Simulations of ICl⁻(CO₂)_nPhotodissociation: Effects of Structure, Excited State Charge Flow, and Solvent Dynamics

Abstract

Experiments on the photodissociation of ICl(-) within size-selected CO(2) clusters [Nadal, M. E.; Kleiber, P. D.; Lineberger, W. C. J. Chem. Phys. 1996, 105, 504.] differ from analogous experiments on homonuclear systems in two major ways: an early onset of caged, ICl(-)-based products with cluster size peaking at n = 5 and a rapid decline of caging after n = 5 in favor of dissociated, Cl(-)-based fragments. Using an effective Hamiltonian model for the solute electronic structure and nonadiabatic MD trajectories, we have simulated the photodissociation of these clusters and found good agreement between experimental and simulated product distributions. The strong preference of the solvent for chlorine in the initial cluster structure, along with favorable kinematics, leads to the efficient caging at small n. At all cluster sizes, the Cl(-)-based fragments are formed by an adiabatic process and are accompanied by spin-orbit excited iodine. Caging of ICl(-) also occurs on the excited state potential surface, and electronic relaxation is not observed during the 100 ps trajectories. For n > 5, the initially caged products appear to be metastable, decaying to solvent-separated Cl(-)...I pairs that subsequently dissociate, leading to the falloff in the caging fraction in the larger clusters.

Photofragmentation dynamics of ICN−(CO2)n clusters following visible excitation

“…This allows the solvated system to evolve adiabatically from an initially excited state, which in the bare anion would dissociate to form I − + Cl, to generate I * + solvated Cl − products. Indeed, molecular dynamics simulations by Faeder and Parson 7 showed that following visible photoexcitation of ICl − (CO 2 ) n nearly all of the observed Cl − -based products are accompanied by I * ( 2 P 1/2 ) products as a result of dissociation to this asymptotic state. The second possible mechanism occurs on the ground-state potential for ICl − (CO 2 ) n .…”

An overview of nonadiabatic dynamics simulations methods, with focus on the direct approach versus the fitting of potential energy surfaces

“…In Table 1 we collected examples of many different simulated processes, all involving nonadiabatic transitions: they include photon adsorption and emission [7,38,46,64,276,307], photoionization [170,211], InterSystem Crossing [155,308,309], excitation energy transfer [310], exciton dissociation [311], ultrafast decay of nucleobabses [312][313][314][315], hole or electron transfer [316][317][318][319], proton transfer [320,321], anelastic and reactive scattering [276,322], photoisomerization [116,133,179,236,237,256,320,323,324] and photodissociation [133,155,179,[325][326][327][328][329][330]. In practically all cases, excited state decay and geometrical relaxation are essential ingredients of the simulation.…”

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

“…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.…”