Femtosecond IR spectroscopy of peroxycarbonate photodecomposition: S1-lifetime determines decarboxylation rate.
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 elucidate the 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) vary from 25 ps in CH2Cl2 to 52 ps in n-heptane. In the case of the tert-butylphenylperoxycarbonate (TBPC) the decomposition takes 5.5 ps in CD2Cl2 and 12 ps in n-heptane. The CO2 fragment is formed vibrationally hot with an excess energy of about 5000 cm(-1). The hot CO2 spectra at high energy can be modeled assuming Boltzmann distributions with initial vibrational temperatures of ca. 2500 K which relax to ambient temperature with time constants of 280 ps in CCl4 and 130 ps in n-heptane. In CCl4 the relaxed spectra at 1.5 ns show 3.5% residual excitation in the n = 1 level of the asymmetric stretch vibration.
Reichardt
Christian
C
Abteilung Spektroskopie und Photochemische Kinetik, Max-Planck-Institut für Biophysikalische Chemie, 37070 Göttingen, Germany.
Schroeder
Jörg
J
Schwarzer
Dirk
D
D002245:Carbon Dioxide / Q000737:chemistry; D002254:Carbonates / Q000737:chemistry*; D002627:Chemistry, Physical / Q000379:methods*; D015643:Erythromycin Ethylsuccinate; D008027:Light; D008956:Models, Chemical; D015233:Models, Statistical; D010777:Photochemistry / Q000379:methods*; D013053:Spectrophotometry / Q000379:methods; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*; D013696:Temperature; D013997:Time Factors; D014732:Vibration
D002254:Carbonates; D015643:Erythromycin Ethylsuccinate; D002245:Carbon Dioxide