Dynamics of Bimolecular Reactions in Solution

Abstract

Mechanisms of bimolecular chemical reactions in solution are amenable to study on picosecond timescales, both by transient absorption spectroscopy and by computer simulation. The dynamics of exothermic reactions of CN radicals and of Cl and F atoms with organic solutes in commonly used solvents are contrasted with the corresponding dynamics in the gas phase. Many characteristics of the gas-phase reaction dynamics persist in solution, such as efficient energy release to specific vibrational modes of the products. However, additional complexities associated with the presence of the solvent are open to investigation. These features of liquid-phase reactions include the role of solvent-solute complexes, solvent caging, coupling of the product motions to the solvent bath, thermalization of internally excited reaction products, incipient hydrogen bond formation, and involvement of charge-separated states that arise from proton transfer.

Femtosecond to microsecond observation of the photochemical reaction of 1,2-di(quinolin-2-yl)disulfide with methyl methacrylate

“…The combination of ultrafast transient vibrational absorption spectroscopy (TVAS) and TEAS is powerful for investigation of bimolecular radical-reaction mechanisms because the formation and decay of the initial radical can be monitored by TEAS, with subsequent reaction intermediates observed by TVAS. [19][20][21] Here, we demonstrate that TVAS at discrete time delays from 200 fs to 50 s in a single set of measurements, combined with TEAS over a more restricted time range from 100 fs -1.3 ns, provides a comprehensive view of a sequence of unimolecular initiation and bimolecular addition steps in a radical chain reaction.…”

Reaction Dynamics of CN Radicals in Acetonitrile Solutions

“…3 These interactions occur on ultrafast (femtosecond to picosecond) timescales, but non-equilibrium reaction dynamics reminiscent of the gas-phase can persist if reactive events are fast enough to compete with the response of the surrounding solvent. [4][5][6][7][8] To investigate some of the effects of solvent on the dynamics and pathways of bimolecular reactions, ultraviolet (UV) photolysis of dissolved cyanogen iodide (ICN) has been employed as a source of CN radicals. 4,5 These reactive radicals typically abstract hydrogen atoms from organic molecules as shown by Equation (1), and the reactions with alkanes have been extensively studied in the gas phase.…”

“…[4][5][6][7][8] To investigate some of the effects of solvent on the dynamics and pathways of bimolecular reactions, ultraviolet (UV) photolysis of dissolved cyanogen iodide (ICN) has been employed as a source of CN radicals. 4,5 These reactive radicals typically abstract hydrogen atoms from organic molecules as shown by Equation (1), and the reactions with alkanes have been extensively studied in the gas phase. [9][10][11][12][13][14] CN + RH → HCN(v1,v2,v3) + R…”

“…15,16 Consequently, the energy released by the reaction excites the HCN in the C-H stretching and bending vibrations. 4 Reactions of the type illustrated by Equation (1) have also been investigated in solution in common chlorinated organic solvents. For example, Orr-Ewing and coworkers explored the reaction dynamics of CN radicals with various organic molecules (cyclohexane, d12-cyclohexane, tetramethylsilane, tetrahydrofuran (THF), acetone) in chloroform and dichloromethane using time-resolved vibrational absorption spectroscopy (TVAS).…”

“…Complete quenching of nascent vibrational excitation by interaction with the solvent bath can take tens to hundreds of picoseconds in chloroform and dichloromethane solutions. 4,5,[17][18][19]22 Modifications of the reaction mechanism by interactions with the solvent bath include formation of complexes between CN radicals and solvent molecules. 23,24 The current study extends these prior investigations to reactions of CN radicals in acetonitrile (CH3CN or CD3CN), which is chosen because it is a more strongly interacting solvent than chloroform and dichloromethane.…”