Vibrational Excitation of Both Products of the Reaction of CN Radicals with Acetone in Solution

Abstract

Transient electronic and vibrational absorption spectroscopy unravel the mechanisms and dynamics of bimolecular reactions of CN radicals with acetone in deuterated chloroform solutions. The CN radicals are produced by ultrafast ultraviolet photolysis of dissolved ICN. Two reactive forms of CN radicals are distinguished by their electronic absorption bands: “free” (uncomplexed) CN radicals, and “solvated” CN radicals that are complexed with solvent molecules. The lifetimes of the free CN radicals are limited to a few picoseconds following their photolytic production because of geminate recombination to ICN and INC, complexation with CDCl₃ molecules, and reaction with acetone. The acetone reaction occurs with a rate coefficient of (8.0 ± 0.5) × 10¹⁰ M^–1 s^–1 and transient vibrational spectra in the CN and CO stretching regions reveal that <i>both</i> the nascent HCN and 2-oxopropyl (CH₃C­(O)­CH₂) radical products are vibrationally excited. The rate coefficient for the reaction of solvated CN with acetone is 40 times slower than for free CN, with a rate coefficient of (2.0 ± 0.9) × 10⁹ M^–1 s^–1 obtained from the rise in the HCN product <i>v</i>₁(CN stretch) IR absorption band. Evidence is also presented for CN complexes with acetone that are more strongly bound than the CN–CDCl₃ complexes because of CN interactions with the carbonyl group. The rates of reactions of these more strongly associated radicals are slower still