Hydrogen Atom Reactivity toward Aqueous <i>tert</i>-Butyl Alcohol

Through a combination of pulse radiolysis, purification, and analysis techniques, the rate constant for the H + (CH₃)₃COH → H₂ + ^•CH₂C­(CH₃)₂OH reaction in aqueous solution is definitively determined to be (1.0 ± 0.15) × 10⁵ M^–1 s^–1, which is about half of the tabulated number and 10 times lower than the more recently suggested revision. Our value fits on the Polanyi-type, rate–enthalpy linear correlation ln­(<i>k</i>/<i>n</i>) = (0.80 ± 0.05)­Δ<i>H</i> + (3.2 ± 0.8) that is found for the analogous reactions of other aqueous aliphatic alcohols with <i>n</i> equivalent abstractable H atoms. The existence of such a correlation and its large slope are interpreted as an indication of the mechanistic similarity of the H atom abstraction from α- and β-carbon atoms in alcohols occurring through the late, product-like transition state. <i>tert</i>-Butyl alcohol is commonly contaminated by much more reactive secondary and primary alcohols (2-propanol, 2-butanol, ethanol, and methanol), whose content can be sufficient for nearly quantitative scavenging of the H atoms, skewing the H atom reactivity pattern, and explaining the disparity of the literature data on the H + (CH₃)₃COH rate constant. The ubiquitous use of <i>tert</i>-butyl alcohol in pulse radiolysis for investigating H atom reactivity and the results of this work suggest that many other previously reported rate constants for the H atom, particularly the smaller ones, may be in jeopardy

Mechanistic Insight into Peroxydisulfate Reactivity: Oxidation of the <i>cis</i>,<i>cis</i>-[Ru(bpy)₂(OH₂)]₂O⁴⁺ “Blue Dimer”

One-electron oxidation of the μ-oxo dimer (<i>cis</i>,<i>cis</i>-[Ru^III(bpy)₂(OH₂)]₂O⁴⁺, <b>{3,3}</b>) to <b>{3,4}</b> by S₂O₈^2– can be described by three concurrent reaction pathways corresponding to the three protic forms of <b>{3,3}</b>. Free energy correlations of the rate constants, transient species dynamics determined by pulse radiolysis, and medium and temperature dependencies of the alkaline pathway all suggest that the rate-determining step in these reactions is a strongly nonadiabatic dissociative electron transfer within a precursor ion pair leading to the <b>{3,4}</b>|SO₄^2–|SO₄^•– ion triple. As deduced from the SO₄^•– scavenging experiments with 2-propanol, the SO₄^•– radical then either oxidizes <b>{3,4}</b> to <b>{4,4}</b> within the ion triple, effecting a net two-electron oxidation of <b>{3,3}</b>, or escapes in solution with ∼25% probability to react with additional <b>{3,3}</b> and <b>{3,4}</b>, that is, effecting sequential one-electron oxidations. The reaction model presented also invokes rapid <b>{3,3}</b> + <b>{4,4}</b> → 2<b>{3,4}</b> comproportionation, for which <i>k</i>_com ∼5 × 10⁷ M^–1 s^–1 was independently measured. The model provides an explanation for the observation that, despite favorable energetics, no oxidation beyond the <b>{3,4}</b> state was detected. The indiscriminate nature of oxidation by SO₄^•– indicates that its fate must be quantitatively determined when using S₂O₈^2– as an oxidant

Water Oxidation Catalyzed by Cobalt(II) Adsorbed on Silica Nanoparticles

A novel, highly efficient, and stable water oxidation catalyst was prepared by a pH-controlled adsorption of Co­(II) on ∼10 nm diameter silica nanoparticles. A <i>lower limit</i> of ∼300 s^–1 per cobalt atom for the catalyst turnover frequency in oxygen evolution was estimated, which attests to a very high catalytic activity. Electron microscopy revealed that cobalt is adsorbed on the SiO₂ nanoparticle surfaces as small (1–2 nm) clusters of Co­(OH)₂. This catalyst is optically transparent over the entire UV–vis range and is thus suitable for mechanistic investigations by time-resolved spectroscopic techniques

Role of Hydrogen Bonding in Photoinduced Electron–Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand

Electron–proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, <b>1­(T)</b>, can be described by a kinetic model that accounts for the H-bonding of <b>1­(T)</b> to phenol, <b>1­(T)</b> to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and <b>1­(T)</b>-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis. Control experiments replacing either <b>1­(T)</b> by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between <b>1­(T)</b> and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT

Mechanism of the Formation of a Mn-Based CO₂ Reduction Catalyst Revealed by Pulse Radiolysis with Time-Resolved Infrared Detection

Using a new technique, which combines pulse radiolysis with nanosecond time-resolved infrared (TRIR) spectroscopy in the condensed phase, we have conducted a detailed kinetic and mechanistic investigation of the formation of a Mn-based CO₂ reduction electrocatalyst, [Mn­(^<i>t</i>Bu₂-bpy)­(CO)₃]₂ (^<i>t</i>Bu₂-bpy = 4,4′-^<i>t</i>Bu₂-2,2′-bipyridine), in acetonitrile. The use of TRIR allowed, for the first time, direct observation of all the intermediates involved in this process. Addition of excess [^<i>n</i>Bu₄N]­[HCO₂] to an acetonitrile solution of <i>fac</i>-MnBr­(^<i>t</i>Bu₂-bpy)­(CO)₃ results in its quantitative conversion to the Mn–formate complex, <i>fac</i>-Mn­(OCHO)­(^<i>t</i>Bu₂-bpy)­(CO)₃, which is a precatalyst for the electrocatalytic reduction of CO₂. Formation of the catalyst is initiated by one-electron reduction of the Mn–formate precatalyst, which produces the bpy ligand-based radical. This radical undergoes extremely rapid (τ = 77 ns) formate dissociation accompanied by a free valence shift to yield the five-coordinate Mn-based radical, Mn^•(^<i>t</i>Bu₂-bpy)­(CO)₃. TRIR data also provide evidence that the Mn-centered radical does not bind acetonitrile prior to its dimerization. This reaction occurs with a characteristically high radical–radical recombination rate (2<i>k</i>_dim = (1.3 ± 0.1) × 10⁹ M^–1 s^–1), generating the catalytically active Mn–Mn bound dimer