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Neutron-diffraction studies on liquid deuterated acetonitrile at 20~C were carried out at neutron wavelengths of 0"5 A and 0.7 A.. For the latter wavelength samples of two different diameters were measured.The data were corrected for background, absorption, multiple scattering and inelastic effects (Placzek correction) and then were normalized to absolute differential cross section by comparison with vanadium standard. The different steps in the correction procedure were experimentally verified.The absolute coherent distinct differential cross section was separated into intermolecular and intramolecular contributions. The variation of molecular structure in passing from the gas phase to the liquid was obtained from this intramolecular contribution. INTRODUCTIONIn molecular liquids as opposed to simple atomic liquids non-central forces are generally important [1] and this results in more complicated expressions for molecular correlation functions which are needed in statistical calculations of thermodynamic properties of liquids. The pair-correlation function which plays a central role within the pair theory of liquids depends for molecules not only on the separation between molecules but also on their relative orientations [2][3][4]. This molecular pair-correlation function may in principle be determined from diffraction experiments [5] but a large number of different experimental data from neutron diffraction using different isotopes and X-ray diffraction are required.The present investigation was carried out in order to provide as much diffraction data as possible for one molecular liquid. Acetonitrile was chosen since X-ray data are already available [6] and at least two different neutrondiffraction studies on the isotopes CD3C14N and CD3CISN are feasible. In the present paper data for CDaCI~N are presented, whereas results for CD3CISN will be published in a second part of this series.Acetonitrile seems to be quite an appropriate example to study. Its large electric dipole moment of 3-92 D t and its comparatively high boiling point indicate strong non-central intermolecular forces and preliminary interpretations of the X-ray data [6] seem to favour strong orientational correlations. On the other hand, the acetonitrile molecule possesses structural symmetry which facilitates evaluation of the molecular pair-correlation function [2] and also its gasphase structure is well established [7][8][9].

Magnetization measurements of well-characterized monodisperse Pt clusters consisting of 13 2 atoms in a zeolite confirm the predicted extraordinary magnetic polarization with up to 8 unpaired electrons on a cluster, corresponding to a magnetic moment of 0:65 5 B per atom. The effect is partly quenched by hydrogen chemisorption. The study provides insight into the electronic structure of the cluster and is fundamental for an understanding of how magnetism develops in small clusters. DOI: 10.1103/PhysRevLett.97.253401 PACS numbers: 36.40.Cg, 36.40.Mr, 61.10.Ht, 61.46.Bc While enhanced magnetism in clusters of elements that are ferromagnetic as bulk solids is well known and has been demonstrated in Stern-Gerlach deflection experiments [1], theoretical studies predicted high-spin ground states for clusters of up to 13 atoms also of Pd and Pt, i.e., of elements which do not show magnetic ordering in the bulk [2 -4]. Nanoparticles comprising several hundred atoms of Au, Pd, and Pt embedded in a polymer revealed magnetic moments corresponding to several unpaired electron spins per entire particle [5,6], but no experimental studies exist for smaller clusters. The electronic structure of small metal clusters is a strong function of size [7]. This is because the average energy spacing of electronic states at the Fermi level, the Kubo gap N, scales as =N, where N is the number of atoms in the cluster. is the width of the valence band which is only a weak function of size and assumes typically values on the order of a few eV. For a given value of N the cluster undergoes a thermal transition from an insulator to a semiconductor at the temperature where the valence electrons can overcome the Kubo gap, providing the band is incompletely filled with electrons. Alternatively, for a given temperature this transition can occur as a function of N. Furthermore, when N is sufficiently small, electrons will occupy the levels with parallel spins, following Hund's rule [3], which leads to a high spin state that is accompanied with a high magnetic moment. But when N exceeds the spin pairing energy the system switches to a low spin state. On this basis, a small cluster could be expected to be in a low spin state, because of its relatively large value of N. However, the picture is complicated further by symmetry. A high symmetry may lead to a highly degenerate HOMO (highest occupied molecular orbital), favoring high spin states. When the symmetry is broken, degeneracy is lifted and the ground state may be one of lower spin, depending on the extent to which the symmetry is broken. Additional complications can arise in applied magnetic fields when the Zeeman or spin-orbit energies compete with level splittings at the Fermi level, leading to magnetic field and also temperature dependent spin states. Furthermore, when the surface is capped, each chemisorbed species engages one of the potential conduction electrons of the cluster and pins them in a localized chemical bond.It is vis-à-vis this complex background that the present results with...

Gold complexes were prepared and investigated as catalysts for the oxidative esterification of aldehydes. Stabilisation by pyridine ligands gave good conversions and the in situ extended X-ray absorption fine structure (EXAFS) study of the reactions indicated that the reaction mixtures contained only mononuclear gold species. Thus, this is the first proof for a homogeneous gold-catalysed oxidation reaction; the presence of nanoparticles could be excluded experimentally.

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Keywords: Nickel / Ligand exchange / EXAFS spectroscopy / XANES / X-ray diffractionThe local structure of nickel and bromine in three squareplanar nickel(II) complexes [(2,2Ј-bipyridyl) (2), [(bromo)(mesityl)bis(triphenylphosphane)nickel(II) (3)] and their ligand exchange behavior towards several solvents has been determined by means of Xray Absorption Near Edge Spectroscopy (XANES), Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS), Xray Diffraction (XRD), multinuclear NMR and UV/Vis spectroscopy. Several square-planar and octahedral nickel(II) complexes were prepared and spectroscopically character-

Although the effect on which X-ray absorption spectroscopy (EXAFS spectroscopy) is based has been known for about 70 years, up until about 20 years ago it had not found any application. The reasons for this were on the one hand the shortage of efficient X-ray sources, and on the other hand the lack of a suitable theoretical description. With the advent of synchrotron radiation the situation has changed completely. Today, EXAFS spectroscopy is applied to a variety of very different problems and used more and more frequently to answer chemical questions. The extraordinary behavior of EXAFS spectroscopy that allows the determination of local structures in almost any mixture, independent of the sample's physical state as well as element-specifically with high sensitivity, carries with it the danger that the amount of the obtainable structural information will be overestimated. A realistic estimate of the capability of EXAFS spectroscopy has in the meantime been achieved for crystalline materials, but not for amorphous materials. In this case the viewpoints range from complete rejection to overinterpretation of the EXAFS spectrum. In this article an attempt is made to show how EXAFS spectroscopy can be applied in the investigation of noncrystalline materials. First, the measurement techniques, data analysis procedures, and the simulation of EXAFS spectra are explained. Starting with the simplest systems. the amorphous metals and nonmetals, and using representative but not necessarily current examples, an explanation is given of the structural information provided by EXAFS spectroscopy, of how the local environment about the two components can be determined in binary systems, and of how even the crystallization process and structural changes can be investigated in chemical reactions. Thereafter, EXAFS studies on solutions, molecular liquids, and melts are presented. The final section is concerned with typical applications of EXAFS spectroscopy, the investigation of homogeneous and heterogeneous catalysts as well as the elucidation of active sites in enzymes. Theoretical FoundationsA monochromatic X-ray beam of energy E is attenuated by the passage through a material of thickness d according to Equation (a).['] ZJE) and Z(E) are the intensity of the incident and transmitted beams, respectively. p ( E ) is the linear absorption coefficient. Its dependence on E is depicted in Figure 1 a. p ( E ) decreases with increasing E of the incident X-rays until a threshold is reached at which the energy is sufficient to remove an electron from an inner shell. At this threshold, p ( E ) increases abruptly. Beyond this point, p ( E ) resumes its steady decrease. More precise measurements of the absorption coefficient indicate that the curve shown in Figure l a atoms. When other atoms are in the vicinity of the absorbing atom, the absorption coefficient shows small oscillations up to about 1000 eV above the absorption edge. This is shown schematically in Figure 1 b. Fig. 1. Graph of the absorption coefficient p ( E ) for an isolated ...

A program package for XAFS data analysis, especially of liquid and amorphous samples, has been developed. For the first time a consequent error propagation is presented for all functions to be calculated in the course of the data analysis. The structural investigation of the Grignard compound CH3MgBr in diethyl ether is taken as an example for the various steps of the data analysis.