Evidence of a Double Surface Crossing between Open- and Closed-Shell Surfaces in the Photodissociation of Cyclopropyl Iodide

Abstract

Gas-phase photodissociations of cyclopropyl iodide were conducted at 266 and 279.7 nm, and the radical products were probed by multiphoton ionization, with imaging of the resulting ions and their corresponding electrons. Solution-phase photodissociations of cyclopropyl iodide were also conducted with TEMPO-trapping of the radical dissociation products. In both gas and solution phases, allyl radical was found to be a direct product of the cyclopropyl iodide photodissociation. CASSCF calculations indicate that the allyl radical could be formed directly from photoexcited cyclopropyl iodide by way of two surface crossings between open- and closed-shell potential energy surfaces. Each surface crossing represents a point of potential bifurcation in the reaction dynamics. Thus, cyclopropyl iodide that is excited to a 1(n,σ*) state can remain on an open-shell surface and generate the cyclopropyl radical and an iodine atom or can cross to a closed-shell (ion-pair) surface. The cyclopropyl cation that results from the surface crossing can undergo barrierless ring opening to the allyl cation before crossing back to an open-shell surface to generate allyl radical and an iodine atom. In this manner, both cyclopropyl radical and allyl radical can be formed as direct products of cyclopropyl iodide photodissociation.

The use of elementary reaction coordinates in the search for conical intersections

“…This system was extensively studied theoretically 24–27 partly as a result of the dilemma raised by Longuet‐Higgins and Abrahamson 24: the reaction (whether conrotatory or disrotatory) appears to be allowed both thermally and photochemically in apparent contradiction to the Woodward–Hoffmann rules 28. A conical intersection is predicted by an early calculation of Merlet et al 29.…”

Transition state region in the A-Band photodissociation of allyl iodide—A femtosecond extreme ultraviolet transient absorption study

“…45,46 However, the photochemistry of the precursor itself is not as well studied as that of allyl radical or even other alkyl halide prototypes. [47][48][49][50][51][52][53][54] The UV absorption spectrum of allyl iodide comprises two broad bands, a stronger band at ∼200 nm and a weaker one at ∼270 nm. 38,50,55 While the absorption at shorter wavelengths is ascribed to transitions involving promotion of an electron to the π * orbital, encompassing both π → π * and n → π * transitions, the latter involves an n → σ * transition, similar to methyl iodide.…”

“…57,58 As already mentioned, the Aband photodissociation of halogenated hydrocarbons is known to produce a halogen atom in both spin-orbit excited ( 2 P 1/2 , I*) and ground ( 2 P 3/2 , I) states, respectively. 53,[59][60][61] The branching ratio of the dual I/I* channels in the photodissociation of allyl iodide was measured by Fan and Pratt by making use of the different wavelength-and electric field-dependent photoionization cross sections of the atoms. 58 They obtained I*/(I + I * ) ratios of 0.61 ± 0.10 at 193 nm and 0.90 ± 0.10 at 266 nm, which is higher than that reported for CH 3 I (0.76 at 266 nm).…”

Photodissociation of C3H5Br and C4H7Br at 234 nm

“…In particular, the photodissociation dynamics of cyclopropyl iodide was investigated at 266 and 280 nm using ion-imaging of nascent iodine atoms. 17 In another study, state-selective photofragment translational spectroscopy of cyclohexyl iodide was performed and the energy difference between the equatorial and axial conformers was determined by monitoring I and I * fragments at several excitation wavelengths. 18 Recently photodissociation dynamics of cyclopentyl bromide has been studied by velocity ion imaging method.…”