Photochemistry and Photophysics of n-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal: Experimental and Theoretical Study

Tropospheric Photochemistry of 2-Butenedial: Role of the Triplet States, CO and Acrolein Formation, and the Experimentally Unidentified Carbonyl Compound-Theoretical Study

Solar irradiation of 2-butenedial in the lower troposphere mainly produces isomeric ketene-enol (a key intermediate product), furanones, and maleic anhydride, the formation pathways of which were investigated in a previous study. The other main products were carbon monoxide and an experimentally unidentified carbonyl compound. This was the subject of the present study. The oxidative reaction mechanisms were studied using DFT calculations. Water intervention is found essential. Its addition and subsequent water-assisted isomerizations (an ene-gem-diol/enol and a carboxylic acid/enol form), followed by cyclization, lead to an interesting cyclic carbonyl compound, but this pathway appears to be rather energy demanding. An alternative implies water cooperation in a ketene-enol + carboxylic acid/enol addition that gives the relevant anhydride. The anhydride is proposed as a candidate for the experimentally unidentified carbonyl product. Regarding CO and acrolein formation, the role of the triplet states, as defined by the probability of intersystem crossing from the excited singlet state S1 to T2 and T1, is discussed. The T1 photolysis pathway connecting butenedial to propenal + CO was then defined.

Mechanism of the Photochemical Isomerization and Oxidation of 2-Butenedial: A Theoretical Study

Under tropospheric conditions, 2-butenedial is photochemically removed to produce secondary organic aerosol. Upon solar irradiation in the lower troposphere, the main photochemical products are ketene-enol (a key intermediate product), furanones, and maleic anhydride. The oxidative reaction mechanism was studied using the multireference method CASSCF to explore the hypersurface of the two most accessible singlet excited states, and by DFT for the ground state. Photoisomerization of 2-butenedial in the first excited state directly produces ground state ketene-enol upon nonradiative relaxation. From this intermediate, furan-2-ol and successively 3H-furan-2-one and 5H-furan-2-one are formed. The cooperative effect of two water molecules is essential to catalyze the cyclization of ketene-enol to furan-2-ol, followed by hydrogen transfers to furanones. Two water molecules are also necessary to form maleic anhydride from furan-2-ol. For this last reaction, in which one extra oxygen must be acquired, we hypothesize a mechanism with singlet oxygen as the oxidant.

Absolute Photoionization Cross Section of the Simplest Enol, Vinyl Alcohol

The absolute photoionization cross section of vinyl alcohol was determined by multiplexed photoionization mass spectrometry of the Norrish type II photodissociation of butanal at 308 nm. The measured cross sections at 10.005 and 10.205 eV are 7.5 ± 1.9 and 8.1 ± 1.9 MB, respectively. A higher signal-to-noise ratio photoionization spectrum of vinyl alcohol was recorded via the pyrolysis of 2-chloroethanol and scaled to the absolute cross sections measured using the Norrish type II method. From a comparison of our spectrum with previously reported photoelectron spectra we conclude that vinyl alcohol is mainly ionized by direct ionization in the energy range of 9-9.6 eV, whereas autoionization is responsible for the steady rise in the photoionization spectrum above the end of the Franck-Condon envelope at 9.9 eV.

Infrared Spectra of Gas-Phase 1- and 2-Propenol Isomers

Fourier transform infrared spectra of isolated 1-propenol and 2-propenol in the gas-phase have been collected in the range of 900-3800 cm^-1, and the absolute infrared absorption cross sections reported for the first time. Both cis and trans isomers of 1-propenol were observed with the trans isomer in greater abundance. Syn and anti conformers of both 1- and 2-propenol were also observed, with abundance consistent with thermal population. The FTIR spectrum of the smaller ethenol (vinyl alcohol) was used as a benchmark for our computational results. As a consequence, its spectrum has been partially reassigned resulting in the first report of the anti-ethenol conformer. Electronic structure calculations were used to support our experimental results and assign vibrational modes for the most abundant isomers, syn-trans-1-propenol and syn-2-propenol.

Photochemical Reactions of Cyclohexanone: Mechanisms and Dynamics

Photochemistry of carbonyl compounds is of major importance in atmospheric and organic chemistry. The photochemistry of cyclohexanone is studied here using on-the-fly molecular dynamics simulations on a semiempirical multireference configuration interaction potential-energy surface to predict the distribution of photoproducts and time scales for their formation. Rich photochemistry is predicted to occur on a picosecond time scale following the photoexcitation of cyclohexanone to the first singlet excited state. The main findings include: (1) Reaction channels found experimentally are confirmed by the theoretical simulations, and a new reaction channel is predicted. (2) The majority (87%) of the reactive trajectories start with a ring opening via C-Cα bond cleavage, supporting observations of previous studies. (3) Mechanistic details, time scales, and yields are predicted for all reaction channels. These benchmark results shed light on the photochemistry of isolated carbonyl compounds in the atmosphere and can be extended in the future to photochemistry of more complex atmospherically relevant carbonyl compounds in both gaseous and condensed-phase environments.

Investigation of the Gas-Phase Photolysis and Temperature-Dependent OH Reaction Kinetics of 4-Hydroxy-2-butanone

Hydroxyketones are key secondary reaction products in the atmospheric oxidation of volatile organic compounds (VOCs). The fate of these oxygenated VOCs is however poorly understood and scarcely taken into account in atmospheric chemistry modeling. In this work, a combined investigation of the photolysis and temperature-dependent OH radical reaction of 4-hydroxy-2-butanone (4H2B) is presented. The objective was to evaluate the importance of the photolysis process relative to OH oxidation in the atmospheric degradation of 4H2B. A photolysis lifetime of about 26 days was estimated with an effective quantum yield of 0.08. For the first time, the occurrence of a Norrish II mechanism was hypothesized following the observation of acetone among photolysis products. The OH reaction rate coefficient follows the Arrhenius trend (280-358 K) and could be modeled through the following expression: k4H2B(T) = (1.26 ± 0.40) × 10(-12) × exp((398 ± 87)/T) in cm(3) molecule(-1) s(-1). An atmospheric lifetime of 2.4 days regarding the OH + 4H2B reaction was evaluated, indicating that OH oxidation is by far the major degradation channel. The present work underlines the need for further studies on the atmospheric fate of oxygenated VOCs.

Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics

Semi-empirical on-the-fly dynamics simulations reveal the importance of cross-molecular reactions in the photochemistry of aldehyde clusters.