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

Greg T Dunning,Thomas J Preston,Gregory M Greetham,Stuart J Greaves,Andrew J Orr-Ewing,Ian P Clark

doi:10.1021/acs.jpca.5b05624

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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 both 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 v1(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.

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Reactions in which a cyano (CN) radical abstracts a hydrogen atom from an organic molecule present an unusual opportunity to contrast chemical reaction dynamics under isolated conditions and in the presence of a liquid solvent.[1,2] These reactions are exothermic, typically releasing more than 100 kJ mol−1; in contrast to the liquid phase where this energy rapidly transfers to the surrounding solvent, the energy in low-pressure gas-phase reactions can only be distributed among the translational and internal degrees of freedom of the two products
The 267 nm wavelength photolysis liberates CN radicals in less than 50 fs,[37] and Time-resolved electronic absorption spectroscopy (TEAS) in the near-UV and visible regions revealed the production of CN radicals, their association with solvent molecules, and their reactive removal
Time-resolved absorption spectroscopy of the exothermic reaction of CN radicals with acetone in CDCl3 solutions demonstrates that a significant fraction of the energy released enters vibrational modes of the HCN and the 2-oxopropyl radical coproduct

Summary

Introduction

Reactions in which a cyano (CN) radical abstracts a hydrogen atom from an organic molecule present an unusual opportunity to contrast chemical reaction dynamics under isolated conditions and in the presence of a liquid solvent.[1,2] These reactions are exothermic, typically releasing more than 100 kJ mol−1; in contrast to the liquid phase where this energy rapidly transfers to the surrounding solvent, the energy in low-pressure gas-phase reactions can only be distributed among the translational and internal degrees of freedom of the two products. Have early and low energy barriers, with a flat angular dependence at the transition state.[3] the energy released by the reaction excites the HCN product modespecifically in the C−H stretching (v3) and bending (v2) vibrations, as confirmed by infrared (IR) emission and absorption spectroscopy measurements,[4−9] as well as trajectory calculations.[3] The rates and mechanisms of reactions of cyano radicals with hydrocarbons have been extensively studied at low pressures and temperatures because of their importance in the chemistry of the atmospheres of Titan, Triton, and Pluto.[10] Crossed molecular beam studies of CN + alkane reactions, in which the alkyl radical products were ionized without quantumstate specificity, showed that approximately 80−85% of the energy of reaction is deposited in internal modes of the products, which are scattered with angular distributions that indicate direct dynamics.[11] Direct H atom abstraction competes with addition and addition−elimination pathways in reactions of CN radicals with alkenes.[12−15]

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