Laser-induced infrared multiphoton isomerization reactions of 2,4- and 1,3-hexadienes

Abstract

Efficient and clean isomerization has been observed in the system consisting of the conjugated 2,4-and 1,3-hexadienes following infrared multiphoton excitation of these species. Variations of both laser fluence and pressure of added inert gas are seen to significantly effect branching ratios and yields. Where a competition exists between a low activation energy, low preexponential factor pathway and one with a high activation energy, high preexponential factor, an increase in fluence is seen to favor the latter pathway. An estimate of the degree of excitation in the molecule is obtainable from product branching ratios and inert gas quenching behavior. Photoacoustic measurements of energy input coupled with observation of product ratios indicate that the production of multiple products in a single laser pulse is compatible with a sequential isomerization mechanism which takes place in an almost vibrationally adiabatic fashion. Thermal and CW laser studies of hexadiene isomerization have been performed. Thermal studies yield rate constants and AH and AS for the various isomers. Thermal studies coupled with multiphoton studies have helped establish the isomerization pathway connecting the cis,trans-2,4-and trans-1,3-hexadiene isomers and have confirmed the other isomerization pathways in the system. The pathways for CW cw laser-induced isomerization have been studied.

Decomposition and hydrocarbon growth processes for hexadienes in nonpremixed flames

“…This mechanism applies to a much wider range of reactants and products, but is much slower than six-center isomerization, since it requires formation of a biradical. (The available six-center isomerization Arrhenius parameters [34,36] predict rates in our temperature regime that are roughly 100 times higher than those for carbene isomerizations [25,40,41].) Presumably carbene isomerizations are important for propadiene and 1,3-butadiene because all of their C-C bonds are double or vinylic, so their C-C fission rates are very low.…”

“…1 shows that 1,3-hexadiene does indeed decompose more slowly than the other hexadienes with alkyl-resonant bonds. Fourth, although Arrhenius parameters have only been measured for the reverse process (1,3 → 2,4 isomerization), and at temperatures below 700 K [34,35], they predict k 1200 ∼ 50,000 s −1 , which is much larger than the values for C-C fission listed in Fig. 2.…”

An experimental and theoretical study of the photoisomerization and thermal reversion on 5-arylmethylene-2-thioxoimidazolidin-4-one

“…Some studies agree with the first theory [30][31][32] whereas other results were more consistent with the second mechanism. [33][34][35] In 1984 Buechele et al 36 suggested an alternative cis/trans isomerization pathway for conjugated dienes which involves the formation of a transient intermediate through tautomerization.…”

Vibrational Dynamics of Terminal Acetylenes: II. Pathway for Vibrational Relaxation in Gas and Solution

“…Because the normal-mode frequencies depend on the molecular structure, reaction control can be obtained through frequency selectivity of the laser pulses. The basic principle of this type of laser chemistry has been demonstrated for reasonably large polyatomic molecules using infrared multiphoton excitation. − In these experiments, collisional-energy relaxation is essential to stabilizing the new isomers. If an isomerization reaction involving several geometries follows a sequential reaction kinetics scheme, then product selectivity can be achieved by properly “timing” the collisional relaxation time (by the variation of solvent density in a supercritical fluid or possibly by using solvents that show a preferential interaction with the different isomers) .…”