NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni 2+ to Ni 3+ , followed by oxidation to a mixed Ni 3+/4+ state at a potential coincident with the onset of OER activity. Calculations on the 25% Fedoped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe 4+ and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixedmetal oxidation states in heterogeneous catalysts.NiFe oxyhydroxide | oxygen evolution reaction | electrocatalysis | spectroelectrochemistry | density functional theory T he photoelectrochemical conversion of water into O 2 and H 2 is a major focus of energy storage and conversion efforts (1-4), with significant attention directed toward development of efficient catalysts for water oxidation and reduction. Such catalysts should operate at low overpotential, exhibit high selectivity, and be composed of earth-abundant materials. Commercial electrolyzers typically use transition-metal-oxide electrocatalysts for the oxygen evolution reaction (OER) (5, 6), and nickel, nickel-iron, and other mixed-metal oxides are especially effective under alkaline conditions (7,8). Despite the importance and potential future impact of these materials, many features of their catalytic mechanism are poorly understood.Nickel oxyhydroxide has long been associated with OER electrocatalysis (9, 10); however, much of the activity in this material has been shown to arise from the presence of Fe impurities (7, 11). This conclusion complements extensive independent studies demonstrating the effectiveness of NiFebased oxide and oxyhydroxide materials as OER electrocatalysts (12-14), including a survey of nearly 3,500 mixed-metal-oxide compositions, which drew attentio...

The rate constants for typical concerted proton-coupled electron transfer (PCET) reactions depend on the vibronic coupling between the diabatic reactant and product states. The form of the vibronic coupling is different for electronically adiabatic and nonadiabatic reactions, which are associated with hydrogen atom transfer (HAT) and electron–proton transfer (EPT) mechanisms, respectively. Most PCET rate constant expressions rely on the Condon approximation, which assumes that the vibronic coupling is independent of the nuclear coordinates of the solute and the solvent or protein. Herein we test the Condon approximation for PCET vibronic couplings. The dependence of the vibronic coupling on molecular geometry is investigated for an open and a stacked transition state geometry of the phenoxyl-phenol self-exchange reaction. The calculations indicate that the open geometry is electronically nonadiabatic, corresponding to an EPT mechanism that involves significant electronic charge redistribution, while the stacked geometry is predominantly electronically adiabatic, corresponding primarily to an HAT mechanism. Consequently, a single molecular system can exhibit both HAT and EPT character. The dependence of the vibronic coupling on the solvent or protein configuration is examined for the soybean lipoxygenase enzyme. The calculations indicate that this PCET reaction is electronically nonadiabatic with a vibronic coupling that does not depend significantly on the protein environment. Thus, the Condon approximation is shown to be valid for the solvent and protein nuclear coordinates but invalid for the solute nuclear coordinates in certain PCET systems. These results have significant implications for the calculation of rate constants, as well as mechanistic interpretations, of PCET reactions.

The benchmarking of the performance for H 2 evolution of cobalt diimine-dioxime catalysts is provided based on a comprehensive study of their catalytic mechanism. The latter follows an ECE'CC pathway with intermediate formation of a Co(II)-hydride intermediate and second protonation possibly at a basic site of the ligand, acting as a proton relay. This suggests an intramolecular coupling between the hydride and protonated ligand as the proton concentration-independent ratedetermining step controlling the turnover frequency for H 2 evolution.

In this work, we present a polarizable frozen density embedding (FDE) method for calculating polarizabilities of coupled subsystems. The method (FDE-pol) combines a FDE method with an explicit polarization model such that the expensive freeze/thaw cycles can be bypassed, and approximate nonadditive kinetic potentials are avoided by enforcing external orthogonality between the subsystems. To describe the polarization of the frozen environment, we introduce a Hirshfeld partition-based density-dependent method for calculating the atomic polarizabilities of atoms in molecules, which alleviates the need to fit the atomic parameters to a specific system of interest or to a larger general set of molecules. We show that the Hirshfeld partition-based method predicts molecular polarizabilities close to the basis set limit, and thus, a single basis set-dependent scaling parameter can be introduced to improve the agreement against the reference polarizability data. To test the model, we characterized the uncoupled and coupled response of small interacting molecular complexes. Here, the coupled response properties include the perturbation of the frozen system due to the external perturbation which is ignored in the uncoupled response. We show that FDE-pol can accurately reproduce both the exact uncoupled polarizability and the coupled polarizabilities of the supermolecular systems. Using damped response theory, we also demonstrate that the coupled frequency-dependent polarizability can be described by including local field effects. The results emphasize the necessity of including local-field effects for describing the response properties of coupled subsystems, as well as the importance of accurate atomic polarizability models.

In recent research [ Chem. Commun. 2014 , 50 , 8667 ], it was found that photoinduced enolization occurred in 1,9-diformyl-5,5-diaryldipyrromethane (DA) by excited-state dual proton transfer resulting in a red-shifted absorption, a phenomena not observed in 1,9-diformyl-5,5-dimethyldipyrromethane (DM) and 1,9-diformyl-5-aryldipyrromethane (MA). The observation was supported by preliminary density functional theoretical (DFT) calculations. In the work reported here, a detailed and systematic study was undertaken considering four molecules, 1,9-diformyldipyrromethane (DH), DM, MA, and DA and their rotational isomers using DFT methods. Different processes, namely, cis-trans isomerization and single and double proton transfer processes, and their mechanistic details were investigated in the ground and excited states. From the simulation studies, it was seen that the presence of different substituents at the meso carbon does not affect the λ values during cis → trans isomerization. However, enolization by proton transfer processes were found to be influenced by the substituents, as seen in the experiments. Enolization was observed to follow a stepwise mechanism, that is, diketo → monoenol → dienol. While monoenols showed negligible substituent effects on the λ values, a large red shift in λ was seen only in DA, in agreement with the experimental findings. This observation can be attributed to the lowering of the keto → enol activation barrier, stabilization of DA in the S state, and the charge transfer nature of the transitions involved in DA.