PubMed Health⌕ Search

Biomedical subjects

G J Kearley

Publications and source records attributed to G J Kearley.

7 recordsLinked to original sources
PubMed2006

Phonon driven proton transfer in crystals with short strong hydrogen bonds.

Recent work on understanding why protons migrate with increasing temperature in short, strong hydrogen bonds is extended here to three more organic, crystalline systems. Inelastic neutron scattering and density functional theory based simulations are used to investigate structure, vibrations, and dynamics of these systems as functions of temperature. The mechanism determined in a previous work on urea phosphoric acid of low frequency vibrations stabilizing average crystal structures, in which the potential energy well of the hydrogen bond has its minimum shifted towards the center of the bond, is found to be valid here. The new feature of the N-H...O hydrogen bonds studied in this work is that the proton is transferred from the donor atom to the acceptor atom. Molecular dynamics simulations show that in an intermediate temperature regime, in which the proton is not completely transferred, the proton is bistable, jumping from one side of the hydrogen bond to the other. In the case of 3,5-pyridine dicarboxylic acid, which has been studied in most detail, specific phonons are identified, which influence the potential energy surface of the proton in the short, strong hydrogen bond.

Journal Article↗
PubMed2006

Intermolecular interactions in solid benzene.

The lattice dynamics and molecular vibrations of benzene and deuterated benzene crystals are calculated from force constants derived from density-functional theory (DFT) calculations and compared with measured inelastic neutron-scattering spectra. A very small change (0.5%) in lattice parameter is required to obtain real lattice-mode frequencies across the Brillouin zone. There is a strong coupling between wagging and breathing modes away from the zone center. This coupling and sensitivity to cell size arises from two basic interactions. Firstly, comparatively strong interactions that hold the benzene molecules together in layers. These include an intermolecular interaction in which H atoms of one molecule link to the center of the aromatic ring of a neighboring molecule. The layers are held to each other by weaker interactions, which also have components that hold molecules together within a layer. Small changes in the lattice parameters change this second type of interaction and account for the changes to the lattice dynamics. The calculations also reveal a small auxetic effect in that elongation of the crystal along the b axis leads to an increase in internal pressure in the ac plane, that is, elongation in the b direction induces expansion in the a and c directions.

Journal Article↗
PubMed2005

Dynamics and lithium binding energies of polyelectrolytes based on functionalized poly(para-phenylene terephthalamide).

Polyelectrolyte materials are an interesting class of electrolytes for use in fuel cell and battery applications. Poly(para-phenylene terephthalamide) (PPTA, Kevlar) is a liquid crystalline polymer that, when sulfonated, is a polyelectrolyte that exhibits moderate ion conductivity at elevated temperatures. In this work, quasi-elastic neutron scattering (QENS) experiments were performed to gain insight into the effect of the presence of lithium counterions on the chain dynamics in the material. It was found that the addition of lithium ions decreases the dynamics of the chains. Additionally, the binding of lithium ions to the sulfonic acids groups was investigated by density functional theory (DFT) calculations. It was found that the local surroundings of the sulfonic acid group have very little effect on the lithium-ion binding energy. Binding energies for a variety of different systems were all calculated to be around 150 kcal/mol. The DFT calculations also show the existence of a structure in which a single lithium ion interacts with two sulfonic acid moieties on different chains. The formation of such "electrostatic cross-links" is believed to be the source of the increased tendency to aggregate and the reduced dynamics in the presence of lithium ions.

Journal Article↗
PubMed2001

Structure of manganese diacetate tetrahydrate and low-temperature methyl-group dynamics.

We have determined the crystal structure of manganese(II) diacetate tetrahydrate at 300 and 14 K by single-crystal neutron diffraction. Proton density distributions for each of the three crystallographically distinct methyl groups have been calculated by Fourier difference. At room temperature the observed densities are those of quasi-free rotors. At low temperature rather well localized protons are observed. Inelastic neutron scattering measurements performed with single crystals allow us to assign each of the three tunnelling lines to a particular crystal site. Classical molecular dynamics simulations give density distributions in qualitative agreement with the observations. With quantum mechanics proton distributions can be represented with rotational wavefunctions convoluted with static distributions of librational coordinates. The effective rotational potentials are temperature dependent.

Journal Article↗
PubMed2000

Quantitative atom-atom potentials from rotational tunneling: their extraction and their use.

Rotational tunneling of small molecular groups has been the subject of considerable theoretical and experimental activity for several decades. Much of this activity has been driven by the promise of exploiting the extreme sensitivity of quantum tunneling to interatomic potentials, but until recently, there was no straightforward means by which quantitative information about these potentials could be extracted. This review explains how a quantitative method, suitable for general application, was developed. It then goes on to show how this has been used to understand tunneling systems for which no previous satisfactory explanation had been found. The application of the methodology, and its results, to other disciplines is discussed.

Journal Article↗
PubMed1994

Inelastic neutron-scattering study of the proton transfer dynamics in polyglycine I at 20 K.

Inelastic neutron-scattering (INS) spectra of three isotopic derivatives of polyglycine I (-COCH2NH-)n, (-COCD2NH-)n, and (-COCH2ND-)n at 20 K are presented from 30 to 4000 cm(-1). The band frequencies are compared to those observed in the infrared and Raman. Assignments in terms of group vibrations are proposed. These mostly resemble previous assignment schemes, except for the amide bands. The INS intensities reveal that the proton dynamics for the (N)H proton are totally different from those proposed previously. They are independent of the molecular frame and the valence bond approach is not consistent with observation. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode has two components at approximately 1377 and 1553 cm(-1). This is a dramatic change compared to all former assignments at approximately 3280 cm(-1) based on infrared and Raman data. These proton-dynamics are associated with a weakening of the NH bond due to the ionic character of the hydrogen bond (N(delta-)...H+...O(delta'-)) and proton transfer. The infrared and Raman spectra are re-examined and a new assignment scheme is proposed for the amide bands; the amide A and B bands are re-assigned to the overtones of the stretching modes. A symmetric double-minimum potential for the proton is consistent with all the observations.

Journal Article↗