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The crystal structure of the AgClO4.C6H6 \pi complex, earlier determined by X-ray diffraction at room temperature, has been redetermined at 18, 78 and 158 K by neutron diffraction. Crystal data: orthorhombic, space group Cmcm, Z = 4, F(000) = 225.318 fm, T: 18, 17, 158 K; D x = 2.591 (2), 2.570 (1), 2.523 (1) g cm−1; \mu n = 0.166, 0.165, 0.162 cm−1; a = 7.913 (1), 7.973 (1), 8.100 (1), 8.336 (1) Å (at 295 K); b = 7.837 (2), 7.857 (1), 7.902 (1), 7.996 (1) Å (at 295 K); c = 11.798 (3), 11.777 (2), 11.739 (2), 11.638 (2) Å (at 295 K); wR(F 2) = 0.037, 0.035, 0.045, S = 1.18, 1.08, 1.10 for 782, 628, 800 reflections and 51 variable parameters. This study confirms the principal features reported in the X-ray investigation and reveals details of structure not observable at room temperature. Distortions of the benzene molecule from D 6h symmetry ascribed to Ag+...C6H6 interactions are small, but significant. At 18 K the two C—C bonds complexed by Ag+ are 1.405 (1) Å in length; the other four are 1.398 (1) Å. The C—H bonds are equal in length at 1.087 (2) (two) and 1.089 (1) Å (four). The H atoms nearest to Ag+ are displaced 0.064 (1) Å from the C6 plane, away from the silver. The shortest Ag+...C distance of the complex is 2.565 (1) Å. This value and bond lengths of the benzene molecule are invariant between 18 and 158 K within 2 e.s.d.'s or less. The nonequivalent bond lengths of ClO^{-}_4, 1.451 (1) (two) and 1.441 (1) Å (two) at 18 K, are foreshortened by −0.007 and −0.005 Å at 158 K by effects of thermal motion. The O—Cl—O angles, 109.08 (7), 109.98 (2) and 107.83 (8)° at 18 K, are virtually unchanged by temperature. The Ag+...ClO^{-}_4 interactions occur at Ag+...O distances of 2.785 (1) and 2.612 (1) Å (18 K), where the shorter values involve ClO^{-}_4 acting as a bidentate group. Rigid-body and riding-motion models do not adequately account for the observed temperature dependence of bond lengths in ClO^{-}_4 nor provide significant corrections to the C6H6 bond lengths at 18, 78 and 158 K beyond their uncertainty limits. A harmonic potential rationalizes the motion of Ag+ at these three temperatures.

The ability of organisms to exercise control over crystal growth is wonderfully exemplified by skeleton formation in echinoderms. A sea urchin spine is a unique composite of a single crystal of calcite and glycoproteins intercalated inside the crystal during its growth. Here we performed a detailed morphological and high-resolution synchrotron X-ray diffraction study of the textures of synthetic and biogenic calcite crystals. We show that the intracrystalline macromolecules from sea urchin spines, when allowed to interact with growing calcite crystals in vitro, selectively reduce the coherence lengths and degrees of alignment of the perfect domains in specific crystallographic directions. These directions also correspond to the newly-developed stable faces. In contrast, the defect distribution of young sea urchin spines composed entirely of spongy stereomic structure is much more isotropic. In mature spines containing secondarily filled-in wedges of calcite, the degree of anisotropy is intermediate betwee...

The crystal structure of squaric acid [3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4, M = 114.1] has been determined from neutron diffraction data at 15 K, T c − 21 K, T c − 13 K and T c + 5 K below and above the phase transition at 374 K. The space group of the material is monoclinic P21/m from 15 K and up to T c and tetragonal I4/m above. After corrections for absorption and thermal diffuse scattering, least-squares refinements were carried out with 69 variable parameters in the model below T c and 25 above T c. The analysis of difference vibrational parameters along the C–O internuclear direction strongly suggest that the high-temperature structure has a disordered C4O4 unit. In agreement with previous findings, the protons are distributed on two principal positions 0.53 Å apart along the O … O hydrogen bond above T c. In the low-temperature phase, an increasing amount of disorder with increasing temperature is found, particularly in the protonic positions. However, the protons in the two different hydrogen bonds show distinctly different behavior, and hence immediately below T c different degrees of order are found along the x- and z-directions. These structural findings are discussed in the light of the 1-D correlations observed in the material.

Crystals formed in biological tissues often adopt remarkable morphologies that are thought to be determined mainly by the shapes of the confined spaces in which they grow. Another possible way of controlling crystal shape, demonstrated only in vitro, is by means of specialized proteins preferentially interacting with certain crystal faces. In so doing, they reduce the rate of growth in these directions and consequently change the overall crystal shape. In an X-ray diffraction study of the distribution of defects within the lattice of calcite crystals produced by certain sponges, we show that a remarkable correlation exists between the defect patterns or crystal texture and the macroscopic morphology of the spicules. This was observed in two cases in which proteins are present within the spicule crystal, but not in a third case where such intracrystalline proteins are absent. Furthermore, one of the spicules exhibited marked differences in texture even within families of structurally identical crystal planes, demonstrating that the organisms exert exquisite control over the microenvironment in which crystals grow. We conclude that highly controlled intercalation of specialized proteins inside the crystals is an additional means by which organisms control spicule growth.

C 22 H 33 D 3 O 2 .CH 3 OH cristallise dans P2 1 avec a=13.345, b=7.729, c=10.471A, β=101.18°, Z=2, T=123K; affinement jusqu'a R=0.0390. La distribution statistique de D entre les deux groupes methyl C20 montre que la chiralite en C20est de 84.0%S, 16.0%R dans les limites de ±0.6 et ±1.0% estimees a partir des moyennes des trois valeurs sur chaque methyl.

Transition-metal _σ-complexes [2] are coordination compounds in which two electrons in an X-H _σ-bond form a dative bond with a transition metal as illustrated in Figure 1. This bond can be further stabilized by π-backbonding from the metal to the X-H σ* antibonding orbital. Transition-metal σ-complexes are typically reactive intermediates that precede oxidative addition of substrates having an X-H bond. σ-complexes are, therefore, intermediates in catalytic hydrogenation (X = H), activation and functionalization of hydrocarbons (X = C), hydro-silylation (X = Si), and hydroboration (X = B) reactions.

Accurate integrated intensities for the Bragg reflection of neutrons from a ZnS single crystal have been measured at temperatures between 285 and 1173 K. After correction for thermal diffuse scattering and extinction effects the data were interpreted with different models that allow anharmonic contributions to the temperature factor. Both the cumulant expansion and the one‐particle potential (OPP) model with quasiharmonic temperature dependence describe the data satisfactorily, although the Gruneisen parameter obtained in the OPP analysis differs greatly from the value calculated from known thermodynamic quantities. Predictions based on Matsubara’s anharmonic formalism are not in accord with the behavior observed at the highest temperatures.

A neutron diffraction study of the amino acid salt L-phenylalanine.HCl(C/sub 9/H/sub 12/NO/sub 2//sup +/Cl/sup -/) has been carried out. Space group P2/sub 1/2/sub 1/2/sub 1/; a = 27.763 (17), b = 7.059 (4), c = 5.380 (5) A. The final conventional R value is 0.084 based on 1026 reflections with F/sub 0//sup 2/ greater than sigma(F/sub 0//sup 2/), and all bond lengths are determined with a precision better than 0.03 A. The structure is composed of hydrogen-bonded double layers of phenylalanine molecules joined by N--H . . . Cl and O--H . . . Cl hydrogen bonds.

The titlecompound, Co3(CO)9CH , crystallizes in space group P1 (Z = 2). The unit-cell dimensions determined from neutron diffraction data at 102 K are a = 7.936(2), b = 13.656(4), c = 6.856 (2).A, a = 101.60 (2), fl = 109.34 (2) and y = 93.96 (2) °. The structure was solved by Patterson methods with X-ray diffraction data obtained at room temperature, and further refined with low-temperature X-ray and neutron diffraction data. R(F) (neutron) = 0.037 for 3781 observations. The molecule possesses an idealized Cap point symmetry. Three Co(CO)a groups and a CH group form a distorted tetrahedron. The high resolution of the present study reveals significant differences between the axial and equatorial Co-C bond lengths, the average Co-C distances for these being respectively 1.837(2) and 1.798 (1)/~. The C-H bond distance as determined by neutron diffraction is 1.084 (1) ,/~. The Co neutron scattering length has been refined to a value of 2.48 (2) fm.

C 5 NH 3 (NH 2 ) 2 cristallise dans le systeme orthorhombique avec le groupe d'espace P2 1 2 1 2 1 . Affinement de la structure jusqu'a 0,093

C/sub 13/H/sub 16/N/sub 4/O/sub 2/, M/sub r/ = 260.92, crystallizes from wet ethanol in the monoclinic space group C2/c, a = 18.140(5), b = 12.796(3), c = 12.332(3) A, ..beta.. = 114.78(1)/sup 0/. D/sub c/ (for eight molecules in the cell of volume 2598.93 A/sup 3/) = 1.169 g cm/sup -3/. The structure was solved with MULTAN and refined by full-matrix least-squares techniques based upon 1838 unique F/sup 2//sub 0/ terms to yield agreement factors R = 0.057 and R/sub w/ = 0.089.

It has been demonstrated, for the first time by diffraction methods, that a solid solution composed of host and guest molecules can exhibit a crystal symmetry lower than that of the host. The study proves that the symmetry of a solid solution is dependent not only upon the host crystal structure and the guest molecular structure but also upon the surface structure and symmetry of the host crystal. The crystal structures of (S)-asparagine monohydrate (D{sub 2}NCOCH{sub 2}CH(ND{sub 3})CO{sub 2} {times} D{sub 2}O) and of the solid solution (0.848:0.152) (S)-asparagine/(S)-aspartic acid (DO{sub 2}CCD{sub 2}CD(ND{sub 3})CO{sub 2}) monohydrate were refined by using neutron diffraction data obtained at 15 K. The space group of the pure host crystal is P2{sub 1}2{sub 1}2{sub 1} (Z = 4), whereas that of the host/guest crystal is monoclinic P12{sub 1}1 with two molecular sites per asymmetric unit. The ratios of guest/host occupancies of the two independent sites are 0.173:0.827 and 0.132:0.868. The reduction in symmetry is in accordance with the preferred adsorption of guest aspartic acid on the (010) crystal face at half of the orthorhombic, symmetry-related surface sites. Aspartic acid mimics, at the preferred (010) surface sites, molecular asparagine, participating in all hydrogen bonds. At themore » less-favored (010) surface sites a normal N-H{hor ellipsis}O(host) hydrogen bond is replaced by O(hydroxyl){hor ellipsis}O(host) repulsion between lone-pair electrons. 21 refs., 5 figs., 6 tabs.« less

4,6-Diamino-1-(p-chlorophenyl)-1,2-dihydro-2,2-dimethyl-s-triazine hydrochloride, C11H15-CIN5+.Cl-, Mr = 288.20, monoclinic, P21/c, a = 8.783 (2), b = 10.267 (2), c = 17.234 (3) A, beta = 115.72 (1) degrees, U = 1400.1 (5) A3, Z = 4, Dx = 1.337 Mg m-3, lambda = 1.15882 (7) A for unit-cell determination and 1.04702 (7) A for collection of intensity data, mu = 0.191 mm-1, T = 15.0 (5) K, final R(F2) = 0.050 and wR(F2) = 0.063 for 3099 independent reflections. Five atoms of the triazine ring are nearly coplanar. The sixth, the quaternary C(2), is displaced from this plane (P1) so that the bond to one of its methyl substituents is nearly perpendicular to P1 while the other methyl substituent lies almost in the plane. The chlorophenyl-ring plane is nearly perpendicular to P1. The heterocycles form cyclic dimers via hydrogen bonds from the 6-amino group to ring atom N(5) of an adjacent molecule. All other N-H units are hydrogen bonded to the Cl- counter ion. The ring is protonated at position N(3).

[NBu 4 ] [Os 6 (μ 3 -H)(CO) 18 ] cristallise dans le systeme monoclinique avec le groupe d'espace I2. Affinement de la structure jusqu'a 0,049 (200 K; diffraction RX) et 0,123 (20 K; diffraction neutrons)

A complex obtained from the reaction of ReH/sub 9//sup 2 -/ with triphos (CH/sub 3/C(CH/sub 2/P(C/sub 6/H/sub 5/)/sub 2/)/sub 3/) is found to be a nonahydride dimer in which ReH/sub 9//sup 2 -/ is coordinated to Re(triphos)/sup +/ via three H atoms and a Re-Re bond. The 10-coordinated Re atom in this compound, (C/sub 2/H/sub 5/)/sub 4/N/sup +/(Re/sub 2/(..mu..-H)/sub 3/H/sub 6/(CH/sub 3/C(CH/sub 2/P(C/sub 6/H/sub 5/)/sub 2/)/sub 3/))/sup -/ x CH/sub 3/CN, is the first example of a transition-metal atom with coordination number greater than 9, in a molecular compound. The structure of the compound was determined at 295 K by X-ray diffraction and at 80 K by neutron diffraction. The molecular crystal structures are reported in detail. 39 references, 5 figures, 8 tables.

The temperature scale used in the analysis of neutron diffraction study of cubic ZnS1, is corrected. The revised temperature scale2 is used to re‐examine the thermal parameters. (AIP)

Anomalous scattering lengths of natural Sm for thermal neutrons with wavelengths between 0.827 and 1.300 A have been determined using a single crystal of a Sm‐complex of known structure. 140 selected reflections were measured at each wavelength and b0+b′ and b″ refined in each case. The values obtained are in good agreement with theoretical values obtained from a Breit‐Wigner calculation using tabulated resonance parameters for 149Sm. A value of b0=4.3±0.2 fm is deduced from the diffraction experiment.

Neutron diffraction data from a colorless 58 mg single crystal of trimethoprim (2,4-diamino-5-(3,4,5-trimethoxybenzyl)pyrimidine (C/sub 14/H/sub 18/N/sub 4/O/sub 3/) were used to obtain a high-precision description of the structure. Crystallographic data: space group P anti 1, a = 10.523 (4) A, b = 11,222 (4) A, c = 8.068 (3) A, ..cap alpha.. = 101.22(1)/sup 0/, ..beta.. = 112.15 (1)/sup 0/, ..gamma.. = 112.65 (2)/sup 0/, Z = 2. A multisolution method was used to solve the phase problem and the structure was refined by least-squares techniques on F/sub 0//sup 2/ (all 4039 reflections) with parameters grouped in two blocks to yield agreement factors, R = 0.045 and R/sub w/ = 0.071. The 2,4-diaminopyrimidine tautomer is confirmed. The bases form an extended hydrogen-bonded ribbon with N(1) and N(2) acting as acceptor and donor respectively across one center of symmetry. Atoms N(3) and N(4) interact similarly across another inversion center. This hydrogen-bonding pattern supports a previously postulated model for binding of the drug to the receptor. No base-stacking interactions of the pyrimidine rings were observed and so no conclusions could be reached regarding the role of this associative mode in the receptor binding. A conformational energy calculation indicates considerable rotational freedom aboutmore » the two bonds linking the aryl groups. The trimethoxybenzyl systems do not participate in any specific nonbonded interactions. (auth)« less

The space group of the pure host crystal of (S)-asparagine monohydrate (D2N-CO-CH2-CH(ND3)-COO·D2O) is P212121 (Z=4), whereas that of the host/guest crystal (0.848:0.152) is P1211.

More than the simple study of crystals, modern crystallography can be defined as the study of the structure of crystalline matter at or near atomic resolution by means of diffraction of X-rays, neutrons or electrons (15). As a science, crystallography throbs with relevancy (8). Crystallography settled the debate over the chemical form of NaCl (1914), served as a basis for the understanding of the chemical bond (1938), and more recently ushered in the now widely popular field of molecular biology (1953). Within the past 10 years the number of small compounds under study has risen to a current level of about 2,000 reports per year, and equally important, the number, size, and variety of macromolecules under investigation is steadily increasing.

Neutron diffraction data from a colorless 58 mg single crystal of trimethoprim (2,4-diamino-5-(3,4,5-trimethoxybenzyl)pyrimidine (C/sub 14/H/sub 18/N/sub 4/O/sub 3/) were used to obtain a high-precision description of the structure. Crystallographic data: space group P anti 1, a = 10.523 (4) A, b = 11,222 (4) A, c = 8.068 (3) A, ..cap alpha.. = 101.22(1)/sup 0/, ..beta.. = 112.15 (1)/sup 0/, ..gamma.. = 112.65 (2)/sup 0/, Z = 2. A multisolution method was used to solve the phase problem and the structure was refined by least-squares techniques on F/sub 0//sup 2/ (all 4039 reflections) with parameters grouped in two blocks to yield agreement factors, R = 0.045 and R/sub w/ = 0.071. The 2,4-diaminopyrimidine tautomer is confirmed. The bases form an extended hydrogen-bonded ribbon with N(1) and N(2) acting as acceptor and donor respectively across one center of symmetry. Atoms N(3) and N(4) interact similarly across another inversion center. This hydrogen-bonding pattern supports a previously postulated model for binding of the drug to the receptor. No base-stacking interactions of the pyrimidine rings were observed and so no conclusions could be reached regarding the role of this associative mode in the receptor binding. A conformational energy calculation indicates considerable rotational freedom aboutmore » the two bonds linking the aryl groups. The trimethoxybenzyl systems do not participate in any specific nonbonded interactions. (auth)« less