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A calculation of nuclear momentum distribution of liquid and solid hydrogen fluoride was performed. In both systems, density functional theory generalized gradient approximation functional of Perdew, Burke, and Ernzerhof was used for the calculation: for liquid hydrogen fluoride, using an atom centered basis set for an isolated molecule with optimized geometry, and for solid hydrogen fluoride using plane-wave basis sets on optimized orthorhombic crystal cell. For liquid hydrogen fluoride, a semiclassical approach was adopted with the vibrational contribution to momentum distribution obtained from the density functional theory calculation and translational and rotational contributions calculated classically. Nuclear momentum distribution in the solid hydrogen fluoride was calculated entirely quantum mechanically using phonon dispersion and vibrational density of states calculated in the framework of plane-wave density functional theory. Theoretical results were contrasted with recently obtained results of Compton (deep inelastic) neutron scattering on liquid and solid hydrogen fluoride. In case of liquid hydrogen fluoride, almost a perfect agreement between theory and experiment was achieved within the harmonic Born-Oppenheimer approximation. For the solid system under investigation, the harmonic approximation leads to small (4%) overestimation of the square root of the second moment indicating that neutron Compton scattering technique is sensitive to proton delocalization due to hydrogen bonding in solid hydrogen fluoride. [ABSTRACT FROM AUTHOR]

A new analytical approach was developed to characterize the properties of water molecules bound to macromolecules in solution using 17O nuclear magnetic resonance (NMR) relaxation. A combination of conventional (single-quantum) and triple-quantum filtered Hahn echo and inversion recovery measurements was employed. From measured relaxation rate constants, the fraction and the correlation time of bound H2 17O molecules and the relaxation rate constant of bulk water in solution were calculated. This was done by solving analytically a set of nonlinear equations describing the overall relaxation rate constants in the presence of chemical exchange between bulk and bound water. The analytical approach shows the uniqueness of the solution for a given set of three relaxation rate constants. This important result sheds light on the data reduction problem from 17O NMR experiments on biological systems. Water bound in photosystem I isolated from the wild type and rubA variant of the cyanobacterium Synechocystis species PCC 7002 was investigated for the first time. The analysis revealed that photosystem I isolated from the wild type binds 1720±110 water molecules, whereas photosystem I isolated from the rubA variant binds only 1310±170. The accuracy of the method proposed can be increased by further 17O enrichment. The methodology, established for the first time in this work, allows the study of a diverse range of biological samples regardless of their size and molecular weight. Applied initially to photosystem I, this novel method has important consequences for the future investigation of the assembly of biological molecules. [ABSTRACT FROM AUTHOR]

The authors have performed neutron Compton scattering measurements on ammonium hexachloropalladate (NH4)2PdCl6 and ammonium hexachlorotellurate (NH4)2TeCl6. Both substances belong to the family of ammonium metallates. The aim of the experiment was to investigate the possible role of electronic environment of a proton on the anomaly of the neutron scattering intensity. The quantity of interest that was subject to experimental test was the reduction factor of the neutron scattering intensities. In both samples, the reduction factor was found to be smaller than unity, thus indicating the anomalous neutron Compton scattering from protons. Interestingly, the anomaly decreases with decreasing scattering angle and disappears at the lowest scattering angle (longest scattering time). The dependence of the amount of the anomaly on the scattering angle (scattering time) is the same in both substances (within experimental error). Also, the measured widths of proton momentum distributions are equal in both metallates. This is consistent with the fact that the attosecond proton dynamics of ammonium cations is fairly well decoupled from the dynamics of the sublattice of the octahedral anions PdCl62- and TeCl62-, respectively. The hypothesis is put forward that proton-electron decoherence processes are responsible for the considered effect. Decoherence processes may have to do rather with the direct electronic environment of ammonium protons and not with the electronic structure of the metal-chlorine bond. [ABSTRACT FROM AUTHOR]