Multiple Proton Transfer Reactions Research Articles

Synthesis and characterization of a series of CpW(CO)2PR3H, [CpW(CO)2PR3]−, [CpW(CO)2PR3(CH3CN)]+, and [CpW(CO)2PR3]2 complexes

A series of tungsten cyclopentadienyl carbonyl complexes were prepared and characterized to quantify their thermochemical properties and explore their reactivity. The PR3 ligand was systematically varied across a series of CpW(CO)2PR3H metal hydride complexes, where PR3 = P(OEt)3, P(Bu)3, and P(Cy)3. These complexes are known to undergo multiple proton, electron, and proton-coupled electron transfer reactions to access a variety of species including [CpW(CO)2PR3]−, [CpW(CO)2PR3(CH3CN)]+, and [CpW(CO)2PR3]2. Cyclic voltammograms of the CpW(CO)2PR3H+/0 and [CpW(CO)2PR3]0/− couples are chemically irreversible, indicating chemical reactivity upon oxidation; the anodic peak potential shifts to lower potentials as the donating ability of phosphine is increased, agreeing with previous literature on similar complexes. Additionally, voltammograms of [CpW(CO)2P(Cy)3]− become chemically reversible at scan rates above 500 mV/s, indicating that the dimerization of the [CpW(CO)2PR3] product, formed by the oxidation of [CpW(CO)2PR3]−, is slower with the sterically bulky phosphine P(Cy)3, and at high scan rates the species can be reduced before dimerization occurs. Further, as the donating ability of the phosphine increases, the pKa of the CpW(CO)2PR3H complexes increases. This work shows how ligand sterics and electronics can tune the thermochemical properties that underpin proton, electron, and proton-coupled electron transfer reactivity of these complexes.

Highly Accurate CCSDT(Q)/CBS Reaction Barrier Heights for a Diverse Set of Transition Structures: Basis Set Convergence and Cost-Effective Approaches for Estimating Post-CCSD(T) Contributions.

The ability to accurately calculate reaction barrier heights is of central importance to many areas of chemistry. We report an extensive study examining the basis set convergence of post-CCSD(T) contributions (up to CCSDT(Q)) for a diverse set of 28 reaction barrier heights. In contrast to previous studies, we focus here on larger transition structures (TSs) involving 4-7 non-hydrogen atoms. The set of reaction barrier heights includes pericyclic, bipolar cycloaddition, cycloreversion, and multiple-proton transfer reactions. We find that in most cases post-CCSD(T) contributions converge rapidly toward the basis set limit, such that even double-ζ and truncated double-ζ basis sets provide useful estimates of the T-(T) and (Q) contributions, respectively. In addition, we find that due to the tendency of these small basis sets to systematically underestimate the T-(T) and (Q) components, scaling is an effective approach for improving performance. For example, scaling the T-(T)/cc-pVDZ contribution by 1.25 results in an RMSD of merely 0.4 kJ mol-1 relative to basis set limit reference values from W3lite-F12 theory. Similarly, calculating the (Q) contribution with a cc-pVDZ basis set without d functions and scaling by 1.6 results in an RMSD of 0.5 kJ mol-1. We also examine the magnitude of post-CCSD(T) contributions for a wide range of TSs. We find that for pericyclic, bipolar cycloaddition, and multiple-proton transfer reactions there is an effective cancellation between the T-(T) and (Q) components (i.e., they have opposite signs and are of similar magnitude), such that overall post-CCSD(T) contributions to the reaction barrier heights are below ∼1 kJ mol-1 (in absolute value). However, for the barrier heights of cycloreversion reactions, the T-(T) and (Q) components are both negative and large and consequentially post-CCSD(T) contributions reduce the reaction barrier heights by significant amounts ranging between 4.1 and 6.7 kJ mol-1.

Controlling Electrons and Protons through Theory: Molecular Electrocatalysts to Nanoparticles.

The development of renewable energy sources that are environmentally friendly and economical is of critical importance. The effective utilization of such energy sources relies on catalysts to facilitate the interconversion between electrical and chemical energy through multielectron, multiproton reactions. The design of effective catalysts for these types of energy conversion processes requires the ability to control the localization and movement of electrons and protons, as well as the coupling between them. Theoretical calculations, in conjunction with experimental validation and feedback, are playing a key role in these catalyst design efforts. A general theory has been developed for describing proton-coupled electron transfer (PCET) reactions, which encompass all reactions involving the coupled transfer of electrons and protons, including sequential and concerted mechanisms for multielectron, multiproton processes. In addition, computational methods have been devised to compute the input quantities for the PCET rate constant expressions and to generate free energy pathways for molecular electrocatalysts. These methods have been extended to heterogeneous PCET reactions to enable the modeling of PCET processes at electrode and nanoparticle surfaces. Three distinct theoretical studies of PCET reactions relevant to catalyst design for energy conversion processes are discussed. In the first application, theoretical calculations of hydrogen production catalyzed by hangman metalloporphyrins predicted that the porphyrin ligand is reduced, leading to dearomatization and proton transfer from the carboxylic acid hanging group to the meso carbon of the porphyrin rather than the metal center, producing a phlorin intermediate. Subsequent experiments isolated and characterized the phlorin intermediate, validating this theoretical prediction. These molecular electrocatalysts exemplify the potential use of noninnocent ligands to localize electrons and protons on different parts of the catalyst and to direct their motions accordingly. In the second application, theoretical calculations on substituted benzimidazole phenol molecules predicted that certain substituents would lead to multiple intramolecular proton transfer reactions upon oxidation. Subsequent experiments verified these multiproton reactions, as well as the predicted shifts in the redox potentials and kinetic isotope effects. These bioinspired molecular systems demonstrate the potential use of multiproton relays to enable the transport of protons over longer distances along specified pathways, as well as the tuning of redox potentials through this movement of positive charge. In the third application, theoretical studies of heterogeneous PCET in photoreduced ZnO nanoparticles illustrated the significance of proton diffusion through the bulk of the nanoparticle as well as interfacial PCET to an organic radical in solution at its surface. These theoretical calculations were consistent with prior experimental studies of this system, although theoretical methods for heterogeneous PCET have not yet attained the level of predictive capability highlighted for the molecular electrocatalysts. These examples suggest that theory will play a significant role in the design of both molecular and heterogeneous catalysts to control the movement and coupling of electrons and protons. The resulting catalysts will be essential for the development of renewable energy sources to address current energy challenges.

Molecular dynamics study of taxadiene synthase catalysis.

Molecular dynamics (MD) simulations have been performed to study the dynamic behavior of noncovalent enzyme carbocation complexes involved in the cyclization of geranylgeranyl diphosphate to taxadiene catalyzed by taxadiene synthase (TXS). Taxadiene and the observed four side products originate from the deprotonation of carbocation intermediates. The MD simulations of the TXS carbocation complexes provide insights into potential deprotonation mechanisms of such carbocations. The MD results do not support a previous hypothesis that carbocation tumbling is a key factor in the deprotonation of the carbocations by pyrophosphate. Instead water bridges are identified which may allow the formation of side products via multiple proton transfer reactions. A novel reaction path for taxadiene formation is proposed on the basis of the simulations. © 2018 Wiley Periodicals, Inc.

Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde

1,8-Dihydroxy-2-naphthaldehyde (DHNA), having doubly intramolecular hydrogen bonds, was strategically designed and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double-proton transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S0 →S1 (π-π*) absorption at ∼400 nm. Upon excitation, two large Stokes shifted emission bands maximized at 520 and 650 nm are resolved, which are ascribed to the tautomer emission resulting from the first and second proton-transfer products, denoted by TA* and TB*, respectively. The first proton transfer (DHNA* → TA*) is ultrafast (< system response of 150 fs), whereas the second proton transfer is reversible, for which the rates of forward (TA* → TB*) and backward (TA* ← TB*) proton transfer were determined to be (1.7 ps)(-1) and (3.6 ps)(-1), respectively. The fast equilibrium leads to identical population lifetimes of ∼54 ps for both TA* and TB* tautomers. Similar excited-state double-proton transfer takes place for DHNA in a single crystal, resulting in TA* (560 nm) and TB* (650 nm) dual-tautomer emission. A comprehensive 2D plot of reaction potential energy surface further proves that the sequential two-step proton motion is along the minimum energetic pathway firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay type of intramolecular double-proton transfer reaction in the excited state, which should gain fundamental understanding of the multiple proton transfer reactions.

Excited-state proton-transfer reactions of 7-azaindole with water, ammonia and mixed water–ammonia: microsolvated dynamics simulation

Dynamics simulations of excited-state multiple proton transfer (ESMPT) reactions in 7-azaindole (7AI) with ammonia, mixed water–ammonia, and water molecules were investigated by quantum dynamics simulations in the first-excited state using RI-ADC(2)/SVP-SV(P) in the gas phase. 7AI(WW), 7AI(WA), 7AI(AW) and 7AI(AA) clusters (W, water and A, ammonia) show very high probability of the excited-state triple proton transfer (ESTPT) occurrence in ranges from 20% for 7AI(WA) to 60% for 7AI(AW), respectively. Furthermore, 7AI(AW) clusters with ammonia placed near N–H of 7AI has the highest probability among other isomers. In 7AI with three molecules of bridged-planar of water, ammonia and mixed water–ammonia clusters, the excited-state quadruple proton transfer reactions occur ineffectively and rearrangement of hydrogen-bonded network on solvents also takes place prior to either ESTPT or excited-state double proton transfer. The role played by mixed-solvent is revealed with replacing H2O with NH3 in which the ESMPT is found to be more efficient corresponding to lower barrier in the excited state. The preferential number of solvent surrounding 7AI that facilitates the proton transfer process is two for methanol and water but this preferential number for ammonia is one.Highlights: (i) replacing H2O with NH3 assists ESPT corresponding to lower barrier in the excited state; (ii) the ESMPT time of 7AI with mixed water–ammonia is in the sub-picosecond timescale; (iii) the PT tends to be concerted process with at least one ammonia, but synchronous without ammonia.

Ground-state and excited-state multiple proton transfer via a hydrogen-bonded water wire for 3-hydroxypyridine

The multiple proton transfer reactions of 3-hydroxypyridine-(H2O)3have been demonstrated, and a perfect proton transfer cycle has been revealed in the ground and excited states.

Theoretical study on excited-state intermolecular proton transfer reactions of 1H-pyrrolo[3,2-h]quinoline with water and methanol

The dynamics of the ultrafast excited-state multiple intermolecular proton transfer (PT) reactions in gas-phase complexes of 1H-pyrrolo[3,2-h]quinoline with water and methanol (PQ(H2O) n and PQ(MeOH) n , where n = 1, 2) is modeled using quantum-chemical simulations. The minimum energy ground-state structures of the complexes are determined. Molecular dynamics simulations in the first excited state are employed to determine reaction mechanisms and the time evolution of the PT processes. Excited-state dynamics results for all complexes reveal synchronous excited-state multiple proton transfer via solvent-assisted mechanisms along an intermolecular hydrogen-bonded network. In particular, excited-state double proton transfer is the most effective, occurring with the highest probability in the PQ(MeOH) cluster. The PT character of the reactions is suggested by nonexistence of crossings between ππ* and πσ* states.

Variation of kinetic isotope effect in multiple proton transfer reactions#

Recently, we had suggested that the motion along the promoter mode in the first part of the IRC of proton transfer reaction enhances the delocalization of n σ electrons on the acceptor atom into the σ* orbital of the donor-hydrogen covalent bond, and as a consequence weakens it. This leads to a reduction of the barrier to the proton transfer as well as the stretching frequency of donor-hydrogen bond. An extension of this to the concerted multiple proton transfer reactions implies that the kinetic isotope effect in such reaction depends exponentially on the number of protons that are being transferred. Computational evidence on three systems, (HF)3, formic acid dimer, and (H2O)n clusters is provided to support this assertion. Kinetic isotope effect in multiple proton transfer reactions is investigated with specific reference to three systems, (HF)3, (H2O)n clusters and formic acid dimer.

Spectroscopic study on the structural isomers of 7-azaindole(ethanol)n (n=1–3) and multiple-proton transfer reactions in the gas phase

The resonance-enhanced two-photon ionization (RE2PI) and laser-induced fluorescence excitation spectra were recorded for the S(1)-S(0)(pipi( *)) region of the 7-azaindole(ethanol)(n) (n=1-3) [7AI(EtOH)(n) (n=1-3)] clusters in the gas phase to investigate the geometrical structures and the multiple-proton/hydrogen atom transfer reaction dynamics. Four and two structural isomers were identified for 7AI(EtOH)(2) and 7AI(EtOH)(3), respectively. Density functional theory calculations at the B3LYP/6-31++G( * *)/6-31G( *) level predicted four different conformations of the ethyl group for 7AI(EtOH)(2), in good agreement with the observation of the four structural isomers in the RE2PI spectra. Visible fluorescence from the tautomeric forms was observed in the S(1) states for all isomers of 7AI(EtOH)(2), but no sign of double-proton/hydrogen atom transfer and quadruple-proton/hydrogen atom transfer has been obtained in the electronic spectra of 7AI(EtOH)(1) and 7AI(EtOH)(3), respectively. These results suggest that the multiple-proton transfer reaction is cluster-size selective, and the triple-proton/hydrogen atom transfer potential is dominated by the cyclic hydrogen-bonded network in 7AI(EtOH)(2). The excitation of the in-phase intermolecular stretching vibration prominently enhances the excited-state triple-proton/hydrogen atom transfer reaction.

The Role of Amino Acid Composition in the Charge Inversion of Deprotonated Peptides via Gas-Phase Ion/Ion Reactions

Ion/ion charge inversion via multiple proton transfer reactions occurs via a long-lived intermediate. The intermediate can be observed if its lifetime is long relative to mechanisms for removal of excess energy (i.e., emission and collisional stabilization). The likelihood for formation of a stabilized intermediate is a function of characteristics of the reagent and analyte ions. This work is focused on the role acidic and basic sites of a deprotonated peptide play in the formation of a stabilized intermediate upon charge inversion with multiply protonated polypropyleniminediaminobutane dendrimers. A group of model peptides based on leucine enkephalin was used, which included YGGFL, YGGFLF, YGGFLK, YGGFLR and YGGFLH as well as methyl esterified and acetylated versions. Results showed that peptides containing basic amino acid residues charge inverted primarily by proton transfer from the DAB dendrimer to the peptide, whereas peptides without basic amino acids charge inverted primarily by complex formation with the DAB dendrimer. The modified versions of the peptides highlighted the importance of the presence of the C-terminus as well as the basicity of the peptide in the observation of a stabilized intermediate. These results provide new insights into the nature of the interactions that occur in the charge inversion of polypeptide anions via ion/ion reactions.

Determination of a Unique Solution to Parallel Proton Transfer Reactions Using the Genetic Algorithm

Kinetic analysis of the dynamics as measured in multiequilibria systems is readily attained by curve-fitting methodologies, a treatment that can accurately retrace the shape of the measured signal. Still, these reconstructions are not related to the detailed mechanism of the process. In this study we subjected multiple proton transfer reactions to rigorous kinetic analysis, which consists of solving a set of coupled-nonlinear differential rate equations. The manual analysis of such systems can be biased by the operator; thus the analysis calls for impartial corroboration. What is more, there is no assurance that such a complex system has a unique solution. In this study, we used the Genetic Algorithm to investigate whether the solution of the system will converge into a single global minimum in the multidimensional parameter space. The experimental system consisted of proton transfer between four proton-binding sites with seven independent adjustable parameters. The results of the search indicate that the solution is unique and all adjustable parameters converge into a single minimum in the multidimensional parameter space, thus corroborating the accuracy of the manual analysis.

Solvent effects on glycine II. Water-assisted tautomerization.

The water-assisted tautomerization of glycine has been investigated at the B3LYP/6-31+G** level using supermolecules containing up to six water molecules as well as considering a 1:1 glycine-water complex embedded in a continuum. The conformations of the tautomers in this mechanism do not display an intramolecular H bond, instead the functional groups are bridged by a water molecule. The replacement of the intramolecular H bond by the bridging water reduces the polarity of the N-H bond in the zwitterion and increases that of the O-H bond in the neutral, stabilizing the zwitterion. Both the charge transfer effects and electrostatic interactions stabilize the nonintramolecularly H-bonded zwitterion conformer over the intramolecularly hydrogen bonded one. The nonintramolecularly H-bonded neutral is favored only by charge transfer effects. Although there is no strong evidence whether the intramolecularly hydrogen bonded or non hydrogen bonded structures are favored in the bulk solution represented as a dielectric continuum, it is likely that the latter species are more stable. The free energy of activation of the water-assisted mechanism is higher than the intramolecular proton transfer channel. However, when the presumably higher conformational energy of the zwitterion reacting in the intramolecular mechanism is taken into account, both mechanisms are observed to compete. The various conformers of the neutral glycine may form via multiple proton transfer reactions through several water molecules instead of a conformational rearrangement.

Water-Mediated Proton Transfer: A Mechanistic Investigation on the Example of the Hydration of Sulfur Oxides

We outline general mechanistic features of “water-mediated proton transfer” in the example of the isomerization reaction in hydrogen-bonded sulfur oxide−water supermolecules containing up to three water molecules. The nucleophilic attack of a water oxygen on the sulfur atom occurs concertedly with the (multiple) proton-transfer event(s). The protons are transferred according to the well-known hydrogen-bond compression mechanism. However, contrary to “pure” multiple proton-transfer reactions, the protons are transferred asynchronously. These mechanistic features force the reaction to be classical rather than quantum-tunneling-dominated down to rather low temperatures. In the quantum-dominated temperature region, tunneling takes place only if all protons tunnel through the barrier. Straight line corner cutting (large curvature tunneling) does not dominate at any temperature, as the reduced mass corresponding to reaction coordinate motion does not drop to values low enough in the reaction barrier region. The...

Concertedness and solvent effects in multiple proton transfer reactions: The formic acid dimer in solution

The issue of multiple proton transfer (PT) reactions in solution is addressed by performing molecular dynamics simulations for a formic acid dimer embedded in a water cluster. The reactant species is treated quantum mechanically, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of more complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The dynamics of the double PT process is studied in a low-barrier case and correlated with solvent polarization fluctuations. Cooperative effects in the motion of the two protons are observed in two different situations: when the solvent polarization does not favor the transfer of one of the two protons and when the motion of the two protons is not synchronized. This body of observations is correlated with local structural and dynamical properties of the solvent in the vicinity of the reactant.

Multistate continuum theory for multiple charge transfer reactions in solution

In this article we present a multistate continuum theory for multiple charge transfer reactions such as proton-coupled electron transfer and multiple proton transfer reactions. The solute is described with a multistate valence bond model, the solvent is represented as a dielectric continuum, and the transferring protons are treated quantum mechanically. This theory provides adiabatic free energy surfaces that depend on a set of scalar solvent variables corresponding to the individual charge transfer reactions. Thus this theory is a multidimensional analog of standard Marcus theory for single charge transfer reactions. For processes involving significant inner-sphere (i.e., solute) reorganization, the effects of solute intramolecular vibrations can be incorporated into the adiabatic free energy surfaces. The input quantities required for this theory are gas phase valence bond matrix elements fit to standard quantum chemistry calculations and solvent reorganization energy matrix elements calculated with standard continuum electrostatic methods. The goal of this theory is to provide insight into the underlying fundamental physical principles dictating the mechanisms and rates of multiple charge transfer reactions.

Effects of Model Protein Environments on the Dynamics of Proton Wires

AbstractThe multiconfigurational molecular dynamics with quantum transitions (MC‐MDQT) method is utilized to study the impact of model protein environments on the dynamics of proton wires. The MC‐MDQT method allows the realtime nonequilibrium quantum dynamical simulation of proton transport along water chains and provides a framework for analyzing the detailed dynamical mechanisms of these multiple proton transfer reactions. In this paper, the protein environment is modeled by applying structural restraints to the oxygen atoms of the chain, by applying external electric fields, and by including solvating water molecules hydrogen‐bonded to the ends of the water chain. Our simulations illustrate that the protein environment could strongly impact the dynamics of proton wires through a combination of structural restraints, fluctuating electric fields, solvation, and hydrogen bonding. Our simulations also indicate that quantum effects such as hydrogen tunneling and nonadiabatic transitions play a significant role under certain nonequilibrium conditions.

Mixed Quantum/Classical Dynamics of Hydrogen Transfer Reactions

This article presents the methodology we have developed for the simulation of hydrogen transfer reactions, including multiple proton transfer and proton-coupled electron transfer reactions. The central method discussed is molecular dynamics with quantum transitions (MDQT), which is a mixed quantum/classical surface hopping method that incorporates nonadiabatic transitions between the proton vibrational and/or electronic states. The advantages of MDQT are that it accurately describes branching processes (i.e., processes involving multiple pathways), is valid in the adiabatic and nonadiabatic limits and the intermediate regime, and provides real-time dynamical information. The multiconfigurational MDQT (MC-MDQT) method combines MDQT with an MC-SCF formulation of the vibrational modes for the simulation of processes involving multiple quantum modes (e.g., for multiple proton transfer reactions). MC-MDQT incorporates the significant correlation between the quantum modes in a computationally practical way and ...

An analytical derivation of MC-SCF vibrational wave functions for the quantum dynamical simulation of multiple proton transfer reactions: Initial application to protonated water chains

This paper presents an analytical derivation of a multiconfigurational self-consistent-field (MC-SCF) solution of the time-independent Schrödinger equation for nuclear motion (i.e. vibrational modes). This variational MC-SCF method is designed for the mixed quantum/classical molecular dynamics simulation of multiple proton transfer reactions, where the transferring protons are treated quantum mechanically while the remaining degrees of freedom are treated classically. This paper presents a proof that the Hellmann–Feynman forces on the classical degrees of freedom are identical to the exact forces (i.e. the Pulay corrections vanish) when this MC-SCF method is used with an appropriate choice of basis functions. This new MC-SCF method is applied to multiple proton transfer in a protonated chain of three hydrogen-bonded water molecules. The ground state and the first three excited state energies and the ground state forces agree well with full configuration interaction calculations. Sample trajectories are obtained using adiabatic molecular dynamics methods, and nonadiabatic effects are found to be insignificant for these sample trajectories. The accuracy of the excited states will enable this MC-SCF method to be used in conjunction with nonadiabatic molecular dynamics methods. This application differs from previous work in that it is a real-time quantum dynamical nonequilibrium simulation of multiple proton transfer in a chain of water molecules.

Multiconfigurational molecular dynamics with quantum transitions: Multiple proton transfer reactions

We present the new method ‘‘multiconfigurational molecular dynamics with quantum transitions’’ (MC-MDQT) for the simulation of processes involving multiple proton transfer reactions. MC-MDQT is a mixed quantum/classical molecular dynamics method that allows the quantum mechanical treatment of the nuclear motion of multiple hydrogen atoms and accurately describes branching processes (i.e., processes involving multiple channels or pathways). MC-MDQT is based on the surface hopping method MDQT, which has already been applied to single proton transfer reactions in solution, where the nuclear motion of only the hydrogen atom being transferred is treated quantum mechanically. The direct extension of MDQT to multiple proton transfer reactions, where many hydrogen atoms must be treated quantum mechanically, is not computationally practical. In MC-MDQT a multiconfigurational self-consistent-field method is combined with MDQT to allow the quantum mechanical treatment of multiple hydrogen atoms while still including the significant correlation. The adiabatic states are expanded in a basis set of single configurations, which are products of one-particle states calculated using effective Hamiltonians derived from the occupied adiabatic state. Thus the one-particle states and the multiconfigurational adiabatic states must be calculated self-consistently. Both the MC-MDQT and the full basis set expansion MDQT methods are applied to a model system comprised of two quantum protons moving in double well potentials and one classical harmonic solvent degree of freedom. The results show that MC-MDQT incorporates the significant correlation and accurately describes branching processes. The MC-MDQT method is also used to study model systems comprised of three quantum protons and one classical solvent degree of freedom.