Measurements of line strengths in the 2nu1 band of the HO2 radical using laser photolysis/continuous wave cavity ring-down spectroscopy (cw-CRDS)

Absolute absorption cross sections of the absorption spectrum of the 2nu1 band of the HO2 radical in the near-IR region were measured by continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to laser photolysis in the wavelength range 6604-6696 cm(-1) with a resolution better than 0.003 cm(-1). Absolute absorption cross sections were obtained by measuring the decay of the HO2 self-reaction, and they are given for the 100 most intense lines. The most important absorption feature in this wavelength range was found at 6638.20 cm(-1), exhibiting an absorption cross section of sigma = 2.72 x 10(-19) cm2 at 50 Torr He. Using this absorption line, we obtain a detection limit for the HO2 radical at 50 Torr of 6.5 x 10(10) cm(-3).     

Absorption cross sections of formaldehyde at wavelengths from 300 to 340 nm at 294 and 245 K

Absorption cross sections for the A1A2-X1A1 electronic transition of formaldehyde have been measured by ultraviolet (UV) laser absorption spectroscopy in the tropospherically significant wavelength range 300-340 nm, over which HCHO is photochemically active. Absorption cross sections are reported at two temperatures, 294 and 245 K and at a spectral resolution of 0.0035 nm (0.35 cm-1). At this resolution, greater peak absorption cross sections are obtained for many of the sharp spectral features than were previously reported. To simulate atmospheric conditions in the troposphere, the effects of adding a pressure of nitrogen of up to 500 Torr and of reduced sample temperature were investigated. The overall magnitudes of peak absorption cross sections are largely unaffected by the added pressure of nitrogen, but a modest degree of pressure broadening (0.2-0.3 cm-1 atm-1) is evident in the line shapes. Computer simulations of spectra have been optimized by comparison with wavelength-dependent formaldehyde absorption cross sections for each major vibronic band in the chosen wavelength range. Experimental and computer simulated spectra at 294 and 245 K are compared to test the reliability of the computer simulations for quantification of the effects of temperature on absorption cross sections. All experimental absorption cross section data and tables of input parameters for spectral simulations are available as Supporting Information.     

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 ?(1)A2–X?(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.     

Kinetic study of the daytime atmospheric fate of (z)-3-hexenal

Rate coefficients for three daytime atmospheric reactions of (Z)-3-hexenal (3HA)-photolysis (J(1)), reaction with OH radicals (k(2)), and reaction with ozone (k(3))-were measured at 760 Torr and 298 K using a 6 m(3) photochemical reaction chamber. The UV absorption cross sections (σ(3HA)(λ)) were obtained in the wavelength range 240-350 nm. The photodissociation rate of 3HA relative to that of NO(2) was measured by a solar simulator at 760 Torr and was determined to be J(1)/J(NO2) = (4.7 ± 0.4) × 10(-3). Using the obtained σ(3HA)(λ) and J(1)/J(NO2), the effective photodissociation quantum yield was calculated to be Φ(3HA) = 0.25 ± 0.06. The rate coefficient for the reaction with OH radicals was measured by the relative rate method with three reference compounds and was determined to be k(2) = (6.9 ± 0.9) × 10(-11) cm(3) molecule(-1) s(-1). The rate coefficient for the reaction with ozone was measured by an absolute method and was determined to be k(3) = (3.5 ± 0.2) × 10(-17) cm(3) molecule(-1) s(-1). Using the obtained rate coefficients, the daytime atmospheric lifetime of 3HA was estimated.     

Kinetic studies of the ClO + ClO association reaction as a function of temperature and pressure

The kinetics of the association reaction of ClO radicals: ClO + ClO + M --> Cl2O2+ M (1), have been investigated as a function of temperature T between 206.0-298.0 K and pressure p between 25-760 Torr using flash photolysis with time-resolved UV absorption spectroscopy. ClO radicals were generated following the photolysis of Br2/Cl2O mixtures in nitrogen diluent gas. Charge coupled device (CCD) detection of time resolved absorptions was used to monitor ClO radicals over a broad wavelength window covering the ClO (A 2Pi<-- X 2Pi) vibronic absorption bands. The high pass filtered ClO absorption cross sections were calibrated as a function of temperature between T = 206.0-320 K, and exhibit a negative temperature dependence. The ClO association kinetics were found to be more rapid than those reported in previous studies, with limiting low and high pressure rate coefficients, in nitrogen bath gas, k0 = (2.78 +/- 0.82) x 10(-32) x (T/300)(-3.99 +/- 0.94) molecule(-2) cm6 s(-1) and k(infinity) = (3.37 +/- 1.67) x 10(-12) x (T/300)(-1.49 +/- 1.81) molecule(-1) cm3 s(-1), respectively, (obtained with the broadening factor F(c) fixed at 0.6). Errors are 2sigma. The pressure dependent ClO association rate coefficients (falloff curves) exhibited some discrepancies at low pressures, with higher than expected rate coefficients on the basis of extrapolation from high pressures (p > 100 Torr). Reanalysis of data excluding kinetic data recorded below p = 100 Torr gave k0 = (2.79 +/- 0.85) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1) and k(infinity) = (3.44 +/- 1.83)x 10(-12) x (T/300)(-1.73 +/- 1.91) molecule(-1) cm3 s(-1). Potential sources of the low pressure discrepancies are discussed. The expression for k(0) in air bath gas is k0 = (2.62 +/- 0.80) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1). These results support upward revision of the ClO association rate coefficient recommended for use in stratospheric models, and the stratospheric implications of the results reported here are briefly discussed.     

Resonance raman excitation profile of a ruthenium(II) complex of dipyrido[2,3-a:3',2'-c]phenazine

The lowest energy transition of [Ru(CN)(4)(ppb)](2-) (ppb = dipyrido[2,3-a:3',2'-c]phenazine), a metal-to-ligand charge transfer, has been probed using resonance Raman spectroscopy with excitation wavelengths (488, 514, 530, and 568 nm) spanning the lowest energy absorption band centered at 522 nm. Wave packet modeling was used to simultaneously model this lowest energy absorption band and the cross sections of the resonance Raman bands at the series of excitation wavelengths across this absorption band. A fit to within +/-20% was obtained for the Raman cross sections, close to the experimental uncertainty which is typically 10-20%. Delta values of 0.1-0.4 were obtained for modes which were either localized on the ppb ligand (345-1599 cm(-1)) or the CN modes (2063 and 2097 cm(-1)). DFT calculations reveal that the resonance Raman bands observed are due to modes delocalized over the entire ppb ligand.     

Absorption cross sections for the A 2A' (0,9(0),0) <-- X 2A' (0,0(1),0) band of the HCO radical

Absorption cross sections for the A 2A' (0,9(0),0) <-- X 2A' (0,0(1),0) band of HCO were determined at 295 K using pulsed laser photolysis combined with cavity ring-down spectroscopy. Formyl radicals (HCO) were produced from the reaction of atomic chlorine, generated by photolysis of Cl2 at 335 nm, with formaldehyde. The concentration of HCO was calibrated using two independent photochemical methods. The peak cross section of the P8 line was determined to be (1.98 +/- 0.36) x 10(-18) cm2, and the intensity of the entire band was normalized to this line. The quoted 2 sigma uncertainty includes estimated systematic errors. Comparisons to previously reported values of HCO cross sections in this band are discussed.     

High-resolution absorption cross sections of formaldehyde in the 30,285-32,890 cm(-1) (304-330 nm) spectral region

Absolute room temperature (294 ± 2 K) absorption cross sections for the ?(1)A(2)-X?(1)A(1) electronic transition of formaldehyde have been measured over the spectral range 30,285-32,890 cm(-1) (304-330 nm) using ultraviolet (UV) laser absorption spectroscopy. Accurate high-resolution absorption cross sections are essential for atmospheric monitoring and understanding the photochemistry of this important atmospheric compound. Absorption cross sections were obtained at an instrumental resolution better than 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 so we were able to resolve all but the most closely spaced lines. Comparisons with previous data as well as with computer simulations have been made. Pressure broadening was studied for the collision partners He, O(2), N(2), and H(2)O and the resulting broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. The pressure broadening coefficient for H(2)O is an order of magnitude larger than the coefficients for O(2) and N(2) and will contribute significantly to spectral line broadening in the lower atmosphere. Spectral data are made available as Supporting Information.     

Ultraviolet photochemistry of trichlorovinylsilane and allyltrichlorosilane: vinyl radical (HCCH2) and allyl radical (H2CCHCH2) production in 193 nm photolysis

The absolute gas phase ultraviolet absorption spectra of trichlorovinylsilane and allyltrichlorosilane have been measured from 191 to 220 nm. Over this region the absorption spectra of both species are broad and relatively featureless, and their cross sections increase with decreasing wavelength. The electronic transitions of trichlorovinylsilane were calculated by ab initio quantum chemical methods and the observed absorption bands assigned to the A(1)A'<-- X[combining tilde](1)A' transition. The maximum absorption cross section in the region, at 191 nm, is sigma = (8.50 +/- 0.06) x 10(-18) cm(2) for trichlorovinylsilane and sigma = (2.10 +/- 0.02) x 10(-17) cm(2) for allyltrichlorosilane. The vinyl radical and the allyl radical are formed promptly from the 193 nm photolysis of their respective trichlorosilane precursors. By comparison of the transient visible absorption and the 1315 nm I atom absorption from 266 nm photolysis of vinyl iodide and allyl iodide, the absorption cross sections at 404 nm of vinyl radical ((2.9 +/- 0.4) x 10(-19) cm(2)) and allyl radical ((3.6 +/- 0.8) x 10(-19) cm(2)) were derived. These cross sections are in significant disagreement with literature values derived from kinetic modeling of allyl or vinyl radical self-reactions. Using these cross sections, the vinyl radical yield from trichlorovinylsilane was determined to be phi = (0.9 +/- 0.2) per 193 nm photon absorbed, and the allyl radical yield from allyltrichlorosilane phi = (0.7 +/- 0.2) per 193 nm photon absorbed.     

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.     

Atmospheric chemistry of n-C(x)F(2)(x)(+1)CHO (x = 1, 2, 3, 4): fate of n-C(x)F(2)(x)(+1)C(O) radicals

Smog chamber/FTIR techniques were used to study the atmospheric fate of n-C(x)F(2)(x)(+1)C(O) (x = 1, 2, 3, 4) radicals in 700 Torr O(2)/N(2) diluent at 298 +/- 3 K. A competition is observed between reaction with O(2) to form n-C(x)()F(2)(x)()(+1)C(O)O(2) radicals and decomposition to form n-C(x)F(2)(x)(+1) radicals and CO. In 700 Torr O(2)/N(2) diluent at 298 +/- 3 K, the rate constant ratio, k(n-C(x)F(2)(x)(+1)C(O) + O(2) --> n-C(x)F(2)(x)(+1)C(O)O(2))/k(n-C(x)F(2)(x)(+1)C(O) --> n-C(x)F(2)(x)(+1) + CO) = (1.30 +/- 0.05) x 10(-17), (1.90 +/- 0.17) x 10(-19), (5.04 +/- 0.40) x 10(-20), and (2.67 +/- 0.42) x 10(-20) cm(3) molecule(-1) for x = 1, 2, 3, 4, respectively. In one atmosphere of air at 298 K, reaction with O(2) accounts for 99%, 50%, 21%, and 12% of the loss of n-C(x)F(2)(x)(+1)C(O) radicals for x = 1, 2, 3, 4, respectively. Results are discussed with respect to the atmospheric chemistry of n-C(x)F(2)(x)(+1)C(O) radicals and their possible role in contributing to the formation of perfluorocarboxylic acids in the environment.     

Photolysis of CH3C(O)CH3 (248 nm, 266 nm), CH3C(O)C2H5 (248 nm) and CH3C(O)Br (248 nm): pressure dependent quantum yields of CH3 formation

The formation of CH(3) in the 248 or 266 nm photolysis of acetone (CH(3)C(O)CH(3)), 2-butanone (methylethylketone, MEK, CH(3)C(O)C(2)H(5)) and acetyl bromide (CH(3)C(O)Br) was examined using the pulsed photolytic generation of the radical and its detection by transient absorption spectroscopy at 216.4 nm. Experiments were carried out at room temperature (298 +/- 3 K) and at pressures between approximately 5 and 1500 Torr N(2). Quantum yields for CH(3) formation were derived relative to CH(3)I photolysis at the same wavelength in back-to-back experiments. For acetone at 248 nm, the yield of CH(3) was greater than unity at low pressures (1.42 +/- 0.15 extrapolated to zero pressure) confirming that a substantial fraction of the CH(3)CO co-product can dissociate to CH(3) + CO under these conditions. At pressures close to atmospheric the quantum yield approached unity, indicative of almost complete collisional relaxation of the CH(3)CO radical. Measurements of increasing CH(3)CO yield with pressure confirmed this. Contrasting results were obtained at 266 nm, where the yields of CH(3) (and CH(3)CO) were close to unity (0.93 +/- 0.1) and independent of pressure, strongly suggesting that nascent CH(3)CO is insufficiently activated to decompose on the time scales of these experiments at 298 K. In the 248 nm photolysis of CH(3)C(O)Br, CH(3) was observed with a pressure independent quantum yield of 0.92 +/- 0.1 and CH(3)CO remained below the detection limit, suggesting that CH(3)CO generated from CH(3)COBr photolysis at 248 nm is too highly activated to be quenched by collision. Similar to CH(3)C(O)CH(3), the photolysis of CH(3)C(O)C(2)H(5) at 248 nm revealed pressure dependent yields of CH(3), decreasing from 0.45 at zero pressure to 0.19 at pressures greater than 1000 Torr with a concomitant increase in the CH(3)CO yield. As part of this study, the absorption cross section of CH(3) at 216.4 nm (instrumental resolution of 0.5 nm) was measured to be (4.27 +/- 0.2) x 10(-17) cm(2) molecule(-1) and that of C(2)H(5) at 222 nm was (2.5 +/- 0.6) x 10(-18) cm(2) molecule(-1). An absorption spectrum of gas-phase CH(3)C(O)Br (210-305 nm) is also reported for the first time.     

High-accuracy measurements of OH(•) reaction rate constants and IR and UV absorption spectra: ethanol and partially fluorinated ethyl alcohols

Rate constants for the gas phase reactions of OH(?) radicals with ethanol and three fluorinated ethyl alcohols, CH(3)CH(2)OH (k(0)), CH(2)FCH(2)OH (k(1)), CHF(2)CH(2)OH (k(2)), and CF(3)CH(2)OH (k(3)) were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions over the indicated temperature intervals: k(0)(220-370 K) = 5.98 × 10(-13)(T/298)(1.99) exp(+515/T) cm(3) molecule(-1) s(-1), k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1) [for atmospheric modeling purposes, k(0)(T) is essentially temperature-independent below room temperature, k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1)], k(1)(230-370 K) = 3.47 × 10(-14)(T/298)(4.49) exp(+977/T) cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 3.87 × 10(-14)(T/298)(4.25) exp(+578/T) cm(3) molecule(-1) s(-1), and k(3)(220-370 K) = 2.48 × 10(-14)(T/298)(4.03) exp(+418/T) cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH(?) were estimated to be 4, 16, 62, and 171 days, respectively, under the assumption of a well-mixed atmosphere. UV absorption cross sections of all four ethanols were measured between 160 and 215 nm. The IR absorption cross sections of the three fluorinated ethanols were measured between 400 and 1900 cm(-1), and their global warming potentials were estimated.     

The Pt2 (1,0) band of System VI in the near infrared by intracavity laser absorption spectroscopy

Intracavity laser absorption spectroscopy has been used to record rotationally resolved electronic spectra of Pt(2) in the near infrared. The metal dimers were created using a 50 mm-long, platinum-lined hollow cathode plasma discharge. The observed transition at 12?937 cm(-1) is identified as the (1,0) band of System VI, with state symmetries Ω = 0 - X Ω = 0.     

Electronic excited states of [Au(2)(dmpm)(3)](ClO(4))(2) (dmpm= bis(dimethylphosphine)methane)

We present studies of the resonance Raman and electronic luminescence spectra of the [Au(2)(dmpm)(3)](ClO(4))(2) (dmpm = bis(dimethylphosphine)methane) complex, including excitation into an intense band at 256 nm and into a weaker absorption system centered about approximately 300 nm. The resonance Raman spectra confirm the assignment of the 256 nm absorption band to a (1)(dsigma --> psigma) transition, a metal-metal-localized transition, in that nu(Au-Au) and overtones of it are strongly enhanced. A resonance Raman intensity analysis of the spectra associated with the 256 nm absorption band gives the ground-state and excited-state nu(Au-Au) stretching frequencies to be 79 and 165 cm(-1), respectively, and the excited-state Au-Au distance is calculated to decrease by about 0.1 A from the ground-state value of 3.05 A. The approximately 300 nm absorption displays a different enhancement pattern, in that resonance-enhanced Raman bands are observed at 103 and 183 cm(-1) in addition to nu(Au-Au) at 79 cm(-1) The compound exhibits intense, long-lived luminescence (in room-temperature CH(3)CN, for example, tau = 0.70 micros, phi(emission) = 0.037) with a maximum at 550-600 nm that is not very medium-sensitive. We conclude, in agreement with an earlier proposal of Mason (Inorg. Chem. 1989, 28, 4366-4369), that the lowest-energy, luminescent excited state is not (3)(dsigma --> psigma) but instead derives from (3)(d(x2-y2,xy --> psigma) excitations. We compare the Au(I)-Au(I) interaction shown in the various transitions of the [Au(2)(dmpm)(3)](ClO(4))(2) tribridged compound with previous results for solvent or counterion exciplexes of [Au(2)(dcpm)(2)](2+) salts (J. Am. Chem. Soc. 1999, 121, 4799-4803; Angew. Chem. 1999, 38, 2783-2785; Chem. Eur. J. 2001, 7, 4656-4664) and for planar, mononuclear Au(I) triphosphine complexes. It is proposed that the luminescent state in all of these cases is very similar in electronic nature.     

Absolute rate coefficient of the OH + CH(3)C(O)OH reaction at T = 287-802 K. The two faces of pre-reactive H-bonding

The rate constants for the reaction OH + CH3C(O)OH --> products (1) were determined over the temperature range 287-802 K at 50 and 100 Torr of Ar or N2 bath gas using pulsed laser photolysis generation of OH by CH3C(O)OH photolysis at 193 nm coupled with OH detection by pulsed laser-induced fluorescence. The rate coefficient displays a complex temperature dependence with a sharp minimum at 530 K, indicating the competition between a reaction proceeding through a pre-reactive H-bonded complex to form CH3C(O)O + H2O, expected to prevail at low temperatures, and a direct methyl-H abstraction channel leading to CH2C(O)OH + H2O, which should dominate at high temperatures. The temperature dependence of the rate constant can be described adequately by k1(287-802 K) = 2.9 x 10(-9) exp{-6030 K/T} + 1.50 x 10(-13) exp{515 K/T} cm3 molecule(-1)(s-1), with a value of (8.5 +/- 0.9) x 10-13 cm3 molecule(-1)(s-1) at 298 K. The steep increase in rate constant in the range 550-800 K, which is reported for the first time, implies that direct abstraction of a methyl-H becomes the dominant pathway at temperatures greater than 550 K. However, the data indicates that up to about 800 K direct methyl-H abstraction remains adversely affected by the long-range H-bonding attraction between the approaching OH radical and the carboxyl -C(O)OH functionality.     

The gas phase tropospheric removal of fluoroaldehydes (CxF2x+1CHO, x = 3, 4, 6)

The rate coefficient of the OH reaction with the perfluoroaldehydes C(3)F(7)CHO and C(4)F(9)CHO have been determined in the temperature range 252-373 K using the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) method: k(C(3)F(7)CHO+OH) = (2.0 +/- 0.6) x 10(-12) exp[-(369 +/- 90)/T] and k(C(4)F(9)CHO+OH) = (2.0 +/- 0.5) x 10(-12) exp[-(356 +/- 70)/T] cm(3) molecule(-1) s(-1), corresponding to (5.8 +/- 0.6) x 10(-13) and (6.1 +/- 0.5) x 10(-13) cm(3) molecule(-1) s(-1), respectively, at 298 K. The UV absorption cross sections of these two aldehydes and CF(3)(CF(2))(5)CH(2)CHO have been measured over the range 230-390 nm at 298 K and also at 328 K for CF(3)(CF(2))(5)CH(2)CHO. The obtained results for C(3)F(7)CHO and C(4)F(9)CHO are in good agreement with two recent determinations but the maximum value of the absorption cross section for CF(3)(CF(2))(5)CH(2)CHO is over a factor of two lower than the single one recently published. The photolysis rates of C(3)F(7)CHO, C(4)F(9)CHO and CF(3)(CF(2))(5)CHO have been measured under sunlight conditions in the EUPHORE simulation chamber in Valencia (Spain) at the beginning of June. The photolysis rates were, respectively, J(obs) = (1.3 +/- 0.6) x 10(-5), (1.9 +/- 0.8) x 10(-5) and (0.6 +/- 0.3) x 10(-5) s(-1). From the J(obs) measurements and calculated photolysis rate J(calc), assuming a quantum yield of unity across the atmospheric range of absorption of the aldehydes, quantum yields J(obs)/J(calc) = (0.023 +/- 0.012), (0.029 +/- 0.015) and (0.046 +/- 0.028) were derived for the photodissociation of C(3)F(7)CHO, C(4)F(9)CHO and CF(3)(CF(2))(5)CHO, respectively. The atmospheric implication of the data obtained in this work is discussed. The main conclusion is that the major atmospheric removal pathway for fluoroaldehydes will be photolysis, which under low NO(x) conditions, may be a source of fluorinated carboxylic acids in the troposphere.     

Overtone dissociation of peroxynitric acid (HO2NO2): absorption cross sections and photolysis products

Band strengths for the second (3nuOH) and third (4nuOH) overtones of the OH stretch vibration of peroxynitric acid, HO2NO2 (PNA) in the gas-phase were measured using Cavity Ring-Down Spectroscopy (CRDS). Both OH overtone transitions show diffuse smoothly varying symmetrical absorption profiles without observable rotational structure. Integrated band strengths (base e) at 296 K were determined to be S(3nuOH) = (5.7 +/- 1.1) x 10(-20) and S(4nuOH) = (4.9 +/- 0.9) x 10(-21) cm(2) molecule(-1) cm(-1) with peak cross sections of (8.8 +/- 1.7) x 10(-22) and (7.0 +/- 1.3) x 10(-23) cm(2) molecule(-1) at 10086.0 +/- 0.2 cm(-1) and 13095.8 +/- 0.4 cm(-1), respectively, using PNA concentrations measured on line by Fourier-transform infrared and ultraviolet absorption spectroscopy. The quoted uncertainties are 2sigma (95% confidence level) and include estimated systematic errors in the measurements. OH overtone spectra measured at lower temperature, 231 K, showed a narrowing of the 3nuOH band along with an increase in its peak absorption cross section, but no change in S(3nuOH) to within the precision of the measurement (+/-9%). Measurement of a PNA action spectrum showed that HO2 is produced from second overtone photodissociation. The action spectrum agreed with the CRDS absorption spectra. The PNA cross sections determined in this work for 3nuOH and 4nuOH will increase calculated atmospheric photolysis rates of PNA slightly.     

Spectrophotometric determination of the pseudohalide 1,2,3,4-thiatriazol-5-thiolate ion, CS(2)N(-)(3)

A critical study of the analytical methods available for the CS(2)N(-)(3) ion is reported. A modification of the argentimetric method is proposed. An oxidative study gave evidence of various steps corresponding to incompletely oxidized intermediates. The absorption maximum of the 1,2,3,4-thiatriazol-5-thiolate ion at 313 nm, with molar absorptivity of 7.4 x 10(-3)1.mole(-1).cm(-1) is utilized to develop a new analytical method. The spectrophotometric procedure is rapid and free from interference by many ions. A value of 1.51 +/- 0.02 was found for pK of HCS(2)N(3) at an ionic strength of 1.00M and at 25 degrees . The spectrum of CS(2)N(-)(3) is changed by increasing acidity of the medium, due to the formation of HCS(2)N(3); an isosbestic point is observed at 251 nm.     

Rate coefficients for the reaction of OH with (E)-2-pentenal, (E)-2-hexenal, and (E)-2-heptenal

Rate coefficients for the gas-phase reaction of the OH radical with (E)-2-pentenal (CH(3)CH(2)CH[double bond]CHCHO), (E)-2-hexenal (CH(3)(CH(2))(2)CH[double bond]CHCHO), and (E)-2-heptenal (CH(3)(CH(2))(3)CH[double bond]CHCHO), a series of unsaturated aldehydes, over the temperature range 244-374 K at pressures between 23 and 150 Torr (He, N(2)) are reported. Rate coefficients were measured under pseudo-first-order conditions in OH with OH radicals produced via pulsed laser photolysis of HNO(3) or H(2)O(2) at 248 nm and detected by pulsed laser-induced fluorescence. The rate coefficients were independent of pressure and the room temperature rate coefficients and Arrhenius expressions obtained are (cm(3) molecule(-1) s(-1) units): k(1)(297 K)=(4.3 +/- 0.6)x 10(-11), k(1)(T)=(7.9 +/- 1.2)x 10(-12) exp[(510 +/- 20)/T]; k(2)(297 K)=(4.4 +/- 0.5)x 10(-11), k(2)(T)=(7.5 +/- 1.1)x 10(-12) exp[(520 +/- 30)/T]; and k(3)(297 K)=(4.4 +/- 0.7)x 10(-11), k(3)(T)=(9.7 +/- 1.5)x 10(-12) exp[(450 +/- 20)/T] for (E)-2-pentenal, (E)-2-hexenal and (E)-2-heptenal, respectively. The quoted uncertainties are 2sigma(95% confidence level) and include estimated systematic errors. Rate coefficients are compared with previously published room temperature values and the discrepancies are discussed. The atmospheric degradation of unsaturated aldehydes is also discussed.