Reaction Dynamics of CN Radicals in Acetonitrile Solutions

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• 作者：Daisuke Koyama ; Philip Coulter ; Michael P. Grubb ; Gregory M. Greetham ; Ian P. Clark ; Andrew J. Orr-Ewing
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：December 31, 2015
• 年：2015
• 卷：119
• 期：52
• 页码：12924-12934
• 全文大小：546K
• ISSN：1520-5215

文摘

The bimolecular reactions that follow 267 nm ultraviolet photolysis of ICN in acetonitrile solution have been studied using transient absorption spectroscopy on the picosecond time scale. Time-resolved electronic absorption spectroscopy (TEAS) in the ultraviolet and visible spectral regions observes rapid production and loss (with a decay time constant of 0.6 ± 0.1 ps) of the photolytically generated free CN radicals. Some of these radicals convert to a solvated form which decays with a lifetime of 8.5 ± 2.1 ps. Time-resolved vibrational absorption spectroscopy (TVAS) reveals that the free and solvated CN-radicals undergo geminate recombination with I atoms to make ICN and INC, H atom abstraction reactions, and addition reactions to solvent molecules to make C₃H₃N₂ radical species. These radical products have a characteristic absorption band at 2036 cm^–1 that shifts to 2010 cm^–1 when ICN is photolyzed in CD₃CN. The HCN yield is low, suggesting the addition pathway competes effectively with H atom abstraction from CH₃CN, but the delayed growth of the C₃H₃N₂ radical band is best described by reaction of solvated CN radicals through an unobserved intermediate species. Addition of methanol or tetrahydrofuran as a cosolute promotes H atom abstraction reactions that produce vibrationally hot HCN. The combination of TEAS and TVAS measurements shows that the rate-limiting process for production of ground-state HCN is vibrational cooling, the rate of which is accelerated by the presence of methanol or tetrahydrofuran.