Solvent Reorganization Controls the Rate of Proton Transfer from Neat Alcohol Solvents to Singlet Diphenylcarbene

Abstract

Femtosecond transient absorption spectroscopy was used to study singlet diphenylcarbene generated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm. Absorption by singlet diphenylcarbene was detected and characterized for the first time. Similar band shapes were observed in acetonitrile and in cyclohexane with lambda(max) approximately 370 nm. The singlet absorption decays by intersystem crossing to triplet diphenylcarbene at rates that agree with previous measurements. The singlet absorption band is completely formed 1 ps after the pump pulse. It is preceded by a strong and broad absorption band, which is tentatively assigned to excited-state absorption by a singlet diazo excited state. In neat alcohol solvents the growth and decay of the diphenylmethyl cation was observed. This species is formed by proton transfer from an alcohol molecule to singlet diphenylcarbene. Since a shell of solvent molecules surrounds each nascent 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 were observed in the longer alcohols. The protonation times correlate reasonably well with solvation times in these alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, the carbocations decay on a somewhat slower time scale to yield diphenylalkyl ethers. In methanol and ethanol the rate of decay is determined by reaction with neutral solvent nucleophiles. There is evidence in 2-propanol that geminate reaction within the initial ion pair is faster than reaction with solvent. No isotope effect was observed for the reaction of the diphenylmethyl carbocation in methanol. Using comparative actinometry the quantum yield of protonation was measured. In methanol, the quantum yield of carbocations reaches a maximum value of 0.18 approximately 18 ps after the pump pulse. According to our analysis, 30% of the photoexcited diazo precursor molecules are eventually protonated. Somewhat lower protonation efficiencies are observed in the other alcohols. Because the primary quantum yield for formation of singlet diphenylcarbene is still unknown, the importance of reaction channels that might exist in addition to protonation cannot be determined at present. Singlet carbenes are powerful, photogenerated bases that open new possibilities for fundamental studies of proton transfer in solution.

Design and Implementation of “Super” Photoacids

“…Similar effects were observed by Peon et al in the symmetrical system, i.e. abstraction of proton from alcohols by super photobases (carbenes) was also rate limited by solvent relaxation [43].…”

Ultrafast time‐resolved studies of the photochemistry of diazo carbonyl compounds

“…The electronic effect preferring the orthogonal structure of the singlet ester carbene affects the ISC rate. A slow ISC rate has been observed for the acyclic carbene ester (<10 9 s −1 , precursor 1 ,6 4 3) compared to carbenes with a distanced carbonyl group (∼10 10 s −1 , precursor 7 21) and without a carbonyl group (>5 × 10 9 s −1 , diphenylcarbene,24 fluorenylidene,25 p ‐biphenylylcarbene,26 and p ‐biphenylylmethylcarbene27). Carbonyl carbenes incorporated within small rings struggle to achieve the preferred orthogonal orientation; they are thereby destabilized, short‐lived and are prone to ultrafast WR 2, 14, 15…”

Persistent Triplet Carbenes

“…125 The experimental ∆E ST values in acetonitrile and isooctane are 2.6 and 4.1 kcal/ mol, respectively. 126 Singlet and triplet DPCs (8a) have bands centered at 370 87 and 300 nm, 127 respectively. The lifetime of 1 8a in cyclohexane and cyclohexane-d 12 is 120 ( 10 and 110 ( 10 ps, respectively, thus suggesting that cyclohexane does not scavenge 1 DPC prior to relaxation to 3 8a.…”