A. Vos

A new form of rubidium–7,7,8,8-tetra­cyanoquinodi­methane

RbH~C ~2N4, tetragonal, a = b = 17.645 (11), c = 3.864 (2)/~, Z -- 4, U= 1203.0 ,~3; F(000) = 564, g(Mo) = 43.3 cm-1; systematic absences: hk0 with h + k odd, hhO with h odd; space group P4/n with twinning according to (100) assumed. The final R is 0.065 for 275 observed reflexions.

Crystal Field Effects, Interchain Charge Transfer and the Phase Transition of DMTM(TCNQ)2

Abstract Preliminary structural data for the low temperature phase of the pseudo one-dimensional conductor DMTM(TCNQ)2 strongly indicate, that the peculiar converse changes in electrical behaviour of this compound at Tc = 272K should be attributed to crystal field effects. The loss of monoclinic symmetry at T c most probably accompanied by an ordering of the DMTM cations, affords an inequivalent crystal potential at successive sheets of TCNQ ions. The potential difference reduces the energy gap for electron transfer in a lateral direction between the successive sheets.

ELECTRON-DENSITY DISTRIBUTIONS ON C-C SINGLE, DOUBLE AND TRIPLE BONDS - PITFALLS AND PROSPECTS OF THE ANALYSIS OF ACCURATE X-RAY-DIFFRACTION DATA

Results obtained so far at Groningen for electron densities of compounds having single, double and triple C-C bonds are compared with each other and with densities from ab initio theoretical calculations. Suggestions for the improvement of further electron density studies and for the determination of physical properties from diffraction data are made.

Accurate structure determination of trans-2,5-dimethyl-3-hexene-2,5-diol hemihydrate, C8H16O2.0.5H2O, at 86 K. II. Discussion of the electron density distribution

Experimental deformation densities for the title compound are compared with a smeared theoretical deformation density based on ab initio quantumchemical calculations for a model compound (9,5,1/5,2 GTO basis set). Around the C-C bonds in the ethylene plane very good agreement with theory is obtained for the experimental D(HO,r) map. This map is based on high-order parameters of a conventional X-ray refinement, but witli scale and overall thermal motion adjusted to full-angle data. Around the O atoms where D(HO,r) is not reliable, the theoretical map is compared with a multipole map not including monopole deformations and based on a multipole refinement with scattering factors. The multipole map clearly shows the influence of the hydrogen bonds on the deformation density.

The influence of profile analysis on structural parameters and deformation density distributions

Nett X-ray diffraction intensities have been determined from measured step-scan profiles of deuterated 2,5- dimethyl-3-hexyne-2,5-diol by four different profile analysis procedures. Least-squares refinements based on the different sets of F values showed that variations in profile analysis method may change the coordinates by 1.5 times the [sigma] of 0.8 x 10-4 A and the thermal parameters by twice the [sigma] of 1.4 x 10-5 A2. The change in scale factor is 1.2% or 4[sigma]. A correct choice of refinement procedure, in which full-angle data are used for the determination of the scale, turned out to be more important for the calculation of reliable deformation densities than the choice of profile-analysis method.