A J Horsewill

University of Nottingham, Nottigham, England, United Kingdom

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Publications (103)387.78 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report an inelastic neutron scattering (INS) study of a H_{2} molecule encapsulated inside the fullerene C_{60} which confirms the recently predicted selection rule, the first to be established for the INS spectroscopy of aperiodic, discrete molecular compounds. Several transitions from the ground state of para-H_{2} to certain excited translation-rotation states, forbidden according to the selection rule, are systematically absent from the INS spectra, thus validating the selection rule with a high degree of confidence. Its confirmation sets a precedent, as it runs counter to the widely held view that the INS spectroscopy of molecular compounds is not subject to any selection rules.
    Physical Review Letters 09/2014; 113(12):123001. · 7.73 Impact Factor
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    ABSTRACT: Inelastic neutron scattering (INS) has been employed to investigate the quantum dynamics of water molecules permanently entrapped inside the cages of C60 fullerene molecules. This study of the supramolecular complex, H2O@C60, provides the unique opportunity to study isolated water molecules in a highly symmetric environment. Free from strong interactions, the water molecule has a high degree of rotational freedom enabling its nuclear spin isomers, ortho-H2O and para-H2O to be separately identified and studied. The INS technique mediates transitions between the ortho and para spin isomers and using three INS spectrometers, the rotational levels of H2O have been investigated, correlating well with the known levels in gaseous water. The slow process of nuclear spin conversion between ortho-H2O and para-H2O is revealed in the time dependence of the INS peak intensities over periods of many hours. Of particular interest to this study is the observed splitting of the ground state of ortho-H2O, raising the three-fold degeneracy into two states with degeneracy 2 and 1 respectively. This is attributed to a symmetry-breaking interaction of the water environment.
    Physical Chemistry Chemical Physics 08/2014; · 4.20 Impact Factor
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    ABSTRACT: In this Letter, we present NMR spin-lattice and relaxometry data for proton transfer in one of the shortest known N-H⋯O hydrogen bonds in a single crystal of 3,5 pyridinedicarboxylic acid (35PDCA). It is widely believed that proton transfer by quantum tunneling does not occur in short hydrogen bonds since the ground state energy level lies above the potential barrier, yet these data show a temperature independent, proton tunneling rate below 77 K and a clear deviation from classical dynamics below 91 K. This study therefore suggests that proton tunneling occurs in all hydrogen bonds at low temperature and the crossover temperature to classical hopping must be determined when evaluating whether proton tunneling persists at higher temperature, for example in enzyme catalysis under physiological conditions.
    Physical Review Letters 07/2014; 113(1):018301. · 7.73 Impact Factor
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    ABSTRACT: The water-endofullerene H2O@C60 provides a unique chemical system in which freely rotating water molecules are confined inside homogeneous and symmetrical carbon cages. The spin conversion between the ortho and para species of the endohedral H2O was studied in the solid phase by low-temperature nuclear magnetic resonance. The experimental data are consistent with a second-order kinetics, indicating a bimolecular spin conversion process. Numerical simulations suggest the simultaneous presence of a spin diffusion process allowing neighbouring ortho and para molecules to exchange their angular momenta. Cross-polarization experiments found no evidence that the spin conversion of the endohedral H2O molecules is catalysed by (13)C nuclei present in the cages.
    The Journal of chemical physics. 05/2014; 140(19):194306.
  • B Zhang, C Sun, A M Alsanoosi, A Aibout, A J Horsewill
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    ABSTRACT: Field-cycling NMR in the solid state at low temperature (4.2 K) has been employed to measure the tunneling spectra of methyl (CH3) rotors in phenylacetone and toluene. The phenomenon of tunnel resonance reveals anomalies in (1)H magnetization from which the following tunnel frequencies have been determined: phenylacetone, νt = 6.58 ± 0.08 MHz; toluene, νt(1) = 6.45 ± 0.06 GHz and νt(2) = 7.07 ± 0.06 GHz. The tunnel frequencies in the two samples differ by three orders of magnitude, meaning different experimental approaches are required. In phenylacetone the magnetization anomalies are observed when the tunnel frequency matches one or two times the (1)H Larmor frequency. In toluene, doping with free radicals enables magnetization anomalies to be observed when the tunnel frequency is equal to the electron spin Larmor frequency. Cross-polarization processes between the tunneling and Zeeman systems are proposed and form the basis of a thermodynamic model to simulate the tunnel resonance spectra. These invoke space-spin interactions to drive the changes in nuclear spin-symmetry. The tunnel resonance lineshapes are explained, showing good quantitative agreement between experiment and simulations.
    The Journal of Chemical Physics 02/2014; 140(8):084302. · 3.12 Impact Factor
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    ABSTRACT: Field-cycling 1H NMR relaxometry has been used to measure the rate of concerted double proton transfer in the hydrogen bonds of 16O and 18O isotopologues of benzoic acid dimers. The experiments have been conducted in the solid state at low temperature 13.3 ≤ T≤ 80K where the dynamics are dominated by incoherent proton tunnelling. The low temperature tunnelling rate in the 16O isotopologue is observed to be approximately 15% faster than in the 18O isotopologue. The difference is attributed to an isotope effect of the heavy atom framework of the benzoic acid dimer resulting from displacements of the oxygen atoms that accompany the proton transfer. Sources of systematic uncertainty have been minimised in the design of the experimental protocols and the experiments are critically appraised in formally assigning the measured differences to an effect of mass on the tunnelling dynamics.
    The Journal of Physical Chemistry B 01/2014; · 3.61 Impact Factor
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    Malcolm H Levitt, Anthony J Horsewill
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    ABSTRACT: This Theo Murphy Meeting Issue contains papers presented at a Discussion Meeting held at the Kavli Centre of the Royal Society in March 2012. The meeting brought together a wide variety of scientists working on different aspects of small-molecule endofullerenes-those intriguing chemical systems in which small molecules such as H2 or H2O are encapsulated in tiny carbon cages.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 08/2013; 371(1998):20130124. · 2.89 Impact Factor
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    ABSTRACT: The quantum dynamics of a hydrogen molecule encapsulated inside the cage of a C60 fullerene molecule is investigated using inelastic neutron scattering (INS). The emphasis is on the temperature dependence of the INS spectra which were recorded using time-of-flight spectrometers. The hydrogen endofullerene system is highly quantum mechanical, exhibiting both translational and rotational quantization. The profound influence of the Pauli exclusion principle is revealed through nuclear spin isomerism. INS is shown to be exceptionally able to drive transitions between ortho-hydrogen and para-hydrogen which are spin-forbidden to photon spectroscopies. Spectra in the temperature range 1.6≤T≤280 K are presented, and examples are given which demonstrate how the temperature dependence of the INS peak amplitudes can provide an effective tool for assigning the transitions. It is also shown in a preliminary investigation how the temperature dependence may conceivably be used to probe crystal field effects and inter-fullerene interactions.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 08/2013; 371(1998):20110627. · 2.89 Impact Factor
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    ABSTRACT: Nuclear magnetic resonance (NMR) techniques are extensively used in many areas of basic and clinical research, as well as in diagnostic medicine. However, NMR signals are intrinsically weak, and this imposes substantial constraints on the amounts and concentrations of materials that can be detected. The signals are weak because of the low energies characteristic of NMR and the resulting very low (typically 0.0001-0.01%) polarization of the nuclear spins. Here, we show that exposure to very low temperatures and high magnetic fields, in conjunction with nanoparticle-mediated relaxation enhancement, can be used to generate extremely high nuclear polarization levels on a realistic timescale; with copper nanoparticles at 15 mK and 14 T, (13)C polarization grew towards its equilibrium level of 23% with an estimated half-time of about 60 hours. This contrasts with a (13)C half-time of at least one year in the presence of aluminium nanoparticles. Cupric oxide nanoparticles were also effective relaxation agents. Our findings lead us to suspect that the relaxation may be mediated, at least in part, by the remarkable magnetic properties that some nanoparticle preparations can display. This methodology offers prospects for achieving polarization levels of 10-50% or more for many nuclear species, with a wide range of potential applications in structural biology and medicine.
    Physical Chemistry Chemical Physics 05/2013; · 4.20 Impact Factor
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    ABSTRACT: Many approaches are now available for achieving high levels of nuclear spin polarization. One of these methods is based on the notion that as the temperature is reduced, the equilibrium nuclear polarization will increase, according to the Boltzmann distribution. The main problem with this approach is the length of time it may take to approach thermal equilibrium at low temperatures, since nuclear relaxation times (characterized by the spin-lattice relaxation time T1) can become very long. Here, we show, by means of relaxation time measurements of frozen solutions, that selected lanthanide ions, in the form of their chelates with DTPA, can act as effective relaxation agents at low temperatures. Differential effects are seen with the different lanthanides that were tested, holmium and dysprosium showing highest relaxivity, while gadolinium is ineffective at temperatures of 20 K and below. These observations are consistent with the known electron-spin relaxation time characteristics of these lanthanides. The maximum relaxivity occurs at around 10 K for Ho-DTPA and 20 K for Dy-DTPA. Moreover, these two agents show only modest relaxivity at room temperature, and can thus be regarded as relaxation switches. We conclude that these agents can speed up solid state NMR experiments by reducing the T1 values of the relevant nuclei, and hence increasing the rate at which data can be acquired. They could also be of value in the context of a simple low-cost method of achieving several-hundred-fold improvements in polarization for experiments in which samples are pre-polarized at low temperatures, then rewarmed and dissolved immediately prior to analysis.
    Physical Chemistry Chemical Physics 04/2013; · 4.20 Impact Factor
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    ABSTRACT: Inelastic neutron scattering, far-infrared spectroscopy, and cryogenic nuclear magnetic resonance are used to investigate the quantized rotation and ortho-para conversion of single water molecules trapped inside closed fullerene cages. The existence of metastable ortho-water molecules is demonstrated, and the interconversion of ortho-and para-water spin isomers is tracked in real time. Our investigation reveals that the ground state of encapsulated ortho water has a lifted degeneracy, associated with symmetry-breaking of the water environment.
    Proceedings of the National Academy of Sciences 07/2012; 109(32):12894-8. · 9.81 Impact Factor
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    ABSTRACT: Over the years, several strategies have been developed for generating highly polarized nuclear spin systems, including dynamic nuclear polarization, optical pumping, and methods exploiting parahydrogen. Here, we present an alternative strategy, using an enhanced 'brute-force' approach (i.e. exposure to low temperatures and high applied magnetic fields). The main problem with this approach is that it may take an excessively long time for the nuclear polarization to approach thermal equilibrium at low temperatures, since nuclear relaxation becomes exceedingly slow due to the loss of molecular motion. We show that low-field thermal mixing can alleviate the problem by increasing the rate at which slowly-relaxing nuclei reach equilibrium. More specifically, we show that polarization can be transferred from a relatively rapidly relaxing (1)H reservoir to more slowly relaxing (13)C and (31)P nuclei. The effects are particularly dramatic for the (31)P nuclei, which in experiments at a temperature of 4.2 K and a field of 2 T show a 75-fold enhancement in their effective rate of approach to equilibrium, and an even greater (150-fold) enhancement in the presence of a relaxation agent. The mixing step is also very effective in terms of the amount of polarization transferred-70-90% of the maximum theoretical value in the experiments reported here. These findings have important implications for brute-force polarization, for the problem becomes one of how to relax the solvent protons rather than individual more slowly-relaxing nuclei of interest. This should be a much more tractable proposition, and offers the additional attraction that a wide range of nuclear species can be polarized simultaneously. We further show that the (1)H reservoir can be tapped repeatedly through a number of consecutive thermal mixing steps, and that this could provide additional sensitivity enhancement in solid-state NMR.
    Physical Chemistry Chemical Physics 03/2012; 14(16):5397-402. · 4.20 Impact Factor
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    ABSTRACT: The quantum dynamics of dihydrogen molecules entrapped inside fullerene cages has been investigated using inelastic neutron scattering (INS). For the endofullerene H 2 @C 60 the low-lying energy levels of the manifold of coupled translational and rotational states have been accurately determined by studying INS spectra recorded in the temperature range 1.5 T 240 K. The majority of transitions observed in the INS spectra interconvert the nuclear spin isomers orthohydrogen and parahydrogen. The cage potential has icosahedral symmetry and splittings observed in the INS spectra reveal the coupling of translational and rotational angular momentum of the H 2 molecules. The effects of nuclear spin symmetry, isotope mass effects, and cage anisotropy have been further investigated by studying HD@C 60 and H 2 inside an open cage endofullerene. The momentum transfer κ arising from the neutron scattering event has also been investigated. The κ-dependence spectra reflect the physical dimensions of the dihydrogen molecule and its confinement in its cage. We show how this may be used as a tool for assigning the INS transitions.
    Physical Review B 01/2012; 20(85). · 3.66 Impact Factor
  • A J Horsewill, S M M Abu-Khumra
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    ABSTRACT: The populations of the tunneling states of CH(3) are manipulated by rf irradiation of weakly allowed sideband transitions within the manifold of tunneling-magnetic levels. Substantial positive and negative CH(3) tunneling polarizations are observed, providing a quantum rotor analogue of dynamic nuclear polarization and the solid effect in NMR. The field-cycling NMR technique used in the experiments employs level crossings between tunneling and Zeeman systems to report on the tunneling polarization. The tunneling lifetimes are measured and the field dependence investigated.
    Physical Review Letters 09/2011; 107(12):127602. · 7.73 Impact Factor
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    ABSTRACT: The supramolecular complex, H2@C60, represents a model of a quantum rotor in a nearly spherical box. In providing a real example of a quantum particle entrapped in a small space, the system cuts to the heart of many important and fundamental quantum mechanical issues. This review compares the predictions of theory of the quantum behaviour of H2 incarcerated in C60 with the results of infrared spectroscopy, inelastic neutron scattering and nuclear magnetic resonance. For H2@C60, each of these methods supports the quantization of translational motion of H2 and the coupling of the translational motion with rotational motion and provides insights to the factors leading to breaking of the degeneracies of states expected for a purely spherical potential. Infrared spectroscopy and inelastic neutron scattering experiments at cryogenic temperatures provide direct evidence of a profound quantum mechanical feature of H2 predicted by Heisenberg based on the Pauli principle: the existence of two nuclear spin isomers, a nuclear spin singlet (para-H2) and a nuclear triplet (ortho-H2). Nuclear magnetic resonance is capable of probing the local lattice environment of H2@C60 through analysis of the H2 motional effects on the ortho-H2 spin dynamics (para-H2, the nuclear singlet state, is NMR silent). In this review we will show how the information obtained by three different forms of spectroscopy join together with quantum theory to create a complementary and consistent picture which strikingly shows the intrinsically quantum nature of H2@C60.
    Coordination Chemistry Reviews 04/2011; · 11.02 Impact Factor
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    ABSTRACT: Double proton transfer in the hydrogen bonds of carboxylic acid dimers has been investigated via measurements of the dispersion of the proton spin-lattice relaxation time, T1, using magnetic field-cycling NMR. A plot of the spin-lattice relaxation rate, T1−1, as a function of magnetic field, provides a direct measurement of the motional spectrum and hence of the proton transfer rate. In the materials studied, these NMR experiments are optimised for the low temperature region where the dynamics are dominated by incoherent quantum tunnelling. Measurements of the proton transfer rate at higher temperatures have been made using quasi-elastic neutron scattering (QENS) and these facilitate the characterisation of the transition between quantum and classical dynamics in the intermediate temperature region. The role of tunnelling in the excited vibrational states of the double minimum potential (DMP) which characterises the system is revealed in these measurements and has been analysed according to the behaviour of a particle in a DMP which is coupled to a bath of phonons. The complementarity between the NMR and QENS techniques is emphasised and discussed, and the dynamic range accessible to the field-cycling NMR technique is explored through studying a range of related materials.
    Berichte der Bunsengesellschaft für physikalische Chemie. 06/2010; 102(3):317 - 324.
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    ABSTRACT: Two-proton exchange along the two hydrogen bonds mediates the tautomerization in benzoic acid (BA) dimers. Optical spectroscopy and quasi-elastic neutron scattering (QENS) have been employed to characterize the proton dynamics in doped and pure BA crystals. The proton motion in BA is governed by a multidimensional potential energy surface (PES), and recent theoretical methods, based on a perturbative instanton approach, to describe tunneling in such PES are presented. This PES is also modulated by the interaction with the solid state environment as manifest by the energy difference between the otherwise equivalent tautomers. The value of this energy difference in pure crystals is an important parameter in the data analysis of NMR and QENS. Both methods give mutually consistent values that differ significantly from earlier determinations via infrared and 13C NMR as well as a recent evaluation using neutron diffraction data. The energy difference between tautomers is altered for dimers in the vicinity of impurity molecules. This is the basis for the optical spectroscopic methods, which enable a direct and accurate determination of the level structure and tautomerization dynamics of these coupled dimers in the limit of very low temperatures, where coherent tunneling is also observed in some cases. Measurements with new impurity molecules make it possible to monitor simultaneously at least 10 different tautomer configurations and prove that the influence of the probe molecules on the proton dynamics is small. The transition to thermally activated barrier crossing at higher temperatures is accessed via the width of the QENS line that is determined by the inverse of the proton correlation time. The quantitative data analysis of the scattered intensity as a function of temperature and scattering angle yields the energy difference, A, between the two wells (A/kB=90+20 K), the length (0.686 Å), and direction of the proton jump vector. These measurements complement NMR investigations presented in the preceding paper.
    Berichte der Bunsengesellschaft für physikalische Chemie. 06/2010; 102(3):325 - 334.
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    ABSTRACT: Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.
    Physical Chemistry Chemical Physics 06/2010; 12(22):5883-92. · 4.20 Impact Factor
  • A. J. Horsewill
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    ABSTRACT: The double proton transfer reaction in the bridging hydrogen bonds of carboxylic acid dimers provides a model system for investigating the fundamental physics underpinning the role of quantum tunnelling in molecular dynamics. The behaviour of this system is used to discuss the possible role that tunnelling plays in the molecular dynamics that characterise more complex biological systems at high temperature. Copyright © 2010 John Wiley & Sons, Ltd.
    Journal of Physical Organic Chemistry 02/2010; 23(7):580 - 585. · 1.58 Impact Factor
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    ABSTRACT: We report an inelastic neutron-scattering (INS) investigation of coupled quantum translation and rotation of hydrogen molecules trapped inside the closed isotropic cages of C60. The low-lying states that characterize the translation-rotation manifold of the hydrogen molecules are accurately determined in our study of the INS peak energies. A comparison between the spectra of H2 and HD isotopomers provides quantitative insight into the coupling between rotational and translational angular momentum with HD exhibiting the strongest effects due to mixing of the rotational eigenstates.
    Physical review. B, Condensed matter 01/2010; 82(8). · 3.77 Impact Factor

Publication Stats

635 Citations
387.78 Total Impact Points

Institutions

  • 1981–2014
    • University of Nottingham
      • School of Physics and Astronomy
      Nottigham, England, United Kingdom
  • 2007–2013
    • University of Southampton
      • • Division of Chemistry
      • • Faculty of Natural and Environmental Sciences
      Southampton, ENG, United Kingdom
  • 2003
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 2000
    • University of Leuven
      Louvain, Flanders, Belgium