Vinoxy radical

Indeed, by using soft El ionization, we have been able to unambiguously detect products from all five reaction pathways (2a)-(2e), determine their branching ratio and characterize their dynamics.34 Here we discuss some of the results that we have obtained on this reaction, which well exemplify the power of soft El ionization. First of all, from measurements of the El efficiency curves at various to/e ratios (15, 42, and 43), we have found that the parent ion at m/e = 43 (CH2CHO+, corresponding to one of the main reaction channels, the vinoxy radical,) is not stable, so measurements of angular and TOF distributions were carried out at m/e = 42. Incidentally, from the El ionization efficiency curve at m/e = 42 we have obtained some direct information on the IE of the vinoxy radical, for which no such information was available till now. The IE should be <11 eV. 

Fig. 28. Schematic of potential energy surfaces of the vinoxy radical system. All energies are in eV, include zero-point energy, and are relative to CH2CHO (X2A//). Calculated energies are compared with experimentally-determined values in parentheses. Transition states 1—5 are labelled, along with the rate constant definitions from RRKM calculations. The solid potential curves to the left of vinoxy retain Cs symmetry. The avoided crossing (dotted lines) which forms TS5 arises when Cs symmetry is broken by out-of-plane motion. (From Osborn et al.67)...

Typical examples of its application to processes of interest for atmospheric chemistry are the measurement of the kinetics of the reaction of the vinoxy radical with 02 (Zhu and Johnston, 1995) and the kinetics of the C2H5 + C2H5 and C2H502 + C2H502 reactions (Atkinson and Hudgens, 1997). In addition, it has been shown to be useful for probing surface processes as well when combined with total internal reflection techniques (Pipino et al., 1997). 

Zhu, L and G. Johnston, Kinetics and Products of the Reaction of the Vinoxy Radical with 02, J. Phys. Chem., 99, 15114 f5119... 

The vinoxy radical has also been observed as an intermediate (Schmidt et al., 1985), suggesting that a portion of the initial adduct isomerizes ... 

The vinoxy radical reacts rapidly with 02 with a high-pressure limiting rate constant kx = (1.9 0.2) X 10 13 cm3 molecule- 1 s- 1 at P > 400 Torr (Zhu and Johnston, 1995). While the reaction leads to glyoxal... 

The primary photochemistry of the vinoxy radical is also of interest. Several relatively low-lying dissociation channels are available [ 10,11] ... 

This work represents the first study of the photodissociation dynamics of the vinoxy radical. We observe predissociation over the entire B 2 A" X 2A"... 

D. M. Neumark We indeed take the translational energy distribution from CH3O dissociation to be evidence for exit channel interactions on a repulsive potential-energy surface. This is in contrast to photodissociation of the vinoxy radical, for which very little variation of the CH3 + CO translational energy distribution occurs over a 0.5-eV range of excitation energy. 

D. M. Neumark Once the vinoxy radical surmounts the isomerization barrier, we don t really know if it passes through the acetyl minimum before dissociating to CH3 + CO, or if instead CO elimination is concerted. This would clearly be an interesting theoretical problem to pursue. 

The vinoxy CH2CHO and 1-methylvinoxy (acetonyl) radicals are key intermediates in the mechanisms of many reactions of importance for atmospheric and combustion chemistry. The formation of vinoxy radicals has been observed in several chemical processes. They may be formed in reactions of OH radicals with ethylene oxide (C2H4O) and with acetylene (C2H2) in the presence of 02.8 They are also produced in reactions of 0(3P) atoms with alkenes and in the reactions of reactive atoms such as F or 0(3P) with acetaldehyde.164,165 The 1-methylvinoxy (acetonyl) radical CH2C(CH3)0 is considered an important intermediate in the atmospheric oxidation of acetone initiated by the OH radical.166171 Spectroscopic studies by Washida et al.164 and Williams et al.m allow estimation of the rate constant for the reaction of acetonyl with 02. 

The electronic structure of the vinoxy radical is usually considered a resonance structure between the two localized electronic states CH2=CHO° and °CH2-CHO. Both quantum mechanical ab initio calculations174"178 182 and microwave spectroscopy172,173 indicate a dominant contribution of the °CH2-CHO form. In consequence, the nitroso-compound ON-CH2CHO is expected to be a major product of the addition reaction of nitric oxide to the vinoxy radical. Delbos et al %2 carried out measurements of the rate constant for the reaction... 

The nitroso-adduct ON-CH2CHO is formed by the attack of nitric oxide on the alkyl carbon atom of the radical. Addition of NO to the oxygen of the vinoxy radical CH2=CHO leads to the formation of CH2=CHONO. Results of ab initio calculations by Delbos et a/.182 show the occurrence of structural isomers of ON-CH2CHO and CH2=CHONO. Three structural stereoisomers of ON-CH2CHO have been found to correspond to the different orientations of the N-... 

Convergence of state-averaged and excited-state CASSCF calculations for the lowest two A" states of the vinoxy radical". 

Natural Bond Orbital Analysis of Photochemical Excitation With Illustrative Applications to Vinoxy Radical... 

The vinoxy radical (CHgCHO) is an important species in combustion and atmospheric chemistry, and its rich photochemistry has been the subject of benchmark studies throughout the modern photochemical era. Early theoretical studies focused on the expected competition between tautomeric ethe-nyloxy (CH2=CH6) and formylmethyl (CH2-CH=0) radical structures, and subsequent theoretical and experimental studies have repeatedly stressed the importance of obtaining accimate structural bonding characterizations of groimd and excited states of vinoxy radical and its derivatives. Qualitative... 

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