Multiphase chemistry in the troposphere: It all starts … and ends … with gases

A kinetic study of the reactions of atomic bromine with the trimethylbenzenes

This work presents the results of kinetic measurements for the reactions of atomic bromine with the three isomers of trimethylbenzene at temperatures from 295 K to 354 K and at pressures close to atmospheric. The atomic bromine was produced by photolysis of Br2 in a thermostated Pyrex chamber as described in our previous work. The reactants were present as dilute mixtures in argon with Br2 in sufficient excess to scavenge the free radical products of the initiation step. Chemical analysis was by gas chromatography and the rate coefficients were calculated from the chromatographic results using the relative rate method. The experimentally measured temperature dependence of the rate coefficients for the reactions of Br with the trimethylbenzenes is given in the units cm3 molec.-1 s-1 by: 1,2,3-trimethylbenzene + Br, log10(k) = -9.57 ± 0.43 - (996 ± 134)/T; 1,2,4-trimethylbenzene + Br, log10(k) = -9.74 ± 0.29 - (968 ± 91)/T; 1,3,5-trimethylbenzene + Br, log10(k) = -9.87 ± 0.26 - (1079 ± 85)/T. The enthalpies and entropies of activation for these reactions are compared to those for the reactions of Br with toluene and the xylenes.

Reduction and Photoreduction of NO2 in Humic Acid Films as a Source of HONO, ClNO, N2O, NO X , and Organic Nitrogen

Atmospheric nitrous acid (HONO), a trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its daytime sources and sinks. HONO is known to accumulate during nighttime and undergo rapid photodissociation during the day to form NO and highly reactive OH radical, making it important to have accurate atmospheric HONO estimations. Despite its rapid photolysis, recent field observations have found quasi-steady-state concentrations of HONO at midday, suggesting photolytic HONO formation pathways to replenish daytime atmospheric HONO. Recent studies suggest that the presence of complex organic photosensitizers in atmospheric aerosols converts atmospheric NO₂ into HONO. To better understand the effect of environmental photosensitizers in daytime mechanisms of HONO formation, we present here laboratory studies on the heterogeneous photolytic reduction of NO₂ by humic acid films, a proxy for organic chromophoric compounds. The effect of pH and Cl^- in the photosensitized formation of HONO and other nitrogen-containing gases is also investigated. A dual Fourier transform infrared (FTIR) system is utilized to simultaneously perform in situ analysis of condensed-phase reactants and gas-phase products. We find that the rate of HONO formation is faster at lower pHs. Nitrogen incorporation in the complex organic chromophore is observed, suggesting a competing pathway that results in suppressed daytime formation of nitrogenous gases. Significantly, the presence of chloride ions also leads to the organic-mediated photolytic formation of nitrosyl chloride (ClNO), a known precursor of HONO. Overall, this work shows that organic acid photosensitizers can reduce adsorbed NO₂ to form HONO, ClNO, and NO while simultaneously incorporating nitrogen into the organic chromophores present in aerosol.

Environment Effects on X-Ray Absorption Spectra With Quantum Embedded Real-Time Time-Dependent Density Functional Theory Approaches

In this work we implement the real-time time-dependent block-orthogonalized Manby-Miller embedding (rt-BOMME) approach alongside our previously developed real-time frozen density embedding time-dependent density functional theory (rt-TDDFT-in-DFT FDE) code, and investigate these methods’ performance in reproducing X-ray absorption spectra (XAS) obtained with standard rt-TDDFT simulations, for model systems comprised of solvated fluoride and chloride ions ([X@(H2O)8−, X = F, Cl). We observe that for ground-state quantities such as core orbital energies, the BOMME approach shows significantly better agreement with supermolecular results than FDE for the strongly interacting fluoride system, while for chloride the two embedding approaches show more similar results. For the excited states, we see that while FDE (constrained not to have the environment densities relaxed in the ground state) is in good agreement with the reference calculations for the region around the K and L1 edges, and is capable of reproducing the splitting of the 1s1 (n + 1)p1 final states (n + 1 being the lowest virtual p orbital of the halides), it by and large fails to properly reproduce the 1s1 (n + 2)p1 states and misses the electronic states arising from excitation to orbitals with important contributions from the solvent. The BOMME results, on the other hand, provide a faithful qualitative representation of the spectra in all energy regions considered, though its intrinsic approximation of employing a lower-accuracy exchange-correlation functional for the environment induces non-negligible shifts in peak positions for the excitations from the halide to the environment. Our results thus confirm that QM/QM embedding approaches are viable alternatives to standard real-time simulations of X-ray absorption spectra of species in complex or confined environments.

The kinetics of the reactions of Br atoms with the xylenes: an experimental and theoretical study

This work reports the temperature dependence of the rate coefficients for the reactions of atomic bromine with the xylenes that are determined experimentally and theoretically. The experiments were carried out in a Pyrex chamber equipped with fluorescent lamps to measure the rate coefficients at temperatures from 295 K to 346 K. Experiments were made at several concentrations of oxygen to assess its potential kinetic role under atmospheric conditions and to validate comparison of our rate coefficients with those obtained by others using air as the diluent. Br₂ was used to generate Br atoms photolytically. The relative rate method was used to obtain the rate coefficients for the reactions of Br atoms with the xylenes. The reactions of Br with both toluene and diethyl ether (DEE) were used as reference reactions where the loss of the organic reactants was measured by gas chromatography. The rate coefficient for the reaction of Br with diethyl ether was also measured in the same way over the same temperature range with toluene as the reference reactant. The rate coefficients were independent of the concentration of O₂. The experimentally determined temperature dependence of the rate coefficients of these reactions can be given in the units cm³ molecule^-1 s^-1 by: o-xylene + Br, log₁₀(k) = (-10.03 ± 0.35) - (921 ± 110)/T; m-xylene + Br, log₁₀(k) = (-10.78 ± 0.09) - (787 ± 92/T); p-xylene + Br, log₁₀(k) = (-9.98 ± 0.39) - (956 ± 121)/T; diethyl ether + Br, log₁₀(k) = (-7.69 ± 0.55) - (1700 ± 180)/T). This leads to the following rate coefficients, in the units of cm³ molecule^-1 s^-1, based on our experimental measurements: o-xylene + Br, k(298 K) = 7.53 × 10^-14; m-xylene + Br, k(298 K) = 3.77 × 10^-14; p-xylene + Br, k(298 K) = 6.43 × 10^-14; diethyl ether + Br, k(298 K) = 4.02 × 10^-14. Various ab initio methods including G3, G4, CCSD(T)/cc-pV(D,T)Z//MP2/aug-cc-pVDZ and CCSD(T)/cc-pV(D,T)Z//B3LYP/cc-pVTZ levels of theory were employed to gain detailed information about the kinetics as well as the thermochemical quantities. Among the ab initio methods, the G4 method performed remarkably well in describing the kinetics and thermochemistry of the xylenes + Br reaction system. Our theoretical calculations revealed that the reaction of Br atoms with the xylenes proceeds via a complex forming mechanism in an overall endothermic reaction. The rate determining step is the intramolecular rearrangement of the pre-reactive complex leading to the post-reactive complex. After lowering the relative energy of the corresponding transition state by less than 1.5 kJ mol^-1 for this step in the reaction of each of the xylenes with Br, the calculated rate coefficients are in very good agreement with the experimental data.