Proton Transfer Potential Research Articles

Composition-Dependent Hydrogen-Bonding Motifs and Dynamics in Brønsted Acid-Base Mixtures.

In recent years the interaction of organophosphates and imines, which is at the core of Brønsted acid organocatalysis, has been established to be based on strong ionic hydrogen bonds. Yet, besides the formation of homodimers consisting of two acid molecules and heterodimers consisting of one acid and one base, also multimeric molecular aggregates are formed in solution. These multimeric aggregates consist of one base and several acid molecules. The details of the intermolecular bonding in such aggregates, however, have remained elusive. To characterize composition-dependent bonding and bonding dynamics in these aggregates, we use linear and nonlinear infrared (IR) spectroscopy at varying molar ratios of diphenyl phosphoric acid and quinaldine. We identify the individual aggregate species, giving rise to the structured, strong, and very broad infrared absorptions, which span more than 1000 cm–1. Linear infrared spectra and density functional theory calculations of the proton transfer potential show that doubly ionic intermolecular hydrogen bonds between the acid and the base lead to absorptions which peak at ∼2040 cm–1. The contribution of singly ionic hydrogen bonds between an acid anion and an acid molecule is observed at higher frequencies. As common to such strong hydrogen bonds, ultrafast IR spectroscopy reveals rapid, ∼ 100 fs, dissipation of energy from the proton transfer coordinate. Yet, the full dissipation of the excess energy occurs on a ∼0.8–1.1 ps time scale, which becomes longer when multimers dominate. Our results thus demonstrate the coupling and collectivity of the hydrogen bonds within these complexes, which enable efficient energy transfer.

Temperature dependent fluorescence spectra arise from change in excited-state intramolecular proton transfer potential of 4′-N,N-dimethylamino-3-hydroxyflavone-doped acetonitrile crystals

The effect of intermolecular interaction on excited-state intramolecular proton transfer (ESIPT) in 4′-N,N-dimethylamino-3-hydroxyflavone (DMHF) doped in acetonitrile crystals was investigated by measuring the temperature dependence of fluorescence excitation and fluorescence spectra. A solid/solid phase transition of DMHF-doped acetonitrile crystals occurred in the temperature between 210 and 218K. Significant differences in the spectral profiles and shifts in the fluorescence spectra were observed in the low- and high-temperature regions of the phase transition. The temperature dependence of the ESIPT potential of DMHF is discussed.

Ultraviolet and infrared spectroscopy of matrix-isolated 7-azaindole dimer: Matrix effect on excited state tautomerization

Photophysics of 7-azaindole homodimer has been studied in Ar and N2 matrixes for the first time. The infrared spectra reveal that a doubly hydrogen bonded dimer is predominantly produced upon annealing the seeded matrixes at 28K. However, unlike in hydrocarbon solutions or a supersonic jet expansion, the matrix-isolated doubly hydrogen bonded dimer emits exclusively in ultraviolet from the locally excited state, and the vital photophysical process, the excited state double proton transfer, is slowed down dramatically. We have interpreted the observation in terms of matrix effect on the double proton transfer potential and promoting vibrational modes.

Methyl Dynamics Flattens Barrier to Proton Transfer in Crystalline Tetraacetylethane

We analyze the interplay between proton transfer in the hydrogen-bond bridge, O···H···O, and lattice dynamics in the model system tetraacetylethane (TAE) (CH(3)CO)(2)CH═CH(COCH(3))(2) using density functional theory. Lattice dynamics calculations and molecular dynamics simulations are validated against neutron scattering data. Hindrance to the cooperative reorientation of neighboring methyl groups at low temperatures gives a preferred O atom for the bridging proton. The amplitude of methyl torsions becomes larger with increasing temperature, so that the free-energy minimum for the proton becomes flat over 0.2 Å. For the isolated molecule, however, we show an almost temperature-independent symmetric double-well potential persists. This difference arises from the much higher barriers to methyl torsion in the crystal that make the region of torsional phase space that is most crucial for symmetrization poorly accessible. Consequently, the proton-transfer potential remains asymmetric though flat at the base, even at room temperature in the solid.

Proton transfer reactions and dynamics in protonated water clusters

Proton transfer reactions and dynamics were theoretically studied using the hydrogen-bond (H-bond) complexes formed from H(3)O(+) and nH(2)O, n = 1-4, as model systems. The investigations began with searching for characteristics of transferring protons in the gas phase and continuum aqueous solution using DFT method at the B3LYP/TZVP level, followed by Born-Oppenheimer molecular dynamics (BOMD) simulations at 350 K. B3LYP/TZVP calculations revealed the threshold asymmetric O-H stretching frequencies (ν(OH)*) for the proton transfers in the Zundel complex (H(5)O) in the gas phase and continuum aqueous solution at 1984 and 1881 cm(-1), respectively. BOMD simulations suggested lower threshold frequencies (ν(OH,MD)* = 1917 and 1736 cm(-1), respectively), with two characteristic ν(OH,MD) being the IR spectral signatures of the transferring protons. The low-frequency band could be associated with the "oscillatory shuttling motion" and the high-frequency band with the "structural diffusion motion". These can be regarded as the spectroscopic evidences of the formations of the shared-proton structure (O···H(+)···O) and the H(3)O(+)-H(2)O contact structure (O-H(+)···O), respectively. Since the quasi-dynamic equilibrium between the Zundel and Eigen complexes was suggested to be the rate-determining step, in order to achieve an "ideal" maximum efficiency of proton transfer, a concerted reaction pathway should be taken. The most effective interconversion between the two proton states, the shared-proton structure and the H(3)O(+)-H(2)O contact structure, can be reflected from comparable intensities of the oscillatory shuttling and structural diffusion bands. The present results iterated the previous conclusions that static proton transfer potentials cannot provide complete description of the structural diffusion process and it is essential to incorporate thermal energy fluctuations and dynamics in the model calculations.

A generalized reactive force field for nonlinear hydrogen bonds: Hydrogen dynamics and transfer in malonaldehyde

Using molecular dynamics (MD) simulations, the spectroscopy and dynamics of malonaldehyde is investigated. To this end, the recently proposed molecular mechanics with proton transfer (MMPT) potential is generalized to nonlinear hydrogen bonds. The calculated properties for malonaldehyde in both gas and condensed phases, including equilibrium geometries, infrared spectra, tunneling splittings, and hydrogen transfer rates, compare well with previous experimental and computational works. In particular, by using a harmonic bath averaged (HBA) Hamiltonian, which is based on a reaction path Hamiltonian, it is possible to estimate the tunneling splitting in an efficient manner. It is found that a zero point corrected barrier of 6.7 kcal/mol and effective masses of 1.234 (i.e., 23.4% larger than the mass of a physical H-atom) and 1.117 (for the physical D-atom) are consistent with the measured splittings of 21.6 and 2.9 cm(-1), respectively. The HBA Hamiltonian also yields a pair of hydrogen transfer fundamentals at 1573 and 1267 cm(-1), similar to results obtained with a reaction surface Hamiltonian on a MP2/6-31G(**) potential energy surface. This amounts to a substantial redshift of more than 1000 cm(-1) which can be rationalized by comparison with weakly (HCO(+): rare gas) and strongly (H(2)O-H(+)-OH(2)) proton-bound systems. Hydrogen transfer rates in vacuum and water were determined from the validated MMPT potential and it is found that the solvent enhances the rate by a factor of 5 at 300 K. The rates of 2.4/ns and 10/ns are commensurate with previous density functional tight binding ab initio MD studies.

Proton transfer reactions and dynamics in CH3OH–H3O+–H2O complexes

Proton transfer reactions and dynamics in hydrated complexes formed from CH(3)OH, H(3)O(+) and H(2)O were studied using theoretical methods. The investigations began with searching for equilibrium structures at low hydration levels using the DFT method, from which active H-bonds in the gas phase and continuum aqueous solution were characterized and analyzed. Based on the asymmetric stretching coordinates (Deltad(DA)), four H-bond complexes were identified as potential transition states, in which the most active unit is represented by an excess proton nearly equally shared between CH(3)OH and H(2)O. These cannot be definitive due to the lack of asymmetric O-H stretching frequencies (nu(OH)) which are spectral signatures of transferring protons. Born-Oppenheimer molecular dynamics (BOMD) simulations revealed that, when the thermal energy fluctuations and dynamics were included in the model calculations, the spectral signatures at nu(OH) approximately 1000 cm(-1) appeared. In continuum aqueous solution, the H-bond complex with incomplete water coordination at charged species turned out to be the only active transition state. Based on the assumption that the thermal energy fluctuations and dynamics could temporarily break the H-bonds linking the transition state complex and water molecules in the second hydration shell, elementary reactions of proton transfer were proposed. The present study showed that, due to the coupling among various vibrational modes, the discussions on proton transfer reactions cannot be made based solely on static proton transfer potentials. Inclusion of thermal energy fluctuations and dynamics in the model calculations, as in the case of BOMD simulations, together with systematic IR spectral analyses, have been proved to be the most appropriate theoretical approaches.

Perturbations of proton transfer potentials caused by polar molecules

The influence of polar molecules on the proton transfer potential within a H-bonded system is studied by ab initio methods, An external HF molecule is placed in various locations in the vicinity of the (H{sub 2}OH {center dot} {center dot} OH{sub 2}){sup +} system. It was found that for most positions the effects on the proton transfer potential could be reproduced very accurately by replacing the full HF molecule by a simple point dipole of magnitude equal to the moment of HF. This result implies that the effects of HF are primarily electrostatic and, further, that attempts to model a large array of solvent molecules by a collection of point dipoles are likely to be successful. The only exceptions occur when H-bonding is possible between HF and (H{sub 2}OH {center dot} {center dot} OH{sub 2}){sup +}. In these cases, the transfer potential is altered to a significantly greater degree by the HF molecule than by the point dipole. It is only when the pertinent H-bonding atoms approach within about 3 {angstrom} that the effects of forces other than electrostatic become noticeable.

Proton transfer potentials in hydrogen-bonded systems: (H5O2)+

Ab initio molecular orbital calculations at the split-valence shell level are used to examine a theory of proton transport across biomembranes involving the transfer of protons along hydrogen-bonded chains. Potentials for central proton motion in (H2OHOH2)+ are calculated using the 4-31G basis set. This procedure is shown to provide results in close agreement with more extended calculations including electron correlation. A potential with a shallow single minimum corresponding to a centrally located proton is obtained for small interoxygen separations. However, for separations greater than about 2.4 A, double-well potentials are calculated. The potentials are relatively insensitive to the inclusion of additional H2O molecules or geometry optimizations. The effects of angular distortions of the hydrogen bond on the barriers separating the two minima are investigated. Small distortions of less than 20d or so increase the barriers slightly, but a sharp rise is noted for greater distortions. In addition, rotations of the two water molecules in opposite directions produce the largest increase in proton transfer barriers.

Additivity of the effects of external ions and dipoles upon the energetics of proton transfer

The perturbations produced in the proton transfer potential of (H2OHOH2)+ by a number of ions and dipoles in various locations around the system are calculated with the 4-31G basis set. The observed perturbations are fully consistent with an electrostatic picture in which the positively charged proton is pushed along the lines of force emanating from the external object. As was observed previously for ions, the effects of the dipoles are magnified by the H bond, relative to their Coulombic interaction with a bare proton. The perturbations produced in the transfer potential by an array of multiple charges or dipoles are found to be almost completely additive.

Tunneling dynamics of double proton transfer in formic acid and benzoic acid dimers

Direct dynamics calculations based on instanton techniques are reported of tunneling splittings due to double proton transfer in formic and benzoic acid dimers. The results are used to assign the observed splittings to levels for which the authors of the high-resolution spectra could not provide a definitive assignment. In both cases the splitting is shown to be due mainly to the zero-point level rather than to the vibrationally or electronically excited level whose spectrum was investigated. This leads to zero-point splittings of 375 MHz for (DCOOH)(2) and 1107 MHz for the benzoic acid dimer. Thus, contrary to earlier calculations, it is found that the splitting is considerably larger in the benzoic than in the formic acid dimer. The calculations are extended to solid benzoic acid where the asymmetry of the proton-transfer potential induced by the crystal can be overcome by suitable doping. This has allowed direct measurement of the interactions responsible for double proton transfer, which were found to be much larger than those in the isolated dimer. To account for this observation both static and dynamic effects of the crystal forces on the intradimer hydrogen bonds are included in the calculations. The same methodology, extended to higher temperatures, is used to calculate rate constants for HH, HD, and DD transfers in neat benzoic acid crystals. The results are in good agreement with reported experimental rate constants measured by NMR relaxometry and, if allowance is made for small structural changes induced by doping, with the transfer matrix elements observed in doped crystals. Hence the method used allows a unified description of tunneling splittings in the gas phase and in doped crystals as well as of transfer rates in neat crystals.

Quantum/Classical Investigation of Amide Protonation in Aqueous Solution

The protonation of N-methylacetamide by a hydronium ion is investigated by means of ab initio methods and statistical-mechanical integral equation theory. The proton-transfer reaction is treated as a function of a two-dimensional coordinate. It is monitored in terms of the distance of the transferred proton to the amide oxygen atom and the distance of the oxygen atoms of the amide and the hydronium. The energy profile of the reaction is calculated for the isolated system as well as for the reaction in aqueous solution. The reaction in a vacuum is investigated by ab initio calculations. The solvent effect is modeled on the basis of a classical approach using the reference interaction site model integral equation theory. Therein reactant and product states are described by classical force fields. A smooth transition between both states is modeled using a continuous switching function. The proton-transfer potential was calculated for the average solvent effect. As a result, the minimum energy path for amide protonation is determined as a function of a one-dimensional reaction coordinate.

The phonon-assisted proton transfer between hydrazinium cations in (N 2H 5) 2HMF 6·2H 2O (M∈{Ga, Al, Fe}) type compounds. FT IR and quantum theoretical study

FT IR spectra of a series of compounds with a general formula (N 2H 5) 2HMF 6·2H 2O (where M∈{Ga, Al, Fe}) were recorded at variable temperatures (from ∼100 to 300 K, at 10 K intervals). The appearance of the spectral region of ν(N–N) modes due to hydrazinium cations further supports the conclusions regarding the N 2H 5 +⋯H +⋯N 2H 5 + hydrogen bond potential well based on Raman spectroscopic data [J. Raman Spectrosc. 28 (1997) 315]. The appearance of two bands corresponding to the ν(N–N) modes in the low temperature FT IR spectra that merge into one upon heating is a clear evidence of a symmetric potential well through which a phonon-assisted proton transfer (PAPT) occurs at higher temperatures. Ab initio MP2/6-311++G(2d,p) quantum chemical study of the proton transfer potential within the N 2H 5 +⋯H +⋯N 2H 5 + cluster confirmed its double-minimum character. The first-order saddle point found on the MP2/6-311++G(2d,p) potential energy hypersurface corresponds to a centrosymmetric structure ( C 2 h symmetry), with the proton placed at the inversion center. The potential energy curve along the tunnelling coordinate was calculated by the intrinsic reaction coordinate (IRC) methodology, leading to an adiabatic PT barrier height of 3.94 kcal mol −1 and a tunneling rate of 1.98 s −1. The corresponding MP4(SDTQ)/6-311++G(2d,p)//MP2/6-311++G(2d,p) value of the adiabatic PT barrier height is 4.26 kcal mol −1.

Theoretical Studies of Excited State Proton Transfer in Small Model Systems

Ab initio calculations that address the problem of excited-state proton transfer across an intramolecular hydrogen bond are reviewed. Small molecules, such as malonaldehyde, containing such a H-bond are first examined. This work reveals that in comparison to the ground state, the H-bond is strengthened and the transfer barrier reduced in the1ππ* state; opposite trends are noted in the triplet ππ* as well as nπ* states. Replacement of the H-bonding O atoms of malonaldehyde by N has only a small effect upon these results, as does enlargement or reduction of the malonaldehyde ring, coupled with anionic charge. The transfer barrier is linearly related to the equilibrium length of the H-bond in the various states of each system. Attachment of a phenyl ring to malonaldehyde introduces a fundamental asymmetry into the proton transfer potential, as the enol and keto tautomers are inequivalent. Whereas the enol is more stable in the ground and nπ* states, a reversal occurs in the ππ* states, which may be understood on the basis of the level of aromaticity within the phenyl ring. Nonetheless, when this asymmetry is accounted for, the phenyl ring affects the intrinsic barrier to proton transfer in the smaller malonaldehyde by a surprisingly small amount. Because of the high transfer barriers in the nπ* states, coupled with low barriers to bond rotation, rotamerization is likely to dominate over proton transfer in these states. This behavior contrasts sharply with the ππ* states, where proton transfer is far more likely than bond rotations. While it is clear that inclusion of electron correlation is essential to a quantitative reproduction of the proton-transfer process in excited states, the most accurate yet affordable method by which to include correlation remains an open question.

The role of quantum effects and ionic defects in high-density ice

The shape of the proton-transfer potential between a donor and an acceptor heavy atom is known to depend strongly on the length of the hydrogen bond. It evolves from a double-well to a single-well shape in the limit of very short hydrogen bonds. Ice at high compression is an ideal candidate to study systematically the consequences of this transmutation on properties of hydrogen-bonded solids. We use a computer simulation technique that treats all nuclei as quantum particles and includes, at the same time, the electrons in the framework of density functional theory. Based on this first principles approach we uncover four different regimes. In the limit of long/weak hydrogen bonds the proton resides covalently bound to a particular heavy atom and zero-point motion broadens its distribution. Ionic defects are shown to play only a minor role. For shorter bonds a regime with strong proton tunneling between donor and acceptor is found. This phase is characterized by a high concentration of ionic defects. At even shorter distances the proton resides midway between donor and acceptor due to zero-point motion, despite the double-well character of the underlying potential. Finally, very short/ultra-strong hydrogen bonds are symmetric because the potential profile is of the single-well type.

Effects of chemical substitution upon excited state proton transfer. Fluoroderivatives of salicylaldimine

Ab initio methods are used to probe the effect of fluorosubstitution upon the excited state proton transfer process in the OH⋯N hydrogen bond contained in the model system salicylaldimine. Two different sites are chosen for substitution, one nearby to the nitrogen on the cycle that contains the intramolecular H-bond, while the other position is located on the aromatic ring close to the oxygen. Many of the computed effects on the proton transfer potential can be considered inductive in nature; the electronegative F makes the proximate N or O atom a stronger acid/weaker base and thereby moderates the preferred position of the bridging proton. The magnitude of the perturbation diminishes as the site of substitution is further removed from the H-bond. This principle also controls the manner in which F affects the geometry and strength of the intramolecular H-bond in both the enol and keto tautomers. There are a number of stretches and contractions that occur in bonds that do not border on the H-bond, patterns that can be understood via the bonding patterns in the orbitals involved in the excitation. Whereas these notions apply fairly consistently to the ground state and excited ππ ∗ singlet and triplet, there are a number of anomalous patterns that emerge in the 1 nπ ∗ state. In general, the effects of fluorosubstitution are smaller in magnitude than the changes that occur in the proton transfer properties as a result of electronic excitation.

Tunnelling and zero-point motion in high-pressure ice

The microscopic structure of ice poses a long-standing challenge to theory1,2,3. Because of their low mass, the protons in the hydrogen bonds that define the structures of crystalline ice are susceptible to quantum-mechanical effects such as tunnelling1,4,5,6,7,8. High pressure provides a means of controlling the length of the hydrogen bonds in order to investigate such effects. In particular, Holzapfel predicted 26 years ago that, under pressure, hydrogen bonds might be transformed from the highly asymmetric O–H···O configuration to a symmetric state in which the proton lies midway between the two oxygens9, leading to a non-molecular symmetric phase of ice, now denoted as ice ‘X’. The existence of this phase has been inferred from spectroscopy10,11,12,13,14, but has still not been observed directly. Here we investigate the role of quantum effects in proton ordering and hydrogen-bond symmetrization within ice at high pressure by using a simulation technique that treats both electrons and nuclei quantum-mechanically15,16,17. We find that the proton-ordered structure at low pressure, with asymmetric hydrogen bonds (ice VIII), transforms on increasing pressure to a proton-disordered asymmetric phase (ice VII) owing to translational proton tunnelling. On further compression, the zero-point fluctuations lead to strongly delocalized protons and hydrogen-bond symmetrization, even though the underlying character of the proton-transfer potential remains a double well. Only at still higher pressures does the double-well potential become transformed into a single well, whereupon the protons again become increasingly localized.

Proton transfer between phenol and ammonia in ground and excited electronic states

Ab initio calculations consider the H-bonded complex between phenol and ammonia. The transfer of a proton in the ground electronic state, which would form PhO − … +HNH 3, is highly disfavored. However, the situation changes dramatically in the first excited singlet state where the proton transfer potential develops a second minimum, competitive in energy with the neutral pair PhOH … NH 3. Following electron ejection, the radical cation complex, PhOH + … NH 3, contains a very flat proton transfer potential connecting PhOH ·+ … NH 3 with PhO · … H +NH 3. Addition of more ammonia molecules further stabilizes the proton-transfered PhO · … H +NH 3 configuration.

A quantum molecular dynamics study of proton conduction phenomena in BaCeO 3

Acceptor doped barium cerate dissolves significant amounts of water thus forming protonic defects. High proton mobility is generally considered to depend strongly on the dynamics of the proton environment. In this paper numerical results using quantum molecular dynamics simulations are presented to study the dynamics of the BaCeO 3 host lattice. At elevated temperatures O-Ce-O bending modes mainly determine the oxygen separation coordinate Q which is considered to be responsible for modulating the proton transfer potential. It is also anticipated that this frequency coincides with the attempt frequency for proton conduction. For low proton excitations the dynamics of the proton is found to be part of a hydroxide ion rotating around the oxygen atom in the plane perpendicular to the Ce-O-Ce axis.

Electrostatic and Non-Electrostatic Contributions to Hydrogen Bonding and Proton Transfer in Solution Phase

A combined methodology of semiempirical density functional (DFT) and Hartree−Fock (HF) theories is used to analyze the solution-phase proton-transfer (PT) process in the H2O···HX (X = F, Cl, and OH) model systems. Gas-phase PT, hydrogen bonding, ion-pairing, dissociation, and solvent effect are considered as the contributing factors to the solution PT reaction. The H-bonded and ion-pair structures are determined from the proton-transfer potential (PTP) profiles and full geometry optimization, using the Amsterdam density functional (ADF) code. These structures are then used as input to the semiempirical SCRF/CNDO method that incorporates solvent effects. The semiempirical SCRF gas-phase results qualitatively reproduce the experimental trend for the gas-phase proton affinities (PA) (OH- > F- > Cl-). The solution-phase results correctly explain the strong acid character of HCl (pKa < 0) and the weak dissociation of HF (pKa > 0) in water.