Reactions of O(³P) with Alkenes:  H, CH₂CHO, CO, and OH Channels

Abstract

The H, CH 2 CHO, CO, and OH products of the reaction of O( 3 P) atom with alkenes were studied by laserinduced fluorescence (LIF) under single-collision conditions. The average kinetic energies of the H atoms were 10-12 kcal/mol. The CO and OH rotational state populations were characterized by near room temperature Boltzmann distributions. The relative LIF intensities of the various products provide vivid proof of the following mechanism for the reaction of O( 3 P) atoms with molecules of the form R′RCdCH 2 , where R′ and R are H or an alkyl group. The O atom attaches itself to the less substituted carbon atom forming a triplet ketocarbene. There is a barrier to the release of an H atom, and the rate of release must compete with the rate of intersystem crossing. If an H atom is not released, following the intersystem crossing an H atom migrates to the adjacent C atom forming an energized aldehyde, R′RCH-CHdO. The aldehyde dissociates unimolecularly forming the pair of radicals R′ • and the substituted vinoxy • RCH-CHdO or the pair R′RCH and HCO. Some of the latter have enough internal energy to dissociate to H and CO. In a side reaction O( 3 P) abstracts H atoms but only from allylic C-H bonds. The most remarkable observation is that chemical reactions that do not involve the side chains such as release of H atoms or breakup of HCO depend sensitively on the length of these chains.

A Theoretical Study of the Reaction of O(3P) with Isobutene

“…7 There are several reaction paths, which have lower barriers, that compete with the dissociation path of 1 IM2 to CO or HCO, which explains why the yield of CO or HCO is very small. 6,8,9 The decomposition of 3 IM2 to H and (CH 3 ) 2 CCHO has a much higher barrier than that of the intersystem crossing, which is the lowest among all paths of 3 IM2 in the triplet state. This supports the experimental findings that the spectrum of (CH 3 ) 2 CCHO was weak.…”

“…7 The direct hydrogen abstraction for O to attack the H atom in a methyl group occurs with a very low barrier, which explains the experimental result that the OH signal was strong. 6 Three reaction channels for the formation of butenols are revealed. The first one is the standard keto-enol tautomerization which has the lowest-energy barrier among all pathways of the decomposition and rearrangement of isobutyraldehyde, whereas the second and third ones are also energetically accessible.…”

“…[2][3][4][5] The reaction of O( 3 P) with isobutene has recently attracted much experimental attentions. [6][7][8][9] Bersohn's group 6 investigated the H, CH 2 CHO, CO, and OH products of the O( 3 P) atom with alkenes by the laser-induced fluorescence (LIF) method under single-collision conditions, and as for O( 3 P) + isobutene, they observed some H product and found that the yield of CO was very small and the OH signal was strong. They also tried to detect the CH 2 C(O)CH 3 radical, but they did not observe any, and consequently they proposed that the attack of O( 3 P) was selective, that is, at the less substituted C atom of the double bond.…”

Vibrational state distribution and relaxation of vinoxy radicals

“…This is not known but can be estimated as the average of the heats of formation of dimethyl ether ͑Ϫ184.1͒ and divinyl ether (Ϫ13.6Ϯ4.0) which is Ϫ98.9Ϯ4.0 kJ mol Ϫ1 . 11 Using 193 nm photons the available energy for reaction ͑3͒ is 366.4 kJ or 30 600 cm Ϫ1 . The rich and complex spectrum of vinoxy photodissociated from EVE extends from the band origin to the red out to 5620 cm Ϫ1 .…”

“…In these reactions H atoms are not formed in appreciable amounts. 11 That means that a 1-2 H atom migration occurs and only then is a radical released…”