Recombination Dynamics and Hydrogen Abstraction Reactions of Chlorine Radicals in Solution

Abstract

We observe chlorine radical dynamics in solution following two-photon photolysis of the solvent, dichloromethane. In neat CH(2)Cl(2), one-third of the chlorine radicals undergo diffusive geminate recombination, and the rest abstract a hydrogen atom from the solvent with a bimolecular rate constant of (1.35 +/- 0.06) x 10(7) M(-1) s(-1). Upon addition of hydrogen-containing solutes, the chlorine atom decay becomes faster, reflecting the presence of a new reaction pathway. We study 16 different solutes that include alkanes (pentane, hexane, heptane, and their cyclic analogues), alcohols (methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol), and chlorinated alkanes (cyclohexyl chloride, 1-chlorobutane, 2-chlorobutane, 1,2-dichlorobutane, and 1,4-dichlorobutane). Chlorine reactions with alkanes have diffusion-limited rate constants that do not depend on the molecular structure, indicating the absence of a potential barrier. Hydrogen abstraction from alcohols is slower than from alkanes and depends weakly on molecular structure, consistent with a small reaction barrier. Reactions with chlorinated alkanes are the slowest, and their rate constants depend strongly on the number and position of the chlorine substituents, signaling the importance of activation barriers to these reactions. The relative rate constants for the activation-controlled reactions agree very well with the predictions of the gas-phase structure-activity relationships.

Photoisomerization and Photoinduced Reactions in Liquid CCl4 and CHCl3

“…Results are presented first for the neat solvents, with analysis using a kinetic scheme based on the Smoluchowski model employed by Crim and co-workers for Cl-atom reactions in solution. [14][15][16] This analysis is then applied to the kinetics of reactions taking place in solutions containing the unsaturated hydrocarbons.…”

“…This discrepancy suggests two different decay processes are being observed in the two experiments that are optimized to observe kinetics on very different timescales. The time dependences of the two bands were fitted using a model based on Smoluchowski theory that was previously used by Crim and co-workers, [14][15][16] and allows for geminate recombination of the radical photoproducts. The photoexcitation of CCl4 is assumed to be followed by prompt dissociation to Cl + CCl3, and the model incorporates the distance, r0, at which the photofragment Cl and CCl3 equilibrate in solution.…”

“…Crim and coworkers [14][15][16] also used a chargetransfer band in the ultraviolet (UV) region, previously assigned to Cl-solvent complexes, 17,18 to follow the reaction kinetics on picosecond to nanosecond timescales, and applied this approach to reactions of Cl atoms with alkanes, alcohols and chloroalkanes. More recently,…”

Velocity map imaging the dynamics of the reactions of Cl atoms with neopentane and tetramethylsilane

“…50 The gas phase studies also serve as benchmarks for comparison with recent investigations of the kinetics and dynamics of reactions of the type summarized by (1) (with a variety of organic molecules RH) in liquid solutions using ultrafast transient absorption measurements of rates of loss of reagents and formation of products. [51][52][53] The greatest detail obtained so far from experimental studies of these important polyatomic-molecule reactions comes from VMI measurements of isolated, gas-phase collisions. Here, we use VMI to compare the reactions of Cl atoms with neopentane and tetramethylsilane (TMS):…”

Velocity map imaging of the dynamics of bimolecular chemical reactions

“…The comparison of dynamics in the gas and condensed phases can, in principle, be used to deduce the effect of a solvent on the features of a reactive PES such as barrier heights and locations, and the couplings between PESs, but experimental studies of the dynamics of bimolecular reactions in liquid solution are sparse. As VMI methods extend to gas phase studies of polyatomic molecules, and ultrafast time-resolved infra-red spectroscopy methods explore further the dynamics of reactions in solution, [118][119][120][121][122][123][124] Illustration of the PHOTOLOC strategy combined with velocity map imaging to study reactive scattering. In the PHOTOLOC experiments,the sequence of events is: (i) expansion of a mixture of reagents through a nozzle into a vacuum chamber to form a molecular beam; (ii) the molecular beam is crossed by two overlapped and counter-propagating lasers, the first of which photodissociates one species to initiate reaction, and the second laser ionizes one reaction product;…”