Methylene, singlet formation

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

Acetylene also plays an important role as intermediate in soot formation in flames, because the reverse of reaction (la) forms singlet methylene,... 

Fig. 11. Center-of-mass translational energy distribution of the CH2 + H product channel of CH3 photodissociation at 193 nm. Arrows indicate the thermodynamic maximum available energies for formation of singlet and triplet methylene. (From North et al.112)...

The formation of the 1,3CH2 methylene radial in either the triplet or singlet electronic states depends on collisions with other molecules such as N2 in the Titan... 

Notes ° Eq. 9, M = CR R AH, (H+) = 365.7 kcal/mol. All data refer to 298 K. b Recommended enthalpies of formation from Ref. 134b 2 kcal/mol. cTo AH, (3 CH-CH=CH2) = 93 kcal/mol was added AEst =10 kcal/mol. The calculated singlet-triplet gap of 13.3 kcal/mol (B3LYP/cc-pVTZ) from Ref. 133 was corrected by 3 kcal/mol, which is the difference between the experimental and computed (B3LYP/cc-pVTZ) AEst for methylene. 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

It has been argued128-124 that singlet methylene adds directly to the carbon double bond to form a cyclopropane ring by a three-center mechanism involving essentially simultaneous formation of two carbon-carbon bonds. We believe, however, that the detailed mechanism must include formation of a short lived diradical as the initial step for both singlet and triplet methylene. The diradical may undergo ring closure or struc-... 

Energy-resolved rate constant measurements near the threshold for diplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by re-election coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

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