Comment on "Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature" by W. Siebrand, Z. Smedarchina, E. Martínez-Núñez and A. Fernández-Ramos, Phys. Chem. Chem. Phys., 2016, 18, 22712.

The article "Methanol dimer formation drastically enhances hydrogen abstraction from methanol by OH at low temperature" proposes a dimer mediated mechanism in order to explain the large low temperature rate coefficients for the OH + methanol reaction measured by several groups. It is demonstrated here theoretically that under the conditions of these low temperature experiments, there are insufficient dimers formed for the proposed new mechanism to apply. Experimental evidence is also presented to show that dimerization of the methanol reagent does not influence the rate coefficients reported under the conditions of methanol concentration used for the kinetics studies. It is also emphasised that the low temperature experiments have been performed using both the Laval nozzle expansion and flow-tube methods, with good agreement found for the rate coefficients measured using these two distinct techniques.

Kinetics and mechanism of the tropospheric reaction of 3-hydroxy-3-methyl-2-butanone with Cl atoms.

The relative rate coefficient for the gas-phase reaction of 3-hydroxy-3-methyl-2-butanone (3H3M2B) with Cl atoms was determined under atmospheric conditions (298 ± 2 K, 720 ± 2 Torr). The products of the reaction were identified and quantified. This work provides the first kinetic and mechanistic determinations of the gas-phase reaction of Cl atoms with 3H3M2B. The rate measurements and the products studies were performed in two simulation chambers coupled to the gas chromatography-mass spectrometer (GC-MS) and the Fourier transform infrared (FTIR) techniques, respectively. The obtained average rate coefficient was (1.13 ± 0.17) × 10(-10) cm(3) molecule(-1) s(-1) using propene and 1,3-butadiene as reference compounds. The major primary reaction products observed in this study were (with % molar yields): acetic acid (42.6 ± 4.8) and 2,3-butanedione (17.2 ± 2.3). Results and mechanism are discussed in terms of the structure-reactivity relationship and compared with the reported reactivity with the other atmospheric oxidants. The atmospheric implications derived from this study are discussed as well.

UV absorption cross-sections of a series of dimethylbenzaldehydes.

The ultraviolet absorption cross-sections of 2,4-, 2,5-, 2,6-, 3,4- and 3,5- dimethylbenzaldehydes are reported in the wavelength range 240-320 nm. The measurements were carried out in the temperature range 318-363 K using two different experimental systems (D 2 lamp-monochromator and D 2 lamp-diode array). The absorption spectra of the five aldehydes have been found to exhibit relatively high absorption cross-sections in the region of the tropospheric interest with maxima around 290 nm. This work provides the first UV cross-section measurements for these aromatic aldehydes. The obtained cross-section values enable us to estimate the tropospheric photolysis lifetimes of these compounds. The results suggest that photolysis could be an important removal process for these species in the troposphere.

Study of benzylperoxy radical using laser photolysis: ultraviolet spectrum, self-reaction, and reaction with HO2 kinetics.

The ultraviolet absorption spectrum of benzylperoxy radical and the kinetics of the reactions 2C(6)H(5)CH(2)O(2) --> products (I) and C(6)H(5)CH(2)O(2) + HO(2) --> products (II) are studied. Experiments are carried out using the laser photolysis technique with time-resolved UV-visible absorption spectroscopy over the temperature range 298-353 K and the pressure range 50-200 Torr. The UV spectrum is determined relative to the known cross section of the ethylperoxy radical C(2)H(5)O(2) at 250 nm. Using factor analysis, the spectrum obtained is refined and the concentrations of the main absorbing species are extracted. The kinetic parameters are determined by analyzing and simulating the temporal profiles of the species concentrations and the experimental optical densities in the spectral region 220-280 nm. These are obtained using the recent UV spectra of the absorbing species existing in our mechanism. The Arrhenius expressions for reactions I and II are (cm(3).molecule(-1).s(-1)) k(I) = 2.50 x 10(-14)e(1562/)(T) and k(II) = 5.70 x 10(-14)e(1649/)(T). Our results are discussed and compared to literature data.