J. Neuefeind

Isotopic quantum effects in water structure measured with high energy photon diffraction

High energy electromagnetic radiation scattering techniques have been used to measure the structural differences between light and heavy water: we have studied both intra- and intermolecular effects. These methods and our data analysis are described in detail. We have observed a maximum isotopic effect of 1.6% relative to the magnitude of the x-ray structure factor. Our uncertainties are an order of magnitude smaller than those of previous -ray measurements (Root J H, Egelstaff P A and Hime A 1986 Chem. Phys. 109 5164) and this has permitted us to test accurately the available quantum simulation results on water. The SPC and TIP4P potentials reproduce the measured results in r -space moderately well for intermolecular effects at distances greater than 2.5 ?. These results show that H2 O is a slightly more disordered liquid than D2 O at the same temperature.

Hydrogen bonding in liquid methanol at ambient conditions and at high pressure

Neutron and hard X-ray diffraction measurements on liquid methanol at room temperature are presented at pressures ranging from 1 bar to 9 kbar. Under ambient conditions a complete partial structure factor analysis has been performed. The partial structures are not all in agreement with simulation results. Notably, the data show no sign of a sharp feature in the HO(O/C) partial, implying that the hydrogen between the hydrogen bonded oxygen is less localized than predicted by the standard simulation potentials. The data have been analysed using a continuous random network approach common in the analysis of the structure of tetrahedral network glasses. The pressure-induced change in the scattering intensity is restricted to the momentum transfer range (0 < Q < 2.5 Å−1), i.e., there is no discernible change in sharp real-space structures. Up to 9 kbar the change in scattering intensity scales to a good approximation with the density.

The structure of water around the compressibility minimum

Here we present diffraction data that yield the oxygen-oxygen pair distribution function, g(OO)(r) over the range 254.2-365.9 K. The running O-O coordination number, which represents the integral of the pair distribution function as a function of radial distance, is found to exhibit an isosbestic point at 3.30(5) Å. The probability of finding an oxygen atom surrounding another oxygen at this distance is therefore shown to be independent of temperature and corresponds to an O-O coordination number of 4.3(2). Moreover, the experimental data also show a continuous transition associated with the second peak position in g(OO)(r) concomitant with the compressibility minimum at 319 K.

Isotope quantum effects in water around the freezing point

We have measured the difference in electronic structure factors between liquid H(2)O and D(2)O at temperatures of 268 and 273 K with high energy x-ray diffraction. These are compared to our previously published data measured from 279 to 318 K. We find that the total structural isotope effect increases by a factor of 3.5 over the entire range, as the temperature is decreased. Structural isochoric temperature differential and isothermal density differential functions have been used to compare these data to a thermodynamic model based upon a simple offset in the state function. The model works well in describing the magnitude of the structural differences above approximately 310 K, but fails at lower temperatures. The experimental results are discussed in light of several quantum molecular dynamics simulations and are in good qualitative agreement with recent temperature dependent, rotationally quantized rigid molecule simulations.

The structure of fluid trifluoromethane and methylfluoride

We present hard x-ray and neutron diffraction measurements on the polar fluorocarbons HCF3 and H3CF under supercritical conditions and for a range of molecular densities spanning about a factor of ten. The Levesque-Weiss-Reatto inversion scheme has been used to deduce the site-site potentials underlying the measured partial pair distribution functions. The orientational correlations between adjacent fluorocarbon molecules - which are characterized by quite large dipole moments but no tendency to form hydrogen bonds - are small compared to a highly polar system like fluid hydrogen chloride. In fact, the orientational correlations in HCF3 and H3CF are found to be nearly as small as those of fluid CF4, a fluorocarbon with no dipole moment.

The structure of liquid carbon dioxide and carbon disulfide

We present neutron and x-ray scattering data (a 2N+X experiment) of liquid CO(2) and CS(2) at a density of about 10 molecules/nm(3). Because the scattering length contrast of the carbon isotope is very small and, in fact, smaller than anticipated from standard scattering length tables, a direct partial structure factor determination via matrix inversion gives unconvincing results. Instead we search for the best representation of the three independent scattering data sets by a simulation of rigid molecules interacting via a 12-6-1 potential, furthermore restricting the pressure p, the density rho, and the temperature T to the experimental values. We show that a 12-6-1 potential is completely adequate to describe the structure of CO(2); for CS(2) we find that the best 12-6-1 potential still slightly overestimates the height of the sulfur-sulfur pair-distribution function g(SS). Orientational correlations reflect the similarities much more than the differences of the two molecular systems. The distinct differences in the atom-atom pair distribution functions of CO(2) and CS(2) do not mean that their structures are radically different and the comparison with the crystalline structures is somewhat deceptive. A linear transformation, wherein all the parameters describing the interaction and the geometry of CS(2) are changed to those of CO(2), allows us to point out the physical parameters which may be responsible for the differences or similarities in thermodynamic behavior (pressure) and structures (orientations) between the two liquids.

Isotopic quantum effects in the structure of liquid methanol: I. Experiments with high-energy photon diffraction

High-energy electromagnetic radiation scattering techniques have been used to measure the structural differences between four isotopic samples of methanol (CH3OH, CD3OD, CH3OD and CD3OH). The first series of experiments employed room temperature and ambient pressure. The carbon-oxygen intramolecular bond length was measured and found to depend more strongly on the isotopic substitution at the hydroxyl site than at the methyl sites. The oscillations in the isotopic difference of the x-ray structure factor, ΔSX(Q), are shown at room temperature to be about 2% as large as the oscillations in the total structure factor. Our uncertainties are an order of magnitude smaller than those of previous gamma ray measurements (Benmore C J and Egelstaff P A 1996 J. Phys.: Condens. Matter 8 9429-32). A second series of experiments was carried out at -80 °C at its vapour pressure in order to study the significant temperature dependence of these effects. The ΔSX(Q) difference at -80 °C is shown to be up to three times larger than the room temperature difference. These studies showed that isotopic structural differences in methanol may be represented as temperature shifts that vary as a function of thermodynamic state and substitution site.

Quantum effects in the structure of liquid benzene at room temperature.

Structural differences between liquid light (protonated) benzene and heavy (deuterated) benzene at room temperature have been measured using high energy electromagnetic radiation scattering techniques. Intra- and intermolecular effects have been examined, and the main quantum contribution is shown to be intramolecular. This is in contrast to the quantum effects measured in liquid water at room temperature, which are primarily intermolecular.

New measurements of the coherent and incoherent neutron scattering lengths of 13C

The techniques of neutron interferometry and neutron diffraction were used to determine the coherent and incoherent neutron scattering lengths of 13C. From a neutron interferometry measurement of the optical path difference in liquid samples, 13CS2 versus natCS2, we obtain a bound coherent scattering length of bcoh,13C = 6.542 ± 0.003 fm, which differs appreciably from the standard tabulated value of 6.19 ± 0.09 fm. The resulting contrast of only 0.106(3) fm with respect to bcoh,natC = 6.6484 ± 0.0013 fm has consequences for neutron diffraction experiments involving 13C isotopic substitution. Combining our result for bcoh,13C with precise neutron diffraction measurements of the self-scattering intensities of liquid samples, 13CS2 versus natCS2, and 13CO2 versus 12CO2, we deduce a bound incoherent scattering length of bincoh,13C = −0.42 ± 0.24 fm that is consistent with the standard tabulated value of −0.52 ± 0.09 fm. The results presented here have required accurate measurements of small effects, for which particular attention has been given to the data analysis.

Isotopic effects in the structure of liquid methanol: II. Experimental data in Fourier space

The high-precision structural measurements of several methanol isotopes described in paper I are Fourier transformed to obtain their corresponding pair correlation functions. At room temperature we have observed a structural isotopic difference depending on the methanol isotopes used ranging between 2 and 5% at intramolecular distances and between 5 and 8% at intermolecular distances relative to the magnitude of (g(r)-1) for CH3OH. For methanol at -80 °C, a maximum effect of 20% has been observed. All these effects may be explained in terms of changes in ground state librational motions and perturbations to the hydrogen bonding structure. The effects are compared with structural changes caused by temperature shifts and are shown to agree with the reciprocal space studies in paper I.