Macroscopic quantum tunnelling of protons in the KHCO3 crystal

Macroscopic quantum entanglement reveals an unforeseen mechanism for proton transfer across hydrogen bonds in the solid state. We utilize neutron scattering techniques to study proton dynamics in the crystal of potassiumhydrogencarbonate (KHCO₃) composed of small planar centrosymmetric dimer entities linked by moderately strong hydrogen bonds. All protons are indistinguishable, they behave as fermions, and they are degenerate. The sublattice of protons is a superposition of macroscopic single-particle states. At elevated temperature, protons are progressively transferred to secondary sites at ≈0.6 Å from the main position, via tunnelling along hydrogen bonds. The macroscopic quantum entanglement, still observed at 300 K, reveals that proton transfer is a coherent process throughout the crystal arising from a superposition of macroscopic tunnelling states.     

Excited State Intramolecular Proton Transfer of 1-Hydroxyanthraquinone

The excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution are investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. Two characteristic bands of excited state absorption and stimu-lated emission are observed in transient absorption spectra with the excitation by the pump wavelength of 400 nm. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S₂ and S₁ states are rever-sal and a conical intersection is demonstrated to exist along the proton transfer coordinate. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with ～200 fs and the following intermolecular energy relaxation with ～16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. From our observations, two proton transfer pathways via a conical intersection are proposed and the dominated one preserves the molecular orbits.     

Inelastic neutron scattering study of hydrogen in d(8)-THFD(2)O ice clathrate

In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deutrated tetrahydrofuran-water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J=0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned since there was residual orthohydrogen at low temperatures (slow conversion to the ground state) resulting in an observable J=1-->0 transition at 5 K (kT=0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV is interpreted as J=1-->2 transitions. In addition to the transitions between rotational states, there are a series of peaks that arise from transitions between center-of-mass translational quantum states of the confined hydrogen molecule. A band at approximately 9 meV can be unequivocally interpreted as a transition between translational states, while broad features at 20, 25, 35, and 50-60 meV are also interpreted to as transitions between translational quantum states. A detailed comparison is made with a recent five-dimensional quantum treatment of hydrogen in the smaller dodecahedral cage in the SII ice-clathrate structure. Although there is broad agreement regarding the features such as the splitting of the J=1 degeneracy, the magnitude of the external potential is overestimated. The numerous transitions between translational states predicted by this model are in poor agreement with the experimental data. Comparisons are also made with three simple exactly solved models, namely, a particle in a box, a particle in a sphere, and a particle on the surface of a sphere. Again, there are too many predicted features by the first two models, but there is reasonable agreement with the particle on a sphere model. This is consistent with published quantum chemistry results for hydrogen in the dodecahedral 5(12) cage, where the center of the cage is found to be energetically unfavorable, resulting in a shell-like confinement for the hydrogen molecule wave function. These results demonstrate that translational quantum effects are very significant and a classical treatment of the hydrogen molecule dynamics is inappropriate under such conditions.     

Hydrogen motions in the alpha-relaxation regime of poly(vinyl ethylene): a molecular dynamics simulation and neutron scattering study

The hydrogen motion in poly(vinyl ethylene) (1,2-polybutadiene) in the alpha-relaxation regime has been studied by combining neutron spin echo (NSE) measurements on a fully protonated sample and fully atomistic molecular dynamics simulations. The almost perfect agreement between experiment and simulation results validates the simulated cell. A crossover from Gaussian to non-Gaussian behavior is observed for the intermediate scattering function obtained from both NSE measurements and simulations. This crossover takes place at unusually low Q values, well below the first maximum of the static structure factor. Such anomalous deviation from Gaussian behavior can be explained by the intrinsic dynamic heterogeneity arising from the differences in the dynamics of the different protons in this system. Side group hydrogens show a markedly higher mobility than main chain protons. Taking advantage of the simulations we have investigated the dynamic features of all different types of hydrogens in the sample. Considering each kind of proton in an isolated way, deviations from Gaussian behavior are also found. These can be rationalized in the framework of a simple picture based on the existence of a distribution of discrete jumps underlying the atomic motions in the alpha process.     

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: an ab initio study of time-resolved photoelectron spectra

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation. Photoelectron kinetic energy distributions arising from wave packets localized in different basins are also distinguishable and could be understood, as expected, on the basis of the strength of the optical couplings in different regions of the ground state potential surface and the Franck-Condon overlaps of the ground state wave packets with the vibrational eigenstates of the ion potential surface.     

Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)-oxazole and -thiazole

The azoles 2-(2′-hydroxyphenyl)oxazole (HPO) and 2-(2′-hydroxyphenyl)-4-methylthiazole (HPT) have been synthesised and studied in order to compare their photophysical characteristics. Their absorption and emission properties are reported in non-polar, alcoholic and aqueous media. Ground and excited state pK data were determined by spectroscopy and a model is proposed to explain the behaviour of HPT and HPO as a function of the pH. Excitation spectra and quantum chemical calculations suggest an equilibrium of ground state conformers. The calculations also predict a small energy barrier for rotation in the first excited singlet state for the proton transferred tautomers. The resulting twisted structure of the tautomer form possesses a biradicaloid nature, and is near-degenerate in energy with the first excited triplet state.     

On some controversy regarding νOH assignments in CsH2PO4

Spectroscopic methods such as Raman scattering and IR absorption, as well as structural methods such as x-ray and n-diffraction, offer rather ambiguous interpretation of the high-frequency phonon spectrum of CsH₂PO₄ (CDP), especially regarding the proper assignment of proton vibrational modes. Empirical lattice dynamics (LD) simulations of the proton vibrational density of states ((H-VDOS)) in CDP also reveal a discrepancy in the frequency assignments of the OH stretching modes of its two non-equivalent hydrogen bonds. This may be resolved by accounting for the LD simulated directional (H-VDOS) along the three Cartesian axes of CDP, from which the corresponding anisotropy of the proton kinetic energy tensor can be deduced. The results may then be tested by a simple deep inelastic neutron scattering (DINS) measurement on a single crystal of para-electric CDP at room temperature.     

A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons

The crystal structures of potassium and cesium bistrifluoroacetates, KH(CF(3)COO)(2) and CsH(CF(3)COO)(2), respectively, were determined at room and cryogenic temperatures with the single crystal neutron diffraction technique. The crystals belong to the monoclinic space groups, I2a and A2a, respectively, and there is no evidence of any structural phase transition. In both crystals, trifluoroacetate entities in centrosymmetric dimers are linked by very short hydrogen bonds lying across a center of inversion. The thermal parameters provide no evidence of any double minimum potential for hydrogen bond protons. Single-minimum potentials were determined via best fitting to the inelastic neutron scattering spectral profiles of the stretching vibrations. They comprise a narrow well for the ground state and a very broad quasiharmonic well for excited states. The spread out of the wave functions of these states shows that protons are no longer confined between the oxygens. Presumably, they are attracted by the lone pairs of oxygen atoms. These potentials emphasize the covalent nature of the OO bond and the ionic character of the hydrogen bond proton.     

On the correspondence between classical and quantum mechanics in macromolecular systems: Too much classical chaos

In a recent study we found the classical dynamics of a polyethylene (PE) chain to exhibit low dimensional chaos at temperatures as low as a few Kelvin. These results strongly suggest that classical molecular dynamic simulations in polymer systems can grossly overestimate vibrational motion, which consequently results in disordered structures. In contrast, quantum mechanical calculations using Internal Coordinate Quantum Monte Carlo (an improved method with an initial conjecture for the correct wave function) indicate that the quantum ground state for a three-dimensional model PE chain is far more rigid than determined from molecular dynamics (MD) simulations, even at energies as low as a small fraction of the ground state energy. This result casts uncertainty on the reliability of MD estimates of dynamical or structural quantities relevant to the study of some macromolecular systems.     

Anomalous neutron Compton scattering cross sections in ammonium hexachlorometallates

The authors have performed neutron Compton scattering measurements on ammonium hexachloropalladate (NH(4))(2)PdCl(6) and ammonium hexachlorotellurate (NH(4))(2)TeCl(6). Both substances belong to the family of ammonium metallates. The aim of the experiment was to investigate the possible role of electronic environment of a proton on the anomaly of the neutron scattering intensity. The quantity of interest that was subject to experimental test was the reduction factor of the neutron scattering intensities. In both samples, the reduction factor was found to be smaller than unity, thus indicating the anomalous neutron Compton scattering from protons. Interestingly, the anomaly decreases with decreasing scattering angle and disappears at the lowest scattering angle (longest scattering time). The dependence of the amount of the anomaly on the scattering angle (scattering time) is the same in both substances (within experimental error). Also, the measured widths of proton momentum distributions are equal in both metallates. This is consistent with the fact that the attosecond proton dynamics of ammonium cations is fairly well decoupled from the dynamics of the sublattice of the octahedral anions PdCl(6) (2-) and TeCl(6) (2-), respectively. The hypothesis is put forward that proton-electron decoherence processes are responsible for the considered effect. Decoherence processes may have to do rather with the direct electronic environment of ammonium protons and not with the electronic structure of the metal-chlorine bond.     

Proton transfer in a polar solvent from ring polymer reaction rate theory

We have used the ring polymer molecular dynamics method to study the Azzouz-Borgis model for proton transfer between phenol (AH) and trimethylamine (B) in liquid methyl chloride. When the A-H distance is used as the reaction coordinate, the ring polymer trajectories are found to exhibit multiple recrossings of the transition state dividing surface and to give a rate coefficient that is smaller than the quantum transition state theory value by an order of magnitude. This is to be expected on kinematic grounds for a heavy-light-heavy reaction when the light atom transfer coordinate is used as the reaction coordinate, and it clearly precludes the use of transition state theory with this reaction coordinate. As has been shown previously for this problem, a solvent polarization coordinate defined in terms of the expectation value of the proton transfer distance in the ground adiabatic quantum state provides a better reaction coordinate with less recrossing. These results are discussed in light of the wide body of earlier theoretical work on the Azzouz-Borgis model and the considerable range of previously reported values for its proton and deuteron transfer rate coefficients.     

Quantum dynamics study of H+NH3-->H2+NH2 reaction

We report in this paper a quantum dynamics study for the reaction H+NH3-->NH2+H2 on the potential energy surface of Corchado and Espinosa-Garcia [J. Chem. Phys. 106, 4013 (1997)]. The quantum dynamics calculation employs the semirigid vibrating rotor target model [J. Z. H. Zhang, J. Chem. Phys. 111, 3929 (1999)] and time-dependent wave packet method to propagate the wave function. Initial state-specific reaction probabilities are obtained, and an energy correction scheme is employed to account for zero point energy changes for the neglected degrees of freedom in the dynamics treatment. Tunneling effect is observed in the energy dependency of reaction probability, similar to those found in H+CH4 reaction. The influence of rovibrational excitation on reaction probability and stereodynamical effect are investigated. Reaction rate constants from the initial ground state are calculated and are compared to those from the transition state theory and experimental measurement.     

Exceptional Isotopic‐Substitution Effect: Breakdown of Collective Proton Tunneling in Hexagonal Ice due to Partial Deuteration

Multiple proton transfer controls many chemical reactions in hydrogen‐bonded networks. However, in contrast to well‐understood single proton transfer, the mechanisms of correlated proton transfer and of correlated proton tunneling in particular have remained largely elusive. Herein, fully quantized ab initio simulations are used to investigate H/D isotopic‐substitution effects on the mechanism of the collective tunneling of six protons within proton‐ordered cyclic water hexamers that are contained in proton‐disordered ice, a prototypical hydrogen‐bonded network. At the transition state, isotopic substitution leads to a Zundel‐like complex, [HO???D???OH], which localizes ionic defects and thus inhibits perfectly correlated proton tunneling. These insights into fundamental aspects of collective proton tunneling not only rationalize recent neutron‐scattering experiments, but also stimulate investigations into multiple proton transfer in hydrogen‐bonded networks much beyond ice.     

How phonons govern the behavior of short, strong hydrogen bonds in urea-phosphoric acid

Recent neutron diffraction data have shown that the hydrogen atom involved in the short, strong hydrogen bond in urea-phosphoric acid migrates toward the midpoint of the hydrogen bond as the temperature increases. With the help of solid state ab initio calculations and inelastic neutron scattering, we have investigated the temperature dependence of the structural and vibrational properties of the system. The potential energy surface of the proton in the short, strong hydrogen bond and the thermal population of the energy levels therein cannot account for the observed proton migration. Ab initio molecular dynamics simulations clearly reveal the migration of the proton. This molecular dynamics result was reported recently by other authors, but they only offered a tentative explanation in terms of a resonance between high-frequency vibrations, which is not supported by the calculations presented here. We explain the proton migration in terms of phonon-driven structural fluctuations and their impact on the temperature-dependent evolution of the potential energy surface of the short hydrogen-bond proton.     

The principles of infrared-x-ray pump-probe spectroscopy. Applications on proton transfer in core-ionized water dimers

In this paper we derive the basic physics underlying infrared-x-ray pump-probe spectroscopy (IR, infrared). Particular features of the spectroscopy are highlighted and discussed, such as dependence on phase of the infrared pulse, duration and delay time of the x-ray pulse, and molecular orientation. Numerical applications are carried out for the water dimer using wave packet techniques. It is shown that core ionization of the donor oxygen of the water dimer results in a drastic change of the potential with the global minimum placed in the proton transfer region. The results of the modeling indicate that IR-x-ray pump-probe spectroscopy can be used to study the dynamics of proton transfer in this core-ionized state, and that, contrary to conventional core level photoelectron spectroscopy, x-ray core-ionization driven by an IR field is a proper method to explore the proton transfer in a system like the water dimer. We observe that the trajectory of the nuclear wave packet in the ground state potential well is strongly affected by the absolute phase of the IR pulse.     

Selective nonlinear response preparation using femtosecond spectrally resolved four-wave-mixing

A novel method is presented to assist the assignment of vibrational coherence in the homodyne degenerate four-wave-mixing technique. The dependence of vibrational coherence dynamics on the interaction sequence of chirped pump and Stokes excitation pulses is exploited to distinguish quantum beating from polarization interference. Moreover, by combining chirped excitation and variable delays between pump and Stokes pulses, it is possible to achieve a controlled excitation of response pathways from a single electronic state and separation of population dynamics and vibrational coherence dynamics within a single response pathway. Numerical simulations are performed in the response function framework, which clearly show that such an approach applies for oscillatory contributions originated in the electronically excited state as well as in the ground state. The approach is experimentally demonstrated in three different polyatomic molecules in solution.     

Advanced solid‐state nuclear magnetic resonance for polymer science

Today, solid‐state nuclear magnetic resonance (NMR) is one of the most powerful and versatile tools for elucidating the structures and dynamics of molecular, macromolecular, and supramolecular systems. It provides information on molecular and collective phenomena over large length scales and timescales and is particularly suited to handle noncrystalline materials. This report describes how developments in solid‐state NMR were triggered by the possibilities that became available about 30 years ago by neutron scattering and synchrotron radiation. Close analogies between NMR spectroscopy and scattering are pointed out to emphasize that the two approaches nicely complement each other. Specific examples applying the new NMR techniques to amorphous polymers and supramolecular systems are described. The findings are related to the mechanical properties of polymers as well as specific functions such as photoconductivity and proton conductivity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5031–5044, 2004     

Geometry of phenylene motion in polycarbonate from NMR spectroscopy and neutron scattering

In view of the importance of molecular dynamics in condensed matter both time scale and geometry of such processes should be determined experimentally. Whereas many techniques are available for the former, only NMR spectroscopy and neutron scattering can provide detailed information on the latter. Because of the different time scales of the dynamics, which the two techniques can detect best, direct comparisons of probing the geometry of the dynamics in the same system are scarce. Here we present such a comparison for the complex rotational motion of the phenylene groups in amorphous polycarbonate based on published (2)H NMR and newly recorded (13)C NMR data covering a wide temperature range, and recent quasielastic neutron scattering (QENS) data. We show that the results of the two techniques are in remarkable agreement, provided the data are consistently analyzed. No evidence is found for additional motions characterized by 90 degrees flips recently deduced from QENS data alone. Instead, the phenylene motion in the glassy state displays a broad heterogeneous distribution of rotational angles, about 80 degrees in width, centered at a flip angle of 180 degrees , which stays essentially constant over a wide temperature range. Thus, the phenylene motion that can consistently be observed in NMR and neutron scattering experiments is sensitive to the local packing.     

Excited State Photophysics of Curcumin and its Modulation in Alkaline Non-Aqueous Medium

Curcumin, a well-known medicinal pigment, has seen limited applications in biology despite having great potential as a therapeutic drug. Deprotonation is one of the possible ways to enhance solubility of curcumin in polar solvent. Here, we have explored the effect of deprotonation on the ultrafast dynamics of this biomolecule with the help of the time-resolved fluorescence spectroscopic measurements using the femtosecond fluorescence upconversion technique. The excited state photophysics of fully deprotonated curcumin significantly differs from that of neutral curcumin. We have observed that the completely deprotonated curcumin not only has higher quantum yield, but also higher excited state lifetime and slower solvation dynamics in comparison to neutral curcumin. We propose solvation dynamics and intramolecular charge transfer as the excited state processes associated with the radiative decay of the completely deprotonated molecule, while ruling out the possibility of excited state proton exchange or proton transfer. Our results are well supported by time-dependent density-functional theory calculations. Lastly, we have also demonstrated the possibility of modulating the ultrafast dynamics of fully deprotonated curcumin using non-aqueous alkaline binary solvent mixtures. We believe our results will provide significant physical insight towards unveiling the excited state dynamics of this molecule.