K. Nygård

Correlation of hydrogen bond lengths and angles in liquid water based on Compton scattering

The temperature-dependent hydrogen-bond geometry in liquid water is studied by x-ray Compton scattering using synchrotron radiation combined with density functional theory analysis. Systematic changes, related to the weakening of hydrogen bonding, are observed in the shape of the Compton profile upon increasing the temperature. Using model calculations and published distribution functions of hydrogen-bond geometries obtained from a NMR study we find a significant correlation between the hydrogen-bond length and angle. This imposes a new constraint on the possible local structure distributions in liquid water. In particular, the angular distortions of the short hydrogen bonds are significantly restricted.

Isotope quantum effects in the electron momentum density of water

The isotope quantum effects in the ground-state electron momentum density of water are studied at temperatures ranging from 5 to 90 degrees C by combining Compton scattering experiments utilizing synchrotron radiation and computational analysis within density functional theory. We observe clear differences in the momentum density between normal and heavy water at room temperature, which are interpreted as predominantly reflecting intramolecular structural differences. The changes in the momentum density upon increasing the temperature are found to be larger for heavy than for normal water, which is attributed primarily to temperature-induced intramolecular structural effects. Both model computations and an ab initio approach qualitatively reproduce the changes in the momentum density as a function of temperature.

Charge localization in alcohol isomers studied by Compton scattering

The isomers of small molecule alcohols propanol (PrOH) and butanol (BuOH) are studied by x-ray Compton scattering experiments with synchrotron radiation and density-functional theory calculations. The lineshape of the measured spectra, i.e., the Compton profile, is a momentum-space property, and its changes reflect changes in the electronic charge density between the isomers. Compared to the linear alcohols (n-PrOH and n-BuOH), the Compton profiles of the branched alcohols (iso-PrOH, iso-BuOH, sec-BuOH) are found to be narrower, which indicates a more delocalized charge for the latter. The calculations are performed for systems consisting of one to three monomer units and are found to reproduce reasonably the experimental spectral features. The influence of the basis set and exchange-correlation scheme is studied in more detail. The results provide new insight into the isomeric differences in small molecule alcohols and show that quantum chemical calculations can be increasingly tested against the x-ray Compton scattering data.

The elemental composition, the microfibril angle distribution and the shape of the cell cross-section in Norway spruce xylem

Relationships between the elemental composition, the microfibril angle (MFA) distribution and the average shape of the cell cross-section of irrigated-fertilised and untreated Norway spruce (Picea abies [L.] Karst.) earlywood were studied. Sample material was obtained from Flakaliden, Sweden. The elemental composition was studied by determining the relative mass fractions of the elements P, S, Cl, K, Ca and Mn by X-ray fluorescence and by determining the mass absorption coefficients for X-rays. X-ray diffraction was used to determine the MFA distribution and the average shape of the cell cross-section. The latter was also determined by light microscopy. In transition from juvenile wood to mature wood, a decrease of the mode of the MFA distribution from 13°–24° to 3°–6° was connected to a change in the shape of the cell cross-section from circular to rectangular. The irrigation-fertilisation treatment caused no change in the MFA distribution or in the shape of the cell cross-section, whereas the mass absorption coefficient was higher and the density was smaller in irrigated-fertilised wood. Larger proportion of the elements S, Cl and K, but smaller proportion of the element Mn, were observed due to the treatment. The results indicate that the shape of the cell cross-section or the MFA distribution are not directly linked to the growth rate of tracheids or to the nutrient-element content in the xylem and only show notable changes as a function of the cambial age.