RESUMO
A new laser-induced fluorescence spectrum was observed in the region of 350 nm-360 nm. The spectrum was observed in the reaction between the CHBrCHBr and OH radicals and in the reaction of CHBrCHBr and CH2CHBr with atomic oxygen O(3P). The spectrum was assigned to the BÌ--XÌ transition of the trans-CHBrCHO (trans-2-bromovinoxy) radical. The BÌ--XÌ electronic transition energy (T0) was 28 542 cm-1, which was 242 cm-1 lower than that of the unsubstituted vinoxy radical (CH2CHO). From an analysis of the laser-induced single vibronic level fluorescence aided by ab initio calculations, some of the vibrational frequencies were assigned to the ground electronic state ν3 (C-O str.) = 1581 cm-1, ν6 (C-C str.) = 1130 cm-1, and ν8 (C-C-O bend.) = 409 cm-1. The fluorescence lifetimes of the excited BÌ state were 35 ns-75 ns, depending on the excited vibrational modes, implying that predissociation had accelerated as the energy level (v') increased.
RESUMO
A pulsed laser photolysis-photoionization mass spectrometer system has been employed to measure the rate constants of HCCO + O2 and HCCCO + O2 over the temperature range 243-423 K in 1.2-8.4 Torr of He or N2. Radicals of HCCO and HCCCO were produced by 193 nm ArF laser photolysis of ethyl ethynyl ether and methyl propiolate, respectively. HCCO was photoionized by a Kr resonance lamp with a CaF2 window (10.03 eV), and HCCCO was ionized by a Xe lamp with a sapphire window (8.44 eV). Both ions were detected as parent ions in a quadrupole mass spectrometer. From analysis of the time profiles of the ion signals for various O2 concentrations, the overall rate constants at 298 K are represented by the values k2 = (6.3 ± 1.0) × 10-13 for HCCO + O2 and k5 = (5.7 ± 0.6) × 10-12 for HCCCO + O2 in the units cm3 molecule-1 s-1. The rate coefficients for the two reactions can be described by k2(T) = (1.5-0.7+1.5) × 10-12 exp[-(225 ± 220)/T] and k5(T) = (1.8-0.9+1.9) × 10-12 exp[(343 ± 228)/T] in the units cm3 molecule-1 s-1 over the temperature range 243-423 K.
RESUMO
A new laser-induced fluorescence spectrum of the 2-fluorovinoxy (CHFCHO) radical was first observed around 335 nm. The radical was produced in the reaction of an OH radical with 1,2-difluoroethylene (CHF=CHF). A single weak band was observed, which was assigned to the 00 0 band of the BÌ-XÌ transition of the trans-CHFCHO radical. The BÌâXÌ electronic transition energy (T0) for trans-CHFCHO was 29 871 cm-1, which was just 3 cm-1 lower than that of its isomer, the 1-fluorovinoxy (CH2CFO) radical. The fluorescence lifetime at 29 871 cm-1 was shorter than 20 ns. This means that strong predissociation is probable at v' = 0 in the excited BÌ state of trans-CHFCHO. From an analysis of the dispersed fluorescence spectrum, some of the vibrational frequencies can be assigned for the ground electronic state: ν3 = 1557 cm-1 (C-O stretch), ν7 = 1162 cm-1 (C-C stretch), and ν8 = 541 cm-1 (CCO bend). These vibrational assignments were supported by ab initio calculations. The structure of the C-C-O skeleton and the spectroscopic character of trans-CHFCHO were close to those of CHClCHO and CH2CHO than those of CH2CFO. For the reaction of CH2=CHF with O(3P), the formation of both the regioisomeric radicals, i.e., 1- and 2-fluorovinoxy radicals, was confirmed. The regioselectivity of the oxygen atom added to the double bond of monofluoroethylene is discussed.
RESUMO
Ground and second excited electronic states of halogen and monomethyl substituted vinoxy radicals were studied by multireference configuration interaction (MRCI) calculation. Optimized geometries, rotational constants and vibrational frequencies of vinoxy and 1-fluorovinoxy showed good agreement with experimental values. Differences in calculated and observed B-X electronic transition energies were less than 0.1 eV and observed trends of blue shift upon increasing the number of substituted halogen atoms were reproduced by MRCI calculation. Observed fluorescence lifetimes of the vibrationless level in B state were in good agreement with calculated values. Rotational profiles of the 0-0 vibronic bands were successfully simulated with calculated rotational constants and transition dipole moments. Energy differences between planar and nonplanar optimized geometries in B state showed good correlation with the onset of fast nonradiative decay in B state, supporting the proposed mechanism of nonradiative decay via avoided crossings from B to A state which is followed by the decay to the ground state via conical intersections.
