An open‐flow helium cryostat for single‐crystal X‐ray diffraction experiments capable of reaching 14 K has been developed using off‐the‐shelf components. Solid/liquid air build‐up is prevented using the transfer‐line helium back‐flow and a heated nozzle. The system has run for over 30 h with no frost build‐up. The effectiveness of the system has been demonstrated using test data for oxalic acid, terbium vanadate and dysprosium vanadate.

Extensive synchrotron (28 K) and conventional sealed-tube (9 K) X-ray diffraction data have been collected on tetrakis(dimethylphosphinodithioato-S,S')thorium(IV), [Th(S(2)PMe(2))(4)]. The use of very low temperatures, well below those obtained with liquid-nitrogen cooling, is crucial for the accuracy of the data. This is due to minimization of temperature-dependent systematic errors such as TDS and anharmonicity, and extension and intensification of the data in reciprocal space. Comparison of structural parameters derived separately from the sealed-tube data and the synchrotron data shows good agreement. The synchrotron data are markedly superior when comparing refinement residuals, standard uncertainties (s.u.'s) of the data and s.u.'s of the derived parameters. However, the study suggests that there are still small uncorrected systematic errors in the data. The very large extent [(sinstraight theta/lambda)(max) = 1.77 Å(-1)] of the synchrotron data and the very low temperature at which they were collected makes it possible to separate anharmonic effects from electron-deformation effects even with only an X-ray data set at a single temperature. The electron density shows a large polarization of the outer Th core of d-type symmetry. This deformation is successfully modelled with contracted multipolar functions, which are only slightly correlated with anharmonic expansions in reciprocal space when using the full extent of the data. In the data collection more than a factor of 100 in speed is gained by use of image-plate area detectors at the synchrotron source compared with conventional sequential measurements. Thus accurate, very low temperature synchrotron-radiation diffraction data can now be measured within days, which makes electron-density studies of compounds beyond the first transition series more frequently within reach.

The electron density and related properties of biguanidinium dinitramide (BIGH)(DN) and biguanidinium bis-dinitramide (BIGH(2))(DN)(2) crystals (space groups P1 and C2/c) have been determined from low-temperature (90(1) K) X-ray diffraction experiments. The Hansen-Coppens multipole model as implemented in the XD program gave R = 0.0247 and 0.0201 (all reflections) which allowed the calculation of the electron density and Laplacian distributions. The bonding (3,-1) critical points were also found. The analysis of the results shows that Bader's topological theory provides a more useful description of the chemical bonding in the studied crystals as compared to the classical analysis of deformation densities. The hydrogen bonding in the crystals was analyzed. The atomic charges were integrated over the atomic basins.

Three biguanidinium salts of the energetic dinitramide anion have been prepared and structurally characterized from room-temperature X-ray diffraction data. Biguanidinium mono-dinitramide, (BIGH)(DN), triclinic, Pi, a = 4.3686(4), b = 9.404(2), c = 10.742(1)A,, c~= 83.54(1), ,;3 = 80.386(9), 7 = 79.93(1) ° , V = 426.8 (1)A, 3, Z = 2, Dx = 1.62 gcm -3. Biguanidinium bis-dinitramide, (BIGH2)(DN)2, monoclinic, C2/c, a= 11.892(2), b = 8.131(1), c = 13.038(2)A,, [2 = 115.79(1) ° , V = 1135.1(3)A, 3, Z = 4, Dx = 1.84 gcm -3. Biguanidinium bis-dinitramide monohydrate, (BIGH2)(DN)2.H20, orthorhombic, P212121, a= 6.4201 (6), b = 13.408(1), c = 14.584 (2) A,, V= 1255.4(4) A, 3, Z = 4, Dx = 1.76 gcm -3. All three structures are characterized by extensive hydrogen bonding. Both the mono-and diprotontated cations consist of two planar halves twisted with respect to each other. The dinitramide anion has a surprisingly variable and asymmetric structure. The two halves of the anion are twisted with respect to each other; however, the twist varies from 5.1 to 28.9 ° . In addition, the two ends of the anion have significantly different geometries, e.g. the 'equivalent' N--N bond lengths differ by up to 0.045/~,.

The electron density, ρ(r), of crystalline ammonium dinitramide (ADN) was determined from low-temperature X-ray diffraction data and electronic structure calculations. Single molecule wave functions were also computed for comparison. Bader's atoms in molecules (AIM) method was used to partition ρ(r). The same number and kinds of critical points in ρ(r) for the dinitramide ion are found in ADN as were found in previous studies of other salts with different amounts of nitro group twist. An atomic interaction line (AIL) is always observed between the two “inner” oxygens. Topological characterization of the negative Laplacian of the charge distribution (−∇2ρ(r)) was also performed to locate (3,−3) critical points in the valence-shell charge concentration (VSCC) region. Such points may be associated with the presence and location of lone pairs of electrons as predicted by the Lewis and VSEPR models. As with ρ(r), the same number of (3,−3) critical points are found in the Laplacian, −∇2ρ(r), with experimental, single-molecule B3LYP, and crystal B3LYP models, however, comparison of experimental and theoretical results show some differences in the location of these points. These differences are shown to arise mostly from limited flexibility in the multipole model used to fit ρ(r) to the experimental data. Nonetheless, both B3LYP modeling and experiment agree that there is a single (3,−3) critical point in the VSCC associated with a lone pair of electrons on the central nitrogen. The hybridization of the central nitrogen of the dinitramide ion is, therefore, sp2-like, as observed in the two biguanidinium salts, [((NH2)2C)2N][N3O4] and [((NH2)2C)2NH][N3O4]2. Despite this robust topology, the dipole moment obtained from both experiment and crystal modeling is larger than that computed for a single dinitramide ion. Significant differences in the direction of the dipole from theory and experiment are found, as are differences in the atomic charges. These are also attributed to the limited flexibility of the multipole model. Electron densities obtained from crystal wave functions demonstrate that strong hydrogen bonding polarizes the dinitramide ion, increasing the negative charge on the most strongly hydrogen bonded oxygen atom. Decomposition of the theoretical molecular dipole moment into atomic charge and atomic dipole contributions reveals that the atomic dipoles are nearly equal in both the crystal and single molecular ion. Changes in the atomic charge contribution to the molecular dipole moment principally account for the induced dipole.