| Abstract
| - Femtosecond transient absorption spectroscopy was used to study singlet diphenylcarbenegenerated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm. Absorption by singletdiphenylcarbene was detected and characterized for the first time. Similar band shapes were observed inacetonitrile and in cyclohexane with λmax ≈ 370 nm. The singlet absorption decays by intersystem crossingto triplet diphenylcarbene at rates that agree with previous measurements. The singlet absorption band iscompletely formed 1 ps after the pump pulse. It is preceded by a strong and broad absorption band, whichis tentatively assigned to excited-state absorption by a singlet diazo excited state. In neat alcohol solventsthe growth and decay of the diphenylmethyl cation was observed. This species is formed by proton transferfrom an alcohol molecule to singlet diphenylcarbene. Since a shell of solvent molecules surrounds eachnascent carbene, the intrinsic rate of protonation in the absence of diffusion could be measured. In methanol,proton transfer occurs with a time constant of 9.0 ps, making this the fastest known intermolecular proton-transfer reaction to carbon. In O-deuterated methanol proton transfer occurs in 15.0 ps. Slower rates wereobserved in the longer alcohols. The protonation times correlate reasonably well with solvation times inthese alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, thecarbocations decay on a somewhat slower time scale to yield diphenylalkyl ethers. In methanol and ethanolthe rate of decay is determined by reaction with neutral solvent nucleophiles. There is evidence in 2-propanolthat geminate reaction within the initial ion pair is faster than reaction with solvent. No isotope effect wasobserved for the reaction of the diphenylmethyl carbocation in methanol. Using comparative actinometrythe quantum yield of protonation was measured. In methanol, the quantum yield of carbocations reachesa maximum value of 0.18 approximately 18 ps after the pump pulse. According to our analysis, 30% of thephotoexcited diazo precursor molecules are eventually protonated. Somewhat lower protonation efficienciesare observed in the other alcohols. Because the primary quantum yield for formation of singletdiphenylcarbene is still unknown, the importance of reaction channels that might exist in addition toprotonation cannot be determined at present. Singlet carbenes are powerful, photogenerated bases thatopen new possibilities for fundamental studies of proton transfer in solution.
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