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This work examines the effect of intermolecular interactions on molecular properties derived from simulated X-ray diffraction data. Model X-ray data are computed from a superposition of ab initio molecular electron densities in the crystal, as well as from periodic crystal Hartree–Fock electron densities, for the hydrogen-bonded systems ice VIII, formamide and urea, as well as the weakly bound acetylene. The effects of intermolecular interactions on the electron density are illustrated at both infinite and finite data resolution, and it is concluded that multipole models are capable of quantitative retrieval of the interaction density, despite the known shortcomings of the radial functions in the model. Multipole refinement reveals considerable enhancement of the molecular dipole moment for hydrogen-bonded crystals, and negligible change in molecular second moments. Electric field gradients at H nuclei are significantly reduced in magnitude upon hydrogen bonding, and this change is also faithfully represented by the rigid pseudoatom model.

The hydrate crystal lithium hydroxide monohydrate LiOH.H2O has been studied by ab initio periodic Hartree-Fock calculations. The influence of the crystalline environment on the local molecular properties (molecular geometry, atomic charges, electron density, molecular vibrations and deuterium quadrupole coupling constants) of the water molecule, the lithium and hydroxide ions has been calculated. A number of crystalline bulk properties are also presented, optimized crystalline structure, lattice energy and electronic band structure. The optimized cell parameters from calculations with a large basis set of triple-zeta quality differ by only 1-3% from the experimental neutron-determined cell, whereas the STO-3g basis set performs poorly (differences of 5-10%). With the triple-zeta basis also the atomic positions and intermolecular distances agree very well with the experiment. The lattice energy differs by approximately 8% from the experimental value, and by at most 3% when a density-functional electron correlation correction is applied. Large electron-density rearrangements occur in the water molecule and in the hydrogen bond and are in qualitative and quantitative agreement with experimental X-ray diffraction results. The quadrupole-coupling constants of the water and hydroxide deuterium atoms are found to be very sensitive to the O-H bond length and are in good agreement with experimental values when the calculation is based on the experimental structure. The anharmonic O-H stretching vibrations in the crystal are presented and found to be very close to results from calculations on molecular clusters. The electronic band and density-of-states spectra are discussed. Model calculations on a hydrogen fluoride chain were used to rationalize the results.

KH(HCOO) 2, Mr=130.144, orthorhombic, Pbca, a=17.703(8), b=7.349(4), c=7.302(3) Aring, V=950.1(8) Aring 3, Z=8, Dx=1.820(1) g cm -3, lambda(Mo Kalpha)=0.71069 Aring, mu x(calc.)=0.973 mm -1, F(000)=528, T=120 K, R( F2)=0.033 for 2109 unique reflexions. The deformation electron density in KH(HCOO) 2 has been studied by means of experimental X- N maps obtained at 120 K. The electron distributions in the two chemically similar but crystallographically nonequivalent formate groups are discussed and correlated with bond order. The density distribution in the short intermolecular O-H. . . bond [2.437(1) Aring] is compared with experimental and theoretical maps in related compounds.

A survey is given of the thermal parameters of 150 water molecules in crystalline hydrates determined by neutron diffraction. The accuracy of the thermal parameters has been examined and rigid-bond tests revealed systematic errors for approximately 25% of the molecules. Considering only the most precise and accurate studies, good agreement is obtained between vibrational amplitudes derived from diffraction and spectroscopy. The influence of the immediate environment on the vibrations of the water molecule has also been investigated. A positive correlation is found between H...O hydrogen-bond distances and librational amplitudes. The coordination geometry around the O atom is shown to influence the vibrational amplitudes of the O atom.

The deformation electron density has been studied in LiOH.H 2O at 295K using a combination of X-ray and neutron diffraction data. Notoriously high extinction was present in the neutron data. Despite an attempt to reduce this by shock-cooling, the subsequent refinement of an extinction correction met with only partial success. Limited use was thus made of the neutron data in the determination of static multipole deformation density maps. The resulting peak maximum in the O-H bonds of the water molecule is 0.66 e Aring -3, and of the OH - ion 0.49 e Aring -3. Lower peaks are observed in the lone-pair regions: 0.22 e Aring -3 for H 2O and 0.27 e Aring -3 for OH -. These are to be compared with the results of theoretical ab initio calculations given by Hermansson and Lunnell (1982).

A method is presented in which parameters obtained from vibrational spectroscopy are used in the least-squares refinement of positional and thermal parameters obtained from diffraction data. The method involves the calculation of those contributions to the temperature factors not well described by the second-rank tensor approximation conventionally used in crystal structure determinations. Expressions are given for the temperature-factor contributions from vibrational motion on an arc and from anharmonic linear motion. The method is applied to the water molecule in dipotassium oxalate monohydrate, 2K+.C2O-4.H2O. Refinements using vibrational data are compared with a conventional refinement; the latter are shown to give considerable systematic errors in the geometrical parameters for the water molecule.

For pt.CXXXVII see ibid., vol.B35, p.2384 (1979). LiNO 3.3H 2O has been studied by neutron diffraction at 120 and 295 K. The final R(w.F 2) values are 4.5% (120K) and 4.9% (295K). The crystal structure found in a previous room-temperature X-ray study is confirmed: one water molecule is involved in six hydrogen bonds; the other independent water molecule in the structure has a tetrahedral environment comprising two Li + ions and two medium-strong hydrogen bonds. The four different O...O hydrogen-bond lengths increase by 0.02-0.03 Aring going from 120 to 295K. The mean-square amplitudes of vibration for the N and O atoms are found to be closely proportional to temperature. For the H and Li atoms the vibrational amplitudes are less sensitive to temperature. Crystal data: space group Cmcm, Z=4; at 120K a=6.713 (7), b=12.669 (4), c=5.968 (5) Aring; at 295K a=6.8018 (4), b=12.7132 (9), c=5.9990 (4) Aring.

Mr=393.26, monoclinic, P2 1/ c, a=13.8937 (14), b=9.6258 (7), c=10.9127 (7) Aring, beta=95.66 (1)deg, V=1452.3 (2) Aring 3, T=295K [the cell parameters were determined from the refinement of 39 theta values from a Guinier-Hagg X-ray powder film (Cr Kalpha radiation, lambda=2.28975 Aring, using a CoP 3 standard: a=7.70778 Aring)], Z=4, Dx=1.80 Mg m -3, final Rw( F2)=0.069 for all 4480 independent reflections collected out to sintheta/lambda=0.69 Aring -1. The crystal structure of Al(NO 3) 3.9D 2O has been studied by three-dimensional neutron diffraction data collected at 295K. The asymmetric unit contains Al(D 2O) 63+ octahedra, NO 3- ions and three D 2O molecules which are not coordinated to Al 3+ ions. All nine water molecules in the formula unit are crystallographically independent and the 19 hydrogen bonds donated have O...O distances in the range 2.650 (2)-3.054 (2) Aring. The hydrogen bonds donated by the D 2O molecules which are coordinated to Al 3+ ions are shorter than the other hydrogen bonds. A distinct correlation exists between the O-D and D...O distances.

For pt.CXIX see Z. Naturforsch. a, vol.32, p.134 (1977). The crystal structure of lithium nitrate trihydrate, LiNO 3.3H 2O, has been determined by X-ray diffraction at 295K. The crystal is orthorhombic, space group Cmcm, a=6.8018 (4), b=12.7132 (9), c=5.9990 (4) Aring, V=518.75 Aring 3, Z=4. The nitrate ions, Li + ions and one of the two crystallographically non-equivalent water molecules lie in a mirror plane and are bonded to atoms in adjacent planes through the second water molecule. The Li + ion is octahedrally surrounded by six O atoms, four belonging to water molecules and two to nitrate ions. The two non-equivalent water molecules are present in the structure in distinctly different bonding situations. One is bonded to two Li + ions and is the donor of two hydrogen bonds. The other is involved in six hydrogen bonds: the O atom accepts two hydrogen bonds, and both protons are involved in bifurcated hydrogen bonds.

The electron density in LiOH.H 2O has been determined by ab initio MO-LCAO-SCF calculations. All nearest neighbours to the H 2O molecule and the OH - ion, respectively, have been included explicitly in the calculations; next-nearest and more-distant neighbours have been simulated by point charges. The theoretical electron density maps are compared with experimental maps with good overall agreement. The influence of intermolecular bonding in the crystal is found to be twofold. Firstly, the overall polarization of the H 2O molecule and the OH - ion is increased significantly. Secondly, the electron density around the O nuclei in H 2O and OH - is rearranged, leading to the decrease of density in the lone-pair directions. The reasons for this decrease are discussed in some detail.