J. Tomkinson

Vibrational spectroscopy with neutrons: with applications in chemistry, biology, materials science and catalysis

The Theory of Inelastic Neutron Scattering Spectroscopy Instrumentation and Experimental Methods Computational Modelling of Inelastic Neutron Scattering Spectra Interpreting Inelastic Neutron Scattering Spectra Dihydrogen and Hydrides Hydrogen Bonding Non-Hydrogenous Materials and Carbon Soft Condensed Matter: Polymers and Biology Vibrations of Atoms in Surfaces and Adsorbed Species Organic and Organometallic Compounds.

A New Look at Proton Transfer Dynamics Along the Hydrogen Bonds in Amides and Peptides

Vibrational spectroscopy with inelastic neutron scattering can provide spectra that are more detailed and easier to interpret than optical spectra. The spectral intensity depends on energy transfer and kinetic momentum transfer, allowing determination of the potential function. Experiments reveal that the proton involved in intermolecular hydrogen bonding in N-methylacetamide and polyglycine I vibrates almost independently. An ionic representation (Nδ-...H+...Oδ-) of the hydrogen bond is more realistic than the normally accepted covalent model (NH...O). For polyglycine I, the proton experiences a local, symmetric double-minimum potential arising from dynamic exchange between the amide-like (CONH) and imidol-like (HOCN) forms of the peptide unit.

Inelastic neutron scattering spectrum of the fullerene C60

Abstract The vibrational spectrum of C 60 has been measured by inelastic neutron scattering at 20 K. All 14 infrared and Raman active vibrational modes can be identifeid together with many optically inactive and low-energy lattice modes. The spectrum extends smoothly to ≈ 200 MeV. Fairly good agreement with existing models of the vibrational properties of C 60 is found.

Proton dynamics in the hydrogen bond. The inelastic neutron scattering spectrum of potassium hydrogen carbonate at 5 K

Abstract We report the inelastic neutron scattering (INS) spectrum from KHCO 3 powder (5 K). The spectrum has been analysed by the frequency and intensity normal coordinate refinement program CLIMAX. The final force field indicates that the proton bending modes in the OH…O bond are independent of other deformation modes, but that the stretching modes interact strongly. This is interpreted in terms of a significant ionic character for the hydrogen bond. Based upon simple calculations for its position and intensity an INS mode at ≈ 220 cm −1 is tentatively assigned to a tunneling transition for the proton.

FULLERENES AND FULLERIDES IN THE SOLID STATE: NEUTRON SCATTERING STUDIES

Abstract Fullerene-60 shows a high molecular symmetry, consistent with the icosahedral point group I h . At low temperature in the solid state, high-resolution powder neutron diffraction reveals that crystalline C 60 (local symmetry S 6 ) adopts a simple cubic crystal structure whose stability is driven by optimisation of the intermolecular electrostatic interactions. Above 90 K, molecular motion is no longer frozen and the molecules shuffle between nearly degenerate orientations, differing in energy by 11.4(3) meV. At 260 K, a first-order phase transition leads to a face-centred cubic structure, characterised by rapid isotropic reorientational motion of the molecules. The phonon spectra of pristine fullerene, superconducting K 3 C 60 and saturation-doped Rb 6 C 60 measured by inelastic neutron scattering in the energy range 20–2000 cm −1 , reveal substantial broadening of fivefold degenerate H g intramolecular vibrational modes both in the lowenergy radial and the high-energy tangential part of the spectrum. This provides strong evidence for a traditional phonon-mediated mechanism of superconductivity in the fullendes but with an electron-phonon coupling strength distributed over a wide range of energies (33–195 meV) as a result of the finite curvature of the fullerene spherical cage.

VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility

The VESUVIO project aims to provide unique prototype instrumentation at the ISIS-pulsed neutron source and to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies. This instrumentation will be specifically designed for high momentum, , and energy transfer inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EU researchers. It will allow to derive single-particle kinetic energies and single-particle momentum distributions, n(p), providing additional and/or complementary information to other neutron inelastic spectroscopic techniques.

An inelastic incoherent neutron scattering study of ice II, IX, V, and VI-in the range from 2 to 140 meV

Inelastic incoherent neutron scattering (IINS) cross sections have been measured for the recovered high‐pressure phases of ice, ices II, V, VI, and IX (H2O) and ices II and IX (D2O) in the energy range from 2 to 150 meV at 20 K using the TFXA (time‐focused crystal analyzer) neutron spectrometer on the ISIS pulsed neutron source at the Rutherford‐Appleton Laboratory. The excellent resolution of the TFXA (less than 2%) covers the whole range of translational and librational modes for the ices investigated and enables us to identify the fine structures in their spectra. Hence a direct comparison between the amplitude‐weighted phonon density of states derived from the experimental spectrum and from a lattice dynamic calculation is possible [E. Whalley and J. E. Bertie, J. Chem. Phys. 46, 1271 (1967); J. E. Bertie and E. Whalley, ibid. 40, 1646 (1964)]. The spectra from these polymorphic ices show many new features in the translational region, which are not visible using infared and Raman techniques. In the li...

Proton transfer dynamics in the hydrogen bond. Inelastic neutron scattering, infrared and Raman spectra of Na3H(SO4)2, K3H(SO4)2 and Rb3H(SO4)2

Abstract Na 3 H(SO 4 ) 2 , K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 crystals are composed of (SO 4 HSO 4 ) −3 dimers linked by rather strong hydrogen bonds ( R O…O =2.43 A for Na 3 H(SO 4 ) 2 , R O…O =2.48 A for Rb 3 H(SO 4 ) 2 and R O…O =2.49 A for K 3 H(SO 4 ) 2 ). Crystallographic data of the salts at room temperature indicate either asymmetric (Na 3 H(SO 4 ) 2 ) or symmetric (K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 ) hydrogen bonds. Inelastic neutron scattering (INS), infrared and Raman spectra of crystal powders at 20 K are reported for these three compounds. The OH bending modes, which give large INS intensities, appear only weakly in the infrared. The two bending modes are degenerate in Na 3 H(SO 4 ) 2 which has the shortest hydrogen bond but are well separated in K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 . The OH stretching band profiles in INS are also quite different from those in the infrared. Strong INS bands at 57 and 44 cm −1 for K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 , respectively, are assigned to 0→1 transitions in quasi-symmetric double-minimum potentials for the OH stretching coordinates. For K 3 H(SO 4 ) 2 the frequency is unaffected by temperature between 2 and 100 K. Potential functions are calculated and the dynamics of the proton transfer are discussed. Infrared spectra are thus dominated by OH stretching transitions in asymmetric double-minimum potentials with low barriers, with relative intensities indicating a large electrical anharmonicity.

Inelastic neutron-scattering study of the proton dynamics in N-methylacetamide at 20 K

Abstract Inelastic neutron-scattering (INS) spectra of four isotopic derivatives of N-methylacetamide (CH 3 CONHCH 3 , CD 3 CONHCH 3 , CH 3 CONHCD 3 and CD 3 CONHCD 3 ) at 20 K are presented from 30 to 4000 cm −1 . The band frequencies are compared with those observed in the infrared and Raman at low temperature. The quantitative simulation of the INS intensities, in the harmonic force field approximation, shows that the proton dynamics for the (N)H proton are totally different from those proposed previously. The valence-bond approach is not consistent with observation and the proton dynamics are independent of the molecular frame. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode is at ∼ 1575 cm −1 . This is a dramatic change compared to all former assignments at ∼ 3250 cm −1 based on the infrared and Raman data. These unforeseen proton dynamics are associated with the weakening of the NH bond due to the ionic character of the hydrogen bond (N δ− …H + …O δ′− ) and proton transfer. The infrared and Raman spectra are reconsidered and a new assignment scheme is proposed for the amide bands in terms of dynamicalproton exchange between the amidic (…OCNH…) and imidolic (…HOCN…) forms in infinite chains of hydrogen-bonded molecules.

Measurement and ab initio modeling of the inelastic neutron scattering of solid melamine: Evidence of the anisotropy in the external modes spectrum

Abstract The inelastic neutron scattering spectrum of melamine has been measured and a normal coordinates analysis has been performed in order to interpret the vibrational dynamics. This study reveals the anisotropy in the external mode spectrum and its important role in the internal modes region. Thus, the Debye–Waller factor has taken a value for the out-of-plane vibrations four times greater than that for the in-plane vibrations. A molecular force field refinement has been carried out in independent symmetry coordinates (D 3h ) in order to confirm the vibrational assignments. The final force field is free of redundancies and therefore the corresponding force constants are unambiguous.