Quenching rate triplet carbenes

Carbonyl oxides are also easily detected by LFP with fast TRUV-vis spectroscopy. For example, LFP of a-diazophenylacetate in deaerated Freon-113 generates a transient absorption band at < 270 nm ascribable to triplet methoxycarbonylphenyl-carbene (53), which shows a pseudo-first-order decay with lifetime of 460 ns. When LFP is carried out in aerated solvents, a new transient band appears at 410 nm at the expense of the transient band due to triplet carbene. The decay rate of triplet car-bene increases as the concentration of oxygen increases. This correspondence indicates that the triplet carbene is trapped by oxygen to form the carbonyl oxides (53-O2). This result confirms that the transient absorption quenched by oxygen... 

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions the singlet carbene inserts into the O—H bond of methanol, whereas the triplet carbene adds to an a I Irene (Scheme 6.173). It has been found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS) and laser flash spectroscopy (Section 3.7 to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20kJ mol J). 

Triplet DPC is readily detectable at 315 nm and is quenched by TEMPOs. The bimolecular rate constant (feq) for carbene scavenging is determined by plotting the pseudo-first-order rate constant for carbene decay as a function of substrate concentration. The values of feq (2-3 x 10 s ) are practically the same for the reac-... 

Fluorescence quenching studies can establish the rate constant at which a certain substrate interacts with the excited carbene, but they cannot provide any independent mechanistic information. Absorption studies are somewhat more informative in that the primary product of reaction can sometimes be detected directly. In the reaction of di-p-tolylcarbene with CCI4, the radical, (MeC6H4)2CCl, obviously formed as a result of abstraction of Cl atom from the substrate, is detected. Its formation can be monitored to give a rate constant of 1.1 x 10 M s for the excited state, which should be compared with a rate of 2 x 10 M s for ground-state triplet DPC. ... 

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