We present an overview of the current status of transition-state
theory and its generalizations. We emphasize
(i) recent improvements in available methodology for calculations on
complex systems, including the interface
with electronic structure theory, (ii) progress in the theory and
application of transition-state theory to condensed-phase reactions, and (iii) insight into the relation of transition-state
theory to accurate quantum dynamics and
tests of its accuracy via comparisons with both experimental and other
theoretical dynamical approximations.
We present a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini-Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini-Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to about 100 u. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N 2 as well as new photodissociation branching ratios for CH 4 and C 2 H 2 . Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions.Main reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan's atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH + ) in the upper ionosphere, (ii) the low mass negative ion densities (CN -, C 3 N -/C 4 H -) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO 2 , H 2 O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.
Isoprene has the highest emission into Earth’s atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO2 and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.
Ozonolysis
of isoprene, one of the most abundant volatile organic
compounds in the earth’s atmosphere, generates the four-carbon
unsaturated methacrolein oxide (MACR-oxide) Criegee intermediate.
The first laboratory synthesis and direct detection of MACR-oxide
is achieved through reaction of photolytically generated, resonance-stabilized
iodoalkene radicals with oxygen. MACR-oxide is characterized on its
first π* ← π electronic transition using a ground-state
depletion method. MACR-oxide exhibits a broad UV–visible spectrum
peaked at 380 nm with weak oscillatory structure at long wavelengths
ascribed to vibrational resonances. Complementary theory predicts
two strong π* ← π transitions arising from extended
conjugation across MACR-oxide with overlapping contributions from
its four conformers. Electronic promotion to the 11ππ*
state agrees well with experiment, and results in nonadiabatic coupling
and prompt release of O 1D products observed as anisotropic
velocity-map images. This UV–visible detection scheme will
enable study of its unimolecular and bimolecular reactions under thermal
conditions of relevance to the atmosphere.
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