Bridging Bond Angle Research Articles

Study of short range structure of amorphous Silica from PDF using Ag radiation in laboratory XRD system, RAMAN and NEXAFS

At present synchrotron and neutron sources are the preferred choices for the Pair Distribution Function (PDF) analysis, but there is a need to explore PDF in a laboratory XRD system for quick feedback about the short range structure of the amorphous materials. Present work considered both crystalline (quartz) and amorphous silica to study the structural differences in silica by PDF analysis using Ag radiations in laboratory XRD. The structural information about short range ordering of the oxygen (O) atoms around silicon (Si) atoms as obtained by the PDF were compared with the results as obtained by Near Edge X-ray Absorption Fine Structure (NEXAFS) and RAMAN experiments. The PDF studies showed that the amorphous silica possessed short range periodicity within the basic unit of (SiO4)4− tetrahedra with a SiO & OO distance are of about 1.622 Å and 2.713 Å while the short range as well as long range ordered structure present in quartz with SiO & OO distance are 1.562 Å and 2.661 Å respectively. Raman spectra showed some asymmetry in amorphous silica which corresponds to the defects present in the lattice and thus forming the n-fold ring structure with Si and O resulting in the wide variation of bridging bond angle SiOSi in amorphous silica. NEXAFS studies revealed the structure of amorphous silica and quartz in the intermediate range (3–5 Å) at the Si L and O K edges. The structural information about short range ordering of the O around Si atoms as obtained by these methods were found to be in good match with the results as obtained by PDF, suggesting this technique may be used as a screening tool for routine PDF studies of amorphous materials.

Novel Silver Cobaltacarborane Complexes with a Linearly Bridging Halide

Great attention has been paid to halide coordination chemistry, not only because it constitutes the fundamentals of inorganic chemistry, but because the versatile coordination abilities of halides can be used to construct many interesting polynuclear complexes, which include halidebridged inorganic polymers, halide-capped metal clusters, halide-templated polynuclear assemblies, and anionic guests. The structural versatility of halides mainly originates from their coordinating abilities of adopting a bridging bond between two or more metal atoms, as well as a terminal bond. Moreover, a halide bridging bond angle is so flexible that thermodynamic stability can be endowed with proper geometry, which conceptually varies from acute to right, obtuse, and linear. In spite of innumerable reports on molecular metal halides, examples of the linearly bridging fashion are very scarce. The reason for the rarity of the linear M–X–M arrangement can be easily explained by the VSEPR (Valence Shell Electron Pair Repulsion) concept. The linear M–X–M formation has only been achieved by adopting a macrocyclic chelate ligand, which is structurally demanding, so that the VSEPR repulsions among lone-pair electrons on the halide atom could be overcome. In the synthetic exploration of new cobaltacarboranes, we found an unexpected ionic product comprising a cation with a linear Ag–X–Ag arrangement. At first, we attempted a reaction between Tl2C2B9H11 and CoCl2(PPh3)2, mimicking the previously reported preparation of a nickellacarborane, [3,3-(PPh3)2–3,1,2–NiC2B9H11]. We found that a new cobalt complex of dicarbollide was generated, but no more information could be obtained. Treatment of the product mixture with AgBr(PPh3)3 afforded a cationic [L3Ag–X–AgL3] (L = triphenylphosphine) with CoSAN as a counterion (CoSAN = [3,3-commo-3,1,2-Co(C2B9H11)2]). Although linear Ag–X–Ag could be achieved, the reaction result was not well reproducible, nor was its mechanism reasonable. Hence, we directed the preparation of the linear complexes in a rationalized synthetic manner, as depicted in the following reaction equation:

Radiation changes in the spectral characteristics of silicon dioxide modifications

The results of a comparative study on the modification of the properties of glassy and crystalline silicon dioxide under the radiation induced structural rearrangement are presented. The patterns of a non-linear change in the spectral parameters of bending vibrations of the Si—O—Si bonds in the function of fast neutron flux are revealed. The radiation kinetics of the peak value of the reflection coefficient and the respective frequency modes in the silicon dioxide modifications are compared. The graphs are found to demonstrate a dose dependence of the spectral parameters of special points. They are considered to be related to a radiation-induced rearrangement of the original structure accompanied by a deformation of the bridging bond angles.

X-ray Crystal Structures of [XeF][MF6] (M = As, Sb, Bi), [XeF][M2F11] (M = Sb, Bi) and Estimated Thermochemical Data and Predicted Stabilities for Noble-Gas Fluorocation Salts using Volume-Based Thermodynamics

The crystal structures of the xenon(II) salts, [XeF][SbF(6)], [XeF][BiF(6)], and [XeF][Bi(2)F(11)], have been determined for the first time, and those of XeF(2), [XeF][AsF(6)], [XeF][Sb(2)F(11)], and [XeF(3)][Sb(2)F(11)] have been redetermined with greater precision at -173 °C. The Bi(2)F(11)(-) anion, which has a structure analogous to those of the As(2)F(11)(-) and Sb(2)F(11)(-) anions, has been structurally characterized by single crystal X-ray diffraction for the first time as its XeF(+) salt. The fluorine bridge between the bismuth atoms is asymmetric with Bi---F(b) bond lengths of 2.092(6) and 2.195(6) A and a Bi---F(b)'---Bi bridge bond angle of 145.3(3)°. The XeF(+) cations interact with their anions by means of Xe---F(b)---M bridges. Consequently, the solid-state Raman spectra of [XeF][MF(6)] (M = As, Sb, Bi) were modeled as the gas-phase ion pairs and assigned with the aid of quantum-chemical calculations. Relationships among the terminal Xe-F(t) and bridge Xe---F(b) bond lengths and stretching frequencies and the gas-phase fluoride ion affinities of the parent Lewis acid that the anion is derived from are considered. The analogous krypton ion pairs, [KrF][MF(6)] (M = As, Sb, Bi) were also calculated and compared with their previously published X-ray crystal structures. The calculated cation-anion charge separations indicate that the [XeF][MF(6)] salts are more ionic than their krypton analogues and that XeF(2) is a stronger fluoride ion donor than KrF(2). The lattice energies, standard enthalpies, and free energies of formation for salts containing the NgF(+), Ng(2)F(3)(+), XeF(3)(+), XeF(5)(+), Xe(2)F(11)(+), and XeOF(3)(+) (Ng = Ar, Kr, Xe) cations were estimated using volume-based thermodynamics (VBT) based on crystallographic and estimated ion volumes. These estimated parameters were then used to predict the stabilities of noble-gas salts. VBT is used to examine and predict the stabilities of, inter alia, the salts [XeF(m)][Sb(n)F(5n+1)] and [XeF(m)][As(n)F(5n+1)] (m = 1, 3; n = 1, 2). VBT also confirms that XeF(+) salts are stable toward redox decomposition to Ng, F(2), and MF(5) (M = As, Sb), whereas the isolable krypton compounds and the unknown ArF(+) salts are predicted to be unstable by VBT with the ArF(+) salts being the least stable.

Chromophore Structure of Cyanobacterial Phytochrome Cph1 in the Pr State: Reconciling Structural and Spectroscopic Data by QM/MM Calculations

A quantum mechanics (QM)/molecular mechanics (MM) hybrid method was applied to the Pr state of the cyanobacterial phytochrome Cph1 to calculate the Raman spectra of the bound PCB cofactor. Two QM/MM models were derived from the atomic coordinates of the crystal structure. The models differed in the protonation site of His260 in the chromophore-binding pocket such that either the δ-nitrogen (M-HSD) or the ɛ-nitrogen (M-HSE) carried a hydrogen. The optimized structures of the two models display small differences specifically in the orientation of His260 with respect to the PCB cofactor and the hydrogen bond network at the cofactor-binding site. For both models, the calculated Raman spectra of the cofactor reveal a good overall agreement with the experimental resonance Raman (RR) spectra obtained from Cph1 in the crystalline state and in solution, including Cph1 adducts with isotopically labeled PCB. However, a distinctly better reproduction of important details in the experimental spectra is provided by the M-HSD model, which therefore may represent an improved structure of the cofactor site. Thus, QM/MM calculations of chromoproteins may allow for refining crystal structure models in the chromophore-binding pocket guided by the comparison with experimental RR spectra. Analysis of the calculated and experimental spectra also allowed us to identify and assign the modes that sensitively respond to chromophore-protein interactions. The most pronounced effect was noted for the stretching mode of the methine bridge A-B adjacent to the covalent attachment site of PCB. Due a distinct narrowing of the A-B methine bridge bond angle, this mode undergoes a large frequency upshift as compared with the spectrum obtained by QM calculations for the chromophore in vacuo. This protein-induced distortion of the PCB geometry is the main origin of a previous erroneous interpretation of the RR spectra based on QM calculations of the isolated cofactor.

Raman characterization of planar waveguides fabricated by electron-beam irradiation of germanium-doped flame hydrolysis deposited silica

The structural changes responsible for the refractive index variations induced in Ge-doped flame-hydrolysis deposited (FHD) silica by electron-beam irradiation have been studied using confocal micro-Raman spectroscopy. Two structural mechanisms, an increase in the concentration of 4- and 3-membered rings in the silica structure and a reduction of the oxygen bridging bond angle between silica tetrahedra have been observed. The formation of small membered rings occurs along the penetration depth of the electrons in the layer, decreasing with depth. The reduction of the angle between silica tetrahedra only occurs on the surface of the material. The results are consistent with previous measurements of the compaction of the material and will determine the shape of the refractive index profile of the waveguides fabricated using this technique.

Molecular orbital study on the onset of the pressure-induced transformations of the metastably compressed germanates

In order to elucidate the onset of the pressure-induced transformations of germanates consisting of GeO4 tetrahedra at room temperature, we have investigated the stability of the crystal structures near the transition pressure in terms of the stability of the linkage of the tetrahedra. The stability of the linkage near the transition pressure is estimated from the results of the molecular orbital calculations for the model cluster H6Ge2O7, which mimics the linkage of tetrahedra, as functions of the bond length d(Ge–Obr) with bridging oxygen and bond angles Ge–Obr–Ge. The calculation shows that the total energy of the linkage becomes minimum at d(Ge–Obr)=1.758 A and <Ge–Obr–Ge=130.4°, and that it increases with the deviation from the energy minimum geometry. From the compression behavior of framework and chain germanates, we find that the linkage of the tetrahedra becomes unstable with increasing pressure, and that these germanates commonly transform into their high-pressure phases when the linkage of the tetrahedra largely departs from the energy minimum geometry. This suggests that the high-pressure transformations of the metastably compressed germanates are induced by the instability of the linkage of tetrahedra.

Structural Rearrangement of SiO2 Films during Their Growth and Annealing

Changes in the refractive index and molar volume of thermally grown films with the growth or annealing (in inert ambients) temperature are characterized by 0.01 eV activation energy. It is suggested here that with increasing temperature the rotational angles between tetrahedra decrease and/or the bridging bond angles increase resulting in increased intertetrahedral distances and molar volume (decreased refractive index). The activation energy of the generation of centers during annealing in vacuum is 0.25 eV, indicating that strained Si‒O bonds with small bridging bond angles (<120°) and enlarged bond lengths (>0.160 nm) are involved. The density of these bonds is reduced by He in the oxide so that annealing in He results in reduced density of centers. © 1999 The Electrochemical Society. All rights reserved.

Growth and characterization of GeO2 films obtained by plasma anodization of epitaxial Ge films

Amorphous GeO2 has been grown by plasma assisted anodic oxidation at temperatures ⩽100 °C. The growth kinetics can be explained in terms of a constant current anodization model. Infrared absorption spectroscopy has been used to characterize the transverse and longitudinal optic vibrational modes of the oxide network. Values are displaced with respect to bulk oxide due to geometrical optic effects and plasma induced network structure variation. In relaxed GeO2 we estimate the mean Ge-O-Ge bridging bond angle to be &amp;gt;123° and the full width at half peak height of the bond angle distribution is ∼14°. Electrical measurements on thin oxide capacitors suggest that in 25 nm oxides there are ⩽2×1011 negative charges cm−2 and negligible interface state densities.

Linear trinuclear and one-dimensional copper(II) complexes containing co-bridging end-on azido and triazole ligands. Crystal structures and magnetic properties of [Cu3(atrz)2(N3)6] and [Cu(atrz)2(N3)]NO3 (atrz = 4-amino-1,2,4-triazole)

New complexes with 1,2,4-triazole and end-on azido bridging ligands, [Cu3(atrz)2(N3)6] 1 and [Cu(atrz)2(N3)]NO32 (atrz = 4-amino-1,2,4-triazole), have been prepared and structurally determined. Complex 1 has a linear trinuclear copper(II) structure with bridging bond angles Cu–N–N (atrz) [119.3(3) and 123.4(3)°], and larger Cu–N–Cu [118.7(2)°]. In 2 the copper(II) ions are linked by one end-on azido and two triazole bridges forming a cationic chain with Cu–N–N (equatorial atrz) [121.0(2) and 122.3(2)°] and Cu–N–Cu 104.05(12)°. The nearest-neighbour Cu· · ·Cu distances are 3.4496(7) A for 1 and 3.5034(6) A for 2. The copper(II) ions are strongly antiferromagnetically coupled with J = –108.8 cm–1 for 1, and weakly with J = –17.7 cm–1 for 2. The nature and the magnitude of the antiferromagnetic exchange of both complexes are discussed based on their structures.

Simultaneous formation of both novel bis(μ-sulfur)- and bis(μ-oxo)bridged copper(II) dimers by reaction of N-benzoyl-N ′,N ′-dimethylthiourea (HL) with CuCl2. Syntheses, crystal structures and magnetic properties of [{CuL(HL)Cl}2] and [{CuL2}2] †

Reaction of N-benzoyl-N′,N′-dimethylthiourea (HL) with copper(II) chloride yielded simultaneously two novel dimers: a pale green bis(µ-sulfur) bridged copper(II) dimer 1, [{CuL(HL)Cl}2], and a dark green bis(µ-oxo) bridged copper(II) dimer 2, [{CuL2}2], instead of the mononuclear copper(II) complexes, [CuL2], usually formed from other dialkyl-substituted benzoylthiourea ligands. The molecular structures of 1 and 2 have been established by single-crystal X-ray analyses. In dimer 1 the central copper(II) ion has a distorted tetrahedral co-ordination environment with one chloride and three sulfur atoms derived from one terminal and two bridging ligands. The two sulfur atoms of both bridging ligands with two copper(II) ions form a strictly planar Cu2S2 bridging core, including two short [2.330(2) A] and two longer [2.549(2) A] Cu–S distances. The dimer 2 may be considered as comprising two mononuclear [CuL2] complexes bridged by one of the acyl oxygen atoms of each to the copper atom of the other, resulting in a distorted tetragonal pyramidal geometry at each copper(II) center, and forming a planar four-membered Cu2O2 bridging unit which consists of two short [1.944(3) A] and two longer Cu–O distances [2.668(3) A], bridging bond angle Cu–O–Cu* [97.9(1)°]. Magnetic measurements for both dimers showed weak antiferromagnetic coupling between the copper(II) centers through bis-sulfur or bis-oxo bridges. The best fitting of the experimental magnetic susceptibility data gave g = 2.110, J = –33 cm–1 for dimer 1 and g = 2.107, J = –4.5 cm–1 for 2.

Low temperature, plasma assisted oxidation of amorphous Si

A preliminary study of plasma assisted oxidation of amorphous, hydrogenated Si (a-Si:H) has been carried out using a microwave frequency, electron cyclotron resonance excited reactor. A ‘constant current through the substrate’ mode was used. Films up to several tens of nm were obtained at room temperature in a time of ⪡ 60 min. The growth kinetics of the SiO 2 on amorphous and crystalline Si substrates were similar and followed the predictions of a constant current, field assisted diffusion model. Infrared absorption spectroscopy on as-grown and chemically etched SiO 2 films suggests that they differ and are inhomogeneous compared to thermal oxides. The differences are attributed to a network having a smaller SiOSi bridging bond angle than that for thermal SiO 2 rather than density or stoichiometric differences. The latter has been confirmed by X-ray photoelectron spectroscopy measurements. Surface roughness in the form of macroscopic bubbles and peeling was observed in samples of a-Si:H following oxidation. This phenomenon, which we attribute to the release and nucleation of the H in the films, may severely limit the applicability of the method for producing low temperature oxides on a-Si.

Oxidation of crystalline Si in an O2 plasma: Growth kinetics and oxide characterization

The growth of anodic oxides of crystalline Si in an O2 plasma has been studied. The kinetics are found to follow those expected when neutral O atoms in the plasma capture an electron at the substrate surface and diffuse through the growing oxide under the influence of an applied positive electric field. The oxides have been characterized using infrared spectroscopy, glancing incidence x-ray reflectometry, and electric capacitance/voltage measurements. Shifts to lower wave numbers of the transverse and longitudinal optic infrared modes associated with the asymmetric stretch of the bridging O’s with respect to thermally grown oxide values are observed. We conclude that these shifts result from structural modifications induced by ultraviolet photons present in the O plasma. A simple model suggests that the plasma grown oxides have a density ∼3.8% larger than thermal SiO2 and an average Si–O–Si bridging bond angle ∼1.6° smaller. The plasma grown oxides are inhomogeneous in the volume, the bond angle shift increasing to ∼−2.3°, and the density decreasing to +2% with respect to thermal oxide near the Si substrate/SiO2 interface. X-ray reflectivity data, which are consistent with film inhomogeneity, enable us to determine the film thickness and surface (air/SiO2) roughness. The latter is always ⩽1 nm. Electrical measurements suggest that the oxides contain fixed oxide charges and interface states which can be reduced by low temperature annealing. The leakage currents remain high, however.

Structural changes and Si redistribution in Si + implanted silica glass

Fine polished SiO 2 glass was implanted with Si + ions of 50 keV in the ion dose range 0.5 to 50 × 10 16 Si +/cm 2. Some samples were annealed in air or in nitrogen atmosphere at temperatures between 400 and 1000°C for one hour. The distribution of excess Si was investigated by RBS and by VIS-NIR transmittance measurements. Information about the structure of the implanted and annealed silica glass layers were obtained by the IRR analysis. After implantation amorphous silicon (a-Si) precipitates were formed the size of which increases with the ion dose. The reduction of the SiOSi bridging bond angle in the silica glass between the a-Si particles up to a constant value of 120° for doses > 2 × 10 17 Si + cm 2 is caused by Si ions which are not incorporated in precipitates and by a change of the density. If annealing is performed in air. at temperatures > 600°C, the formation of a surface SiO 2 layer is observed. We assume that small precipitates occurring in the wings of the distribution close to the surface react with the oxygen of the air to form the detected SiO 2 layer. This process is remarkably reduced during annealing in nitrogen atmosphere. After annealing at temperatures ≥ 900°C the existence of nano-crystalline Si is observed by Raman spectroscopy. Heat treatment at 1000°C in nitrogen atmosphere is combined with a redistribution of Si from the wings to the maximum of the profile. This process can be understood in terms of a Ostwald ripening process.

Magnetic Susceptibility Trends in Oxo-Bridged, Dinuclear Chromium(III) Complexes. Crystal Structure of [(tmpa)Cr(&mgr;-O)(&mgr;-CO(3))Cr(tmpa)](ClO(4))(2).2H(2)O.

The synthesis and physical characterization of oxo-bridged [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) complexes (tmpa = tris(2-pyridylmethyl)amine) containing a variety of complementary ligands (X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-)) are described, with the objective of understanding factors underlying variations in the antiferromagnetic coupling constant J. We also present the crystal structure of [(tmpa)Cr(&mgr;-O)(&mgr;-CO(3))Cr(tmpa)](ClO(4))(2).2H(2)O, for comparison with previous findings on [(tmpa)Cr(&mgr;-O)(&mgr;-CH(3)CO(2))Cr(tmpa)](ClO(4))(3). The carbonate-bridged complex crystallizes in the monoclinic space group P2(1)/c with a = 11.286(10) Å, b = 18.12(2) Å, c = 20.592(12) Å, beta = 95.99(5) degrees, and V = 4190 Å(3) and Z = 4. Asymmetric tmpa ligation pertains, with apical N atoms situated trans to bridging oxo and acido O atoms. Key structural parameters include Cr-O(b) bond lengths of 1.818(6) and 1.838(6) Å, Cr-OCO(2) distances of 1.924(7) and 1.934(7) Å, and a bridging bond angle of 128.3(3) degrees. Several attempts to prepare oxo, amido-bridged dimers were unsuccessful, but the nearlinear [Cr(tmpa)(N(CN)(2))](2)O(ClO(4))(2).3H(2)O complex was isolated from the reaction of dicyanamide ion with [Cr(tmpa)(OH)](2)(4+). In contrast to the behavior of analogous diiron(III) complexes, antiferromagnetic coupling constants of [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) dinuclear species are highly responsive to the X group. Considering the complexes with X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-), SO(4)(2)(-), and RCO(2)(-) (10 R substituents), we find a reasonably linear, empirical relationship between J and oxo bridge basicity, as measured by pK(a) (Cr(OH)Cr) values in aqueous solution. While there is no theoretical basis for such a correlation between solid-state and solution-phase properties, this relationship demonstrates that CrOCr pi-bonding contributes significantly to antiferromagnetic exchange. Thus, J tends to become less negative with increasing &mgr;-O(2)(-) basicity, showing that greater availability of a bridging oxo group lone pair toward the proton, with decreasing CrOCr pi-interaction, reduces the singlet-triplet gap.

Structural properties of low temperature plasma enhanced chemical vapor deposited silicon oxide films using disilane and nitrous oxide

The structural properties of low temperature plasma enhanced chemical vapor deposited SiO2 films using Si2H6 and N 2O have been studied. It is observed that the degree of compaction of as-deposited SiO2 films, upon subsequent annealing, increases up to 4%. The shift of Si-O-Si stretching peak wave number of the as-deposited SiO 2 films (Δω=−20 cm−1) compared to the undensified SiO 2 films is attributed to 9.4% increase in the film density, resulting in smaller Si-O-Si bridging bond angle of 138°. It is also believed that the high temperature annealing results in the reduction of hydroxyl containing species in the film and in turn drives the dielectric constant towards that of thermal SiO2 films.

Solid-State 17O Magic-Angle and Dynamic-Angle Spinning NMR Study of the SiO2 Polymorph Coesite

Five distinctly resolved I7O solid-state NMR resonances in room temperature coesite, an Si02 polymorph, have been observed and assigned using dynamic-angle spinning (DAS) at 11.7 T along with magic-angle spinning (MAS) spectra at 9.4 and 11.7 T. The I7O quadrupolar parameters for each of the five oxygen environments in coesite are correlated with the Si-0-Si bridging bond angles determined by diffraction experiments. The sign of e2qQ/h along with the orientation of the electric field gradient for oxygen in the Si-0-Si linkage were determined from a Townes-Dailey analysis of the data.

Macroscopic and microscopic effects of radiation in amorphous SiO2

Microscopic and macroscopic changes in the a-SiO2 network induced by radiation are reviewed. In the former case, ten intrinsic network defects are identified, most having been examined structurally by electron spin resonance. The presence of the different defects is dependent upon manufacturing processes. For example, it is demonstrated that oxygen-vacancy-like and non-bridging hole center defects may be produced in a correlated manner as a result of bond scission in strained networks. Other defects, such as Si microclusters, are observed in oxygen deficient materials again resulting from preparation conditions. Macroscopic density variations ≤ 3% are induced by radiation. It is demonstrated that this densification cannot be attributed to stable defect creation and cannot be likened to pressure/temperature induced plastic densification. Analysis of refractive index, Raman scattering and infrared absorption data indicates that radiation induces a unique “structure” in which there is a substantial reduction in the mean Si-O-Si bridging bond angle (of ∼ 10°) but in which the network density is only marginally changed. This is the exact opposite of the situation observed in pressure/temperature induced plastically densified a-SiO2. We argue that the structural modification involves the creation of large voids or open spaces analagous to the cage networks found in the crystalline phases known as porosils.

Ion implantation- and radiation-induced structural modifications in amorphous SiO 2

Data on the asymmetric infrared stretching mode of the oxygen in the SiOSi bridge and on the refractive index are examined for crystalline SiO 2 polymorphs and compacted, amorphous SiO 2. A set of experimentally based relationships is established between the infrared peak position and bridging bond angle and the refractive index and density. Using these relationships, results on radiation- and ion implantation-induced densification in amorphous SiO 2 are examined. It is demonstrated that the saturation densification involves reduction in the bridging bond angle by ∼ 10° and a densification ≤3%. This result is inconsistent with the observed bond angle/density relationship established for pressure/temperature compacted amorphous SiO 2. It is suggested that the overall densification results from a decrease in the bond angle and a simultaneous generation of both small and large rings.

Single crystal X-ray diffraction studies of aromatic oligomers: resolution of the bond-angle anomaly in poly(aryletherketone)s

To resolve long-standing uncertainties surrounding the bridge-bond angles of crystalline poly(aryletherketone)s, the structure of an oligomeric ether-ketone, ClArCOArOArCOArCl (Ar = 1,4-phenylene), has been determined by single crystal X-ray methods. The average torsion angle ø of the two ‘ether-ketone’ rings, relative to a plane defined by the bridging oxygen and carbonyl-carbon atoms is 31°. The bond angles at carbonyl-carbon and ether-oxygen are 121.9(3)° and 121.2(3)°, respectively, values which are substantially lower than those consistently reported for poly(aryletherketone)s on the basis of X-ray powder- and fibre-diffraction studies. [An average bridge-bond angle of 126.5(6)° has, for example, been proposed to account for the c-axis length of 10.09(2) Å in the prototype poly(etherketone) (PEK) (-OArCOAr-) n.] Despite this apparent difference in bridge-bond angles, a PEK c-axis distance of 10.11(1) Ǎ, entirely consistent with that of the polymer, is found in the present oligomer structure. More detailed analysis of the oligomer structure reveals a pattern of sterically induced bond angle distortions at the aromatic carbon atom linked to an ether or ketone bridge, a pattern that is repeated in the X-ray structure of a second oligomer ArOArCOArArCOArOAr (Ar = phenyl or 1,4-phenylene). These findings strongly suggest that bridge-bond angles in poly(aryletherketone)s actually lie in the region of 121–122°, and that the range 125–127° consistently reported in the literature is an artifact deriving from the assumption of ideal geometry elsewhere in the polymer chain.