Photochemistry and photophysics of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal: experimental and theoretical study.

Dilute mixtures of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal in synthetic air, different N(2)/O(2) mixtures, and pure nitrogen (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K. The main photooxidation products were ethene (n-butanal), propene (3-methylbutanal) or i-butene (3,3-dimethylbutanal), CO, vinylalcohol, and ethanal. The photolysis rates and the absolute quantum yields were found to be dependent on the total pressure of synthetic air but not of nitrogen. At 100 Torr, the total quantum yield Φ(100) = 0.45 ± 0.01 and 0.49 ± 0.07, whereas at 700 Torr, Φ(700) = 0.31 ± 0.01 and 0.36 ± 0.03 for 3-methylbutanal and 3,3-dimethybutanal, respectively. Quantum yield values for n-butanal were reported earlier by Tadic et al. (J. Photochem. Photobiol. A2001143, 169-179) to be Φ(100) = 0.48 ± 0.02 and Φ(700) = 0.32 ± 0.01. Two decomposition channels were identified: the radical channel RCHO → R + HCO (Norrish type I) and the molecular channel CH(3)CH(CH(3))CH(2)CHO → CH(2)CHCH(3) + CH(2)═CHOH or CH(3)C(CH(3))(2)CH(2)CHO → CHC(CH(3))CH(3) + CH(2)═CHOH, (Norrish type II) having the absolute quantum yields of 0.123 and 0.119 for 3-methybutanal and 0.071 and 0.199 for 3,3-dimethylbutanal at 700 Torr of synthetic air. The product ethenol CH(2)═CHOH tautomerizes to ethanal. We have performed ab initio and density functional quantum (DFT) chemical computations of both type I and type II processes starting from the singlet and triplet excited states. We conclude that the Norrish type I dissociation produces radicals from both singlet and triplet excited states, while Norrish type II dissociation is a two-step process starting from the triplet excited state, but is a concerted process from the singlet state.

Photooxidation of n-octanal in air: experimental and theoretical study.

Dilute mixtures of n-octanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K. The main photooxidation products were 1-hexene, CO, vinyl alcohol, and acetaldehyde. The photolysis rates and the absolute quantum yields were found to be slightly dependent on the total pressure. At 100 Torr, Φ(100) = 0.41 ± 0.06, whereas at 700 Torr the total quantum yield was Φ(700) = 0.32 ± 0.02. Two decomposition channels were identified: the radical channel C(7)H(15)CHO → C(7)H(15) + HCO and the molecular channel C(7)H(15)CHO → C(6)H(12) + CH(2)═CHOH, having absolute quantum yields of 0.022 and 0.108 at 700 Torr. The product CH(2)═CHOH tautomerizes to acetaldehyde. Carbon balance data lower than unities suggest the existence of unidentified decomposition channel(s) which substantially contributes to the photolysis. On the basis of experimental and theoretical evidence, n-octanal photolysis predominantly proceeds to form Norrish type II products as the major ones.

Photolysis of acetaldehyde in air: CH4, CO and CO2 quantum yields.

The quantum yields for CO and CH(4), formed in the photolysis of about 200 µmol mol(-1) of acetaldehyde in air at atmospheric pressure, were determined at 15 wavelengths in the range: 250-330 nm. The quantum yields for CO(2) were determined at nine wavelengths in the range: 250-315 nm. The products are mainly assigned to three primary processes: I) CH(3)CHO*→CH(3)+HCO, II) CH(3)CHO*→CH(4)+CO, III) CH(3)CHO*→CH(3)CO+H, with dissociation occurring from the initially populated S(1) singlet state of acetaldehyde. The pressure dependence of the quantum yields at wavelengths of 270, 304.4, and 313 nm exhibits a Stern-Volmer behavior in all cases. Collisional quenching is shown to deactivate the singlet state S(1) by pressure-induced intersystem crossing to the neighboring triplet state, which is subsequently rapidly quenched by oxygen. Quenching coefficients as a function of excitation energy are obtained by comparison with literature data; the wavelength dependence of individual primary quantum yields is also derived. CO(2) quantum yields at 304.4 and 313 nm were found to increase with rising CH(3)CHO concentrations due to secondary reactions. The reaction mechanism responsible for this effect was explored by means of computer simulations.

The gas-phase ozonolysis of beta-caryophyllene (C(15)H(24)). Part I: an experimental study.

The gas phase reaction of ozone with beta-caryophyllene was investigated in a static glass reactor at 750 Torr and 296 K under various experimental conditions. The reactants and gas phase products were monitored by FTIR-spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Aerosol formation was monitored with a scanning mobility particle sizer (SMPS) and particulate products analysed by liquid chromatography/mass spectrometry (HPLC-MS). The different reactivity of the two double bonds in beta-caryophyllene was probed by experiments with different ratios of reactants. An average rate coefficient at 295 K for the first-generation products was determined as 1.1 x 10(-16) cm(3) molecule(-1) s(-1). Using cyclohexane as scavenger, an OH-radical yield of (10.4 +/- 2.3)% was determined for the ozonolysis of the more reactive internal double bond, whereas the average OH-radical yield for the ozonolysis of the first-generation products was found to be (16.4 +/- 3.6)%. Measured gas phase products are CO, CO(2) and HCHO with average yields of (2.0 +/- 1.8)%, (3.8 +/- 2.8)% and (7.7 +/- 4.0)%, respectively for the more reactive internal double bond and (5.5 +/- 4.8)%, (8.2 +/- 2.8)% and (60 +/- 6)%, respectively from ozonolysis of the less reactive double bond of the first-generation products. The residual FTIR spectra indicate the formation of an internal secondary ozonide of beta-caryophyllene. From experiments using HCOOH as a Criegee intermediate (CI) scavenger, it was concluded that at least 60% of the formed CI are collisionally stabilized. The aerosol yield in the ozonolysis of beta-caryophyllene was estimated from the measured particle size distributions. In the absence of a CI scavenger the yield ranged between 5 and 24%, depending on the aerosol mass. The yield increases with addition of water vapour or with higher concentrations of formic acid. In the presence of HCHO, lower aerosol yields were observed. This suggests that HCOOH adds to a Criegee intermediate to form a low-volatility compound responsible for aerosol formation. The underlying reaction mechanisms are discussed and compared with the results from the accompanying theoretical paper.

Determination of higher carboxylic acids in snow samples using solid-phase extraction and LC/MS-TOF.

The objective of this work was to develop a method to determine the concentrations of higher organic acids in snow samples. The target species are the homologous aliphatic alpha,omega-dicarboxylic acids from C(5) to C(13), pinonic acid, pinic acid and phthalic acid. A preconcentration procedure utilizing solid phase extraction was developed and optimized using solutions of authentic standards. The influences of different parameters such as flow rate during extraction and the concentration of the eluent on the efficiency of the extraction procedure were investigated. The compounds of interest were separated by HPLC and detected by a quadrupole time-of-flight mass spectrometer (qTOF-MS). The recovery rate (extraction efficiency) of the extraction procedure was found to vary between 41% for tridecanedioic acid and 102% for adipic acid. The limits of detection were determined for all compounds and were between 0.9 nmol/L (dodecanedioic acid) and 29.5 nmol/L (pinonic acid). An exception is pinic acid, for which a considerably higher detection limit of 103.9 nmol/L was calculated. Snow samples were collected in December 2006 and January 2007 at the Fee glacier (Switzerland) from locations at heights from 3056 to 3580 m asl and from different depths within the snow layer. In total, the analysis of 61 single snow samples was performed, and the following compounds could be quantified: homologous aliphatic alpha,omega-dicarboxylic acids with 5-12 carbon atoms and phthalic acid. Tridecanedioic acid, pinonic and pinic acid were identified in the samples but were not quantified due to their low concentrations. The three most abundant acids found in the molten snow samples were glutaric acid (C(5)-di; 3.90 nmol/L), adipic acid (C(6)-di; 3.35 nmol/L) and phthalic acid (Ph; 3.04 nmol/L).

Mass spectrometric characterization of 4-oxopentanoic acid and gas-phase ion fragmentation mechanisms studied using a triple quadrupole and time-of-flight analyzer hybrid system and density functional theory.

4-Oxopentanoic acid was characterized experimentally by electrospray ionization using a triple quadrupole and time-of-flight analyzer hybrid system. This compound was chosen as a model substance for small organic compounds bearing an acetyl and a carboxyl group. Collision-induced dissociation experiments at different activation energies were performed to elucidate possible fragmentation pathways. These pathways were also studied on the theoretical level using density functional theory (DFT) B3LYP/6-311++G(3df,3pd)//B3LYP/6-31+G(d)+ZPVE calculations. CO2 ejection from the [M-H](-) anion of 4-oxopentanoic acid was observed and the fragmentation pathway studied by DFT reveals a new concerted mechanism for CO2 elimination accompanied by an intramolecular proton transfer within a pentagonal transition state structure. Successive elimination of water and CO from the [M-H](-) anion of 4-oxopentanoic acid was also observed. A rearrangement in the primary deprotonated ketene anion produced after water elimination was found on the theoretical level and leads to CO elimination from the primary product anion [M-H-H2O](-). Energy diagrams along the reaction coordinates of the fragmentation pathways are presented and discussed in detail. Mulliken charge distributions of some important structures are presented.

Mass spectrometric characterization of beta-caryophyllene ozonolysis products in the aerosol studied using an electrospray triple quadrupole and time-of-flight analyzer hybrid system and density functional theory.

The components of the organic aerosol formed due to gas-phase beta-caryophyllene ozonolysis were characterized by the use of a triple quadrupole and time-of-flight analyzer hybrid system coupled to an electrospray ionization source operated in the negative ion mode. A reversed-phase high-performance liquid chromatography (HPLC) column was used to achieve chromatographic separations at neutral pH which has been proved to induce ionization of organic compounds bearing aldehyde moieties. In addition to the detected oxo- and dicarboxylic acids, isomeric oxidation products, which bear multi-functional groups such as aldehyde, carbonyl and hydroxyl groups, could be differentiated by examining their corresponding collision-induced dissociation (CID) fragmentation pathways. Proposed fragmentation mechanisms were drawn for the experimentally observed fragmentation pathways in all the CID experiments. Cyclic oxidation products could also be discerned and their fragmentation behaviour under low energy collisional conditions was studied in detail. Gas-phase deprotonation potentials were calculated by the use of DFT B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d) + ZPVE to estimate the most thermodynamically favourable deprotonation site for efficient negative ion formation in the ion source. The optimized gas-phase geometries for the most prominent oxidation products reveal a strong intramolecular interaction between the upper and lower C4 carbon chains, which are formed after the decomposition of the primary ozonide generated by ozone attack of the reactive endocyclic C==C bond.

Benzene, alkylated benzenes, chlorinated hydrocarbons and monoterpenes in snow/ice at Jungfraujoch (46.6 degrees N, 8.0 degrees E) during CLACE 4 and 5.

Benzene, alkylated benzenes, chlorinated hydrocarbons and monoterpenes were measured in snow/ice collected directly in-cloud at Jungfraujoch (3580 m asl) in February and March 2005 and 2006 during the CLoud and Aerosol Characterization Experiments CLACE 4 and CLACE 5. Melted snow/ice samples were analyzed by headspace-solid-phase-dynamic-extraction (HS-SPDE) followed by gas chromatography/mass spectrometry (GC/MS). Generally, there was a tendency in the results that higher concentrations were found after longer precipitation-free periods, suggesting that higher concentrations in snow/ice may be caused by the washout effect of precipitation. High concentration variations in snow/ice samples taken at the same time at the same place highlight the heterogeneous nature of snow/ice. Air concentrations calculated by scavenging ratios and measured snow/ice values markedly exceed the typically reported concentrations of benzene and alkylbenzenes in air (Li Y, Campana M, Reimann S, Schaub KS, Staehlin J, Peter T. Hydrocarbon concentrations at the alpine mountain sites Jungfraujoch and Arosa. Atmos Environ 2005;39:1113-27). This argues for an efficient snow/ice scavenging of those compounds from the atmosphere during precipitation formation.