Energy Barrier For Proton Transfer Research Articles

Photofragmentation of gas-phase acetic acid and acetamide clusters in the vacuum ultraviolet region.

Photofragmentation of gas-phase acetamide and acetic acid clusters produced by a supersonic expansion source has been studied using time-of-flight mass spectrometry and the partial ion yield (PIY) technique combined with tunable vacuum-ultraviolet synchrotron radiation. Appearance energies of the clusters and their fragments were experimentally determined from the PIY measurements. The effect of clusterization conditions on the formation and fragmentation of acetic acid clusters was investigated. Ab initio quantum mechanical calculations were performed on both samples' dimers to find their neutral and ionized geometries as well as proton transfer energy barriers leading to the optimal geometries. In the case of the acetamide dimer, the reaction resulting in the production of ammoniated acetamide was probed, and the geometry of the obtained ion was calculated.

Toward Exploring Novel Organic Materials: MP4-DFT Properties of 4-Amino-3-Iminoindene.

Tautomerism links with many applications and remains an attracting feature in exploring novel systems. In this regard, properties of indene-based HNCCCN segments have not received any considerable attention. In this computational organic chemistry study, first, to calculate the proton transfer energy barrier at a reasonable cost, the study identified an accurate forth order Møller–Plesset perturbation theory-density functional theory (MP4-DFT) protocol equivalent to the outstanding pioneering benchmark calculations. The calculations illustrate that the two tautomers of the 4-amino-3-iminoindene nucleus are separated by a considerable energy barrier while featuring different molecular orbital characteristics; frontier orbital distribution, λmax, and energies, which are known basic requirements in molecular switching and logic circuit applications. The N-H/BH2 substitution was found to have significant influence on the electronic structure of the skeleton. Similarities in the two tautomers and the boron derivative to properties of known molecular materials have been found.

Quantum Mechanical Investigation of Proton Transport in Imidazolium Methanesulfonate Ionic liquid

Protic ionic liquids (PIL) are promising anhydrous proton conductors for fuel cell applications especially at higher temperature. In this study, quantum chemistry calculations using density functional theory (DFT) were performed to investigate proton conduction in imidazolium methanesulfonate (IMMSA) PIL on interaction with varying imidazole (IM) concentration. These investigations include the extent of proton transfer (PT) from methanesulfonic acid to the imidazole base and calculation of various energy barriers of PT. The results show that the difference in gas phase proton affinity (ΔPA) of the anion and base is a reliable predictor of the extent of proton transfer from acid to base, and ΔPA values <90 kcal/mol indicates facile PT. These gas phase DFT calculations show that, on addition of at least one IM to IMMSA, proton dissociates from MSA to IM leading to the generation of charged species, cations and anions, essential for proton conduction. The PT barrier from IMH+ to IM reduces with IM molecules ...

A theoretical prediction on the shear-induced phase transformation of TKX-50.

Dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) is a new and attractive energetic material that outperforms numerous common explosives because of its excellent properties and performance, and is thus a promising candidate to replace some of them. Nevertheless, knowledge of its physico-chemical properties, in particular, the underlying mechanism for it undergoing external stimuli for complete decay still remains poor. In the present study, we ascertain a preferred slip system of (010)/[101] and a shear-induced phase transition of TKX-50 with the aid of theoretical calculations. In other words, a new phase of TKX-50, γ-TKX-50, is observed to be formed by shearing TKX-50 along a slip system of (010)/[101] or (010)/[101[combining macron]] with a space group of P21/a, elevated energy of 9.4 kcal mol-1 and a unit cell expanded 4%, relative to the original TKX-50. Moreover, γ-TKX-50 can most readily be formed by shearing TKX-50 along (010)/[101] with a lowest energy barrier of 18.6 kcal mol-1, which is much below that for TKX-50 decay. The predicted elastic constants of γ-TKX-50 verify its mechanical stability with decreased mechanical anisotropy relative to the original TKX-50. In addition, we find that, after phase transition, the hydrogen bonding is weakened, while the electrostatic repulsion of Hδ+Hδ+ increases, which disfavors the proton transfer from NH3OH+ to C2O2N82- to facilitate the thermal decay of TKX-50. This suggests that the shear-induced transition from TKX-50 to γ-TKX-50 can enhance thermal stability by elevating the energy barrier for proton transfer, potentially contributing to the low mechanical sensitivity of TKX-50. Hopefully, this study would enrich the insight into the underlying mechanism of TKX-50 against external thermal-mechanical stimuli. Moreover, in combination with the newly found heat-induced phase, the shear-induced phase observed in the present study and the original one, there are at least three phases for TKX-50.

Comment on "Proton transport in barium stannate: classical, semi-classical and quantum regimes" by G. Geneste, A. Ottochian, J. Hermet and G. Dezanneau, Phys. Chem. Chem. Phys., 2015, 17, 19104.

In a recent paper in this journal, proton transport in oxides was considered in terms of density functional theory and the non-adiabatic Flynn-Stoneham approach of small polaron type proposed much earlier for metals. Also, regimes of hydrogen diffusion relevant to oxides were reviewed, but the straightforwardly observable channel of low-temperature over-barrier jumps has passed unnoticed. We offer this latter possibility, together with some additional arguments, to make our objection more compelling. There are two major contentious findings in the article. First, in discussing the phonon-assisted quantum regime and the adiabatic coincidence configuration in barium stannate, the article claimed that the models based on a fully non-adiabatic picture for metals cannot be generalized to proton-conducting oxides. It is difficult to agree with such a viewpoint because such generalizations are being published. By means of a counterexample, this comment illustrates the real efficacy of using Flynn-Stoneham-like models in studying these oxides. Second, we have strong grounds for supposing that the main claim of the paper being commented on about the adiabatic nature of the proton transfer is in conflict with general interpretation of small polaron hopping. The exact knowledge of energy barriers for proton transfer is needed to confirm the validity of assuming an adiabatic regime. Since the most likely influence of the functional on the adiabaticity criterion formulation is certainly evident, comparison of the results of Geneste et al. to results obtained with higher functionals may check the validity of the present GGA-PBE scheme.

Theoretical insights into photoinduced proton transfer of 7-hydroxyquinoline via intermolecular hydrogen-bonded wire of mixed methanol and water

Proton transfer (PT) reactions of 7-hydroxyquinoline (7HQ) via intermolecular hydrogen-bonded wire of methanol, water and mixed methanol–water in ground (S0) and first excited (S1) states, [7HQ(X)3 when X = M-methanol and W-water], have been studied by using density functional theory (DFT) at B3LYP and its time-dependent DFT with 6-31+G(d,p) basis set. For all complexes, the intermolecular hydrogen bonds become shorter and the O–H stretching vibrational frequencies shift to lower frequencies in the S1 state, which confirm that the hydrogen bonding interaction is stronger in the S1 state. Moreover, the frontier molecular orbitals of all complexes were analyzed to confirm the PT reactions (ππ*). The simulated absorption and emission spectra of 7HQ(MMM) are in good agreement with the experimental data. In addition, the potential energy curves along the PT reaction coordinates of all complexes both in S0 and S1 states were scanned by constrained optimizations fixing the O–H bond distance (a proton donor site of 7HQ) to investigate the effect of different intermolecular hydrogen-bonded solvent wires surrounding 7HQ. PT reactions are found to be favorable in S1 state due to the low PT energy barrier. For pure solvent, the excited-stated proton transfer (ESPT) occurs faster via methanol than water. For mixed solvent, when replacing methanol with one up to three water molecules, PT energy barrier is found to be higher than that of 7HQ(MMM); therefore, water may block the ESPT reaction.

Hydration effect on proton transfer in melamine-cyanuric acid complex.

Self-assembly of melamine-cyanuric acid (MC) leads to urinary tract calculi and renal failure. The hydration effects on molecular geometry, the IR spectra, the frontier molecular orbital, the energy barrier of proton transfer (PT), as well as the stability of MC were explored by density functional theory (DFT) calculations. The intramolecular PT breaks the big π-conjugated ring of melamine or converts the p-π conjugation (:N-C'=O) to π-π conjugation (O=C-N=C') of cyanuric acid. The intermolecular PT varies the coupling between melamine and cyanuric acid from pure hydrogen bonds (Na…HNd and NH…O) to the cooperation of cation…anion electrostatic interaction (NaH(+)…Nd (-)) and two NH…O hydrogen bonds. Distinct IR spectra shifts occur for Na…HNd stretching mode upon PT, i.e., blue-shift upon intramolecular PT and red-shift upon intermolecular PT. It is expected that the PT would inhibit the generation of rosette-like structure or one-dimensional tape conformer for the MC complexes. Hydration obviously effects the local geometric structure around the water binding site, as well as the IR spectra of NH…O and N…HN hydrogen bonds. Hydration decreases the intramolecular PT barrier from ~45 kcal mol(-1) in anhydrous complex to ~11.5 kcal mol(-1) in trihydrated clusters. While, the hydration effects on intermolecular PT barrier is slight. The relative stability of MC varies slightly by hydration due to the strong hydrogen bond interaction between melamine and cyanuric acid fragments. Graphical Abstract Hydration effect on proton transfer in melamine-cyanuric acid complex.

Proton transfer in the molecular complexes of phosphorus acids with DMSO

Proton transfer processes in various H-bonded complexes of phosphoric (H3PO4), phosphorous (H3PO3) and methylphosphonic (H2MePO3) acids with dimethyl sulfoxide (DMSO), i.e., (acid)2–DMSO, acid–(DMSO) n for n = 1, 2 and H3PO4–(DMSO)3 have been studied. The potential energy surface (PES) for proton transfer was investigated using the B3LYP level of theory, and the solvent effect on the PES was included using the conductor polarized continuum model. The energy curves for proton transfer from acid to DMSO oxygen atom in all investigated complexes represent single-well potentials, if no constraints are imposed on the system. The PES obtained by optimizing the geometry of each complex at fixed O···O distances has two nonsymmetric minima with respect to the energy barrier. In these cases, the distance at which a barrier starts to rise is slightly different. The energy barrier for proton transfer in all considered complexes increases in the series of acids: H3PO4 < H3PO3 < H2MePO3. The energies associated with proton transfer in the complexes of all investigated acid with DMSO become higher with increasing number of DMSO molecules. For all complexes, the effect of DMSO environment favors a proton transfer.

Tipping the Balance between Concerted versus Sequential Proton-Coupled Electron Transfer.

We use quantized molecular dynamics simulations to investigate the competition between concerted and sequential proton-coupled electron-transfer (PCET) reaction mechanisms in inorganic catalysts. By analyzing reactive nonadiabatic PCET trajectories and computing both concerted and sequential rate constants, we characterize various molecular features that govern inorganic PCET reactions, including the solvent polarity, ligand-mediated electron-proton interactions, and intrinsic proton-transfer (PT) energy barrier. Using atomistic simulations with over 1200 atoms, we find that the symmetric iron biimidazoline system is extremely biased toward the concerted mechanism because of the strong ligand-mediated electron-proton interaction and the short PT distance. However, by investigating system-bath models in which electron-proton interactions are shielded, which are representative of ruthenium terpyridylbenzoates and iron (tetraphenylporphyrin)benzoates, we predict that a crossover between the concerted and sequential PCET mechanisms may be possible either by increasing the polarity of the solvent or by increasing the intrinsic PT energy barrier. In addition, we predict the possibility of a crossover in the PCET mechanism by directly varying the strength of the ligand-mediated electron-proton interactions. The results presented here reveal new strategies for altering the competition between the competing PCET mechanisms and design principles for controlling PCET in catalytic systems.

Theoretical Investigation of Proton Transfer in Thiazolidine-2-thione and Oxazolidine-2-thione via Direct Transition and Self-Assisted and Water-Assisted Tautomerization

Thione–thiol transformation of thiazolidine-2-thione and oxazolidine-2-thione via direct transition, as well as self-assisted and water-assisted tautomerization was investigated using the quantum-chemical calculations. The thione complexes are more stable than the corresponding thiols. The calculations revealed that the proton transfer energy barriers are significantly lower in the presence of water molecules. Direct transition is more difficult than the water-assisted and self-assisted processes. The cooperative effects in terms of additive interaction energies and cooperativity factors of the clusters are measured and discussed. The electronic properties of the complexes are analyzed using parameters derived from atoms-in-molecules and natural bond orbital methodologies. Also, the influence of the hydrogen bond on the molecular and topological properties, as well as nuclear magnetic resonance one- and two-bond spin–spin coupling constants are investigated.

Ab initio calculations of proton transfer in dimethylformamide-phosphoric acid complexes of 1 : 1 composition

The energy parameters of the reactions that result from adding protons to phosphoric acid, its dimer, and dimethylformamide; the reactions of the formation of phosphoric acid dimers and dimethylformamide-phosphoric acid complexes; and reactions involving the protonated forms of dimethylformamide and acid are calculated by means of DFT B3LYP using the 6–31++G(d,p) basis set. The structural characteristics of the complexes and transitional states are calculated along with the change in energy upon proton transfer. The effect media have on the energy characteristics of proton transfer is studied. It is found the energy barrier of proton transfer grows upon an increase in the O⋯O distance. It is concluded that the lowest energy barriers of proton transfer are expected for DMFH+⋯DMF and H3PO4⋯H2PO4− complexes.

Theoretical investigations on Zundel cation present inside boron-nitride nanotubes: Effect of confinement and hydrogen bonding

The properties of protonated water systems in nano-confinement differ significantly from its bulk counterparts mainly due to the geometric constraint and nature of water–medium interactions. Herein, we have investigated the structure, vibrational spectra and proton transfer energetics of Zundel cation (ZC) under the confinement of boron–nitride nanotubes (BNNTs) by varying the degree of confinement in a systematic manner. Our results based on dispersion-corrected density functional theory (DFT) based methods reveal that the structure of ZC is significantly modified under the confinement and the nature of interaction largely depends on the diameter of BNNT. The ZC effectively forms hydrogen bonds through Hwater⋯NBNNT with BNNT. Among the BNNTs considered in the present study, the maximum interaction energy of ZC and minimum energy barrier for proton transfer are observed for the case of BNNT(5,5). This implies that this particular nanotube can facilitate the proton to easily hop between water molecules of a confined protonated water system.

Effect of confinement on the structure and energetics of Zundel cation present inside the hydrophobic carbon nanotubes: an ab initio study

Protonated water clusters confined to the carbon nanotube (CNT) channels of sub-nanometer diameter is beyond the realm of a continuum description. The steric compulsions offered by the narrow geometry can restrict the formation of hydrogen bond between the water molecules but enhance water and channel interactions. Herein, we have made an attempt to investigate the factors, which are strongly affecting the structure and energy barrier for proton transfer in Zundel cation under the confinement of CNT by employing dispersion-corrected density functional theory-based methods. Our results reveal that the diverse nature of water–water and water–wall interaction inside the nanotube for different size of CNT channels can have remarkable effects on the energetics of the proton transfer process, geometrical parameters, oscillatory shuttling motion of the proton and various energy components, viz. interaction energy, hydrogen bond energy, etc. Due to these factors, the proton oscillation in Zundel cation is shown to be nonmonotonic in nature with respect to the degree of confinement. Finally, we have demonstrated that the effect of confinement rendered by CNT(6,6) on Zundel cation can be the best suitable candidate among the series of CNTs considered in the present study, for assisting the proton transfer in the Zundel cation easily. These conclusions can have important implications and motivate further investigations to understand the fluidics under confined nanomaterials.

Ab initio study on transition states of formohydroxamic acid tautomerization in the presence of water molecules

Keto-iminol transformation of formohydroxamic acid in the presence of water molecules is investigated using ab initio quantum mechanics calculations. Calculations revealed that the proton transfer energy barriers are significantly lowered in the presence of water molecules. The proton transfer in Z-keto form occurs via a pathway involving two transition states and a charge separated intermediate. The proton transfer in E-keto tautomer occurs via a single transition state. A third pathway, which is not feasible in the absence of water, becomes feasible when there are hydrogen bonded water molecules. The stability order of formohydroxamic acid tautomers was found to be E-keto>Z-keto>Z-iminol>E-iminol and remains the same even after hydrogen bonded to water.

Hydration and Proton Transfer in Highly Sulfonated Poly(phenylene sulfone) Ionomers: An Ab Initio Study

The need to operate proton exchange membrane fuel cells under hot and dry conditions has driven the synthesis and testing of sulfonated poly(phenylene) sulfone (sPSO(2)) ionomers. The primary hydration and energetics associated with the transfer of protons in oligomeric fragments of two sPSO(2)ionomers were evaluated through first-principles electronic structures calculations. Our results indicate that the interaction between neighboring sulfonic acid groups affect both theconformation and stability of the fragments. The number of water molecules required to affect the transfer of a proton in the first hydration shell was observed to be a function of the hydrogen bonding in proximity of the sulfonic acid groups: three H(2)O for the meta- and four H(2)O for the ortho-conformations. Calculations of the rotational energy surfaces indicate that the aromatic backbones of sPSO(2) are much stiffer than the polytetrafluoroethylene (PTFE) backbones in perfluorosulfonic acid (PFSA) ionomers: the largest energy penalty for rotating phenylene rings (i.e., 15.5 kcal/mol for ortho-ortho-sPSO(2)) is nearly twice that computed for the rotation of a CF(2) unit in a PTFE backbone. The energetics for the transfer of various protons in proximity to one or two sulfonate groups (-SO(3)(-)) was also determined. The computed energy barrier for proton transfer when only one sulfonic acid group is present is approximately 1.9 kcal/mol, which is 2.1 kcal/mol lower than similar calculations for PFSA systems. When two sulfonic acid groups are bridged by water molecules, a symmetric bidirectional transfer occurs, which gives a substantially small energy barrier of only 0.7 kcal/mol.

Intra-octahedral proton transfer in bulk orthorhombic perovskite barium cerate

Intra-octahedral proton transfer in bulk orthorhombic perovskite barium cerate was investigated in order to understand the proton transfer mechanism using density functional theory. Since Ce-centered octahedrons tilt in the orthorhombic perovskite structure to accommodate the tensile strain between Ba and O ions, the CeOCe unit is bent. A proton attached to an O ion can transfer intra-octahedrally to a neighboring O ion in the structure. An energy barrier of 1.06eV is required as the bent CeOCe unit is straightened and bent in the opposite direction during proton transfer. When the bent CeOCe unit rotates without being straightened during proton transfer, a much lower energy barrier of 0.26eV is required. The energy barrier for proton transfer by rotating the bent CeOCe unit increases to 0.45eV, when the proton transfers near a Y ion that is substituted for a Zr ion as a dopant. Therefore, the proton transfers by rotating the bent CeOCe unit in bulk orthorhombic barium cerate, resulting in better agreement with experimentally measured energy barriers (0.5–0.54eV).

Structure and energetics of a ferroelectric organic crystal of phenazine and chloranilic acid

We report first-principles calculations for a ferroelectric organic crystal of phenazine and chloranilic acid molecules. Weak intermolecular interactions are properly treated by using a second version of the van der Waals density functional known as vdW-DF2 [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. Lattice constants, total energies, spontaneous electric polarizations, phonon modes and frequencies, and the energy barrier of proton transfer are calculated and compared with PBE and experiments whenever possible. We show that the donation of one proton from a chloranilic acid molecule to a neighboring phenazine molecule is energetically favorable. This proton transfer is the key structural change that breaks the centrosymmetry and leads to the ferroelectric structure. However, there is no unstable phonon associated with the proton transfer, and an energy barrier of 8 meV is found between the paraelectric and ferroelectric states.

Migration and Interaction of Multi-protons in Zinc-doped Barium Zirconate

Migration and interaction of multi-protons in a zinc-doped barium zirconate (Zn-doped BaZrO3) super cell were investigated using a density functional theory. O ions in the super cell form interconnected octahedrons with Zr or Zn ions positioned in their centers and Ba ions positioned among the eight octahedrons. When one proton was added to the super cell, the energy barrier of 0.80 eV for proton transfer from the first to second nearest O ion sites from the Zn ion reached its highest value. When two protons were added to the super cell, the two protons preferred the first nearest O ions from the Zn ion. The two protons were accommodated by pushing the neighboring Zn ion further away from the center of the octahedron. Energy barriers for proton transfer from the Zn-octahedron to the neighboring Zr-octahedron were spread in the range of 0.36 ~ 1.02 eV. (Received September 22, 2011)

Energy barrier of proton transfer at ice surfaces

We estimated the energy barrier of proton transfer on ice film surfaces through the measurement of the H/D exchange kinetics of H(2)O and D(2)O molecules. The isotopomeric populations of water molecules and hydronium ions on the surface were monitored by using the techniques of reactive ion scattering and low energy sputtering, respectively, along the progress of the H/D reaction. When hydronium ions were externally added onto an ice film at a temperature of 70 K, a proton was transferred from the hydronium ion mostly to an adjacent water molecule. The proton transfer distance and the H/D exchange rate increased as the temperature increased for 90-110 K. The activation energy of the proton transfer was estimated to be 10+/-3 kJ mol(-1) on a polycrystalline ice film grown at 135 K. The existence of a substantial energy barrier for proton transfer on the ice surface agreed with proton stabilization at the surface. We also examined the H/D exchange reaction on a pure ice film surface at temperatures of 110-130 K. The activation energy of the reaction was estimated to be 17+/-4 kJ mol(-1), which was contributed from the ion pair formation and proton transfer processes on the surface.

Water Deficient Environment Accelerates Proton Exchange: Acetone−Water Reaction Catalyzed by Calix[4]hydroquinone Nanotubes

Calix[4]hydroquinone nanotubes possess the unique property to catalyze proton exchange between water and acetone. Since concerted proton transfer mechanisms could be excluded previously, stepwise proton transfer via ionic intermediates created by predissociation of CHQ OH groups is studied using state-of-the-art quantum chemical methodology. In fact, the presence of charged species, protonated acetone or deprotonated hydroquinone, leads to a substantial decrease of the proton transfer energy barrier and to calculated reaction rates that provide an explanation for the experimentally observed proton exchange. Furthermore, our quantum chemical investigation demonstrates that the catalytic activity of CHQ aggregates is not based on a reduction of the energy barrier connected with proton transfer but on the desolvation of acetone and prevention of solvent water cluster formation.