A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

The surface–bulk partitioning of small saccharide and amide molecules in aqueous droplets was investigated using molecular dynamics. The air–particle interface was modeled using a 80 Å cubic water box containing a series of organic molecules and surrounded by gaseous OH radicals. The properties of the organic solutes within the interface and the water bulk were examined at a molecular level using density profiles and radial pair distribution functions. Molecules containing only polar functional groups such as urea and glucose are found predominantly in the water bulk, forming an exclusion layer near the water surface. Substitution of a single polar group by an alkyl group in sugars and amides leads to the migration of the molecule toward the interface. Within the first 2 nm from the water surface, surface-active solutes lose their rotational freedom and adopt a preferred orientation with the alkyl group pointing toward the surface. The different packing within the interface leads to different solvation shell structures and enhanced interaction between the organic molecules and absorbed OH radicals. The simulations provide quantitative information about the dimension, composition, and organization of the air–water interface as well as about the nonreactive interaction of the OH radicals with the organic solutes. It suggests that increased concentrations, preferred orientations, and decreased solvation near the air–water surface may lead to differences in reactivities between surface-active and surface-inactive molecules. The results are important to explain how heterogeneous oxidation mechanisms and kinetics within interfaces may differ from those of the bulk

Effect of Relative Humidity on the OH-Initiated Heterogeneous Oxidation of Monosaccharide Nanoparticles

The OH-initiated heterogeneous oxidation of solid methyl β-d-glucopyranoside nanoparticles (a cellulose oligomer surrogate) is studied in an atmospheric pressure gas flow reactor coupled to an aerosol mass spectrometer. The decay of the solid reactant relative concentration is measured as a function of OH exposure over a wide range of ambient relative humidities (RHs). The kinetic traces display an initial fast exponential decay followed by a slower decay. For long OH exposure, the fraction of a particle that reacts decreases from 90% at RH = 30% to 60% at RH = 20% and to 40% at RH = 10%. A computational model based on the diffusion and reaction of the radical, monosaccharide, and water is developed in order to further examine the experimental data. The model parameters and validity are discussed on the basis of previous literature data. The experimental data are consistent with a diffusion-controlled heterogeneous oxidation. These findings are discussed toward a better understanding of mass transport in semisolid organic material and their effect on chemical change, in particular during the thermal transformation of cellulosic materials to useful chemicals

Kinetics of the OH Radical Reaction with Fulvenallene from 298 to 450 K

Self-recombination and cross-reactions of large resonant stabilized hydrocarbon radicals such as fulvenallenyl (C₇H₅) are predicted to form polycyclic aromatic hydrocarbons in combustion and the interstellar medium. Although fulvenallenyl is likely to be present in these environments, large uncertainties remain about its formation mechanisms. We have investigated the formation of fulvenallenyl by reacting the OH radical with fulvenallene (C₇H₆) over the 298 to 450 K temperature range and at a pressure of 5 Torr (667 Pa). The reaction rate coefficient is found to be 8.8(±1.7) × 10^–12 cm³ s^–1 at room temperature with a negative temperature dependence that can be fit from 298 to 450 K to <i>k</i>(<i>T</i>) = 8.8(±1.7) × 10^–12 (<i>T</i>/298 K)^{−6.6(±1.1)} exp[−(8.72(±3.03) kJ mol^–1)/(<i>R</i>((1/<i>T</i>) – (1/298 K)))] cm³ s^–1. The comparison of the experimental data with calculated abstraction rate coefficients suggests that over the experimental temperature range, association of the OH radical to fulvenallene plays a significant role likely leading to a low fulvenallenyl branching fraction

Product Branching Fractions of the CH + Propene Reaction from Synchrotron Photoionization Mass Spectrometry

The CH­(X²Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C₄H₆ (<i>m</i>/<i>z</i> 54) + H. By fitting experimental photoionization spectra to measured spectra of known C₄H₆ isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05), and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to C₄H₆ (<i>m</i>/<i>z</i> 54) + D and C₄H₅D (<i>m</i>/<i>z</i> 55) + H, formed at a ratio of 0.4 (<i>m</i>/<i>z</i> 54) to 1.0 (<i>m</i>/<i>z</i> 55). The D elimination channel forms almost exclusively 1,2-butadiene (0.97 ± 0.20) whereas the H elimination channel leads to the formation of deuterated 1,3-butadiene (0.89 ± 0.18) and 1-butyne (0.11 ± 0.02); photoionization spectra of undeuterated species are used in the fitting of the measured <i>m</i>/<i>z</i> 55 (C₄H₅D) spectrum. The results are generally consistent with a CH cycloaddition mechanism to the CC bond of propene, forming 1-methylallyl followed by elimination of a H atom via several competing processes. The direct detection of 1,3-butadiene as a reaction product is an important validation of molecular weight growth schemes implicating the CH + propene reaction, for example, those reported recently for the formation of benzene in the interstellar medium (Jones, B. M. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 452−457)

Product Detection of the CH Radical Reaction with Acetaldehyde

The reaction of the methylidyne radical (CH) with acetaldehyde (CH₃CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr₃ and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH₂), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH₃), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD₃CHO) provides fine details about the reaction mechanism. The CH₂ elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C–H bond of the methyl group of acetaldehyde is likely to lead to a C₃H₅O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH₃, which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein

Isomer Specific Product Detection in the Reaction of CH with Acrolein

The products formed in the reaction between the methylidene radical (CH) and acrolein (CH₂CHCHO) are probed at 4 Torr and 298 K employing tunable vacuum-ultraviolet synchrotron light and multiplexed photoionization mass-spectrometry. The data suggest a principal exit channel of H loss from the adduct to yield C₄H₄O, accounting for (78 ± 10)% of the products. Examination of the photoionization spectra measured upon reaction of both CH and CD with acrolein reveals that the isomeric composition of the C₄H₄O product is (60 ± 12)% 1,3-butadienal and (17 ± 10)% furan. The remaining 23% of the possible C₄H₄O products cannot be accurately distinguished without more reliable photoionization spectra of the possible product isomers but most likely involves oxygenated butyne species. In addition, C₂H₂O and C₃H₄ are detected, which account for (14 ± 10)% and (8 +10, −8)% of the products, respectively. The C₂H₂O photoionization spectrum matches that of ketene and the C₃H₄ signal is composed of (24 ± 14)% allene and (76 ± 22)% propyne, with an upper limit of 8% placed on the cyclopropene contribution. The reactive potential energy surface is also investigated computationally, and specific rate coefficients are calculated with RRKM theory. These calculations predict overall branching fractions for 1,3-butadienal and furan of 27% and 12%, respectively, in agreement with the experimental results. In contrast, the calculations predict a prominent CO + 2-methylvinyl product channel that is at most a minor channel according to the experimental results. Studies with the CD radical strongly suggest that the title reaction proceeds predominantly via cycloaddition of the radical onto the CO bond of acrolein, with cycloaddition to the CC bond being the second most probable reactive mechanism

Radical–Radical Reactions in Molecular Weight Growth: The Phenyl + Propargyl Reaction

The mechanism for hydrocarbon ring growth in sooting environments is still the subject of considerable debate. The reaction of phenyl radical (C6H5) with propargyl radical (H2CCCH) provides an important prototype for radical–radical ring-growth pathways. We studied this reaction experimentally over the temperature range of 300–1000 K and pressure range of 4–10 Torr using time-resolved multiplexed photoionization mass spectrometry. We detect both the C9H8 and C9H7 + H product channels and report experimental isomer-resolved product branching fractions for the C9H8 product. We compare these experiments to theoretical kinetics predictions from a recently published study augmented by new calculations. These ab initio transition state theory-based master equation calculations employ high-quality potential energy surfaces, conventional transition state theory for the tight transition states, and direct CASPT2-based variable reaction coordinate transition state theory (VRC-TST) for the barrierless channels. At 300 K only the direct adducts from radical–radical addition are observed, with good agreement between experimental and theoretical branching fractions, supporting the VRC-TST calculations of the barrierless entrance channel. As the temperature is increased to 1000 K we observe two additional isomers, including indene, a two-ring polycyclic aromatic hydrocarbon, and a small amount of bimolecular products C9H7 + H. Our calculated branching fractions for the phenyl + propargyl reaction predict significantly less indene than observed experimentally. We present further calculations and experimental evidence that the most likely cause of this discrepancy is the contribution of H atom reactions, both H + indenyl (C9H7) recombination to indene and H-assisted isomerization that converts less stable C9H8 isomers into indene. Especially at low pressures typical of laboratory investigations, H-atom-assisted isomerization needs to be considered. Regardless, the experimental observation of indene demonstrates that the title reaction leads, either directly or indirectly, to the formation of the second ring in polycyclic aromatic hydrocarbons