Search

This contribution explains the origin of dramatic rate accelerations in chemical reactions taking place in/on aqueous electrosprays. We combine experiments with electrosprays and proton-nuclear magnetic resonance with quantum mechanics to systematically decouple genuine interfacial effects from non-equilibrium conditions., The recent application of electrosprays to characterize the air–water interface, along with the reports on dramatically accelerated chemical reactions in aqueous electrosprays, have sparked a broad interest. Herein, we report on complementary laboratory and in silico experiments tracking the oligomerization of isoprene, an important biogenic gas, in electrosprays and isoprene–water emulsions to differentiate the contributions of interfacial effects from those of high voltages leading to charge-separation and concentration of reactants in the electrosprays. To this end, we employed electrospray ionization mass spectrometry, proton nuclear magnetic resonance, ab initio calculations and molecular dynamics simulations. We found that the oligomerization of isoprene in aqueous electrosprays involved minimally hydrated and highly reactive hydronium ions. Those conditions, however, are non-existent at pristine air–water interfaces and oil–water emulsions under normal temperature and pressure. Thus, electrosprays should be complemented with surface-specific platforms and theoretical methods to reliably investigate chemistries at the pristine air–water interface.

We develop the grand canonical potential kinetics (GCP-K) formulation based on thermodynamics from quantum mechanics calculations to provide a fundamental basis for understanding heterogeneous electrochemical reactions. Our GCP-K formulation arises naturally from minimizing the free energy using a Legendre transform relating the net charge of the system and the applied voltage. Performing this macroscopic transformation explicitly allows us to make the connection of GCP-K to the traditional Butler–Volmer kinetics. Using this GCP-K based free energy, we show how to predict both the potential and pH dependent chemistry for a specific example, the hydrogen evolution reaction (HER) at a sulfur vacancy on the basal plane of MoS_2. We find that the rate-determining steps in both acidic and basic conditions are the Volmer reaction in which the second hydrogen atom is adsorbed from the solution. Using the GCP-K formulation, we show that the stretched bond distances change continuously as a function of the applied potential. This shows that the main reason for the higher activity in basic conditions is that the transition state is closer to the product, which leads to a more favorable Tafel slope of 60 mV/dec. In contrast if the transition state were closer to the reactant, where the transfer coefficient is less than 0.5 we would obtain a Tafel slope of almost 150 mV/dec. Based on this detailed understanding of the reaction mechanism, we conclude that the second hydrogen at the chalcogenide vacant site is the most active toward the hydrogen evolution reaction. Using this as a descriptor, we compare it to the other 2H group VI metal dichalcogenides and predict that vacancies on MoTe_2 will have the best performance toward HER.

The recent application of electrosprays to characterize the air-water interface, along with the reports on dramatically accelerated chemical reactions in aqueous electrosprays, have sparked a broad interest. Herein, we report on complementary laboratory and in silico experiments tracking the oligomerization of isoprene, an important biogenic gas, in electrosprays and isoprene-water emulsions to differentiate the contributions of interfacial effects from those of high voltages leading to charge-separation and concentration of reactants in the electrosprays. To this end, we employed electrospray ionization mass spectrometry, proton nuclear magnetic resonance, and quantum mechanical simulations. We found that the oligomerization of isoprene in aqueous electrosprays involved minimally hydrated and highly reactive hydronium ions. Those conditions, however, are non-existent at pristine air-water interfaces and oil-water emulsions under normal temperature and pressure. Thus, electrosprays should be complemented with surface-specific platforms and theoretical methods to reliably investigate chemistries at the pristine air-water interface., 16 pages and 6 figures in the main paper; 30 pages and 14 figures in total including supporting information

The oxygen rebound mechanism, proposed four decades ago, is invoked in a wide range of oxygen and hetero-atom transfer reactions. In this process, a high-valent metal-oxo species abstracts a hydrogen atom from the substrate to generate a carbon-centered radical, which immediately recombines with the hydroxometal intermediate with very fast rate constants that can be in the ns to ps regime. In addition to catalyzing C-O bond formation, we found that manganese porphyrins can also directly catalyze C-H halogenations and pseudohalogenations, including chlorination, bromination and fluorination as well as C-H azidation. For these cases, we showed that long-lived substrate radicals are involved, indicating that radical rebound may involve a barrier in some cases. In this study, we show that axial ligands significantly affect the oxygen rebound rate. Fluoride, hydroxide and oxo ligands all slow down the oxygen rebound rate by factors of 10-40 fold. The oxidation of norcarane by a manganese porphyrin coordinated with fluoride or hydroxide leads to the formation of significant amounts of radical rearranged products. Cis-decalin oxidation afforded both cis- and trans-decalol. Xanthene afforded dioxygen trapped products and the radical dimer product, bixanthene, under aerobic and anaerobic conditions, respectively. DFT calculations probing the rebound step show that the rebound barrier increases significantly (by 3.3, 5.4 and 6.0 kcal/mol, respectively) with fluoride, hydroxide and oxo as axial ligands.

How to efficiently oxidize H_2O to O_2 (Oxygen Evolution Reaction –OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO_2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work we performed first principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the IrO_2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the micro-kinetic models. This allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.

It has been demonstrated experimentally that electrochemical CO_2 reduction catalyzed by (POCOP)IrH_2 ([C_6H_3-2,6-[OP(tBu)_2]_2]IrH_2) produces formate without significant H_2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from Ir^(III) dihydride explains the selectivity for formate over H_2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)_2^+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of Ir^(III)(H)(NCMe)_2^+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both Ir^(III)(H)_2 and Ir^I–H^– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO_2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from Ir^I and Ir^(III) intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H_2/H^+ and HCOO^–/CO_2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO_2. The effect of solvation on calculated driving forces for hydride transfer is also discussed.

Experimentally, [(L)Mn(CO)_3]− (where L = bis-alkyl-substituted bipyridine) has been observed to catalyze the electrochemical reduction of CO_2 to CO in the presence of trifluoroethanol (TFEH). Here we report the atomistic level mechanism of complete catalytic cycles for this reaction, on the basis of DFT calculations (B3LYP-D3 with continuum solvation) of the free energies of reaction and activation, as well as reduction potentials for all catalytically relevant elementary steps. The highly exergonic homoconjugation and carbonation of TFE– play critical roles in reaction thermodynamics and kinetics, the overall half-reaction being 3CO_2 + 2TFEH + 2e– → CO + H_2O + 2[F_3CCH_2OCO_2]− (calculated standard reduction potential: −1.49 V vs SCE). In the catalytic cycle for CO formation, CO_2 coordinates to [(L)Mn(CO)_3]− (1a, L = bpy), and the adduct is then protonated to form [(L)Mn(CO)_3(CO_2H)] (3a). 3a subsequently reacts to form [(L)Mn(CO)_4]0 (5a) via one of two pathways: (a) TFEH-mediated dehydroxylation to [(L)Mn(CO)_4]+ (4a), followed by one-electron reduction to 5a, or (b) under more reducing potentials, one-electron reduction to [(L)Mn(CO)_3(CO_2H)]− (3′a), followed by dehydroxylation to 5a. Pathway b has a lower activation energy by 2.2 kcal mol^(–1). Consequently, the maximum catalytic turnover frequency (TOF_(max)) is achieved at ∼−1.75 V vs SCE (∼0.25 V overpotential). For the analogous bipyrimidine compound (not yet studied experimentally), reduction of 3b to 3′b occurs at a potential 0.5 V more positive than that of 3a, and the overpotential required to achieve TOF_(max) is predicted to be lower by ∼0.25 V. This improvement is, however, achieved at the price of a lower TOF_(max), and we predict that 1b has superior TOF at potentials above ∼−1.6 V vs SCE. In addition, the various factors contributing to product selectivity (CO over H_2) are discussed.

The Rh(I) complexes [(^(Fl)DAB)Rh(coe)(TFA)] (1) and [(BOZO)Rh(coe)(TFA)] (2) [^(Fl)DAB = N,N-bis-(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene, coe = cyclooctene, TFA = trifluoroacetate, BOZO = bis(2-oxazolin-2-yl)] are efficient catalyst precursors for H/D exchange between arenes and DTFA. Catalyst precursor 1 exhibits a TOF of 0.06 s^(−1) at 150 °C for benzene H/D exchange. DFT calculations revealed that H/D exchange through reversible oxidative addition or internal electrophilic substitution of benzene is a viable pathway.

Traditional C–H bond activation by a concerted metalation–deprotonation (CMD) mechanism involves precoordination of the C–H bond followed by deprotonation from an internal base. Reported herein is a “through-arene” activation of an uncoordinated benzylic C–H bond that is 6 bonds away from a Rh^(III) ion. The mechanism, which was investigated by experimental and DFT studies, proceeds through a dearomatized xylene intermediate. This intermediate was observed spectroscopically upon addition of a pyridine base to provide a thermodynamic trap.

The mechanism of oxidation by O_2 of (dpms)Pt^(II)Me(OH_2) (1) and (dpms)Pt^(II)Me(OH)^− (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4–14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C_1-symmetric monomethyl Pt^(IV) complex (dpms)Pt^(IV)Me(OH)_2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt^(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O_2 activation at the Pt^(II) center to form a Pt^(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O_2 than neutral complex 1 (pK_a = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH^– ions; the reaction order in [Pt^(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt^(IV) hydroperoxide 4. At pH ≥ 12, formation of a C_1-symmetric dimethyl complex 6, (dpms)Pt^(IV)Me_2(OH), along with [(dpms)Pt^(II)(OH)_2]^− (7) becomes the dominant reaction pathway (50–70% selectivity). This change in the product distribution is explained by the formation of a C_s-symmetric intermediate (dpms)Pt^(IV)Me(OH)_2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; k_H/k_D = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A S_N2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.