, a diffusion-controlled recombination rate constant in acetonitrile of (4.3 ± 0.3) × 10 10 10 M −1 sec −1 (ca. 1.2× 10 10 m −1 in n -propanol) is obtained.
TL;DR: In this paper, the ultraviolet absorption spectra were recorded for the transient radicals CdCH3, ZnCH3 and TeCH3 produced in the flash photolysis of the appropriate dimethyl metals.
Abstract: Ultraviolet absorption spectra were recorded for the transient radicals CdCH3, ZnCH3, and TeCH3 produced in the flash photolysis of the appropriate dimethyl metals. For CdCH3 and ZnCH3 two systems were observed and assigned to the two transitions A 2E← x2 A1 and B 2E← x2 A1. For the TeCH3 radical only one system was observed which was assigned to the B← x transition. The progressions in the spectra are assigned to the upper state carbon‐metal stretching vibration (ν′3) and CH3 symmetric deformation vibrations (ν′2).
TL;DR: In this paper, the ground state vibrational constant of ClO2 has been measured for the first time, and the extinction coefficients for a number of the ClO transitions have been measured.
TL;DR: The mechanism of photo-substitution of ferrocene in haloalkane-ethanol solutions was investigated by the flash photolysis technique and other chemical methods mainly on the Ferrocene-Carboxylate-FeCl3 system.
Abstract: The mechanism of photo-substitution of ferrocene in haloalkane–ethanol solutions was investigated by the flash photolysis technique and other chemical methods mainly on the ferrocene–carbon tetrachloride–ethanol system. The photoreaction is initiated by the excitation of charge transfer state (electron transfer from ferrocene to carbon tetrachloride). In the absence of ethanol, CT-excitation leads to the decomposition of ferrocene to FeCl3 with first order reaction kinetics of k=(1.6±0.5)×105 s−1. The formation of FeCl3 was inhibited by ethanol effectively to give ethyl ferrocenecarboxylate. Diethylamine quenched the formation of FeCl3, whereas dimethyl sulfoxide, a stronger base than ethanol, is a less effective quencher than ethanol. These facts suggest that the most important role of ethanol is to ethanolyse trichloromethylferrocene, which otherwise decomposes to FeCl3.
TL;DR: In this paper, a rapidly decaying signal was detected in reaction center preparations from the R26 blue-green mutant of Rhodopseudomonas spheroides, which is essentially the same as that seen previously in iron-free preparations.
Abstract: — Using the technique of flash photolysis-electron spin resonance, we have detected, by means of a kinetic analysis, a rapidly decaying signal in reaction center preparations from the R26 blue-green mutant of Rhodopseudomonas spheroides. This signal, which we designate Signal B3, is essentially the same as that seen previously in iron-free preparations. Signal B3 decays at 20°C with a 1/e time of ˜ 3 ms and exhibits an activation energy of 5 ± 1 kcal mol-1 over the temperature range 0–30°C. Extraction with isooctane completely eliminates Signal B3, whereas readdition of exogenous ubiquinone-30 completely restores the signal. o-Phenanthroline has no effect on Signal B3. We discuss these results in terms of a model in which the primary acceptor is an iron-ubiquinone complex with excess ubiquinone serving as a secondary electron acceptor pool.
TL;DR: The first observable transient (lifetime > 50 ns) on flash photolysis of Cr(CO)6 was observed in this paper, which is the precursor of two species previously assigned to the D3h and C4v forms of the same form.
Abstract: The first observable transient (lifetime > 50 ns) on flash photolysis of Cr(CO)6 is Cr(CO)5, which is the precursor of two species previously assigned to the D3h and C4v forms of Cr(CO)5, and now shown to arise from reaction with impurities.
TL;DR: Ionic photodissociation of electron donor-acceptor (EDA) complexes of s-tetracyanobenzene (TCNB) and pyromellitic dianhydride (PMDA) with 2-methyltetrahydrofuran (MTHF) has been investigated by means of nsec flash photolysis and transient photocurrent measurements as mentioned in this paper.
Abstract: Ionic photodissociation of electron donor-acceptor (EDA) complexes of s-tetracyanobenzene (TCNB) and pyromellitic dianhydride (PMDA) with 2-methyltetrahydrofuran (MTHF) has been investigated by means of nsec flash photolysis and transient photocurrent measurements. The solvent-shared ion-pair of TCNB anion and MTHF cation is formed from the excited charge-transfer (CT) singlet state of a TCNB–MTHF complex, dissociating into solvated ions. TCNB complexes with aromatic hydrocarbons dissociate directly into ions from the lowest excited CT singlet state. Dissociation of PMDA complexes occurs in the CT triplet state in nonpolar solvents and mainly in the excited CT singlet state in polar solvents. The relative rate constant of radiationless process, which competes with ionic dissociation in the excited singlet state, can be deduced from the effect of solvent on ionic photodissociation. It has been confirmed for the first time that the ionic dissociation of a TCNB–benzene–1,2-dichloroethane (DCE) system occurs ...
TL;DR: The enhanced second‐order decay of the flavin radicals in oxygen‐containing solutions is interpreted in terms of their reaction with the peroxy radicals, which account for the production of hydroxylated alloxazines.
Abstract: — The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λmax 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with Λmax 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH2*) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines.
TL;DR: In this article, a good correlation was found between the frequency of maximum absorption and the ionization potential of the solvent molecules, and the 385 nm transient absorption previously attributed to these charge transfer species was reassigned to I−2 radical anions.
TL;DR: In this paper, the laser flash photolysis of ozone at ≈ 6000 A has been used to generate a clean kinetic source of ground state atomic oxygen, O( 3 P), which was monitored via resonance fluorescence at 1300 A, under static reaction cell conditions.
TL;DR: In this paper, the rate constants for the reactions OH(X 2 Π, ν = O) + NH 3 → k1 H 2 O + NH 2 → HO 2 + O 2 were measured at 298°K by the flash photolysis resonance fluorescence technique.
TL;DR: It is shown that the overall quantum yield of photoreduction is governed by the second step and can be calculated from the ratio of the rate constants of the two elementary processes involved in this step.
Abstract: — The photoreduction of thiazine dyes by ethylene diamine tetraacetic acid (EDTA) was investigated by Rash photolysis. This reaction was found to occur according to a three-step mechanism. the first being the formation of the dye triplet state followed, in weakly acid solutions, by protonation. During the second step, the triplet state of the dye disappears through two competing processes: spontaneous deactivation and reaction with EDTA, which leads to the semireduced dye. The third step leads to the leucodye. It is shown that the overall quantum yield of photoreduction is governed by the second step and can be calculated from the ratio of the rate constants of the two elementary processes involved in this step. This ratio was measured over a wide pH range.
TL;DR: In this article, the ultraviolet spectrum of the transient HS 2 radical produced in the flash photolysis of H 2 S 2 was recorded and the spectrum observed in the λ = 3070-3800 A region is assigned to the A 2 A′ ← X 2 A″ electronic transition.
TL;DR: In this paper, the authors used time-resolved mass spectroscopy to study the kinetics of methyl radical association and determined the rate coefficient for the association process by following the time dependence of ethane formation after flash photolysis of CH3I.
Abstract: Time-resolved mass spectroscopy has been utilized to study the kinetics of methylradical association. The rate coefficient for the association process was determined by following the time dependence of ethane formation after flash photolysis of CH3I in the presence of N2. The net rate coefficient for the process 2CH3 (+M) → C2H6(+M) has a high-pressure limit of (4.0 ± 0.3) × 10−11 cm3 molecule−1 sec−1 at 313°K. This rate coefficient has found to be insensitive to the third-body number densities for pressures ranging from 13 Torr down to below 0.5 Torr, indicating that the lifetime for dissociation of the intermediate C2H6* species is greater than 2 × 10−7 sec at 313°K for a gas pressure of 0.5 Torr.
TL;DR: In this paper, the rate constant for CH3 was determined by means of vacuum ultraviolet flash photolysis and time-resolved kinetic spectroscopic observations of the 1504-A absorption band of CH3, and the measurements made using three different sources of methyl radicals (azomethane, dimethylmercury, and ketene-hydrogen) were in accord and yielded a value of k1 = (9.53 ± 1.17) × 10−11 cc molec−1 sec−1.
Abstract: The rate constant for the reaction
has been determined by means of vacuum ultraviolet flash photolysis and time-resolved kinetic spectroscopic observations of the 1504-A absorption band of CH3. The measurements made using three different sources of methyl radicals (azomethane, dimethylmercury, and ketene-hydrogen) were in accord and yielded a value for the rate constant of k1 = (9.53 ± 1.17) × 10−11 cc molec−1 sec−1. A detailed error analysis is presented. The f-value for the 1504-A band of CH3 is determined to be (2.5 ± 0.7) × 10−2.
TL;DR: In this article, the flash photolysis of aqueous solutions of tyrosine has been studied in the presence of various concentrations of the cyclic disulfide sodium lipoate (thioctic acid, Na+ salt).
Abstract: — The flash photolysis of aqueous solutions of tyrosine has been studied in the presence of various concentrations of the cyclic disulfide sodium lipoate (thioctic acid, Na+ salt). In addition to the formation of phenoxyl radicals and hydrated electrons (and possibly H atoms) from the photoionization of tyrosine, the characteristic spectrum of the radical anion RSSR- of lipoate was also observed in neutral as well as in alkaline solutions. From the dependence of these yields upon the concentration of lipoate, it was found that a long–lived triplet excited state of tyrosine, rather than the singlet excited state, is involved in these reactions. The negative radical ions RSSR- are formed by two distinct pathways: (a) Na+–lipoate reacts with the solvated electrons which are ejected from the tyrosine triplets 3Tyr RO.+e-aq+ H+ followed by e-aq+ RSSR RSSR-, and (b) by direct interaction of lipoate with triplet excited tyrosine, resulting in the transfer of a negative charge from tyrosine to the disulfide linkage. At high lipoate concentrations, the singlet excited state of lipoate is quenched, k4= 1.6 × 1010M-1 sec-1, but this reaction does not lead to the formation of RSSR- radical ions.
TL;DR: In this paper, the lower pressure flash photolysis of divinyl ether has been monitored by time-of-flight mass spectrometry, and the results favor a rapid isomerization, followed by decomposition C2H3O⇄CH3CO→CH3+CO.
Abstract: The lower pressure flash photolysis of divinyl ether has been monitored by time‐of‐flight mass spectrometry. The primary photodecomposition process is restricted essentially to the formation of vinyl and vinoxy radials: C2H3OC2H3→C2H3+C2H3O. The vinyl radical (m/e=27) has been observed directly and its 2nd order decay constant measured. The decay is primarily by disproportionation 2C2H3→C2H4+C2H2 and k10 has an average value of 5.3± 0.5× 10−12cc molecule−1·sec−1 over the pressure range studied (65–200 mtorr). The vinoxy radical has not been observed directly in this work. Our results favor a rapid isomerization, followed by decomposition C2H3O⇄CH3CO→CH3+CO. A mass balance is achieved within 400 μsec of the photolysis flash and a mechanism proposed to account for the major (C2H4, C2H2, CO) and the minor products (CH4, C2H6, 2‐butenal, acetaldehyde, and ketene).