Gordon J. Kearley

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.

Hydrogen in Porous Tetrahydrofuran Clathrate Hydrate

The lack of practical methods for hydrogen storage is still a major bottleneck in the realization of an energy economy based on hydrogen as energy carrier.1 Storage within solid-state clathrate hydrates,2-4 and in the clathrate hydrate of tetrahydrofuran (THF), has been recently reported.5, 6 In the latter case, stabilization by THF is claimed to reduce the operation pressure by several orders of magnitude close to room temperature. Here, we apply in situ neutron diffraction to show that-in contrast to previous reports([5, 6])-hydrogen (deuterium) occupies the small cages of the clathrate hydrate only to 30 % (at 274 K and 90.5 bar). Such a D(2) load is equivalent to 0.27 wt. % of stored H(2). In addition, we show that a surplus of D(2)O results in the formation of additional D(2)O ice Ih instead of in the production of sub-stoichiometric clathrate that is stabilized by loaded hydrogen (as was reported in ref. 6). Structure-refinement studies show that [D(8)]THF is dynamically disordered, while it fills each of the large cages of [D(8)]THF17D(2)O stoichiometrically. Our results show that the clathrate hydrate takes up hydrogen rapidly at pressures between 60 and 90 bar (at about 270 K). At temperatures above approximately 220 K, the H-storage characteristics of the clathrate hydrate have similarities with those of surface-adsorption materials, such as nanoporous zeolites and metal-organic frameworks,7, 8 but at lower temperatures, the adsorption rates slow down because of reduced D(2) diffusion between the small cages.

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.

What is the structure of kaolinite? Reconciling theory and experiment.

Density functional modeling of the crystalline layered aluminosilicate mineral kaolinite is conducted, first to reconcile discrepancies in the literature regarding the exact geometry of the inner and inner surface hydroxyl groups, and second to investigate the performance of selected exchange-correlation functionals in providing accurate structural information. A detailed evaluation of published experimental and computational structures is given, highlighting disagreements in space groups, hydroxyl bond lengths, and bond angles. A major aim of this paper is to resolve these discrepancies through computations. Computed structures are compared via total energy calculations and validated against experimental structures by comparing computed neutron diffractograms, and a final assessment is performed using vibrational spectra from inelastic neutron scattering. The density functional modeling is carried out at a sufficiently high level of theory to provide accurate structure predictions while keeping computational requirements low enough to enable the use of the structures in large-scale calculations. It is found that the best functional to use for efficient density functional modeling of kaolinite using the DMol3 software package is the BLYP functional. The computed structure for kaolinite at 0 K has C1 symmetry, with the inner hydroxyl group angled slightly above the a,b plane and the inner surface hydroxyls aligned close to perpendicular to that plane.

The structure and dynamics of crystalline durene by neutron scattering and numerical modelling using density functional methods

Abstract Inelastic neutron scattering (INS) and single crystal diffraction measurements of tetramethylbenzene (durene) are reported along with first-principles calculations, based on density functional theory (DFT), of structure and dynamics. Atomic positions obtained from refinement of the neutron scattering data and from three different DFT methodologies are in excellent agreement. Normal modes and INS spectra are calculated within the harmonic approximation using the direct cell finite displacement technique. DFT affords a reliable description of intramolecular and intermolecular interactions with the result that the vibrational spectra are well reproduced by all calculations. The advantage over traditional ab initio, single molecule calculations is the improved description of the low frequency vibrations that are influenced by intermolecular interactions. No refinement of force constants has been undertaken. This structural and vibrational analysis is discussed in the context of optical work in durene host lattices.

Methyl group dynamics in paracetamol and acetanilide: probing the static properties of intermolecular hydrogen bonds formed by peptide groups

Abstract Measurements of tunnelling and librational excitations for the methyl group in paracetamol and tunnelling excitations for the methyl group in acetanilide are reported. In both cases, results are compared with molecular mechanics calculations, based on the measured low temperature crystal structures, which follow an established recipe. Agreement between calculated and measured methyl group observables is not as good as expected and this is attributed to the presence of comprehensive hydrogen bond networks formed by the peptide groups. Good agreement is obtained with a periodic quantum chemistry calculation which uses density functional methods, these calculations confirming the validity of the one-dimensional rotational model used and the crystal structures. A correction to the Coulomb contribution to the rotational potential in the established recipe using semi-emipircal quantum chemistry methods, which accommodates the modified charge distribution due to the hydrogen bonds, is investigated.

Structure and vibrational dynamics of the strongly hydrogen-bonded model peptide: N-methyl acetamide

Density functional theory-based methods have been used to calculate the vibrations, in the harmonic approximation, of n-methyl acetamide in the solid state. Good agreement is obtained with previously published inelastic neutron scattering spectra. The starting point for the calculation is the crystal structure, which has to be measured at the same temperature as the vibrational spectra. Unit cell and atomic coordinates have been obtained using powder neutron diffraction on the methyl-deuterated material at 2 K. The controversial assignment of the N–H stretch mode at ∼1600 cm−1, made in the original analysis of the vibrational spectra, is not supported by the calculations presented here. Neither is evidence found for the proposed double-well potential for the proton in the hydrogen bond.

Negative Thermal Expansion in LnCo(CN)6 (Ln=La, Pr, Sm, Ho, Lu, Y): Mechanisms and Compositional Trends†

Negative thermal expansion (NTE) is a comparatively rare phenomenon that is found in a growing number of materials. The discovery of new NTE materials and the elucidation of mechanisms underpinning their behavior is important both in extending the field and enabling tailored thermal expansion properties. NTE has been found throughout a broad family of cyanide coordination frameworks, arising from thermal population of low-energy transverse vibrations of the cyanide bridges, which reduce the average metal–metal distances, and thus the lattice parameters, with increasing temperature. More complex mechanisms have been established in metal– organic framework materials, in which both local and longrange modes contribute to NTE. The low-energy dynamics of metal-based materials are often modeled in terms of rigid unit modes (RUMs), wherein the metal-centered polyhedra are treated as rigid, with only the linkage being flexible. Most NTE cyanide frameworks are members of two cubic structural types: Zn(CN)2 analogues, [2a,b,5] containing tetrahedral metal centers in the diamondoid topology; and Prussian blue analogues, with octahedral metal centers in the a-Po topology. NTE has recently been observed in a framework of a different structural type: ErCo(CN)6, [7] possessing hexagonal symmetry (P63/mmc) owing to the combination of ErN6 trigonal prisms alternating with CoC6 octahedra. ErCo(CN)6 displays near-isotropic NTE with axial coefficients of thermal expansion (CTEs) aa= da/adT= 8 10 6 K , ac= 9 10 6 K 1 and effective linear CTE, al= 1/3 dV/VdT= 9 10 6 K . Herein we probe in detail the novel mechanism for NTE in this structure type through a comprehensive approach combining synthesis, structural and dynamic analysis, and modeling. Substitution of other trivalent lanthanoids for Er yields an extended series, LnCo(CN)6, of which representative members have been selected for characterization (Ln= La, Pr, Sm, Ho, Lu, and Y). Topotactic dehydration of the parent framework hydrates LnCo(CN)6·nH2O (n= 4, 5) yields an extended isostructural series with the trigonal prismatic LnN6 coordination geometry (Figure 1a, inset), which is a rare example of an isostructural

Extreme compressibility in LnFe(CN) 6 coordination framework materials via molecular gears and torsion springs

The mechanical flexibility of coordination frameworks can lead to a range of highly anomalous structural behaviours. Here, we demonstrate the extreme compressibility of the LnFe(CN)6 frameworks (Ln = Ho, Lu or Y), which reversibly compress by 20% in volume under the relatively low pressure of 1 GPa, one of the largest known pressure responses for any crystalline material. We delineate in detail the mechanism for this high compressibility, where the LnN6 units act like torsion springs synchronized by rigid Fe(CN)6 units performing the role of gears. The materials also show significant negative linear compressibility via a cam-like effect. The torsional mechanism is fundamentally distinct from the deformation mechanisms prevalent in other flexible solids and relies on competition between locally unstable metal coordination geometries and the constraints of the framework connectivity, a discovery that has implications for the strategic design of new materials with exceptional mechanical properties.