The dynamical role of solvent on the ICN photodissociation reaction: connecting experimental observables directly with molecular dynamics simulations

Abstract

The ICN photodissociation reaction is the prototype system for understanding energy disposal and curve crossing in small molecule bond-breaking. The wide knowledge base on this reaction in the gas phase makes it an excellent test case to explore and understand the influence of a liquid solvent on the photo-induced reaction dynamics. Molecular dynamics simulations that include surface-hopping have addressed numerous aspects of how the solvent should influence non-adiabatic transitions and energy flow and ultimately determine product branching for this reaction system. In this paper, we report femtosecond transient absorption work directly combined with new molecular dynamics simulations that make direct connection with the spectroscopic observables. The full spectral evolution after initiating ICN photodissociation at 266 nm in water and ethanol is recorded with unprecedented time resolution, fast enough to see the nascent products emerge before interacting with the solvent cage. Use of a 266 nm pump maximizes the probability of subsequent caging on the upper diabat while launching large rotational energy release for trajectories emerging on the lower diabat. The 2D dataset yields a map of the different products and how they interconvert. In particular, information on the branching ratio and spectral evolution of the product bands is revealed as the products relax their electronic and rotational degrees of freedom. An evolution from rotationally hot gas-phase like CN (sharp band, at 390 nm) to equilibrated and solvated CN radicals (broad, at 326 nm in water and 415 nm in ethanol) is clearly observed in both solvents, and signals assignable to I* are also captured. The non-adiabatic molecular dynamics simulations focus on identifying when trajectories curve cross, filtering the trajectory ensemble into spectroscopically distinct sub-populations and analyzing the rotational energy for the CN product population. The experimental results, taken together with the MD simulations, establish the initial surface crossing probability and suggest multiple passes through the curve crossing region determine the final product yields and provide a source of freshly torqued CN radicals that continues to top up the population of rotationally hot photoproduct over the first few picoseconds.

The Study of Reactive Intermediates in Condensed Phases

“…These additional spectral features can be advantageous; for example, bands corresponding to charge-transfer from/to the solvent, or distinct spectral signatures of solvent-complexed radicals help to characterize the solvation environment of a reactive species and its influence on reaction rates. [71][72][73] The time dependence of IR or UV/vis bands provides further evidence to support proposed assignments. A recent study of fluorine atom reactions in d-acetonitrile illustrates application of TVA and TEA methods to chemical dynamics in solution.…”

Recombination, Solvation and Reaction of CN Radicals Following Ultraviolet Photolysis of ICN in Organic Solvents

“…2(f)). The interpretation of the broad overlapping bands was previously discussed by both Bradforth and co-workers 39 and Keiding and co-workers. 40 Despite this prior analysis, the decomposition of such spectra and the subsequent assignment In the decomposition, the feature peaking at 340 nm (red line) has contributions from both solventto-solute I( 2 P3/2) CT and CN-radical complex absorption bands.…”

“…To a lesser extent, these solvents also destabilize the electronically excited states of ICN contributing to the broad UV absorption band. [38][39]47 However, the band maxima do not show a straightforward correlation with either solvent molecular dipole moments or relative permittivities, suggesting a more complex interaction of the solvents with the electronic states of ICN. In each case, absorption at 267 nm is on the long-wavelength side of the ICN band and favours excitation to the 3 0+ and 3 1 states correlating adiabatically to the I* + CN and I + CN dissociation limits respectively.…”

“…A combination of these experimental measurements and dynamics simulations using the potential energy surfaces of Morokuma and co-workers 4,6 showed that CN and I radicals will undergo recombination as a result of the solvent cage in ~60 fs and diffusive recombination in ~16 ps. 39 Furthermore Bradforth and coworkers demonstrated that nascent, rotationally hot CN radicals will retain their rotational coherence for up to a few picoseconds in solution. 38,49 The photochemistry of ICN has also been studied in cryogenic matrices, with Fraenkel and Haas assigning an infrared band that developed to lower wavenumber than the ICN fundamental C≡N stretching mode to the INC isomer.…”

Competitive reaction pathways in vibrationally induced photodissociation of H2SO4

“…Several interesting experimental studies which provided fragmentation patterns following the excitation of small molecules in solution have recently appeared. 63,64 As the present and previous investigations have shown, simulations can provide a deeper understanding of energy migration pathways in such complex systems. 64,65 …”