Search

No Abstract Available.

The crystal structure and electron density in sodium vanadium oxide bronze, β-Na x V2O5 [x = 0.282 (3)], have been studied by accurate Mo Kα X-ray diffraction measurements at 9.6 (3) K. No noticeable difference in the crystal structures at room temperature and 9.6 K has been observed. No superstructure reflections, previously found by Kanai, Kagoshima & Nagasawa [(1982), J. Phys. Soc. Jpn, 51, 697–698], have been detected at low temperature. Analysis of the deformation electron density has revealed the presence of the quasi-two-dimensional sheets of the —V—O—V—O— bonds in the structure. The electron density in the different chemical bonds within each of the three crystallographically independent VO6 polyhedra noticeably varies, although there is no clear evidence that the three crystallographically independent V atoms have different valence states.

Spin-orbit interactions play an essential part in elucidating the magnetic structures which are measured by the polarized neutron diffraction technique. This work extends our previous Hartree–Fock theory, with a one-electron spin-orbit term limitation, to incorporate both one- and two-electron terms exactly. This new theory, which is based on the current density, has been applied to calculate the low-temperature magnetic structure factors of the Cs3CoCl5 crystal. The crystal is assumed to be assembled from noninteracting molecular fragments. The calculated structure factors were compared directly with those observed from experiment. The agreement between these theoretical results and the experimental data shows great improvement compared with those from the usual unrestricted Hartree–Fock theory and with those from the previous generalized Hartree–Fock theory including one-electron terms. To examine the electron correlation effects, we use ab initio wave-function-based correlation methods, unrestricted se...

The crystal structures of the α-alums rubidium chromium bis(­sulfate) dodecahydrate, RbCr(SO4)2·12H2O, and caesium chromium bis[tetraoxoselenate(VI)] dodecahydrate, CsCr(SeO4)2·12H2O, have been determined by X-ray diffraction at 293 and 12 K. The metal atoms lie on {\overline 3} sites and the anions lie on threefold rotation axes. The accurate and extensive data sets lead to much more precise determinations than are available from earlier work, particularly at 12 K. The changes in the atomic displacement parameters between 293 and 12 K correspond to the respective predominances of intermolecular and intramolecular vibrational effects.

The crystal structures of the Tutton salts (NH4)2-[Cu(H2O)6](SO4)2, diammonium hexaaquacopper disulfate, formed with normal water and isotopically substituted H2 18O, have been determined by X-ray diffraction at 9.5 K and are very similar, with Cu—O(7) the longest of the Cu—O bonds of the Jahn–Teller distorted octahedral [Cu(H2O)6]2+ complex. It is known that structural differences accompany deuteration of (NH4)2[Cu(H2O)6](SO4)2, the most dramatic of which is a switch to Cu—O(8) as the longest such bond. The present result suggests that the structural differences are associated with hydrogen-bonding effects rather than with increased mass of the water ligands affecting the Jahn–Teller coupling. The Jahn–Teller distortions and hydrogen-bonding contacts in the compounds are compared with those reported for other Tutton salts at ambient and high pressure.

Comparison has been made between theoretically obtained structure factors and those obtained from very low temperature X-ray and polarised neutron diffraction experiments on single crystals of Ni(NH3)4(NO2)2. This work has emphasised an approach in which interactions between neighbour molecules have been ignored and the unit cell charge density has been determined from single molecule calculations by summing the densities of the molecules in the unit cell. Agreement between experiment and theory has been assessed using the χ2 and goodness of fit (Rw) statistics. For this molecule the agreement with experiment is little altered by change of basis up to the level of a valence triple-zeta set with polarisation functions (VTZP). The importance of the crystalline environment was examined using the CRYSTAL95 package. At the level of theory available in this program the effect of intermolecular interactions on structure factors has been shown to be negligible. Including electron correlation also made little change to the structure factors. The best of the single molecule calculated structure factors, using five parameters, fitted the X-ray experiment almost as well as 83 or 126 parameter multipole expansions. A similar result was obtained for the polarised neutron diffraction structure factors.

Powder neutron diffraction measurements have been made at 2 and 20 K for the complex cis-[ReCl4(NHCMeNHPh)2]. The changes in diffracted intensities on magnetic ordering at 9.7(1) K were fitted in the Shubnikov group Pc′c′n to an antiferromagnetically ordered magnetic structure in which the spins are aligned along c in this orthorhombic crystal. A moment of 2.58(6) µB resides on the rhenium site in each molecule, while each chlorine site has a spin population of 1.03(8) µB due to covalent spin delocalisation. The N–C–N region of the other two ligands each has a spin population of –1.86(10) µB, indicating a substantial spin polarisation resulting from electron correlation effects. The antiferromagnetic structure is consistent with a reinterpretation of earlier single crystal magnetisation data. The very large amounts of spin transfer observed within the molecule are not unexpected in this heavy metal complex, and qualitatively reasonable given a synergistic interaction between the two ligand types. The previous observation of a metamagnetic transition in this crystal is explained as being driven by a change in canting angles within the molecule, from alignment closely along c to an arrangement in which opposite halves of the molecule are differently canted, with small but significant +b and –b components. The direction of magnetisation within the molecule is thus not all parallel.

A polarised neutron diffraction (PND) experiment on [Ru(acac)3] at 4.18 K and 5 T along the b and c axes yielded 28 and 17 flipping ratios respectively. Single-crystal X-ray structure determinations at 293, 92 and 10.5 K, together with powder neutron diffraction experiments, provided auxiliary data allowing magnetic structure factors to be deduced from the PND experiment. The structure is disordered at all temperatures. There are ordered stacks of molecules along b, which stacks are arranged in the ac plane in two sites of relative displacement 0.4 A along b. The molecular geometries at the sites are similar and approximately D3 in symmetry, but the magnetisation data indicate C2 symmetry. The PND data showed significant spin delocalisation by covalence onto the acetylacetonato rings, dominated by spin-polarisation effects, and of C2 symmetry. However the two data sets differed, with the field along b indicating a 2A and that along c a 2B dominated state. This cannot be explained in the ligand-field model which fits the magnetic susceptibility data, indicating that vibronic effects must be explicitly considered. Good-quality ab-initio local density calculations reproduced the molecular geometry but grossly underestimated the spin correlation between metal and rings.

Single-crystal magnetisation and polarised neutron diffraction (PND) measurements have been made for the complex cis-[ReCl 4 (NHCMeNHPh) 2 ]. At higher temperatures the magnetic behaviour is typical of such a rhenium-(IV) complex, with three unpaired spins (µ = 3.6 µ B ), an almost isotropic g-tensor of 1.86(1) and a large zero-field splitting, 2D, of -20(2) cm -1 . At very low temperatures the magnetisation behaviour as a function of temperature and field strength and direction indicates two magnetically ordered structural phases when H||b, but only one for a and c, with T N = 9.7(1) K. A tricritical point at T = 5.8(2) K and H = 4.40(5) T was established on the transition line in H vs. T space between the two phases when H||b. This behaviour is metamagnetic. The two-dimensional form of the stronger magnetic exchange implied, with weaker interlayer interaction, can be correlated with the observed structural feature of ab planes of ReCl 4 units separated by layers of phenyl rings. The PND experiment showed that about half of the spin of the system is delocalised away from the Re by covalent bonding to the Cl atoms and the amidine ligand, accompanied by considerable effects resembling spin polarisation. The magnetic space groups are P2 1 /c at low magnetic fields and Pc2 1 n at high fields along b, compared with the nuclear structural space group of Pccn.

Polarised neutron diffraction from two orientations of tetragonal single crystals of [AsPh 4 ][TcNCl 4 ] at 1.5 K and a magnetic field of 4.6 T gave a combined data set of 116 magnetic structure factors. These were fitted by a 26-parameter model which includes proton nuclear-spin orientation to give a spin-density model for the formally 4d 1 [TcNCl 4 ] - ion. This model revealed substantial anistropic π bonding between Tc and Cl resulting in 29(2)% of the spin being delocalised onto the chlorine in-plane 3p π orbitals, -15(1)% onto the nitrogen as a result of spin-polarising the short Tc–N bond, and substantial spin polarisation on the technetium site of 4d and more diffuse density. Three unconstrained Hartree–Fock and density functional calculations with good basis sets failed to reproduce these observations adequately, but an unconstrained density functional calculation with gradient corrections and a relativistic treatment of the core gave encouraging agreement with experiment.

A quantitative description of transition-metal bonding has been obtained through combined analysis of 9(1) K X-ray and 13(1) K time-of-flight neutron diffraction data. It is shown that a simple valence-orbital model is too crude an approximation adequately to describe the electron-density distribution of Ni(ND 3 ) 4 (NO 2 ) 2 . To exhaust more fully the information present in the very-low-temperature diffraction data, a more flexible electron-density model was used. Quantitative measures describing the bonding in the complex have been achieved through topological analysis of the derived static model density. To study the effects of co-ordination and intermolecular interactions, comparisons were made with good-quality wavefunctions calculated for free nitrite and ammonium ions. Both ligands appear co-ordinated through predominantly electrostatic interactions. Contrary to previous studies of Ni(ND 3 ) 4 (NO 2 ) 2 , the topological analysis revealed that the metal–ligand interactions, besides cylindrical σ contributions, also have non-cylindrical π contributions to the covalent part of the bonding. Plots of the Laplacian of the electron density were used to locate regions of charge concentration and charge depletion in the valence regions of the atoms in the molecule. For all atoms, maxima in the valence-shell charge concentration are found in accord with the simple Lewis electron-pair concept of bonded and non-bonded charge concentrations. The study demonstrates that X-ray diffraction data measured carefully at very low temperatures have sufficient precision to allow for a reliable and detailed topological analysis of transition-metal electron-density distributions.

Structural parameters derived from 9 1) K X-ray diffraction data and 13 (1) K time-of-flight neutron diffraction data on perdeuterated tetraamminedinitronickel(II), Ni(ND3)4(NO2)2, are compared. It is shown that excellent agreement can be obtained for both positional and thermal parameters derived separately from the two experiments, provided that great care is taken in all steps of the process, including data collection, data reduction, and nuclear and electronic structure refinement. The mean difference in the thermal parameters, ΔUij |>, is as low as 0.00034 Å2 and ΔUij/σ)2>1/2 = 1.92, showing that, even without any form of scaling between the parameters, the same values can be obtained. This, compared with other such studies, indicates that time-of-flight neutron diffraction data can give structural information of a quality comparable to monochromatic neutron diffraction. The excellent correspondence between the thermal parameters derived separately from X-ray and neutron diffraction data gives confidence in the deconvolution of the thermal motion from the X-ray diffraction data, which is necessary for any study of a static electron density distribution.

Hexaaammineruthenium(III) tristhiocyanate has space group P2 1 /n at 20K with typical bond lengths and angles, with two of the six ammine groups rotationally disordered. Eight further limited data sets from 100 to 290K show a reversible, probably second-order, phase transition to a C2/c cell at 283 (2) K. The C2/c cell contents are derived from those of P2 1 /n by orientationally disordering one thiocyanate site together with small but well defined displacements of the other atoms.

The measurement of X-ray diffraction from crystals of (ND 4 ) 2 Cu(SO 4 ) 2 ·6D 2 O at 9 K gives a set of Bragg intensities as the basic observable. It is shown that ab initio quantum-mechanical calculations on individual isolated ions, near the Hartree—Fock limit, based on nuclear positions from a neutron diffraction experiment, when assembled into the crystal unit cell, reproduce the data set for the complex at a level close to the experimental error limits. This is accomplished using only four adjustable parameters related to the experimental details. Our procedure avoids the biasing effects of multipole models, of which a typical one used one hundred and eighty four parameters.

EXAFS and vibrational spectra of acetone solutions of zinc and lead(II) ethylxanthate have been recorded in order to correlate structural data with absorption bands in the mid-infrared region. Two ethylxanthate ions are monodentately coordinated to both zinc and lead(II) in acetone solution. Two acetone molecules are weakly coordinated to zinc to complete a tetrahedral configuration round zinc, while no acetone molecules were found in the inner sphere of the lead(II) complex. A band containing a large contribution from the C-S stretching, observed just above 1000 cm(-1) in solid zinc and lead(II) ethylxanthate, where the ethylxanthate ion is bridgingly and bidentately coordinated, respectively, shifts upwards by 20 and 17 cm(-1) in the corresponding zinc and lead(II) complexes, respectively, in acetone solution. An upward shift by about 8 cm(-1) is found in the nickel(II) and arsenic(III) complexes, where the coordination of the ethylxanthate ion remains unchanged but acetone molecules solvate the central ion to complete an octahedral configuration. No shift was found in iron(III) ethylxanthate, where the coordination remains unchanged and no acetone molecules solvate the iron(III) ion. (Less)

In a polarized neutron diffraction experiment on the magnetically highly anisotropic ammonium cobalt Tutton salt (ND4)2Co(SO4)2·6D2O, 70 and 22 flipping ratios were measured in the hk0 and h01 zones respectively. These, and the underlying magnetization distribution around Co, more closely resemble those predicted by a model with a cubic crystal field with CoO6 axes, than those predicted from a generally orientated ellipsoidal crystal field, even though the latter provides a good fit to ESR and magnetic susceptibility data. However, all the real space magnetization distributions are substantially isotropic. For an examination of covalence as the experimental objective, these results indicate that a suitably scaled spin-only model for the magnetization distribution on the metal may suffice, even in highly anisotropic situations where a large orbital moment is present, such as the present case.

Analysis of asymmetric line shapes of the synchrotron-generated x rays diffracted at 12 K from ca. 85% deuterated single-crystal copper ammonium Tutton salt, (ND 4 ) 2 Cu(OD 2 ) 6 (SO 4 ) 2 , shows a breakdown in translational order that we interpret as regions of two monoclinic phases segregated on a scale of 100-200 unit cells in fixed relative orientation. The separation and orientation can be understood as the best epitactic fit between crystals differing very slightly in Jahn-Teller distortion at the Cu site. Previous studies have shown two distinct much larger Jahn-Teller distortions at the Cu site to be close in free energy. At 180 K more perfect order is regained, and the process is reversible

Diammonium hexaaquacopper(II) disulfate-d 20 , [ND 4 ] 2 °u(D 2 O) 6 ](SO 4 ) 2 , M r =419.7, monoclinic P2 1 /a, a=9.393 (2), b=12.666 (2), c=6.061 (1) A, β=107.16 (1) o , V=689.0 (4) A 3 , Z=2, Mo Kα radiation, λ=0.71069 A, μ=2.01 mm -1 , F(000)=415.3 (414.0 without anomalous dispersion), T=9 (1) A, R(I)=0.013, R(F)=0.0095, χ 2 =0.89, 5134 reflections. Refinement of the X-ray data, using a radially augmented multipolar charge model, gave thermal and positional parameters agreeing with those derived from neutron diffraction data and the correct cell content of electrons

Flipping ratios for 923 reflections were measured in a polarized neutron diffraction experiment at 1·5 K with a magnetic field of 4·6 T applied along the [100] and [011] directions of a crystal of Cs2KFe(CN)6, containing the low-spin [2T2g] hexacyanoferrate(III) ion. Various multipolar models of the noncollinear magnetization density were fitted to this and to the earlier [010] data. The expected transfer of spin by π covalence and σ spin polarization to the cyanide groups is observed. The field applied along [011] breaks the magnetic monoclinic symmetry of the crystal to give two independent anions. The direction of the net magnetization on each of the four independent ions in the three orientations can be accounted for by a crystal field of uniaxial symmetry with polar angles θ = 95·4(16)°, φ = 92·5(16)° relative to ac*b. The magnetization is almost collinear with the applied field for the [100] and [010], but distinctly noncollinear for the [011] data. In addition, there are significant anisotropic non...

The results of a polarized neutron diffraction (pnd) experiment on [Co(NH 3 ) 5 (OH 2 )][Cr(CN) 6 ] carried out for two crystal orientations at 1.5 K with a magnetic field of 4.6 T are reported. Local density functional (ldf) calculations on models containing (a) the [Co(NH 3 ) 6 ] 3+ cation and one [Cr(CN) 6 ] 3- anion and (b) one such cation and two such anions a presented. These models contain the major features of the hydrogen-bonding interactions in the crystal. Surprisingly, the pnd experiment shows the presence of substantial amounts of spin in the cation, located on both the cobalt atom and the ammine protons

The crystal structure of the title compound, [Co(NH3)6][S2O3]Cl.H2O, consists of discrete ions, and water molecules of hydration. The [Co(NH3)6]3+ cation in the asymmetric unit is distributed between two different symmetry centres. The water molecule and the ions are linked in the crystal by weak hydrogen bonds and interionic contacts.

The crystal structure of trisodium hexanitro­cobaltate(III) has been determined by X-ray diffraction at 293 and 10 K. It contains the slightly distorted octahedral Co(NO2)63− anion.

(ND 4 ) 2 [Fe(D 2 O) 6 ](SO 4 ) 2 cristallise a 85K dans P2 1 /a avec a=9,170, b=12,419, c=6,297A, β=106,69°, Z=2; affinement jusqu'a R=0,018. L'analyse de la population des orbitales de valence fournit pour le fer la configuration 3d du champ cristallin proche. Quand les resultats pour les trois metaux sont compares, les charges deduites pour les unites M, D 2 O, S, SO 4 et ND 4 sont reduites d'environ 35% par rapport aux valeurs formelles. Les charges et populations des ions sulfate et complexe metallique different dans le cas du cuivre par rapport aux cas du fer et du chrome. Ces differences indiquent un transfert de charge important du sulfate vers le metal, via les liaisons hydrogene pour le fer et le chrome.

Diammonium hexaaquacopper(II) disulfate-d 20, [ND4]2[Cu(D2O)6](SO4)2, Mr = 419.7, monoclinic, P21/a, a = 9.399 (2), b = 12.673 (2), c = 6.071 (1) Å, β= 107.13 (1)°, V= 691.1 (4) Å3, Z = 2, Dx = 2.02 Mg m−3, Mo Kα radiation, λ = 0.71069 Å, μ = 2.008 mm−1, F(000) = 415.3, T= 85 (2) K, R(I) = 0.020,R(F) = 0.014 for 7287 reflections. The CuO6 octahedron has a large Jahn–Teller distortion; Cu—O(8) 2.301 (1), Cu—O(7) 2.010 (1), Cu—O(9) 1.960 (1) Å. It was necessary to use quartic anharmonic thermal parameters at the CuII site. These modelled potential softening associated with the Jahn–Teller distortions, important even at this temperature. At the ammonium, the sulfate and the hexaaqua-ion sites the charge densities, at this experimental accuracy, can be described by simple valence functions which reflect only local symmetry. Large lower-symmetry densities are not observed; any small such effects are comparable with uncertainties in the treatment of the thermal motion. The valence refinement gave Cu 3d populations of 3d^{1.58 (6)}_{xy}3d^{2.12 (3)}_{xz, yz}3d^{2.18(6)}_{z^{2}}3d^{0.82 (7)}_{x^{2} - y^{2}}. This shows the hole expected in 3d_{x^{2}_y^{2}}, modified, taking overlap into account, by a covalent or acceptance of 0.52 (1) and π back donation of 0.18 (10) e. The rhombic distortion causes 3d_{z^2}/3d_{x^{2} - y^{2}} mixing corresponding to a 3d_{z^2} coefficient of −0.21 (7) in a spin-hole wavefunction. Ionic charges from the model are Cu(OD2)^{17+}_{6}, ND^{6+}_{\xB0} and SO^{1.5-}_{4}. These are more ionic than for the isomorphous CrII salt, where σ and π charge flows are both to the metal, not opposed as here. Qualitatively these results agree with the conclusions from ESR, NMR and optical spectra.

Polarized neutron diffraction experiments on deuterated ammonium chromous Tutton salt at 1.6 K and 4.6 T, in two orientations of the crystal in the magnetic field, are reported. 216 ( b //H) and 212 ( a //H) data were refined using a model of the magnetization density involving only the spin distribution on the [Cr(OD 2 ) 6 ] 2+ ion, extinction and six canting parameters, three for each data set, to give a goodness-of-fit of 1.4. The canting parameters describe the projection of the total ion spin moment onto the a , b and c * directions. The spin density corresponds closely to a ‘spin hole’ of 1.03(4) spins in the 3d x 3 - y 2 orbital of this Jahn-Teller distorted Cr 2+ ion, with a small but significant contribution from covalence from the coordinated water molecules, and spin polarization of both those molecules and the Cr 11 ion. The two water molecules attached by the long Cr-O(8) bonds behave differently from the four shorter bonded ligands, reflecting their lesser covalence in, respectively, spin occupied and unoccupied Cr 11 3d orbitals. The canting parameters, which correspond to angles of 24° and 29° for the two datasets, and further data on their field and temperature dependencies, are fitted well by a tetragonal crystal field model in which only variation in the spin-orbit coupling constant is important. This quantity has a value 17(5)% greater than for the free Cr 2+ ion. Experiment and theory both confirm that orbital magnetization effects are important only in determining the canting of the magnetic moments and are otherwise small compared with the spin moment in this compound.

Calculations on a V(OH2)2+6 ion in the experimental geometry have been performed with conventional ab initio and with discrete-variational local-density functional approximation unrestricted Hartree–Fock methods. In each case a variety of basis sets was employed. The charge and spin distributions produced were compared with each other, and the latter with experimental results from a polarized neutron diffraction experiment on (ND4)2V(SO4)2 6D2O. If the DZP ab initio results are taken as the benchmark, the charge-difference and the spin-density maps of the LDF methods are in good agreement, certainly within the errors associated with the relevant experiments. However, when Mulliken population analyses are performed, rather different results are obtained from different basis sets and methods, and this emphasizes the difficulty of comparing theoretical atomic orbital populations with those obtained by least-squares modelling of the experimental data.

The crystal structure of Na2Fe(CN)5(NO)·2D2O, disodium penta­cyano­nitro­syl­ferrate(III) bis­(dideuterium oxide), has been determined by X-ray diffraction at 11 and 293 K, and by neutron diffraction at 15 K. The accurate and extensive data sets lead to more precise determinations than are available from earlier work. The agreement in atomic positional and displacement parameters between the determinations at low temperature is very good.

The crystal structure of dicaesium potassium hexacyanomanganese(III) has been determined by X-ray diffraction at 293, 85 and 10 K. The Mn and K atoms lie on inversion centres and the Cs atom is in a general position. The accurate and extensive data sets collected should be suitable for charge–density analysis studies.

Analyse des donnees de diffraction magnetique de neutrons existantes pour l'antiferromagnetique La 2 CuO 4 -δ en utilisant un modele empirique de la densite d'aimantation

Diammonium hexaaquachromium(II) disulfate, [NH412[Cr(H20)6](SO4)2, Mr = 388"29. At 84(2)K, the structure is monoclinic, P2~/a, a = 9.490 (2), b 12.816 (3), c = 6.104 (1) A, /3 = 107.09 (2) °, V = 710 (1) A 3, Z = 2, Dx = 1"82 Mg m -3, a(Mo Ka) = 0"71069 ]1, /x = 1"196 mm -~, F(000) = 405.2, R(/) = 0-022, R(F)= 0-030 for 7490 reflections. The CrO6 octahedron has a large Jahn-Teller distortion with a tetragonal elongation of Cr--O(8) 2.389(1)]t; Cr--O(7) 2.079 (1), Cr--O(9) 2.053 (1) ]1. At 295 (2)K, the structure is monoclinic, P2/a, a = 9.424 (2), b = 12.699 (3), c = 6.204 (1) ]1, /3 = 106-60 (1) °, V= 712 (1) ]13, Z= 2, Dm = 1.80 (1), Dx = 1.81 Mg m -3, A(Mo K~r) = 0.71069 ]1, /1, = l'199mm -I, F(000)=405.2, R(/) = 0.020, R(F)= 0.024 for 4339 reflections. The frO6 octahedron has a slightly smaller Jahn-Teller distortion with tetragonal elongation Cr---O(8) 2.323(2)]1; Cr---O(7) 2.125 (2), Cr--O(9) 2-054 (1) ]1. The CrO6 geometry has an unusual temperature dependence. The valence-orbital populations from the refinement give, in the Cr(OH2)6 unit, Cr-atom populations of 3dl.26 (3) qd 2.04 (4) 2,,/0.88 (4)2,40-28 (3)A,~- 0.32 (16)4pO-39 (15) xy ~ XZ, yZ Jlzt 22 "3t~ x2--y2 "l'~-I x, y which show the substantial differences expected on the grounds of the large Jahn-Teller effect present. The two short Cr--O bonds show much greater covalent charge transfer from the O lone pairs to the Cr atom than does the longer bond. For the shorter bonds distinct accummulation of charge in the midbond region is observed. The populations within the ammonium and sulfate ions present the expected features and preserve ideal tetrahedral symmetry accurately, and the charge distributions within them are unremarkable. The ionic charges depart from the ideal values, apparently owing to charge transfers associated with the strong hydrogen bonding in the crystal. They are, approximately, [Cr(OH2)6] z2+, [SO4] ~5- and [NH4] °'9+, with some variation according to the method of analysis, as expected.

Analyse des donnees relatives a la diffraction magnetique de neutrons en utilisant un modele qui incorpore la covalence dans les interactions des liaisons Cu-Cl et Cu-O; examen de la validite du modele par des calculs, dans l'approximation de la densite locale, sur la molecule et sur son dimere. En ce qui concerne la fonction d'onde de l'etat fondamental, le modele conduit a des conclusions distinctement differentes de celles obtenues anterieurement qui considerent un modele entierement ionique perturbe par le couplage spin-orbite

The polarized neutron diffraction data for deuterated trans-Ni(NH3)4(NO2)2 have been reanalysed using an improved method for accounting for the oribital component in the scattering of the polarized neutrons. In this case, where the applied magnetic field was not along a principal crystal or molecular magnetic susceptibility direction, there seemed to be a possibility that the usual ‘dipole approximation’ correction might not have been fully adequate within the accuracy of the data. However, because of the smallness of the orbital contribution to the angular momentum associated with the Ni2+ ion in the compound, the effects of the ‘full correction’ do not lead to a significantly different valence orbital population analysis of the spin density than does the dipole approximation.

In order to assess whether the effects of intramolecular dynamic electron correlation on the electron density would be experimentally detectable, X-ray structure factors which include thermal averaging effects have been calculated from the electron densities of a range of small-molecule molecular crystals [C(2)H(6), C(2)H(4), C(2)H(2), BH(3)NH(3), NH(3), NH(2)CN, OCl(2), CO(NH(2))(2)] using the procrystal, Hartree-Fock, B3LYP and QCISD wavefunction models with the superposition-of-independent-molecules method to create the electron density in the crystal. A naive R-factor-like criterion of 1% has been used to assess detectability, as well as a more sophisticated method based on real X-ray data for estimating experimental errors. Correlation effects on the density are found to be only marginally above the 1% detectability threshold, and are about one to two orders of magnitude smaller than deviations from the procrystal model. Further, only 10% of the data up to 1.2 A(-1) are significant for detecting correlation effects; and of those 10%, many are at low intensity and therefore difficult to measure. Another method to estimate the experimental errors indicates that the intramolecular correlation effects would not be measurable. Although thermal averaging effects are important for the absolute value of the calculated structure factors, the use of different thermal averaging models does not change our overall conclusion of detectability. Likewise, calculations using the B3LYP method for some molecules do not show significant changes in the amount of, or distribution of, the changes that would be detectable by experiment.

A polarized neutron diffraction experiment on a salt containing the 4d1 TcNCl4− ion (S=1/2) yielded the flipping ratios of six reflections confined to the hk0 plane. Using nuclear structure factors, FN, extrapolated from the 123 and 295 K x‐ray structures, magnetic structure factors, FM, were obtained from the flipping ratios. The values of FM were fitted to a model of spin populations in the Tc‐4d and the Cl‐3(sp) orbitals. It was found that 46(5)% of the spin density is located on the Cl atoms, corresponding to exceptionally high covalence in the Tc–Cl bonding. The amount is distinctly higher than found indirectly by ESR measurements, but is to some extent supported by a local density functional DV–Xα calculation. Because the data is limited to the one projection, it is not possible to separate the Tc and N spin density contributions.

Constrained Hartree–Fock calculations have been performed to obtain wavefunctions that reproduce experimental X-ray structure-factor magnitudes for crystalline NH3 to within the limits of experimental error. Different model densities using both a single molecule and clusters of NH3 in the calculation of X-ray structure-factor magnitudes have been examined. The effects of the crystalline lattice on the experimental wavefunction of the NH3 unit can be reproducibly recovered. The construction of structure-factor magnitudes based on normally distributed random perturbations of the experimental values has also been used to gauge the accuracy of integrated atomic properties obtained from the wavefunctions, the point at which the constraint procedure should be terminated, and the approximate error in the experimental \sigma({\bf k}) values.

Total charge densities ρ(r) of solid NH3 have been derived using an ab initio crystalline molecular-orbital approach and also from multipole refinement of the structure factors obtained from the same charge density. Comparison of the topological features of these charge densities, as defined by the quantum theory of atoms in molecules, has been used to probe the ability of the multipole analysis to reproduce exactly known total charge-density distributions. For the most part, multipole refinement satisfactorily returns the features of the original density, although the fit to theoretical data is not as good as that to the experimental data. The one topological parameter that is poorly reproduced is the Laplacian \nabla^2\rho({\bf r}_b) at NH bond critical points.

Precise X-ray determinations of the crystal structure of the 1:2 complex of N-ethyl-N-methylmorpholinium and 7,7,8,8-tetracyano-p-quinodimethanide, abbreviated as MEM-TCNQ or MEM(TCNQ)(2) (C(7)H(16)NO(+).2C(12)H(4)N(4)(0.5-)), have been performed at 293 and at 10 K. Evidence for the expected spin-Peierls transition at 19 K is not found, and this may follow from radiation damage to the crystal or from insufficient equipment sensitivity.

The crystal structure of trisodium hexanitrocobaltate(III) has been determined by X-ray diffraction at 293 and 10 K. It contains the slightly distorted octahedral Co(NO(2))(6)(3-) anion. The accurate and extensive data sets collected should be suitable for charge-density analysis studies.

The crystal structure of the title compound, [ReCl(4)(C(8)H(10)N(2))(2)], has been determined by X-ray diffraction at 11 K and by neutron diffraction at 20 K. The accurate and extensive data sets lead to more precise determinations than are available from earlier work. The agreement in atomic positional and displacement parameters at these very low temperatures is good. The results will facilitate re-examination of the magnetic structure of the complex. The Re atom lies on a special position and the molecule has twofold crystallographic symmetry.

The crystal structure of Na2Fe(CN)5(NO)·2D2O, disodium penta­cyano­nitro­syl­ferrate(III) bis­(dideuterium oxide), has been determined by X-ray diffraction at 11 and 293 K, and by neutron diffraction at 15 K. The accurate and extensive data sets lead to more precise determinations than are available from earlier work. The agreement in atomic positional and displacement parameters between the determinations at low temperature is very good.