Complex dynamics of proton and deuteron transfer in double hydrogen bond of benzoic acid isotopes

This paper deals with spin-lattice relaxation due to classical jumps and incoherent tunnelling of protons and deuterons in hydrogen bonds in solids. An analysis of experimental spin-lattice relaxation data for carboxylic acids suggests that tunnelling does not contribute to spin-lattice relaxation above the temperature at which the thermal energy of molecules and the potential barrier height are equal. It is also shown that contributions to the spin-lattice relaxation rate due to classical motion and incoherent tunnelling in excited vibrational states are negligible for fast proton transfer. However, for deuterons this contribution to spin-lattice relaxation is significant.     

Phonon driven proton transfer in crystals with short strong hydrogen bonds

Recent work on understanding why protons migrate with increasing temperature in short, strong hydrogen bonds is extended here to three more organic, crystalline systems. Inelastic neutron scattering and density functional theory based simulations are used to investigate structure, vibrations, and dynamics of these systems as functions of temperature. The mechanism determined in a previous work on urea phosphoric acid of low frequency vibrations stabilizing average crystal structures, in which the potential energy well of the hydrogen bond has its minimum shifted towards the center of the bond, is found to be valid here. The new feature of the N-H...O hydrogen bonds studied in this work is that the proton is transferred from the donor atom to the acceptor atom. Molecular dynamics simulations show that in an intermediate temperature regime, in which the proton is not completely transferred, the proton is bistable, jumping from one side of the hydrogen bond to the other. In the case of 3,5-pyridine dicarboxylic acid, which has been studied in most detail, specific phonons are identified, which influence the potential energy surface of the proton in the short, strong hydrogen bond.     

Variation of atomic charges during proton transfer in hydrogen bonds

The point atomic charges in a number of ionic H-bonded systems are studied by ab initio calculations as functions of the proton transfer coordinate. In the proton-bound complexes of water–water, ammonia–ammonia, formamide–water, formamide–ammonia, and dimethylether–ammonia, the net atomic charges were obtained using Mulliken population analysis and from the diagonal elements of the atomic polar tensors calculated at the HF/4–31G and MP2/6–31 + G** levels. The dependence of the atomic charges upon the coordinate of the transferring proton was found to be close (within an error of 0.02 e) to a linear function for intermolecular distances in the 2.5–2.8 Å range. The obtained charge and charge flux dependencies highlight the electron redistribution during the proton transfer process and provide insights into the source of the high infrared (IR) intensities of stretching modes of N? H and O? H bonds undergoing hydrogen bonding. © 1994 by John Wiley & Sons, Inc.     

Compression effect on structure and proton transfer in crystalline benzoic acid

Geometry optimizations for an isolated dimer and a crystal of benzoic acid were performed in order to evaluate the equilibrium geometries and the energy difference between the dimers in isolated and crystalline states using model potentials. The optimization in the crystal field results in a shortening of the O⋯O distance in comparison with that in an isolated dimer. The magnitude of the shortening agrees well with the difference between the observed values of the O⋯O distance in the gaseous (2.703 Å) and crystalline (2.64 Å) states. The energy increase due to this shortening is estimated to be about 0.24–0.40 kcal mol⁻¹ and is found to be one of the causes of the discrepancy between the barrier height of 1–2 kcal mol⁻¹ measured by NMR for crystalline carboxylic acids and that of 7.1–9.1 kcal mol⁻¹ calculated by the ab initio method for the isolated dimer.     

The substituent effect on the single and double hydrogen atom transfer reactions in para-substituted benzoic acid isobutyl esters

The substituent effect on the single and double hydrogen atom transfer reactions in para-substituted benzoic acid isobutyl esters has been investigated by electron impact mass spectrometry. Electron-donating substituents favour formation of the [M? C₄H₈]⁺˙ ion generated by single hydrogen atom transfer reaction (McLafferty rearrangement), whereas electron-withdrawing substituents favour formation of the [M? C₄H₇]⁺ ion generated by double hydrogen atom transfer reaction. In the case of the latter compounds, the m/z56 ([C₄H₈]⁺˙) ion, which is generated by single hydrogen atom transfer reaction with charge migration, is very intense, while in the former compounds, the m/z56 ion is very weak. These observations can be reasonably explained on thermochemical grounds based on the sum of the standard heats of formation of the fragments.     

An ab initio calculation on proton transfer in the benzoic acid dimer

We have made an ab initio calculation of the barriers for proton transfer in the hydrogen-bonded dimers of benzoic acid and acetic acid. Geometrical optimization values which are closer to the experiment one.     

Unraveling the spectroscopy of coupled intramolecular tunneling modes: a study of double proton transfer in the formic-acetic acid complex

The rotational spectrum of the hetero dimer comprising doubly hydrogen-bonded formic acid and acetic acid has been recorded between 4 and 18 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Each rigid-molecule rotational transition is split into four as a result of two concurrently ongoing tunneling motions, one being proton transfer between the two acid molecules, and the other the torsion/rotation of the methyl group within the acetyl part. We present a full assignment of the spectrum J = 1 to J = 6 for the ground vibronic states. The transitions are fitted to within a few kilohertz of the observed frequencies using a molecule-fixed effective rotational Hamiltonian for the separate A and E vibrational species of the G(12) permutation-inversion symmetry group. Interpretation of the motion problem uses an internal-vibration and overall-rotation angular momentum coupling scheme and full sets of rotational and centrifugal distortion constants are determined. The tunneling frequencies of the proton-transfer motion are measured for the ground A and E methyl rotation states as 250.4442(12) and -136.1673(30) MHz, respectively. The slight deviation of the latter tunneling frequency from being one half of the former, as simple theory otherwise predicts, is due to different degrees of mixing in wavefunctions between the ground and excited states.     

The proton potential in short hydrogen bonds

The state of knowledge of the proton potential in systems for which double minimum and symmetrical, single minimum type curves have been envisaged is critically reviewed. In particular, the proposed model potentials for carboxylic acids, for KDP type ferroelectrics and for the shortest hydrogen bonds are confronted with experimental data, particularly from vibrational spectra. The early attempts at explaining the complex features in the AH stretching region by level splitting in one-dimensional double minimum potentials have not been successful. The interest in the profile of the potential surface is shifted from this aspect to its role in anharmonic interaction based general band-shaping mechanisms.     

Symmetric double proton tunneling in formic acid dimer: a diabatic basis approach

A model of double proton tunneling in formic acid dimer is developed using a reaction surface Hamiltonian. The surface includes the symmetric OH stretch plus the in-plane stretch and bend interdimer vibrations. The surface Hamiltonian is coupled to a bath of five A1g and B3g normal modes obtained at the D2h transition state structure. Eigenstates are calculated using Davidson and block-Davidson iterative methods. Strong mode specific effects are found in the tunneling splittings for the reaction surface, where splittings are enhanced upon excitation of the interdimer bend motion. The results are interpreted within the framework of a diabatic representation of reaction surface modes. The splitting patterns observed for the reaction surface eigenstates are only slightly modified upon coupling to the bath states. Splitting patterns for the bath states are also determined. It is found that predicting these splittings is greatly complicated by subtle mixings with the inter-dimer bend states.     

NMR studies of double proton transfer in hydrogen bonded cyclic N,N'-diarylformamidine dimers: conformational control, kinetic HH/HD/DD isotope effects and tunneling

Using dynamic NMR spectroscopy, the kinetics of the degenerate double proton transfer in cyclic dimers of polycrystalline (15)N,(15)N'-di-(4-bromophenyl)-formamidine (DBrFA) have been studied including the kinetic HH/HD/DD isotope effects in a wide temperature range. This transfer is controlled by intermolecular interactions, which in turn are controlled by the molecular conformation and hence the molecular structure. At low temperatures, rate constants were determined by line shape analysis of (15)N NMR spectra obtained using cross-polarization (CP) and magic angle spinning (MAS). At higher temperatures, in the microsecond time scale, rate constants and kinetic isotope effects were obtained by a combination of longitudinal (15)N and (2)H relaxation measurements. (15)N CPMAS line shape analysis was also employed to study the non-degenerate double proton transfer of polycrystalline (15)N,(15)N'-diphenyl-formamidine (DPFA). The kinetic results are in excellent agreement with the kinetics of DPFA and (15)N,(15)N'-di-(4-fluorophenyl)-formamidine (DFFA) studied previously for solutions in tetrahydrofuran. Two large HH/HD and HD/DD isotope effects are observed in the whole temperature range which indicates a concerted double proton transfer mechanism in the domain of the reaction energy surface. The Arrhenius curves are non-linear indicating a tunneling mechanism. Arrhenius curve simulations were performed using the Bell-Limbach tunneling model. The role of the phenyl group conformation and hydrogen bond compression on the barrier of the proton transfer is discussed.     

Hydrogen transfer between sulfuric acid and hydroxyl radical in the gas phase: competition among hydrogen atom transfer, proton-coupled electron-transfer, and double proton transfer

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid, the hydrogen transfer between H2SO4 and HO* in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations, as well as classical transition state theory computations. The first step of the H2SO4 + HO* reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol(-1) below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO* and a degenerate double hydrogen-exchange between H2SO4 and HO* may occur. The single hydrogen transfer, yielding HSO4* and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is associated with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate constant of 1.50 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K is obtained for the overall reaction H2SO4 + HO*. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate constant at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO* yielding HSO4* and H2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.     

Vibrational gating of double hydrogen tunneling in porphycene

A procedure that enables determining the reaction rate from the analysis of fluorescence anisotropy is described and applied to the investigation of double hydrogen transfer between inner-cavity nitrogen atoms in electronically excited porphycene. Tautomerization proceeds as a thermally activated synchronous double hydrogen tunneling. The barrier to the reaction is dynamically modulated by a vibration that simultaneously changes the strength of two intramolecular hydrogen bonds. Different mechanisms of tautomerization in porphycene and its parent isomer, porphyrin, can be understood by analyzing the potentials for hydrogen transfer.     

A Schiff-base porphyrin complex with double intramolecular hydrogen bonds

We have designed a porphyrin with a Schiff-base substituent as a model to study intramolecular hydrogen-bonding. The corresponding complex [Zn(SATPP)(CH₃OH)] has been synthesized and characterized by X-ray crystallography, ¹H NMR, and UV-Vis spectroscopy. The structure shows that there are three phenyl groups and one salicylideneaminophenyl group at the meso positions of the porphyrin, and the phenol oxygen is involved in double hydrogen bonds, one within the salicylideneaminophenyl and the other between coordinated methanol and phenol oxygen. ¹H NMR spectra suggest that the binding of methanol to zinc is an equilibrium process in solution and the equilibrium constant has been determined by UV-Vis measurements. The intramolecular hydrogen bond stabilizes the structure, and the binding affinity increases 10 times over the corresponding TPP (TPP, dianion of meso-5,10,15,20-tetraphenylporphyrin).     

Tunnelling effects in the double proton transfer reaction of 2,5-dihydroxy-1,4-benzoquinone

Two dynamic models were used to investigate the double proton transfer reaction of 2,5-dihydroxy-1, 4-benzoquinone. The stationary points of the potential energy surface were located using the AM1 method. A model two-dimensional potential surface based on the Cartesian coordinates of the protons is constructed. According to the first model, the synchronous and asynchronous mechanisms of the reaction are treated separately. In the second model the two-dimensional vibrational problem is solved on the corresponding potential energy surface. The unimolecular rate constants are calculated in terms of the RRKM theory and the one- and two-dimensional approximations are compared. The second model predicts the dominant role of tunnelling and the synchronous and asynchronous processes can hardly be separated.     

Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols

The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)(3)](3+) oxidant in aqueous solution was investigated. The reaction followed a concerted electron-proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sj?din, M. et al. J. Am. Chem. Soc. 2000, 122, 3932-3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (lambda), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer-proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.     

Theoretical analysis on the hydrogen bonding and reactivity that associated with the proton transfer reaction of carboxylic acid dimers and their monosulfur derivatives

Carboxylic acid dimers and their monosulfur derivatives are investigated by density functional theory calculations. Basis set superposition error (BSSE) counterpoise correction is included to compare the influence of BSSE on the interaction energies as well as on the geometries. The nature of hydrogen bond is determined on the basis of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Good correlations have been established between H‐bond length versus AIM topological parameter, orbital interaction, and barrier height for proton transfer. The reactivity behavior along the reaction path of the double proton transfer reaction has also been studied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011