Femtosecond IR Spectroscopy of Peroxycarbonate Photodecomposition: S1-Lifetime Determines Decarboxylation Rate

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• 作者：Christian Reichardt ; Jö ; rg Schroeder ; Dirk Schwarzer
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：October 11, 2007
• 年：2007
• 卷：111
• 期：40
• 页码：10111 - 10118
• 全文大小：290K
• 年卷期：v.111,no.40(October 11, 2007)
• ISSN：1520-5215

文摘

The ultrafast photofragmentation of arylperoxycarbonates R-O-C(O)O-O-tert-butyl (R = naphthyl, phenyl)is studied using femtosecond UV excitation at 266 nm and mid-infrared broadband probe pulses to elucidatethe dissociation mechanism. Our experiments show that the rate of fragmentation is determined by the S₁-lifetime of the peroxide, i.e., the time constants of S₁ decay and of CO₂ and R-O formation are identical.The fragmentation times are solvent dependent and for tert-butyl-2-naphthylperoxycarbonate (TBNC) varyfrom 25 ps in CH₂Cl₂ to 52 ps in n-heptane. In the case of the tert-butylphenylperoxycarbonate (TBPC) thedecomposition takes 5.5 ps in CD₂Cl₂ and 12 ps in n-heptane. The CO₂ fragment is formed vibrationally hotwith an excess energy of about 5000 cm^-1. The hot CO₂ spectra at high energy can be modeled assumingBoltzmann distributions with initial vibrational temperatures of ca. 2500 K which relax to ambient temperaturewith time constants of 280 ps in CCl₄ and 130 ps in n-heptane. In CCl₄ the relaxed spectra at 1.5 ns show3.5% residual excitation in the n = 1 level of the asymmetric stretch vibration.