Simulation of UV photodissociation of I2−(CO2)n: Spin-orbit quenching via solvent mediated electron transfer

Abstract

We simulate the 395 nm photodissociation of I2− embedded in clusters of 6 to 22 CO2 molecules. In the isolated molecule, photodissociation at this wavelength leads exclusively to spin-orbit excited iodine (I*) plus I−. In the larger clusters we observe efficient electronic relaxation, leading both to dissociated products containing ground-state iodine and to recombined products containing I2−. The time scale and cluster size dependence of the spin-orbit quenching process agree well with experimental determinations of Sanov et al. (companion paper). The simulation trajectories show that spin-orbit quenching occurs by resonant charge transfer from solvated I− to a nascent I* atom. A model derived from the theory of electron transfer reactions in solution illustrates that this resonance arises when the I spin-orbit energy is compensated by the difference between the solvation energies of the ion and the neutral.

Spin-orbit relaxation and recombination dynamics in I2−(CO2)n and I2−(OCS)n cluster ions: A new type of photofragment caging reaction

“…As discussed by Parson and co-workers in the companion paper, 65 solvation effects involving the anion can compensate for the energy gap between the two spin-orbit asymptotes. Because of the substantial binding energy of OCS and CO 2 to the cluster ͑ϳ0.2 eV per molecule͒, 5,10,16 the relative state energies are greatly affected by asymmetries of the solvent and charge distributions.…”

“…IV, we rule out the mechanisms that would explain the observed spin-orbit quenching without explicitly considering the perturbed I 2 Ϫ potentials. We then use the solvent asymmetry mediated electron transfer model, first suggested by Maslen et al 37 and discussed in detail in the companion paper, 65 to interpret the experimental results. Favorable comparison of the experimental and calculated 65 results yields confidence in the proposed mechanism and allows for improved understanding of quenching dynamics.…”

The interaction of negative charge with carbon dioxide – insight into solvation, speciation and reductive activation from cluster studies

“…This feature is a direct result of caging through interaction with the CO 2 solvent [131,132]. Theoretical work [145,147,148] International Reviews in Physical Chemistry 503…”

“…There has been an extensive body of work over more than two decades on the interaction of dihalide anions with CO 2 , experimentally mostly performed by Lineberger and coworkers [129][130][131][132][133][134][135][136][137][138][139][140][141][142][143], with theoretical work notably done by Parson and coworkers [135,[137][138][139][140][142][143][144][145][146][147][148][149][150] and by McCoy and coworkers [137,142,143,151,152]. In this series of papers, CO 2 had the role of a solvent that could react to electronic excitation of a solvated ion and modify the solute ion's photophysics as well as its vibrational characteristics.…”

“…Parson and coworkers [144][145][146][147][148][149]156] used non-adiabatic molecular dynamics simulations to trace the high observed caging efficiency in I À 2 Á ðCO 2 Þ n clusters to the charge distribution of the excited ion and its polarisation by the solvent. In electronic states with primarily bonding character, the solvent polarises the charge into the most favourably solvated I atom, while the opposite is true in electronic states with primarily antibonding character.…”