The wavelength dependence of the neutron absorption cross section of hydrogen in [ReH3(C2H4) 2-{P(i-Pr)2Ph}2] was investigated by recording the decrease in Bragg peak intensity from a standard NaC1 crystal caused by attenuation of the incident beam by the Re sample. Data were collected on the Single Crystal Diffractometer (SCD) at the Intense Pulsed Neutron Source (IPNS, Argonne National Laboratory) which enabled concurrent measurements at a selection of wavelengths using the time-of-flight technique.To calculate the optimal absorption correction in a single-crystal neutron diffraction experiment, the linear absorption coefficient p of a sample should be determined directly by transmission measurements. In practice, however, it is common to calculate # from tabulated values of atomic absorption cross sections and knowledge of the size and contents of the unit cell.In this context hydrogen poses a problem, since its contribution to the attenuation of intensity is appreciable and not well characterized, although its cross section for true absorption is relatively small. This attenuation effect depends on the chemical environment of the bound hydrogen atom. It is hoped that the new results will be an improvement on the estimates currently used, and thereby be useful for future accurate structure determinations in areas such as electron density deformation experiments, where accurate corrections are of paramount importance (Coppens & Hall, 1983).The linear absorption coefficient is usually calculated from the expressionwhere #(2) is the linear absorption coefficient, V is the volume of the unit cell, n~ is the number of the ith type of atom in the unit cell and a,(2) is the effective absorption cross section (the sum of the true absorption and the incoherent scattering cross sections). The *And Chemistry and Materials Science and Technology Divisions, Argonne National Laboratory, Argonne, Illinois 60439, USA. summation runs over all elements present in the sample. The effective absorption cross section of hydrogen can be removed from this summation to givei~H where nn is the number of hydrogen atoms in the unit cell and a n is the effective absorption cross section of hydrogen. The summation now runs over all non-H elements, whose absorption cross sections are tabulated for a fixed wavelength and taken to be proportional to wavelength (Bacon, 1975) if the incoherent scattering cross sections are small, as is the case here.Hence (2) can be rewritten asi~H where ai (20) is the tabulated absorption cross section for the ith element at a wavelength 2o. Equation (3) is of the form where a = V/n n,i¢H These a and b are constants for a given compound and can be evaluated from crystallographic information and tabulated absorption cross sections.In this experiment p(2) is calculated from the measured attenuation through a known thickness of material using the Beer-Lambert law:where I is the path length in the sample. This principle has been used previously (Koetzle & McMullan, 1980) for crystals of organic molecules ...

Crystallographic observations of eight-membered ring conformations, retrieved from the Cambridge Structural Database, have been mapped and classified using symmetry-adapted deformation coordinates, principal component analysis and cluster analysis. Seven subsets of eight-membered rings, containing 11-32 conformational observations, have been analysed: cyclooctane (dataset 8C1), cyclooctene (8C2), cycloocta-l,3-diene (8C3), mono-exo-unsaturated carbocycles (8C4), monohetero (8A1), 1,5-dihetero (8A2) and 1,3,5,7-tetrahetero rings (8A3). The energetically preferred (by ~7 kJ mo1-1) boat-chair form is adopted by 26 of the 32 examples of 8C 1, although varying degrees of twist are induced by fusion to rings of sizes three, four and five. Crystallographic results for other subsets also populate the lower-energy areas of the appropriate potential energy hypersurface, but the analyses are complicated by the effects of ring fusion and by the small numbers of relevant crystal structures in some cases.

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Occurrence, properties and utilization of natural zeolites. Edited by D. Kall6 and H. S. Sherry. Pp. xiii +857. Budapest: