Use Of Laser Flash Photolysis Research Articles

New Measurements and Calculations on the Kinetics of an Old Reaction: OH + HO2 → H2O + O2.

Literature rate coefficients for the prototypical radical-radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO2 radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + H2O2 reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of k1,298K ∼1 × 10-11 cm3 molecule-1 s-1, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, k1,H2O,298K = (2.17 ± 0.09) × 10-28 cm6 molecule-2 s-1, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of k1,298K. Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of k1,298K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO2 → HCl + O2. The implications of these results in atmospheric models are discussed.

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H.

The reaction of CH radicals with H2 has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k1 shows complex behavior: at low pressures, the main route for the CH3* formed from the insertion of CH into H2 is the formation of 3CH2 + H, and as the pressure is increased, CH3* is increasingly stabilized to CH3. The kinetic data on CH + H2 have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, 3CH2 + H → CH + H2. The resulting parametrization is kCH2+H(T) = (1.69 ± 0.11) × 10-10 × (T/298 K)(0.05±0.010) cm3 molecule-1 s-1, where the errors are 1σ.

Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunneling.

A generally accepted principle of chemical kinetics is that a reaction will be very slow at low temperatures if there is an activation barrier on the potential energy surface to form products. However, this Account shows that the reverse is true for gas-phase hydrogen abstraction reactions of the hydroxyl radical, OH, with organic molecules with which it can form a weakly bound (5-30 kJ mol-1) hydrogen-bonded complex. For hydrogen atom abstraction reactions of OH with volatile organic compounds (VOCs) containing alcohol, ether, carbonyl, and ester functional groups, the reaction accelerates rapidly at very low temperatures, with rate coefficients, k, that can be up to a 1000 times faster than those at room temperature, despite the barrier to products. The OH radical is a crucial intermediate in Earth's atmosphere, combustion processes, and the chemistry of the interstellar medium, where temperatures can reach as low as 10 K, so this behavior has very important implications for gas-phase chemistry in space. The key point is that at low temperatures the lifetime of the OH-VOC complex against re-dissociation back to reactants becomes much longer, and hence the probability of quantum mechanical tunneling under the reaction barrier to form products becomes much higher. These observations were made possible by using Laval nozzles to generate uniform supersonic flows at extremely low temperatures so that condensation of the reagents at reactor walls is avoided. In this Account, the use of laser flash-photolysis combined with laser-induced fluorescence spectroscopy within Laval flows is described to study the unusual kinetics of this type of reaction at temperatures down to 21 K and demonstrate the rapid upturn in the rate coefficient. For the reaction of OH with CH3OH, further evidence for the precomplex and tunneling mechanism comes from observation of the CH3O reaction product at very low temperatures, despite it being formed over the higher barrier to reaction. The experimental observations are supported by theoretical calculations using the MESMER master equation package to calculate k( T) and product yields as a function of temperature and which make use of potential energy surfaces determined using ab initio methods. The CH3O product is formed over a narrower barrier with a larger imaginary frequency and is calculated to be the sole product at very low temperatures. The kinetics of the OH reaction with CH3OH were measured to be independent of pressure, consistent with a tunneling mechanism rather than any collisional stabilization of the prereactive complex. In this Account, we collate available kinetic data and show that this newly discovered mechanism for H atom transfer reactions appears to be generally applicable for reactions of OH with organic molecules containing oxygenated functional groups, which have been observed in space by radio-astronomy. Rather than being ignored for a range of interstellar environments, these OH reactions are now being included in chemical networks in space and have been shown to significantly influence the abundance of OH, the organic molecules themselves, and reaction products and provide novel routes to forming even more complex functional groups, for example, precursors to prebiotic molecules.

Interligand electron transfer as a reason of very weak red luminescence of Eu((i-Bu)2PS2)3Phen and Eu(C4H8NCS2)3Phen complexes

Nanosecond laser flash photolysis and time resolved luminescence were used to study the photophysical processes for Eu((i-Bu)2PS2)3Phen (1) and Eu(C4H8NCS2)3Phen (2) complexes in acetonitrile. These complexes show a very weak red Eu3+ luminescence in spite of the fact that the phenanthroline molecule in triplet state is a good antenna to excite the red luminescence of many Eu(III)-Phen complexes. To determine the reasons of this effect the photoprocesses in solutions, containing the (i-Bu)2PS2− or C4H8NCS2− ions and free phenanthroline molecule, have been studied with the use of laser flash photolysis (266nm). It was shown that the phenanthroline in triplet excited state (TPhen*) deprives the electron from these dithiolate ions with a high rate constants close to 109M−1s−1. The transient spectra of phenanthroline anion-radical and dithiolate radicals were recorded which are in a good agreement with literature data. Since the effective concentration of dithiolate ions (L−) in the coordination sphere of 1 and 2 complexes is close to 10M the time of electron transfer between L− and TPhen* is in the range of 100ps or less. As the laser flash photolysis of solutions of 1 and 2 complexes with a 10ns time resolution failed to detect the spectra of phenanthroline anion-radical and dithiolate radicals, it indicates that the time of back electron transfer is less than 10−8s. Thus, the very weak red luminescence of 1 and 2 complexes is due to the electron transfer between ligands in the coordination sphere which successfully suppresses the energy transfer from the phenanthroline triplet state to Eu3+ ion.

Kinetic studies of the reaction between pesticides and hydroxyl radical generated by laser flash photolysis.

Due to contamination of the environment by pesticides and their mishandling, there is the need for treatment of contaminated sites and correct disposal of materials containing them. Thus, studies with advanced oxidation processes are expanding and can determine the rate constant of the hydroxyl radical with organic compounds of great importance in environmental contamination. In this context, the use of laser flash photolysis has been shown to be viable for the determination of these constants. The reaction rate constants of different pesticides with HO(•) in degassed acetonitrile have been determined. They were 1.6 × 10(9) M(-1) s(-1), 0.6 × 10(9) M(-1) s(-1), 1.2 × 10(9) M(-1) s(-1), 2.4 × 10(9) M(-1) s(-1) and 2.2 × 10(9) M(-1) s(-1) for the pesticides carbaryl, propoxur, fenoxycarb, ethoxysulfuron and chlorimuron-ethyl, respectively. These values are about an order of magnitude smaller than the diffusion controlled rate and correlate with the relative rates of disappearance of the pesticides in the photo-Fenton reaction in water. The correlation of the relative rate constants determined by laser flash photolysis with the relative rates of photo-Fenton degradation of the pesticides is compelling evidence for the participation of the hydroxyl radical in the degradation of these pesticides in the latter system.

A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet α-naphthoflavone

The hydrogen abstraction (HA) reaction by the triplet of alpha-naphthoflavone (1) has been investigated experimentally by the use of laser flash photolysis (LFP) and theoretically with density functional theory (DFT) and atoms in molecules (AIM). The triplet excited state of 1, in acetonitrile, has an absorption maximum at 430 nm and lifetime of 10 micros. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, substituted phenols and amines were determined. The low reactivity of this ketone with respect to HA from 1,4-cyclohexadiene is in accord with a pi,pi* excited state. HA from phenols in acetonitrile is proposed to occur in a diffusion controlled reaction from free phenol based upon the determination of the Abraham beta(H)(2) value for acetonitrile and correction of the quenching rate constants for hydrogen bonding of the phenols to acetonitrile. A molecular orbital analysis of the triplet (SOMO and SOMO-1) of 1 reveals contributions from the carbonyl oxygen atom, but principally from the alpha-carbon and the associated pi-bond network, consistent with a pi,pi* excited state. From a thermodynamic point of view, the triplet HA from phenol to oxygen of the carbonyl group is 17 kcal mol(-1) less demanding than the transfer to the alpha-carbon, consistent with the acidic nature of the phenolic hydrogen atom. DFT and AIM analysis of the hydrogen abstraction reaction reveals that the transition state (TS) is pseudo-symmetrically polarized and that HA in the hydrogen bonded exciplex occurs in a concerted manner but not necessarily by simultaneous electron and proton transfer.

Evidence of the hydroxyl radical formation upon the photolysis of an iron-rich clay in aqueous solutions

With the use of laser flash photolysis, the formation of hydroxyl radicals upon the photolysis of an iron-rich clay (montmorillonite KSF) was demonstrated. The •OH radical was shown to be formed by the photolysis of the Fe(OH)aq 2+ complex that escaped from the clay into the solution bulk.

Study of the phototransformation of imazaquin in aqueous solution: a kinetic approach

The photodegradation of imazaquin, [2-(4,5-dihydro-4-methyl-4-(methylethyl)-5-oxo-1-imidazol-2-yl]-3-quinolinecarboxylic acid (CAS RN: [81335-37-7], an herbicide belonging to the family of imidazolinone, was investigated using steady-state irradiation in aqueous solutions at different pH values, different wavelengths and different oxygen concentrations. Using spectrometry techniques (UV–vis and fluorescence), p K a values for the ground state, p K a ( S 0 ) , and for the lowest excited singlet state p K a ( S 1 ) , were found to be 3.6 and 7.5, respectively. The degradation of imazaquin appeared to be more effective at pH > p K a ( S 0 ) and in the absence of oxygen. The use of laser flash photolysis has shown the presence of a triplet state in the region from 320 to 550 nm with a maximum centred at 380 nm. By comparing the quenching rate constant, k q, obtained from the quenching of the triplet state by molecular oxygen ( k q = 9.31 × 10 8 mol −1 L s −1) and from the Stern–Volmer relation ( k q = 1.13 × 10 8 mol −1 L s −1), we deduced that the singlet state also seems to play a role in the photodegradation process. According to the experimental results, it has been found that imazaquin is a rather stable molecule in the dark or at low pH. In pure water, imazaquin degrades slowly and the photodegradation leads to the formation of four photoproducts. Three photoproducts were tentatively identified by HPLC–MS analyses and another one was proposed by taking into account the literature.

Diels−Alder Reactions of Anthracenes with Dienophiles via Photoinduced Electron Transfer

Photochemical Diels−Alder reaction of anthracene and its derivatives with dienophiles (p-benzoquinone and fumaronitrile) occurs in competition with the dimerization of anthracenes in chloroform at 298 K. The dependence of the quantum yields on the concentrations of dienophiles has revealed that the photochemical Diels−Alder reaction proceeds via electron transfer from the singlet excited states of anthracenes to dienophiles. The rates of photoinduced electron transfer are diffusion limited, agreeing with the largely negative free energy change of electron-transfer judging from the more negative one-electron oxidation potentials of the singlet excited states of anthracenes than the one-electron reduction potentials of dienophiles. The radical ion pair produced in the photoinduced Diels−Alder from the singlet excited states of anthracenes to p-benzoquinone and fumaronitrile has been detected as the transient absorption spectrum at 298 K with use of laser flash photolysis. The diradical intermediates prior t...

Furanochromone radical cations: generation, characterization and interaction with DNA

The radical cations of naturally occurring furanochromones visnagin (VI) and khellin (KH) have been generated and identified for the first time by use of laser flash photolysis and pulse radiolysis techniques. The lifetimes of VI ·+ and KH ·+ are determined as ∼6 and ∼35 μs under these conditions, respectively. Direct 308-nm excitation of VI in aqueous buffer at physiological pH results in monophotonic photoionization to generate VI ·+, with a quantum yield of 0.075, which is much higher than that of 8-methoxypsoralen and KH under identical conditions. Though VI ·+ is a more powerful oxidant than KH ·+, both of them react with guanosine mononucleotide ( k=1.2×10 9 and 3.8×10 7 dm 3 mol −1 s −1, respectively) via electron transfer to give the guanine radical cation. Furthermore, selective oxidation of guanine in single and double strand DNA by VI ·+ was also observed. These novel findings suggest that electron transfer reactions involving furanochromone radical cations may be of considerable importance in furanochromone photochemotherapy.

Solvent-dependent C-OH homolysis and heterolysis in electronically excited 9-fluorenol: the life and solvation time of the 9-fluorenyl cation in water.

The primary pathways of the photodecomposition of 9-fluorenol (FOH) were studied in polar and nonpolar solvents by use of laser flash-photolysis with a resolution time of 10 ps. In solvents of high polarity, that is, in 1,1.1,3,3,3-hexafluoroisopropanol (HFIP), 2,2,2-trifluoroethanol (TFE), formamide or water, the fluorenyl cation, F+, forms by heterolytic C-O bond cleavage. In H2O, the initial (10 ps) spectrum of F+ has lambdamax at <460 nm. This absorption red-shifts with T = 25 ps to the "classical" spectrum with lambdamax = 510-515 nm. This process is assigned to the solvation of the initial "naked" cation, or rather, the contact ion pair. The lifetime of the solvated fluorenyl cation in H2O (or D2O) and TFE was measured to be tau 20 ps and 1 ns, respectively. In solvents of lower polarity such as alkanes, ethers and alcohols, the long-lived (tau 1/2 1 micros) fluorenyl radical, F., (lambdamax = 500 nm) forms through homolytic C-O cleavage. In addition to the radical and the cation, the vibrationally relaxed excited singlet state of FOH is seen with its absorption at approximately 640 nm; its lifetime is strongly dependent on the solvent, from 10 ps for formamide to 1.7 ns for cyclohexane. The rate constant for singlet decay increases exponentially with the polarity of the solvent (as expressed by the Dimroth-Reichardt ET value) or with the Gutmann solvent acceptor number. The relaxation of S1 to S0 is accompanied by homolytic C9-O bond cleavage (except in HFIP, TFE, and water, where S1 is not seen).

A pulse radiolysis and pulsed laser study of the pyrromethene 567 triplet state

The triplet state of pyrromethene 567, a molecule with potential as a solid state laser dye, has been characterized in benzene by pulse radiolysis in terms of its absorption spectrum, lifetime, self-quenching, electronic excitation energy, triplet–triplet extinction coefficient and oxygen quenching rate constant. The use of laser flash photolysis has then allowed determination of the triplet quantum yield, efficiency of formation of singlet oxygen (1Δg), and the rate constant for reaction of the latter species with the ground state. The affects of oxygen on the fluorescence and triplet yields demonstrate that oxygen-induced intersystem crossing is important, the sum of these parameters being unity within experimental error. The mechanism of reaction of P-567 with 1Δg in benzene is predominantly physical in character with only a small (∼6%) contribution from chemical reaction.

Radical adduct of trichloromethyl to C 70: A laser flash photolysis of C 70 in carbon tetrachloride

The addition reaction of trichloromethyl radical to C 70 has been investigated by use of laser flash photolysis. The difference absorption spectrum of radical adduct, [C 70CCl 3], exhibits broad absorption band in the visible region. The rate constant for reaction of trichloromethyl radical with C 70 is determined.

A photochemical retro-Friedel–Crafts alkylation. Rapid rearrangement of cyclohexadienyl cations

This paper reports the use of laser flash photolysis (LFP) techniques to show that cyclohexadienyl cations (σ complexes) of the Friedel–Crafts reaction of 1,3-dimethoxybenzene and the diphenylmethyl cation rearrange on the ns time scale without separating the aromatic compound and the electrophile. This is demonstrated through a study of the photochemical behaviour of 2-diphenylmethyl-1,3-dimethoxybenzene (4) in 1,1,1,3,3,3-hexafIuoroisopropyl alcohol (HFIP). Derivatives of 1,3-dimethoxybenzene have previously been found to selectively protonate at C2 upon excitation in HFIP, and indeed the principal products with 4 are 1,3-dimethoxybenzene (6) and Ph2CHOCH(CF3)2 (7), the species expected if the cyclohexadienyl cation formed in the C2 protonation cleaved Ph2CH+. These products are, however, accompanied by 4-diphenylmethyl- 1,3-dimethoxybenzene (8), a rearranged isomer of 4. A portion of this product is explained by the combination of Ph2CH+ and 1,3-dimethoxybenzene as the latter accumulates during the irradiation. However, 11.5% of 8 is also seen upon extrapolation to zero time. LFP experiments on the ps time scale reveal that the C2 protonated cation, the 1-diphenylmethyl-2,6-dimethoxybenzenium ion (5), is formed within 100–200 ps, and reacts with k = 9 × 108 s−1, with absorbance for Ph2CH+ growing in as 5 decays. LFP studies on the ns time scale reveal that there is a second quantity of Ph2CH+ that grows in, with k = 5.0 × 105 s−1. The precursor for this has been identified as the 1-diphenylmethyl-2,4-dimethoxybenzenium ion (10), the thermodynamically more stable isomer of 5. A mechanistic model is proposed in which excited 4 is C2 protonated in HFIP with k ≥ 1 × 1010 s−1 to form 5, which loses Ph2CH+ with k = 3 × 108 s−1 in competition with rearrangement to 10 with k = 6 x 108 s−1. The cation 10 serves as the second source of Ph2CH+, losing Ph2CH+ with k = 4 × 105 s−1; in competition 10 is deprotonated by HFIP to give 8 with k = 8 × 104 s−1. The 11.5 % of the rearranged 8 that is observed at zero conversion is thus shown to come from an intramolecular pathway in which the key step is the migration of a diphenylmethyl group without separation: 4 → 4* → 5 → 10 → 8. Key words: cyclohexadienyl, photoprotonation, Friedel–Crafts, diphenylmethyl, 1,3-dimethoxybenzene.

Mechanism of ligand binding to hemes and hemoproteins. A high-pressure study

The effect of pressure on the recombination kinetics of small ligands binding to sperm whale myoglobin, protoheme dimethyl ester, and monochelated protoheme was studied with use of laser flash photolysis. The volumes of activation observed indicate that in both the protein and the models bond formation is the rate-determining step only for carbon monoxide, while for oxygen, isocyanides, and 1-methylimidazole almost no bond formation occurs in the transition state of the observed reaction. The effect of pressure on the escape of carbon monoxide, oxygen, and methyl isocyanide from the heme pocket of sperm whale myoglobin was also investigated

ChemInform Abstract: Reactions of Polypyridylchromium(II) Ions with Oxygen: Determination of the Self‐Exchange Rate Constant of O2/O‐ 2.

AbstractThe rapid and irreversible formation of CrL32+ in the reductive quenching of the 2E excited states of CrL33+ complexes (L: bipyridine, phenanthroline and their substituted analogues) by ethylenediaminetetraacetate and oxalate is studied by use of laser flash photolysis.

Use of laser flash photolysis to study the interaction of the triplet state of duroquinone with transition-metal compounds

1. Values of the rate constants for quenching of the triplet state of duroquinone by transition-metal salts have been obtained. Quenching follows an exhange—resonance mechanism. 2. The durosemiquinone radical can form either from the triplet, or the excited singlet, state of the duroquinone.