Photodissociation Reaction of 1,2-Diiodoethane in Solution:  A Theoretical and X-ray Diffraction Study

Abstract

Various molecular species are known to form during the photoreaction of C2H4I2 in the gas phase and in solution. We have studied all species involved in this reaction by ab initio and density functional theory (DFT) calculations: Geometries, energies, and vibrational frequencies of C2H4I2, bridged C2H4I*, anti C2H4I*, C4H4, I2, I3-, and the isomer C2H4I-I were calculated. The absorption peaks and oscillator strengths of selected species along the potential energy surface (PES) were calculated using time-dependent DFT and were compared with available experimental results. The calculated PES satisfactorily describes the observed reactions of the photoexcited C2H4I2 molecule. In the gas phase, there is only one reaction pathway: the first C-I bond ruptures followed by a secondary C-I breakage in the haloethyl radical C2H4I*. In solution, by contrast, another reaction channel, which is energetically more favored over the secondary dissociation, is switched on due to a solvation effect: the bridged C2H4I* can bind to the free iodine atom to form a C2H4I-I isomer without any energy barrier. The isomer can then break into C2H4 and I2. The rotational barriers in the gas phase and in solution were also calculated and compared. To provide experimental data on the structure of C2H4I2 in solution, the ground state structure of C2H4I2 in methanol was determined from static X-ray diffraction data using 88 keV (lambda = 0.14 A) X-rays. The structural parameters are compared with those from the theoretical results.

Case of the Missing Isomer: Pathways for Molecular Elimination in the Photoinduced Decomposition of 1,1-Dibromoethane

“…(27) Moreover, Ihee and co-workers have identified iso-1,2-C2H4I2 as a solution phase intermediate in time-resolved X-ray diffraction studies of the photolysis of 1,2-C2H4I2, and their calculations indicate that the isomer might be best described as an ethylene-I2 van der Waals complex derived from I atom addition to a bridged iodoethyl radical. (28)(29)(30) This is reminiscent of our studies of CF2I2, where iso-CF2I2 corresponds to a shallow minimum on the CF2I2 potential energy surface (PES) that is nearly isoenergetic with (and separated by a small barrier from) a van der Waals complex between CF2 and I2. (20) Like their iodo-analogues, the 1,1-and 1,2-dibromoethanes are prototypical systems for understanding the photochemistry of larger halons.…”

When isomerisation is electron transfer: the intriguing story of the iso-halocarbons

“…Are the isomers relevant for the chemistry of larger polyalkyhalides? Ihee and co-workers have identified iso-1,2-C 2 H 4 I 2 as a solution-phase intermediate in time-resolved X-ray diffraction studies of the photolysis of 1,2-C 2 H 4 I 2 ; however, their calculations indicate that the isomer is best described as an ethylene -I 2 van der Waals complex derived from I atom addition to a bridged iodoethyl radical [139][140][141]. However, time-resolved studies of 1,2-dihaloethanes have been reported by several groups.…”

New torsion potential expression for molecules without rotational symmetry

“…Table III. [42][43][44][46][47][48][49][50][51][52][53][54][55][56][57] It is shown that our calculations give very good results for the partially staggered conformation compared to the experimental values. For example, the calculated energy for the partially staggered conformation of CH 2 ICH 2 I is 2.54 kcal/ mol and the experimental value is 2.6 kcal/ mol.…”