Time-Resolved Studies of CN Radical Reactions and the Role of Complexes in Solution

Abstract

Time-resolved studies using 100 fs laser pulses generate CN radicals photolytically in solution and probe their subsequent reaction with solvent molecules by monitoring both radical loss and product formation. The experiments follow the CN reactants by transient electronic spectroscopy at 400 nm and monitor the HCN products by transient vibrational spectroscopy near 3.07 microm. The observation that CN disappears more slowly than HCN appears shows that the two processes are decoupled kinetically and suggests that the CN radicals rapidly form two different types of complexes that have different reactivities. Electronic structure calculations find two bound complexes between CN and a typical solvent molecule (CH(2)Cl(2)) that are consistent with this picture. The more weakly bound complex is linear with CN bound to an H atom through the N atom, and the more strongly bound complex has a structure in which the CN bridges Cl and H atoms of the solvent. Fitting the transient absorption data with a kinetic model containing two uncoupled complexes reproduces the data for seven different chlorinated alkane solvents and yields rate constants for the reaction of each type of complex. Depending on the solvent, the linear complex reacts between 2.5 and 12 times faster than the bridging complex and is the primary source of the HCN reaction product. Increasing the Cl atom content of the solvents decreases the reaction rate for both complexes.

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.…”

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.…”

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

“…instead, strong complexing interactions of CN-radicals with individual solvent molecules are thought to be responsible, as proposed by Crowther et al [55][56] We explored the spectra of CN-solvent complexes in chloroform, TEAS measurements subsequent to ICN photolysis in dichloromethane show similar spectral features to those observed in chloroform. However, the band assigned to free CN is only visible at the earliest time delays in acetonitrile, ethanol and methanol; for these solvents, the spectra are dominated by broad absorption bands.…”

“…39 Although the reaction of CN radicals with solvent molecules to make HCN may be relatively slow in chlorinated solvents, [55][56] in most other organic solvents the CN radicals are expected to react on pico-to nanosecond timescales, producing an organic radical co-product. The corresponding reaction is endothermic by ~ 24.8 kJ mol -1 with water, 67 and is therefore unfavourable.…”