Gas Phase UV Absorption Research Articles

Gas-phase UV absorption spectra of a series of of terpenic oxygenated VOC: Nopinone, Myrtenal, Ketolimonene, Limononaldehyde and Caronaldehyde

The UV absorption spectra of nopinone, myrtenal, ketolimonene, caronaldehyde and limononaldehyde were investigated in the spectral range 210-370 nm at 353 K using a D2 lamp coupled to a monochromator-PM. All the spectra show an important absorption band beyond 280 nm. Below 230 nm, a much stronger absorption band is also observed. The cross-section values reported in this work were used to estimate the lifetime of the studied compounds with respect to photolysis. Results suggest that photolysis is potentially an important process for the removal of these compounds from the troposphere.

Photostability of the deprotonated forms of the UV filters homosalate and octyl salicylate: molecular dissociation versus electron detachment following UV excitation.

While common molecular anions show a strong propensity to undergo electron detachment upon UV excitation, this process often occurs in competition with molecular ion dissociation. The factors that affect the balance between these two major possible decay pathways have not been well understood to date. Laser photodissociation spectroscopy of the deprotonated forms of the UV filter molecules, Homosalate (HS) and Octyl Salicylate (OS), i.e. [HS − H]− and [OS − H]−, was used to acquire gas-phase UV absorption spectra for [HS − H]− and [OS − H]−via photodepletion from 3.0–5.8 eV. No photofragmentation (i.e. dissociation of the ionic molecular framework) was observed for either [HS − H]− and [OS − H]− following photoexcitation, revealing that electron loss entirely dominates the electronic decay pathways for these systems. High-level quantum chemical calculations were used to map out the excited states associated with [HS − H]− and [OS − H]−, revealing that the minimum-energy crossing points (MECPs) between the S1 and S0 states are located in elevated regions of the potential energy surface, making internal conversion unlikely. These results are consistent with our experimental observation that electron detachment out-competes hot ground state molecular fragmentation. More generally, our results reveal that the competition between molecular dissociation and electron detachment following anion photoexcitation can be determined by the magnitude of the energy gap between the excitation energy and the MECPs, rather than being a simple function of whether the excitation energy lies above the anion's vertical detachment energy.

Gas-phase UV absorption spectra of pyrazine, pyrimidine and pyridazine

The gas phase UV absorption spectra of pyrazine, pyrimidine and pyridazine were determined by using a D2-lamp coupled to a monochromator-PM system between 200 and 380 nm, at 353 ± 3 K and at pressures lower than their vapor pressure (0.08–0.5 torr). The obtained spectra consist of a broad continuum with low intense absorption bands at 200–380 nm. Absorption of the investigated compounds at wavelengths>290 nm is non-negligible (σλ > 290 nm in the order of 10−20 cm2 molecule−1). These results indicate that, pyrazine, pyrimidine and pyridazine could be removed from the troposphere via photolysis reactions.

UV absorption spectra of trans-2-pentenal, trans-2-hexenal and 2-methyl-2-pentenal

C5 and C6 unsaturated aldehydes are a category of BVOCs released by plants in response to their wounds and stress. In this work, the gas phase UV absorption spectra of trans-2-pentenal (t-2P), trans-2-hexenal (t-2H) and 2-methyl-2-pentenal (2M2P) are determined between 240 and 400 nm, at room temperature and at pressures (1–9) torr. The reported UV absorption cross sections are (cm2molecule−1): σt-2P, 330 = (4.86 ± 0.53) × 10−20, σt-2H, 330 = (5.43 ± 0.57) × 10−20 and σ2M2P, 320 = (6.62 ± 0.50) × 10−20. Their photolysis constants as well as their atmospheric lifetimes toward photolysis were then estimated.

Gas-phase UV absorption spectra and OH-oxidation kinetics of 1H-1,2,3-triazole and pyrazole.

In this work, we report the gas phase UV absorption spectra and the kinetics of the OH-oxidation of 1H-1,2,3-triazole and pyrazole. UV spectra were determined between 200 and 250 nm, at 350 ± 2 K and at pressures between 0.09 and 0.3 Torr. The reported maximal UV absorption cross sections are (cm2 per molecule): σ206 nm, 1H–1H-1,2,3-triazole = 2.04 × 10−18 and σ203 nm, pyrazole = 5.44 × 10−18. The very low absorption capacity of these compounds beyond 240 nm indicates that their atmospheric photodissociation is negligible. The OH-oxidation of these species was performed in an atmospheric simulation chamber coupled to an FTIR spectrometer and to a GC/MS over the temperature range 298–357 K and at atmospheric pressure. Experiments were conducted in relative mode using benzaldehyde, trans-2-hexenal and heptane as references. The obtained rate constants at 298 K were (×10−11 cm3 per molecule per s): k(OH + 1H-1,2,3-triazole) = 2.16 ± 0.41; k(OH + pyrazole) = 2.94 ± 0.42. These results were compared to those available in the literature and discussed in terms of structure-reactivity and temperature dependency. Their tropospheric lifetimes with respect to reaction with OH radicals were then estimated.

Gas-phase UV absorption cross-sections and photolysis kinetics of 4-hydroxy-3-hexanone: Atmospheric implications

The UV absorption spectrum and the photolysis reaction kinetics of 4-hydroxy-3-hexanone (4H3H) in the 275–330nm region have been investigated for the first time. The photolysis frequency of this compound in the atmosphere was evaluated using actinometry and found to be J4H3H,atm=(5.9±3.1)×10−3h−1. An atmospheric effective quantum yield was estimated to about 10% and a photolysis lifetime of about 7days was determined. The comparison with the reactivity of other hydroxyketones suggests that the kinetics of hydroxyketones photolysis depends on their structure, mainly the branching and the proximity of the two functions.

Gas phase UV absorption cross-sections for a series of hydroxycarbonyls

The UV absorption cross-sections of hydroxyacetone, 3-hydroxy 2-butanone, 4-hydroxy 2-butanone and 3-hydroxy 3-methyl 2-butanone have been measured. The experiments have been carried out between 250 and 363K using a D2 lamp coupled to a monochromator. This work provides the first UV cross-section measurements for 3-hydroxy 2-butanone, 4-hydroxy 2-butanone and 3-hydroxy 3-methyl 2-butanone. The obtained cross-section values are used to calculate the photolysis rates and to estimate the tropospheric lifetimes of the studied compounds. The results suggest that photolysis could be an important removal process for these species in the troposphere.

ChemInform Abstract: A Flash Photolysis Investigation of the Gas Phase UV Absorption Spectrum and Self-Reaction Kinetics of the Neopentylperoxy Radical

The ultraviolet absorption spectrum of the neopentylperoxy radical, (CH 3 ) 3 CCH 2 O 2 (or C 5 H 11 O 2 ), and the kinetics of its self-reaction have been studied in the gas phase using a flash photolysis technique. The room temperature absorption cross-section at 250 nm was determined to be σC 5 H 11 O 2 (250 nm)=(5.18±0.78)×10 −18 cm 2 molecule −1 and was used to normalize the radical absorption spectrum between 210 and 300 nm. Detailed modeling of the self-reaction system was used to interpret the transient absorption kinetic decay curves over the temperature range 228-380 K, at total pressures between 25 and 100 torr. The results are discussed in relation to previous measurements of alkylperoxy radical spectra and kinetics

Stark-Effect Studies of 1-Phenylpyrrole in the Gas Phase. Dipole Reversal upon Electronic Excitation

High-resolution fluorescence excitation experiments on 1-phenylpyrrole in a molecular beam, both in the absence and presence of an applied electric field, show that the electronic state responsible for the strong features in the gas-phase UV absorption spectrum is the 1Lb state, that this state is significantly more planar than the ground state, and that the direction of the permanent electric dipole moment in the 1Lb state is reversed relative to that of the ground state. Implications of these findings for intramolecular charge transfer in the isolated molecule are discussed.

UV Absorption Spectrum of the ClO Dimer (Cl2O2) between 200 and 420 nm

The UV photolysis of Cl(2)O(2) (dichlorine peroxide) is a key step in the catalytic destruction of polar stratospheric ozone. In this study, the gas-phase UV absorption spectrum of Cl(2)O(2) was measured using diode array spectroscopy and absolute cross sections, sigma, are reported for the wavelength range 200-420 nm. Pulsed laser photolysis of Cl(2)O at 248 nm or Cl(2)/Cl(2)O mixtures at 351 nm at low temperature (200-228 K) and high pressure (approximately 700 Torr, He) was used to produce ClO radicals and subsequently Cl(2)O(2) via the termolecular ClO self-reaction. The Cl(2)O(2) spectrum was obtained from spectra recorded following the completion of the gas-phase ClO radical chemistry. The spectral analysis used observed isosbestic points at 271, 312.9, and 408.5 nm combined with reaction stoichiometry and chlorine mass balance to determine the Cl(2)O(2) spectrum. The Cl(2)O(2) UV absorption spectrum peaks at 244.5 nm with a cross section of 7.6(-0.5)(+0.8) x 10(-18) cm(2) molecule(-1) where the quoted error limits are 2sigma and include estimated systematic errors. The Cl(2)O(2) absorption cross sections obtained for wavelengths in the range 300-420 nm are in good agreement with the Cl(2)O(2) spectrum reported previously by Burkholder et al. (J. Phys. Chem. A 1990, 94, 687) and significantly higher than the values reported by Pope et al. (J. Phys. Chem. A 2007, 111, 4322). A possible explanation for the discrepancy in the Cl(2)O(2) cross section values with the Pope et al. study is discussed. Representative, atmospheric photolysis rate coefficients are calculated and a range of uncertainty estimated based on the determination of sigma(Cl(2)O(2))(lambda) in this work. Although improvements in our fundamental understanding of the photochemistry of Cl(2)O(2) are still desired, this work indicates that major revisions in current atmospheric chemical mechanisms are not required to simulate observed polar ozone depletion.

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. E-2-Hexenal and E,E-2,4-hexadienal were found to undergo rapid isomerization to produce Z-2-hexenal and a ketene-type compound (probably E-hexa-1,3-dien-1-one), respectively. Both isomerization processes were reversible with formation of the reactant slightly favoured. Values of j(E-2-hexenal)/j(NO(2)) = (1.80 +/- 0.18) x 10(-2) and j(E,E-2,4-hexadienal)/j(NO(2)) = (2.60 +/- 0.26) x 10(-2) were determined. The gas phase UV absorption cross-sections of E-2-hexenal and E,E-2,4-hexadienal were measured and used to derive effective quantum yields for photoisomerization of 0.36 +/- 0.04 for E-2-hexenal and 0.23 +/- 0.03 for E,E-2,4-hexadienal. Although photolysis appears to be an important atmospheric degradation pathway for E-2-hexenal and E,E-2,4-hexadienal, the reversible nature of the photolytic process means that gas phase reactions with OH and NO(3) radicals are ultimately responsible for the atmospheric removal of these compounds. Atmospheric photolysis of Z-3-hexenal produced CO, with a molar yield of 0.34 +/- 0.03, and 2-pentenal via a Norrish type I process. A value of j(Z-3-hexenal)/j(NO(2)) = (0.4 +/- 0.04) x 10(-2) was determined. The results suggest that photolysis is likely to be a minor atmospheric removal process for Z-3-hexenal.

Gas phase UV absorption cross-sections for a series of amides

The ultra violet absorption cross-sections of N, N-dimethyl formamide, N, N-dimethyl acetamide, N, N-dimethyl proprionamide, and 1-methyl-2-pyrrolidinone are reported. The measurements were carried out over the wavelength 210–270 nm in the temperature range 298–363 K using two different experimental systems (D 2 lamp – monochromator and D 2 lamp – diode array). This work provides the first UV cross-section measurement for these amides.

Gas phase UV absorption spectra for a series of alkyl sulfides

Absorption cross-sections have been measured for a series of alkyl sulfides between 209.0 and 235.0 nm. Measurements are reported for dimethylsulfide ((CH 3) 2S, h 6 -DMS) and its deuterated isotopomer ((CD 3) 2S, d 6 -DMS), methylethylsulfide (CH 3SC 2H 5, MES), diethylsulfide ((C 2H 5) 2S, DES), dipropylsulfide ((C 3H 7) 2S, DPS) and dibutylsulfide ((C 4H 10) 2S, DBS).

Gas phase UV absorption spectra for peracetic acid, and for acetic acid monomers and dimers

Absorption cross sections have been determined in the UV spectral region (205–350 nm) for gas phase acetic acid (CH 3C(O)OH) in its monomeric and dimeric forms (270–345 K), as well as for peracetic acid (CH 3C(O)OOH, 248 and 298 K). Analysis of acetic acid spectra recorded over a range of pressures (0.12–3.6 Torr), coupled with a prior knowledge of the dimerization equilibrium constant, allowed for the deconvolution of the monomer and dimer absorption spectra. The 298 K monomer spectrum consists of a maximum near 207 nm ( σ=1.4×10 −19 cm 2 per molecule), with monotonically decreasing cross sections at longer wavelengths. The dimer spectrum possesses a maximum located short of 205 nm, with a peak intensity about double that of the monomer. The first gas phase spectrum of peracetic acid is also reported, which is similar to a previously published solution-phase spectrum. Measurable absorption extends to 340 nm at 298 K, and it is determined that the tropospheric photolysis of this species will be a reasonably rapid process (lifetime ≈ weeks), which will play at least a minor role in its atmospheric destruction.

UV absorption spectra of HO2, CH3O2, C2H5O2, and CH3C(O)CH2O2 radicals and mechanism of the reactions of F and Cl atoms with CH3C(O)CH3

AbstractPulse radiolysis techniques were used to measure the gas phase UV absorption spectra of the title peroxy radicals over the range 215–340 nm. By scaling to σ(CH3O2)240 nm = (4.24 ± 0.27) × 10−18, the following absorption cross sections were determined: σ(HO2)240 nm = 1.29 ± 0.16, σ(C2H5O2)240 nm = 4.71 ± 0.45, σ(CH3C(O)CH2O2)240 nm = 2.03 ± 0.22, σ(CH3C(O)CH2O2)230 nm = 2.94 ± 0.29, and σ(CH3C(O)CH2O2)310 nm = 1.31 ± 0.15 (base e, units of 10−18 cm2 molecule−1). To support the UV measurements, FTIR‐smog chamber techniques were employed to investigate the reaction of F and Cl atoms with acetone. The F atom reaction proceeds via two channels: the major channel (92% ± 3%) gives CH3C(O)CH2 radicals and HF, while the minor channel (8% ± 1%) gives CH3 radicals and CH3C(O)F. The majority (>97%) of the Cl atom reaction proceeds via H atom abstraction to give CH3C(O)CH2 radicals. The results are discussed with respect to the literature data concerning the UV absorption spectra of CH3C(O)CH2O2 and other peroxy radicals. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 283–291, 2002

UV-absorption cross sections of a series of monocyclic aromatic compounds

Aromatic hydrocarbons are of increasing importance as components of urban volatile organic compounds. Besides gas-chromatographic techniques, the detection of aromatic hydrocarbons by Differential Optical Absorption Spectroscopy (DOAS) is a promising alternative technique. The absolute calibration of DOAS-instruments is straightforward: Only the absorption cross sections of the species of interest and the instrument function are required. In this work we present measurements of high-resolution gas phase UV-absorption cross section spectra of benzene (C 6H 6), toluene (C 7H 8), p-, m-, o-xylene (C 8H 10), phenol (C 6H 5OH), benzaldehyde (C 7H 6O) and the cresol isomers (C 7H 8O), p-, m- and o-cresol in the wavelength interval between 230 and 290 nm. The spectral resolution is 0.11 nm (FWHM), except for o- and m-cresol, where the resolution is 0.32 nm (FWHM). These spectra can be used to calibrate DOAS-instruments. From these absorption cross sections typical detection limits for the atmospheric measurements with Long-Path-Differential Optical Absorption Spectroscopy are calculated to be in the range of a few ppt (phenol), to a few 10 ppt (cresol, benzaldehyde, benzene, p-xylene), up to 200 ppt ( o-xylene). The attainability of these detection limits is demonstrated by measurements of atmospheric phenol and benzaldehyde.

Absorption Cross Sections of Gaseous Dimethylnitramine at Selected Wavelengths between 185 and 325 nm

Dimethylnitramine (DMNA), (CH3)2N-NO2, has been extensively studied as a simple analog expected to exhibit some of the processes important in cyclic nitramine decomposition. Although many of these studies include its gas-phase dissociation following excitation with UV radiation, a reference for its gas-phase UV absorption cross sections has not been established. Since this is an important parameter in analyzing the results of DMNA dissociation, we report our results for room-temperature DMNA. Absorption cross sections at selected wave-lengths between 185 and 325 nm have been calculated from UV absorbance spectra with the use of Beer-Lambert's law.

A flash photolysis investigation of the gas phase uv absorption spectrum and self‐reaction kinetics of the neopentylperoxy radical

AbstractThe ultraviolet absorption spectrum of the neopentylperoxy radical, (CH3)3CCH2O2 (or C5H11O2), and the kinetics of its self‐reaction have been studied in the gas phase using a flash photolysis technique. The room temperature absorption cross‐section at 250 nm was determined to be and was used to normalize the radical absorption spectrum between 210 and 300 nm. Detailed modeling of the self‐reaction system was used to interpret the transient absorption kinetic decay curves over the temperature range 228–380 K, at total pressures between 25 and 100 torr. The results are discussed in relation to previous measurements of alkylperoxy radical spectra and kinetics.

The gas phase UV absorption spectrum of CH 3O 2 radicals: A reinvestigation

Computational analyses of the effects of possible HO 2 interferences on our earlier reported results for the CH 3O 2 UV spectrum and self-reaction kinetics are presented. New absorption cross-section data for CH 3O 2 are derived from experiments in which secondary formation of HO 2 is virtually excluded. The results are discussed in terms of the literature data.

Gas phase studies of substituted methylperoxy radicals: the UV absorption spectrum and self-reaction kinetics of CH 3OCH 2O 2 — the reaction of CF 2ClO 2 with Cl atoms

A flash photolysis technique has been used to measure the gas phase UV absorption cross-sections for the methoxy-methylperoxy radical, CH 3OCH 2O 2, over the wavelength range 210 – 290 nm. Reaction (1) was then studied by kinetic absorption spectroscopy ▪ and the rate constant, defined as —d[CH 3OCH 2O 2]/d t = 2 k 1[CH 3OCH 2O 2] 2, was derived using σ CH 3OCH 2O 2 (240 nm) = (3.65 ± 0.35) × 10 −18 cm 2 molecule −1. The self-reaction rate constants were observed to be in the fall-off region over the temperature range 228 – 380 K at pressures between 25 and 800 Torr (using N 2) and were fit using the formulation developed by Troe for association reactions. Attempts at similar studies for CF 2ClO 2 radicals were hindered by the apparent secondary formation of ClO. An interpretation of the experimental observations in this radical formation system is offered which is consistent with the occurrence of a rapid reaction between CF 2ClO 2 and Cl yielding ClO ▪