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 CDCl<sub>3</sub> molecules,
and reaction with acetone. The acetone reaction occurs with a rate
coefficient of (8.0 ± 0.5) × 10<sup>10</sup> M<sup>–1</sup> s<sup>–1</sup> and transient vibrational spectra in the CN
and CO stretching regions reveal that <i>both</i> the nascent HCN and 2-oxopropyl (CH<sub>3</sub>C(O)CH<sub>2</sub>) 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) × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup> obtained from
the rise in the HCN product <i>v</i><sub>1</sub>(CN
stretch) IR absorption band. Evidence is also presented for CN complexes
with acetone that are more strongly bound than the CN–CDCl<sub>3</sub> complexes because of CN interactions with the carbonyl group.
The rates of reactions of these more strongly associated radicals
are slower still