Further Information:

Quantum Molecular Tunnelling and Field-Cycling NMR 

Background to Quantum Tunnelling Research Group

My principal research interests lie in the study of quantum tunnelling phenomena in molecular systems in the solid state. The dynamics of atoms and molecular groups can be described in the context of a displacement along a reaction coordinate connecting two or more potential wells which are separated by an interceding potential barrier. At high temperature, for particles with large mass, the motion may often be described as barrier hopping using the principles of classical mechanics. However, when the mass of the particles is small and the barrier region is narrow then the dynamics reveal the underlying quantum mechanical and wavelike nature of the particles. The motion at low temperature may be dominated by tunnelling, however with increasing temperature the interaction with the external degrees of freedom spoils the coherence of the tunnelling regime and the dynamics evolve smoothly towards more classical behaviour. This is the quantum to classical transition and one of our aims is to investigate the interface between quantum and classical mechanics.

Examples of systems studied in our laboratory include symmetrical rotors (eg coherent tunnelling in the methyl group) and proton transfer reactions in the hydrogen bond which exhibit incoherent tunnelling. Recently we have investigated the quantum motion of hydrogen molecules trapped inside C60 bucky balls.

In Nottingham we have developed specialised magnetic field-cycling Nuclear Magnetic Resonance (NMR) techniques for measuring tunnelling frequencies and rates. These are conducted using a custom designed superconducting field-cycling magnet. >

Complementary investigations are undertaken using neutron scattering at the Institut Laue-Langevin in Grenoble, France. NMR and neutron scattering are receptive in different energy windows and together the two techniques provide a complete picture of the tunnelling dynamics and the molecular modes with which the tunnelling sub-system interacts.

Field-cycling NMR Spectrometer

The construction of our field cycling NMR spectrometer was funded by research grants from 'The Royal Society: Paul Instrument Fund' and the 'Engineering and Physical Sciences Research Council'. This custom built spectrometer, designed primarily for solid state NMR, provides a facility that is unique in the world, combining the ability to record NMR properties as a function of magnetic field with a low temperature sample environment. The specification of this spectrometer is summarised below:

• Magnet Coil: Superconducting solenoid with inductance L = 0.02 H powered continuously by a fast ramping power supply so that the B-field is determined by the instantaneous current.
• B-field: Maximum 2.5 Tesla with current Imax = 160 Amps, available continuously without restriction on the duty cycle. The B-field is linear with current and is set using a control voltage derived from a 20-bit DAC.
• B-field homogeneity: Better than 1:104 over sample volume 1cm3.
• Magnetic field-ramping rate: Maximum 10 T s -1.
• Sample Temperature: 3 < T < 300 K with stability better than 0.05 K.
• NMR Console: TecMag Apollo
• Automation: All functions of the spectrometer are fully automated using a VisualBasic control program as the user interface to the TecMag Apollo NMR console software.

The superconducting magnet and low temperature helium cryostats were manufactured by Cryogenic Ltd.

Publications

There are two reviews that summarise the specialised field-cycling NMR techniques that have been developed for the study of molecular tunnelling processes in our laboratory:

'Quantum tunnelling in the hydrogen bond' by A.J. Horsewill,
Progress in Nuclear Magnetic Resonance Spectroscopy 52 (2008) 170-196
http://dx.doi.org/10.1016/j.pnmrs.2007.09.002

'Quantum tunnelling aspects of methyl group rotation' by A.J. Horsewill,
Progress in Nuclear Magnetic Resonance Spectroscopy 35 (1999) 359-389.
http://dx.doi.org/10.1016/S0079-6565(99)00016-3

Some selected recent papers:

Horsewill AJ, Panesar KS, Rols S, Johnson MR, Murata Y, Komatsu K, Mamone S, Danquigny A, Cuda F, Maltsev S, Grossel M,C, Carravetta M, Levitt MH. Quantum Translator-Rotator: Inelastic Neutron Scattering of Dihydrogen Molecules Trapped inside Anisotropic Fullerene Cages, Physical Review Letters 102 (2009) 013001
http://link.aps.org/doi/10.1103/PhysRevLett.102.013001

This article has also been independently reviewed in: Physical Review Focus, http://focus.aps.org/story/v23/st1

Horsewill AJ, Sun C. ‘Tunnelling magnetic resonances: dynamic nuclear polarisation and the diffusion of methyl group tunnelling energy’ Journal of Magnetic Resonance (2009)
http://dx.doi.org/10.1016/j.jmr.2009.03.013

Horsewill AJ, Wu W 'Proton tunneling in a hydrogen bond measured by cross-relaxation field-cycling NMR'
Journal of Magnetic Resonance 179 (2006) 169-172
http://dx.doi.org/10.1016/j.jmr.2005.11.010

Wu W, Noble DL, Owers-Bradley JR, Horsewill AJ
'A C-13 field-cycling NMR relaxometry investigation of proton tunnelling in the hydrogen bond: Dynamic isotope effects, the influence of heteronuclear interactions and coupled relaxation' Journal of Magnetic Resonance 175 (2005) 210-221
http://dx.doi.org/10.1016/j.jmr.2005.04.003

Xue Q, Horsewill AJ, Johnson MR, Trommsdorff HP
'Isotope effects associated with tunneling and double proton transfer in the hydrogen bonds of benzoic acid' Journal of Chemical Physics 120 (2004) 11107-11119
http://dx.doi.org/10.1063/1.1738644

Jenkinson RI, Ikram A, Horsewill AJ, Trommsdorff HP.
'The quantum dynamics of proton transfer in benzoic acid measured by single crystal NMR spectroscopy and relaxometry' Chemical Physics 294 (2003) 95-104
http://dx.doi.org/10.1016/j.chemphys.2003.07.002

Horsewill AJ, Xue Q
'Magnetic field-cycling investigations of molecular tunnelling' Physical Chemistry Chemical Physics 4 (2002) 5475-5480
http://dx.doi.org/10.1039/b206263a

Further references to our work can be found therein or by email to A.Horsewill@nottingham.ac.uk.