Photodissociation reaction of 1,2-diiodoethane in solution: A theoretical and X-ray diffraction study

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 C ₂H₄I•, anti C₂H₄I•, C ₂H₄, 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 C ₂H₄I•. 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 C₂H ₄I• can bind to the free iodine atom to form a C ₂H₄I-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 C₂H₄I ₂ in solution, the ground state structure of C₂H ₄I₂ in methanol was determined from static X-ray diffraction data using 88 keV (A = 0.14 Å) X-rays. The structural parameters are compared with those from the theoretical results.