Photochemistry of Simple Olefins: Chemistry of Electronic Excited States or Hot Ground State?

Collin, Guy J.

doi:10.1002/9780470133446.ch3

Cited by 37 publications

(14 citation statements)

References 127 publications

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“…Considering the studies of Borrell et al (1971) at 184.9 nm, of Collin and coworkers (Collin et al, 1979;Collin, 1988) at 163.3, and of Niedzielski and coworkers (Niedzielski et al, 1982;Naroznik and Niedzielski, 1986;Gierczak et al, 1988) at 123.6 and 147.0 nm, we independently come to the conclusion…”

Section: J-valuementioning

confidence: 87%

Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model

Vuitton

Yelle

Klippenstein

et al. 2019

Icarus

163

362

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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.

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Section: J-valuementioning

confidence: 87%

Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model

Vuitton

Yelle

Klippenstein

et al. 2019

Icarus

163

362

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“…In view of the pyramidal distortion on surrounding the R3s/pp* CI, the momentum model would predict that from the Rydberg state there is more carbene formation than from the pp* state. In fact this is experimentally well known [119][120][121][122][123]. The previous interpretation [119] invoked the positively charged core of the Rydberg state.…”

Section: Predistortion By Momenta Generated In Preceding Statesmentioning

confidence: 86%

“…They also found that at least part of the ring cleavage takes place via intermediate formation of cyclopropyl carbenes (that also deliver methylene cyclopropanes as end products) by ring contraction (alkyl 1,2-migration, Scheme 13) [118] (it is also conceivable that carbene formation and ring splitting result from the same conical intersection). This carbene path strongly reminds of the enhanced carbene formation from olefins (Scheme 13), if a Rydberg state is excited instead of the pp* state [119][120][121][122][123]. It can be attributed to the temporary pyramidalization (Scheme 12) on passing around the Rydberg/pp* CI, as explained in the following.…”

Section: Predistortion By Momenta Generated In Preceding Statesmentioning

confidence: 91%

“…It can be attributed to the temporary pyramidalization (Scheme 12) on passing around the Rydberg/pp* CI, as explained in the following. Simple alkenes not only isomerize their double bond on irradiation but also form carbenes (by hydrogen or alkyl 1,2-migration, Scheme 13) and consecutive products and, where possible, undergo also sigmatropic hydrogen or alkyl 1,3-migration (reviews [119][120][121][122][123]). In ethylene, the former two reactions pass through the same CI [72,109] or branch just above it (see, e.g., [124,125]; in cyclic alkenes, a common CI was suggested [65,71]).…”

Section: Predistortion By Momenta Generated In Preceding Statesmentioning

confidence: 99%

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Predistortion amplified in the excited state

Fuß¹

2015

Journal of Photochemistry and Photobiology A: Chemistry

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“…the discussion in ref. 58: in contrast to the latter, a buffer gas suppresses them partially, and in the condensed phase they are typically only detectable in traces; only with small molecules such as ethylene they still contribute substantially to the products (H and H 2 elimination in this example 58 ). The crucial parameters are the initial temperature and the rate of cooling compared to the reaction rate.…”

Section: Reactions In the Hot Ground Sate: Expectations And Some Obsementioning

confidence: 91%

Previtamin D:Z–Ephotoisomerizationviaa Hula-twist conical intersection

Fuß¹

2019

Phys. Chem. Chem. Phys.

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On photoisomerization of previtamin D -a steroid Z-triene -produced in situ by ring opening of 7-dehydrocholesterol in a cold matrix, it was found in A. M. Müller et al. [Angew. Chem., Int. Ed., 1998, 37, 505-507] that the product (tachysterol) had rotated not only its central double bond but also an adjacent single bond. This is called a Hula twist (HT) due to the alternative description, in which it is just one central CH group that rotates. It was pointed out that the results directly support the calculated molecular structure at a conical intersection, which mediates the Z-E isomerization of polyenes. With a more sophisticated technique, Saltiel et al. (J. Phys. Chem. Lett., 2013, 4, 716-721) confirmed this tachysterol rotamer as the main product but found two additional conformers. They believed to have seen also three previtamin D conformers, suggested to be a result of hot-ground-state reactions from the primary rotamer, and interpreted all tachysterol products to be a result of a double-bond twist (DBT), not a HT. On the basis of published circular dichroism data and consideration of other reactions, it is here shown that under these conditions hot-ground-state reactions are unimportant or even negligible and that there is practically only a single conformer of previtamin D after ring opening. All products can be easily understood on the basis of an HT-type conical intersection, which is thus further supported. Invoking a published pretwist model even rationalizes product ratios. The two twists in HT are concerted. Furthermore HT is fully consistent with the NEER principle (nonequilibration of excited rotamers) and even offers additional possibilities for conformer control.Scheme 1 Structures of 7-dehydrocholesterol (Dehy), lumisterol (Lumi), previtamin D (Pre) and tachysterol (Tachy). c and t denote s-cis and s-trans arrangement around the 5-6 and 7-8 single bonds. Obviously the molecules are not planar; the chromophores in Dehy and its primary ring-opening product cZc-Pre form a left-handed screw ((À)cZ(À)c-Pre), whereas in Lumi and its cZc-Pre product ((+)cZ(+)c-Pre) they correspond to a right-handed screw.Perspective PCCP

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Photochemistry of Simple Olefins: Chemistry of Electronic Excited States or Hot Ground State?

Cited by 37 publications

References 127 publications

Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model

Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model

Predistortion amplified in the excited state

Previtamin D:Z–Ephotoisomerizationviaa Hula-twist conical intersection

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