Quantum Yield Of Radicals Research Articles

Kinetics and mechanisms of cylindrospermopsin destruction by sulfate radical-based advanced oxidation processes

Cylindrospermopsin (CYN) is a potent cyanobacterial toxin frequently found in water bodies worldwide raising concerns over the safety of drinking and recreational waters. A number of technologies have been investigated to remove and/or degrade cyanotoxins with advanced oxidation processes (AOPs) being among the most promising and effective for water detoxification. In this study, the degradation of CYN by sulfate radical-based UV-254 nm-AOPs was evaluated. The UV/S2O82− (UV/peroxydisulfate) was more efficient than UV/HSO5− (UV/peroxysulfate) and UV/H2O2 (UV/hydrogen peroxide) processes when natural water samples were used as reaction matrices. The observed UV fluence based pseudo-first-order rate constants followed the expected order of radical quantum yields. The presence of 200 μM natural organic matter (NOM) as carbon slightly inhibited the destruction of CYN; 1.24 mg L−1NO3− (nitrate) had no significant influence on the removal efficiency and 50 μg L−1 Fe2+ [iron (2+)] or Cu2+ [copper (2+)] improved the performance of UV/S2O82−. The addition of tert-butyl alcohol (t-BuOH; hydroxyl radical scavenger) in the reaction yielded byproducts that indicated specific sites in CYN preferentially attacked by sulfate radicals (SRs). The predominant CYN degradation byproduct was P448 consistent with fragmentation of the C5C6 bond of the uracil ring. The subsequent formation of P420 and P392 through a stepwise loss of carbonyl group(s) further supported the fragmentation pathway at C5C6. The byproduct P432 was identified exclusively as mono-hydroxylation of CYN at tricyclic guanidine ring, whereas P414 was detected as dehydrogenation at the tricyclic ring. The elimination of sulfate group and the opening of tricyclic ring were also observed. The possible degradation pathways of CYN by SR-AOP were presented.

Photolysis of CH₃CHO at 248 nm: evidence of triple fragmentation from primary quantum yield of CH₃ and HCO radicals and H atoms.

Radical quantum yields have been measured following the 248 nm photolysis of acetaldehyde, CH3CHO. HCO radical and H atom yields have been quantified by time resolved continuous wave Cavity Ring Down Spectroscopy in the near infrared following their conversion to HO2 radicals by reaction with O2. The CH3 radical yield has been determined using the same technique following their conversion into CH3O2. Absolute yields have been deduced for HCO radicals and H atoms through fitting of time resolved HO2 profiles, obtained under various O2 concentrations, to a complex model, while the CH3 yield has been determined relative to the CH3 yield from 248 nm photolysis of CH3I. Time resolved HO2 profiles under very low O2 concentrations suggest that another unknown HO2 forming reaction path exists in this reaction system besides the conversion of HCO radicals and H atoms by reaction with O2. HO2 profiles can be well reproduced under a large range of experimental conditions with the following quantum yields: CH3CHO + hν(248nm) → CH3CHO*, CH3CHO* → CH3 + HCO ϕ(1a) = 0.125 ± 0.03, CH3CHO* → CH3 + H + CO ϕ(1e) = 0.205 ± 0.04, CH3CHO*[Formula: see text]CH3CO + HO2 ϕ(1f) = 0.07 ± 0.01. The CH3O2 quantum yield has been determined in separate experiments as ϕ(CH₃) = 0.33 ± 0.03 and is in excellent agreement with the CH3 yields derived from the HO2 measurements considering that the triple fragmentation (R1e) is an important reaction path in the 248 nm photolysis of CH3CHO. From arithmetic considerations taking into account the HO2 and CH3 measurements we deduce a remaining quantum yield for the molecular pathway: CH3CHO* → CH4 + CO ϕ(1b) = 0.6. All experiments can be consistently explained with absence of the formerly considered pathway: CH3CHO* → CH3CO + H ϕ(1c) = 0.

Kinetic and mechanism investigation on the photochemical degradation of atrazine with activated H2O2, S2O82− and HSO5−

Abstract Degradation of atrazine was investigated under UV-254 nm irradiation, alone or in combination with peroxides, i.e., hydrogen peroxide, persulfate (PS) or peroxymonosulfate (PMS). UV/PS was found to be the most efficient process in this study, which was probably due to its higher radical quantum yield under UV irradiation, considering atrazine’s comparable second order rate constant of 2.59 × 10 9 M −1 s −1 and 2.25 × 10 9 M −1 s −1 with sulfate radical and hydroxyl radical, respectively. The pH values showed no significant effect on direct photolysis of atrazine, with observed UV fluence based pseudo-first-order rate constant ( k obs ) of 7.59 × 10 −4 , 7.73 × 10 −4 and 6.72 × 10 −4 cm 2 mJ −1 at pH 3.0, 5.7 and 11.0, respectively. Performance of UV/H 2 O 2 and UV/PMS were more independent of initial pH values while that of UV/PS appeared to be more efficient at neutral pH condition. UV/PS was observed as a less energy consumption process. Eight new degradation by-products were identified in this study, namely, 2-chloro-4-acetamido-6-(2-hydroxy-isopropylamino)-s-triazine (CDHT), 2-hydroxy-4-(2-hydroxy-ethylamino)-6-isopropylamino-s-triazine (ONIT), 2-chloro-4-vinylamino-6-isopropylamino-s-triazine (CVIT), 2-hydroxy-4-vinylamino-6-(2-hydroxy-isopropylamino)-s-triazine (OVHT), 2-hydroxy-4-acetamido-6-isopropenylamino-s-triazine (ODPT), 2-hydroxy-4-(2-hydroxy-ethylamino)-6-vinylamino-s-triazine (ONVT), 2-hydroxy-4-(2-hydroxy-ethylamino)-6-methylamino-s-triazine (ONMT) and 2-chloro-4-vinylamino-6-amino-s-triazine (CVAT). Potential degradation pathways were further proposed.

Laser flash photolysis study of photocatalytic properties of pillared interlayered clays and Fe,Al-silica mesoporous catalysts

Laser flash photolysis was applied to determine the primary photochemical processes over iron-containing clay (montmorillonite KSF), pillared interlayered clays (PILCs) and mesoporous mesophase iron silicate materials (MMMs). For KSF, the homogeneous photochemical reaction of Fe(III) leached from the clay material resulted in the formation of OH radicals, which were monitored by means of their reaction with methyl viologen dication (MV(2+)). For PILCs and MMMs, no leaching of Fe(III) to the solution nor hydroxyl radical formation were observed. Nevertheless, these catalysts were found to exhibit a sufficient effect on phenol photoionization. The increase in quantum yields of PhO radicals is caused by the effect of PILCs and MMMs and is explained by heterogeneous processes on the surface of catalyst particles.

Effect of the interface on the photophysics of eosin-Y in reverse miceles

Abstract The photophysics of the xanthene dye eosin-Y (Eos) was studied in reverse micelles (RMs) of sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) and benzylhexadecyldimethylammonium chloride (BHDC). Special interest was assigned to the effect of organization on the triplet state properties. In the reverse micellar media both the absorption and emission spectra of the dye present a red shift with respect to water. It is concluded that Eos in both reverse micellar systems is localized in the interface. Although this is predictable in BHDC due to the positive charge at the interface and the negative charge of the dye, it is notable in the case of the negative interface of AOT. In this case it may be concluded that Eos resides in the interface co-micellizing with the surfactant, although more exposed to water than in the case of BHDC. Triplet state decay kinetics and absorption spectra were determined by laser flash photolysis. The triplet lives longer in RMs than in homogeneous solvents. The compartmentalization of the dye precludes the auto-quenching which shorts the lifetime in homogeneous solvents. The quenching by aliphatic amines was also investigated. The hydrophilic triethanolamine (TEOA) is localized in the water pool, and the triplet quenching is more efficient than in homogeneous solvents. On the other hand the quenching by tributylamine (TBA) which is located in the bulk organic solvent is much less efficient. The quenching by TEOA in RMs can be understood in terms of an exchange mechanism between micelles. Quantum yields of radicals produced in the triplet quenching process were determined.

The role of effluent nitrate in trace organic chemical oxidation during UV disinfection

Most conventional biological treatment wastewater treatment plants (WWTPs) contain nitrate in the effluent. Nitrate undergoes photolysis when irradiated with ultraviolet (UV) light in the 200–240 and 300–325 nm wavelength range. In the process of nitrate photolysis, nitrite and hydroxyl radicals are produced. Medium pressure mercury lamps emitting a polychromatic UV spectrum including irradiation below 240 nm are becoming more common for wastewater disinfection. Therefore, nitrified effluent irradiated with polychromatic UV could effectively become a de facto advanced oxidation (hydroxyl radical) treatment process. UV-based advanced oxidation processes commonly rely on addition of hydrogen peroxide in the presence of UV irradiation for production of hydroxyl radicals. This study compares the steady-state concentration of hydroxyl radicals produced by nitrate contained in a conventional WWTP effluent to that produced by typical concentrations of hydrogen peroxide used for advanced oxidation treatment of water. The quantum yield of hydroxyl radical production from nitrate by all pathways was calculated to be 0.24 ± 0.03, and the quantum yield of hydroxyl radicals from nitrite was calculated to be 0.65 ± 0.06. A model was developed that would estimate production of hydroxyl radicals directly from nitrate and water quality parameters. In effluents with >5 mg-N/L of nitrate, the concentration of hydroxyl radicals is comparable to that produced by addition of 10 mg/L of H2O2. Nitrifying wastewater treatment plants utilizing polychromatic UV systems at disinfection dose levels can be expected to achieve up to 30% degradation of some micropollutants by hydroxyl radical oxidation. Increasing UV fluence to levels used during advanced oxidation could achieve over 95% degradation of some compounds.

Radical Quantum Yields from Formaldehyde Photolysis in the 30 400–32 890 cm–1 (304–329 nm) Spectral Region: Detection of Radical Photoproducts Using Pulsed Laser Photolysis–Pulsed Laser Induced Fluorescence

The relative quantum yield for the production of radical products, H + HCO, from the UV photolysis of formaldehyde (HCHO) has been measured using a pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) technique across the 30,400–32,890 cm(–1) (304–329 nm) spectral region of the Ã(1)A2–X̃(1)A1 electronic transition. The photolysis laser had a bandwidth of 0.09 cm(–1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (0.07 cm(–1)), and the yield spectrum shows detailed rotational structure. The H and HCO photofragments were monitored using LIF of the OH radical as a spectroscopic marker. The OH radicals were produced by rapid reaction of the H and HCO photofragments with NO2. This technique produced an “action” spectrum that at any photolysis wavelength is the product of the H + HCO radical quantum yield and HCHO absorption cross section at the photolysis wavelength and is a relative measurement. Using the HCHO absorption cross section previously obtained in this laboratory, the relative quantum yield was determined two different ways. One produced band specific yields, and the other produced yields averaged over each 100 cm(–1). Yields were normalized to a value of 0.69 at 31,750 cm(–1) based on the current recommendation of Sander et al. (Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Jet Propulsion Laboratory: Pasadena, CA, USA, 2011). The resulting radical quantum yields agree well with previous experimental studies and the current JPL recommendation but show greater wavelength dependent structure. A significant decrease in the quantum yield was observed for the 5(0)(1) + 1(0)(1)4(0)(1) combination band centered at 31,125 cm(–1). This band has a low absorption cross section and has little impact on the calculated atmospheric photodissociation rate but is a further indication of the complexity of HCHO photodissociation dynamics.

Charge separation in a covalently-linked phthalocyanine-oligo(p-phenylenevinylene)-C60 system. Influence of the solvent polarity

A photo- and redoxactive system ZnPc-oPPV-C602, in which the photoexcited state electron donor – zinc phthalocyanine – and the ground state electron acceptor – C60 – are connected by a oligo(p-phenylenevinylene) (oPPV) spacer, has been synthesized in a multi-step synthesis by means of two consecutive Wadsworth–Horner–Emmons and a dipolar 1,3-cycloaddition reactions as key steps. The simpler system ZnPc-C601 has also been prepared as a reference model for photophysical studies. In this regards, the photophysical investigations by means of fluorescence, flash photolysis, and transient-absorption spectroscopy have manifested a clear dependence between charge transfer kinetics and spatial arrangement. In both systems, intramolecular charge separation evolves from the photoexcited ZnPc and yields the ZnPc·+/C60·- radical ion pairs. Interestingly, the ZnPc·+/C60·- radical ion pair lifetimes and quantum yields are strongly impacted by the solvent polarity and the distance. To this end, maximum radical ion pair lifetimes of 2900 and 5530ps were found in anisol for 1 and 2, respectively.

Enhancing photocatalytic activity of titania materials by using porous structures and the addition of gold nanoparticles

Titanium dioxide is a photocatalyst that has attracted considerable attention for tackling pollution in liquid or gaseous environments. Titania has the benefits of high stability and low toxicity, it is abundant and therefore is relatively cheap. However, intrinsic issues in the material, in particular the recombination between the photon induced electron and hole pair, the wide band gap (∼3.2 eV), and the associated issues of nanoparticle separation (generally nanoparticle samples are required to achieve high surface areas) have hampered the full potential of this photocatalytic (PC) material. Here, recent progress in producing porous titania materials, the addition of gold nanoparticles (Au NPs) to the titania and the coupling of these two approaches to improve the PC properties are reviewed. Incorporating porosity within the titania material affords large surface areas without associated nanoparticulate separation issues, and increased accessibility for the organic pollutant to the active sites on the titania, thereby enhancing PC activity. Au NPs act as electron sinks to enhance the charge separation between the e−/h+ produced on photon absorption, hence improving the quantum yield of superoxide radicals, resulting in improved PC activity. Further enhancement can be achieved by coupling the porous structure of the TiO2 and the addition of Au NPs.

Hydroxyl Radical Quantum Yields from Isopropyl Nitrite Photolysis in Air

Alkyl nitrites photolyze in air to yield alkoxy radicals and NO which, through secondary reactions, generate OH radicals. This photochemistry is important in the atmosphere and in laboratory studies where nitrites are often used as a source of OH. The overall quantum yield for hydroxyl radical formation from irradiation of isopropyl nitrite (i-C(3)H(7)ONO) between 300 and 425 nm in 1 atm air at 296 ± 2 K is reported for the first time. The OH radical was scavenged by reaction with CF(3)CF═CF(2) and the formation of CF(3)CFO and CF(2)O monitored as a function of time using Fourier transform infrared spectrometry. The quantum yield was found to be 0.54 ± 0.07 (2σ) and is independent of whether or not NO was added (up to 3 × 10(14) molecules cm(-3)) prior to photolysis to increase NO concentrations above those due to the photolysis of the nitrite. Ultraviolet-visible and infrared cross sections of i-C(3)H(7)ONO are also reported. These data on the OH quantum yields as well as the UV-visible and infrared cross sections for isopropyl nitrite are critical for quantitatively interpreting the results of laboratory studies where i-C(3)H(7)ONO is employed as an OH source as well as for assessing the role of alkyl nitrites in the chemistry of the troposphere.

Enhancement of the Photocatalytic Activity of TiO2 Nanoparticles by Water-Soluble Complexes of Carotenoids

Photoirradiation of TiO(2) nanoparticles by visible light in the presence of the water-soluble natural polysaccharide arabinogalactan complexes of the hydrocarbon carotenoid β-carotene leads to enhanced yield of the reactive hydroxyl (OH) radicals. The electron paramagnetic resonance (EPR) spin-trapping technique using α-phenyl-N-tert-butyl nitrone (PBN) as the spin-trap has been applied to detect this intermediate by trapping the methyl and methoxy radicals generated upon reaction of the hydroxyl radical with dimethylsulfoxide (DMSO). The free radicals formed in this system proceed via oxygen reduction and not via the reaction of holes on the TiO(2) surface. As compared with pure carotenoids, carotenoid-arabinogalactan complexes exhibit an enhanced quantum yield of free radicals and stability toward photodegradation. The observed enhancement of the photocatalytic efficiency for carotenoid complexes, as measured by the quantum yield of the desired spin adducts, arises specifically from the decrease in the rate constant for the back electron transfer to the carotenoid radical cation. These results are important for a variety of TiO(2) applications, namely, in photodynamic therapy, and in the design of artificial light-harvesting, photoredox, and catalytic devices.

Effects of bromide on UV/chlorine advanced oxidation process

High quantum yield (1.4 mol Es−1) of hydroxyl radicals (∙OH) from photolysis of chlorine under typical disinfection conditions indicates the potential of UV/chlorine coexposure in serving as both disinfection and advanced oxidation processes (AOP). In this study, photolysis of chlorine and bromine was explored in buffer and simulated natural water solutions under low-pressure UV (LPUV) and medium-pressure UV (MPUV) lamps. At pH 6.5 and 8.5, the quantum yields of bromine photolysis were 3.8 and 0.6 for MPUV, and 4.4 and 0.8 for LPUV, respectively. At pH 6.5, the photolysis of bromine was faster than that of chlorine under either MPUV or LPUV source, while at a higher pH of 8.5, the contrary was found. For all conditions tested, the presence of bromide did not significantly change the observed photolysis rate of total free halogen during the UV/chlorine process in the presence or absence of natural organic matter. Upon UV irradiation, chlorine always produces higher ∙OH concentration than bromine does. The presence of bromide results in considerable decrease of ∙OH concentration at pH 6.5, compared to that obtained from the UV/chlorine process in the absence of bromide.

Photophysics and Photochemistry of an Asymmetrically Substituted Diazene: A Suitable Cage Effect Probe

The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene were thoroughly studied by laser flash photolysis from the picosecond to the microsecond time domain. The compound has favorable features as a radical photoinitiator and as a probe for cage effect studies in liquids, supercritical fluids, and compressed gases. The biphenyl moiety acts as an antenna efficiently transferring electronic energy to the dissociative (1)n,pi* state centered on the azo moiety. By picosecond experiments irradiating at the biphenyl- and at the azo-centered transitions, we were able to demonstrate this fact as well as determine a lifetime of 0.7 ps for the buildup of 1-biphenyl-4-yl-1-methyl-ethyl radicals (BME*). The sum of in-cage reaction rate constants of BME* radicals by combination and disproportionation is 5 x 10(10) s(-1). The free radical quantum yield in solution is 0.21 (phi(BME*)) in n-hexane at room temperature, whereas the dissociation quantum yield approaches 50%. The symmetric ketone, 2,4-bis-biphenyl-4-yl-2,4-dimethyl-pentan-2-one, was used as a reference compound for the production and reaction of BME* radicals. Transient IR measurements show CO stretching bands of the excited (3)pi,pi* and (1)n,pi* states but no dissociation up to 0.5 ns. A fluorescence lifetime of 1 ns for this ketone is consistent with this observation. By transient actinometry and kinetic decays in the microsecond time range, we measured epsilon(BME*) = (2.3 +/- 0.2) x 10(4) M(-1) cm(-1) at 325 nm and a second-order rate constant of 5.8 x 10(9) M(-1) s(-1) for the consumption of BME* radicals.

Ultraviolet Photolysis of HCHO: Absolute HCO Quantum Yields by Direct Detection of the HCO Radical Photoproduct

Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, Phi(HCO), have been measured for the tropospherically relevant range of wavelengths (lambda) between 300 and 330 nm. The HCO photoproduct was directly detected by using a custom-built, combined ultra-violet (UV) absorption and cavity ring down (CRD) detection spectrometer. This instrument was previously employed for high-resolution (spectral resolution approximately 0.0035 nm) measurements of absorption cross-sections of HCHO, sigma(HCHO)(lambda), and relative HCO quantum yields. Absolute Phi(HCO) values were measured at seven wavelengths, lambda = 303.70, 305.13, 308.87, 314.31, 320.67, 325.59, and 329.51 nm, using an independent calibration technique based on the simultaneous UV photolysis of HCHO and Cl(2). These Phi(HCO) measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for Phi(HCO). The absolute Phi(HCO)(lambda) determinations and previously measured sigma(HCHO)(lambda) were used to scale an extensive set of relative HCO yield measurements. The outcome of this procedure is a full suite of data for the product of the absolute radical quantum yield and HCHO absorption cross-section, Phi(HCO)(lambda)sigma(HCHO)(lambda), at wavelengths from 302.6 to 331.0 nm with a wavelength resolution of 0.005 nm. This product of photochemical parameters is combined with high-resolution solar photon flux data to calculate the integrated photolysis rate of HCHO to the radical (H + HCO) channel, J(HCO). Comparison with the latest NASA-JPL recommendations, reported at 1 nm wavelength resolution, suggests an increased J(HCO) of 25% at 0 degrees solar zenith angle (SZA) increasing to 33% at high SZA (80 degrees). The differences in the calculated photolysis rate compared with the current HCHO data arise, in part, from the higher wavelength resolution of the current data set and highlight the importance of using high-resolution spectroscopic techniques to achieve a complete and accurate picture of HCHO photodissociation processes. All experimental Phi(HCO)(lambda)sigma(HCHO)(lambda) data are available for the wavelength range 302.6-331.0 nm (at 294 and 245 K and under 200 Torr of N(2) bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of Phi(H(2)+CO)(lambda)sigma(HCHO)(lambda) for the molecular (H(2) + CO) photofragmentation channel, produced using the measured Phi(HCO)(lambda) sigma(HCHO)(tau) values, are also provided at 0.1 and 1.0 nm resolution.

Competitive photosensitized oxidation of tyrosine and methionine residues in enkephalins and their model peptides

Laser flash photolysis was used to investigate the oxidation of methionine (Met) and tyrosine (Tyr) residues in methionine-enkephalin (MetEnk, TyrGlyGlyPheMet) by the triplet state of 4-carboxybenzophenone (4CB). Quenching of the 4CB triplet by model amino acids and peptides (Tyr, TyrGly, TyrGlyGly, PheMet, TyrMet, MetTyr, MetEnk and TyrGlyGlyPheLeu) occurred with rate constants close to the diffusion-controlled limit, k q = (1 − 3) × 10 9 M −1 s −1. Experimental transient spectra, that were resolved into components, revealed the presence of various electron-transfer intermediates, i.e. ketyl radicals (CBH ) and ketyl radical anions (CB −) of 4CB, tyrosyl radicals (TyrO ), and (S∴N) + radical cations. Based on the concentration profiles obtained from spectral deconvolutions, quantum yields of the transients were determined. For the quenchers containing only the tyrosine residue, the quantum yields of the tyrosyl radicals were found to be close to unity and equal to the sum of the quantum yields for the formation of 4CB ketyl radicals and 4CB ketyl radical anions. For quenchers containing both tyrosine and methionine residues, the formation quantum yields of tyrosyl radicals were decreased from the value of 1 to approximately 0.7–0.8 which was equal to the sum of the quantum yields for the formation of CBH and CB −. The above observations are discussed in terms of competitive oxidation reactions involving Tyr and Met followed by the intramolecular electron transfer from the tyrosine residue to the sulfur-centered radical cation on the methionine residue.

Wavelength Dependence of Nitrate Radical Quantum Yield from Peroxyacetyl Nitrate Photolysis: Experimental and Theoretical Studies

The photolysis wavelength dependence of the nitrate radical quantum yield for peroxyacetyl nitrate (CH(3)C(O)OONO(2), PAN) is investigated. The wavelength range used in this work is between 289 and 312 nm, which mimics the overlap of the solar flux available in the atmosphere and PAN's absorption cross section. We find the nitrate radical quantum yield from PAN photolysis to be essentially invariant; Phi(NO3)(PAN) = 0.30 +/- 0.07 (+/-2sigma) in this region. The excited states involved in PAN photolysis are also investigated using ab initio calculations. In addition to PAN, calculations on peroxy nitric acid (HOONO(2), PNA) are performed to examine general photochemical properties of the -OONO(2) chromophore. Equation of motion coupled cluster calculations (EOM-CCSD) are used to examine excited state energy gradients for the internal coordinates, oscillator strengths, and transition energies for the n --> pi* transitions responsible for the photolysis of both PNA and PAN. We find in both molecules, photodissociation of both O-O and O-N bonds occurs via excitation to predissociative electronic excited states and subsequent redistribution of that energy as opposed to directly dissociative excitations. Comparison and contrast between experimental and theoretical studies of HOONO(2) and PAN photochemistry from this and other work provide unique insight on the photochemistry of these species in the atmosphere.

Can OH Radicals Diffuse from the UV-Irradiated Photocatalytic TiO2 Surfaces? Laser-Induced-Fluorescence Study

Diffusion of OH radicals from UV-irradiated TiO2 surface to the gas phase was successfully detected using a laser-induced-fluorescence technique for various types of TiO2 powders. The diffusion time of OH radicals was found to vary with the types of TiO2 powders and to be affected by the heat treatments of these powders, depending on the treatment temperatures. The diffusion mechanism was discussed based on the characteristic OH-LIF intensities for individual TiO2 powders and the observations of OD-LIF after the exposure of D2O vapors over the TiO2 powders. The quantum yield of OH radicals diffused from the TiO2 surface was estimated to be about 5 × 10-5 by comparing the OH-LIF intensities produced by the 266-nm photolysis of HNO3.

Production of Free Radicals and Triplets from Contact Radical Pairs and from Photochemically Generated Radical Ions

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.

Reactivity of halides with triplets of 6-chloro and 6-bromopicolinic acids: contrasting effects of bromide and chloride

The reactivity of Br(-) and Cl(-) with triplet of anionic 6-chloropicolinic acid (pH = 5.4) and with triplets of 6-chloro and 6-bromopicolinic acids in zwitterionic forms (pH = 0.9) was studied by laser flash photolysis and steady-state irradiation. Br(-) was found to trap the three triplets. Triplet lifetime measurements gave quenching rate constants of 8 x 10(8) mol(-1) dm(3) s(-1) for the zwitterion of 6-chloropicolinic acid and of 3.4 x 10(5) mol(-1) dm(3) s(-1) for the anionic counterpart. No secondary transient species were observed indicating that the charge transfer intermediates are subject to dissipative processes. Cl(-) trapped triplet of zwitterions only, and reactions were found to be associated with a high quantum yield of radicals. The photolysis of 6-bromopicolinic acid photolysis was drastically enhanced by Cl(-), 6-chloropicolinic acid being produced with a chemical yield of about 90%. The 6-bromo-2-carboxypyridinyl radical could be characterized (lambda(max)/nm = 318 with shoulder at 370 nm and epsilon/mol(-1) dm(3) cm(-1) = 8100).

Nitrate Radical Quantum Yield from Peroxyacetyl Nitrate Photolysis

Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) is a ubiquitous pollutant that is primarily destroyed by either thermal or photochemical mechanisms. We have investigated the photochemical destruction of PAN using a combination of laser pulsed photolysis and cavity ring-down spectroscopic detection of the NO3 photoproduct. We find that the nitrate radical quantum yield from the 289 nm photolysis of PAN is Phi(NO3)PAN = 0.31 +/- 0.08 (+/-2 sigma). The quantum yield is determined relative to that of dinitrogen pentoxide, which is assumed to be unity, under identical experimental conditions. The instrument design and experimental procedure are discussed as well as auxiliary experiments performed to further characterize the performance of the optical cavity and photolysis system.