Multiple-Site Concerted Proton-Electron Transfer Reactions of Hydrogen-Bonded Phenols are Non-adiabatic and Well Described by Semi-Classical Marcus Theory

摘要

Photo-oxidations of hydrogen-bonded phenols using excited state polyarenes are described, to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol-bases having −CPh2NH2, −Py, and −CH2Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (>HOAr-NH2, >HOAr-Py, and >HOAr-CH2>Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-^tBu3C6H2OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical kCPET values with those from prior thermal stopped-flow kinetic studies gives datasets for the oxidations of >HOAr-NH2 and of >HOAr-CH2>Py that span over 10⁷ in kCPET and nearly 0.9 eV in driving force (ΔG^o′). Plots of log(kCPET) vs. ΔG^o′ define a single Marcus parabola in each case, each including both excited state anthracenes and ground state aminium radical cations. These two datasets are thus well described by semi-classical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give λCPET ≅ 1.15–1.2 eV and Hab ≅ 20–30 cm⁻¹. These experiments represent the most direct measurements of Hab for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 10¹² s⁻¹. Thus, this is a very clear demonstration that the reactions are non-adiabatic. The non-adiabatic character slows the reactions by a factor of ~45. Results for the oxidation of >HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-^tBu₃C₆H₂OH, which lacks a base, show that both have substantially lower λ and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.