Crystalline Model Compounds Research Articles

WITHDRAWN: Ultra wideline dipolar NMR spectroscopy: Combining SEDOR and WURST-CPMG

A method to obtain dipolar information for nuclei exhibiting ultra wideline NMR spectra in disordered solids is presented. This is achieved via a constant time version of SEDOR (spin echo double resonance) employing WURST (wideband uniform rate smooth truncation) pulses and CPMG (Carr Purcell Meiboom Gill) detection. The method – coined constant time WUDOR (WURST SEDOR) NMR spectroscopy – is tested on two crystalline model compounds (31P{19F}-CT-WUDOR NMR and 31P{19F}-CT-WUDOR CPMG NMR on Na2PO3F, 125Te{31P}-CT-WUDOR CPMG NMR on Ba2(PO4)2TeO) and applied to obtain structural information on TeO2 – NaPO3 – NaF glasses.

Persistent Digermenes with Acyl and α-Chlorosilyl Functionalities.

We report the preparation of α‐chlorosilyl‐ and acyl‐substituted digermenes. Unlike the corresponding transient disilenes, these species with a Ge=Ge double bond show an unexpectedly low tendency for cyclization, but in turn are prone to thermal Ge=Ge bond cleavage. Triphenylsilyldigermene has been isolated as a crystalline model compound, and is the first fully characterized example of a neutral digermene with an A2GeGeAB substitution pattern. Spectroscopic and computational evidence prove the constitution of 1‐adamantoyldigermene as a first persistent species with a heavy double bond conjugated with a carbonyl moiety.

Continuous Drying of Pharmaceutical Powders Using a Twin-Screw Extruder

Continuous manufacturing of pharmaceuticals offers such benefits as production flexibility, reduced drug product costs, and improved product quality. Moving toward continuous manufacturing requires suitable small-scale equipment, either by development of new equipment or optimization of existing equipment. In primary manufacturing, particle properties are often altered during crystallization and have to be restored during subsequent processing. Drying a crystallized product is one of the most challenging steps, especially since attrition and agglomeration can occur. To that end, we investigated the drying behavior of a crystalline model compound with moisture levels of up to 10 wt % in a corotating twin-screw extruder. The feed mass flows on a piece of small-scale equipment used for pharmaceutical production varied between 0.5 and 2.0 kg/h. Experiments were conducted to evaluate the drying performance in various process settings. Because of a very narrow and consistent residence time distribution, extrusi...

Recoupling dipolar interactions with multiple I=1 quadrupolar nuclei: A 11B{6Li} and 31P{6Li} rotational echo double resonance study of lithium borophosphate glasses

The case of rotational echo double resonance (REDOR) experiments on the observe nuclei 11B and 31P interacting with multiple I=1 quadrupolar nuclei is analyzed in detail by SIMPSON simulations and experimental studies. The simulations define the region within the parameter space spanned by nutation frequency, quadrupolar coupling constant and spinning frequency where the parabolic analysis of the initial REDOR curve in terms of dipolar second moments has validity. The predictions are tested by experimental studies on the crystalline model compounds lithium diborate and lithium pyrophosphate, which are subsequently extended to measure dipolar second moments M2(11B{6Li}) and M2(31P{6Li}) in three borophosphate glasses. The data indicate that the lithium cations interact significantly more strongly with the phosphate than with the borate species, despite the formally anionic character of four-coordinate boron and the formally neutral character of the ultraphosphate (P(3)) units to which they are linked.

Detection of “free” oxide ions in low-silica Ca/Mg silicate glasses: Results from 17O → 29Si HETCOR NMR

Silicon and oxygen are the two most abundant elements in most technological and geological glasses and glass-forming liquids, and extensive studies of the structures of these materials have been made using 29Si and 17O high-resolution solid-state NMR. Double resonance NMR experiments hold great potential for revealing new details about the network structures of such important materials but have not yet been done for this particular pair of spins because of several technical challenges. Using a crystalline model compound, Ca3SiO5 (calcium trisilicate or the ‘alite’ phase of Portland cement), which contains oxide ion sites bonded either to one Si4+ or only to Ca2+, we demonstrate that the 17O→29Si Cross-Polarization Heteronuclear Correlation (CP-HETCOR) experiment can clearly distinguish between these two types of oxygen sites. The latter species, so-called ‘free’ oxide ions that are bonded to no network cations such as Si4+, play an important role in models of glass and melt structure but have been difficult to directly detect in many systems. In two Ca/Mg silicate glasses near to the orthosilicate composition (O/Si≥4), the HETCOR experiment clearly shows the presence of such ‘free’ oxide ions, whose 1D 17O NMR signals are actually completely hidden under those for the predominant non-bridging oxygens. Their observed chemical shifts suggest coordination by predominantly Mg2+ cations, an ordering that may be expected from the smaller size and stronger bonding interactions of this cation relative to those of Ca2+. The estimated abundances of ‘free’ oxide ions in these glasses are consistent with previous, indirect estimates based on analyses of 1D 29Si NMR spectra.

Measurement of homonuclear magnetic dipole–dipole interactions in multiple 1/2-spin systems using constant-time DQ-DRENAR NMR

A new pulse sequence entitled DQ-DRENAR (Double-Quantum based Dipolar Recoupling Effects Nuclear Alignment Reduction) was recently described for the quantitative measurement of magnetic dipole–dipole interactions in homonuclear spin-1/2 systems involving multiple nuclei. As described in the present manuscript, the efficiency and performance of this sequence can be significantly improved, if the measurement is done in the constant-time mode. We describe both the theoretical analysis of this method and its experimental validation of a number of crystalline model compounds, considering both symmetry-based and back-to-back (BABA) DQ-coherence excitation schemes. Based on the combination of theoretical analysis and experimental results we discuss the effect of experimental parameters such as the chemical shift anisotropy (CSA), the spinning rate, and the radio frequency field inhomogeneity upon its performance. Our results indicate that constant-time (CT-) DRENAR is a method of high efficiency and accuracy for compounds with multiple homonuclear spin systems with particular promise for the analysis of stronger-coupled and short T2 spin systems.

Flow-through cross-polarized imaging as a new tool to overcome the analytical sensitivity challenges of a low-dose crystalline compound in a lipid matrix

Assessing the physical state of a low-dose active compound in a solid lipid or polymer matrix is analytically challenging, especially if the matrix exhibits some crystallinity. The aim of this study was first to compare the ability of current methods to detect the presence of a crystalline model compound in lipid matrices. Subsequently, a new technique was introduced and evaluated because of sensitivity issues that were encountered with current methods. The new technique is a flow-through version of cross-polarized imaging in transmission mode. The tested lipid-based solid dispersions (SDs) consisted of β-carotene (BC) as a model compound, and of Gelucire 50/13 or Geleol mono- and diglycerides as lipid matrices. The solid dispersions were analyzed by (hyper) differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), and microscopic techniques including atomic force microscopy (AFM). DSC and XRPD could analyze crystalline BC at concentrations as low as 3% (w/w) in the formulations. However, with microscopic techniques crystalline particles were detected at significantly lower concentrations of even 0.5% (w/w) BC. A flow-through cross-polarized imaging technique was introduced that combines the advantage of analyzing a larger sample size with high sensitivity of microscopy. Crystals were detected easily in samples containing even less than 0.2% (w/w) BC. Moreover, the new tool enabled approximation of the kinetic BC solubility in the crystalline lipid matrices. As a conclusion, the flow-through cross-polarized imaging technique has the potential to become an indispensable tool for characterizing low-dose crystalline compounds in a lipid or polymer matrix of solid dispersions.

Strontium complexation in aqueous solutions and silicate glasses: Insights from high energy-resolution fluorescence detection X-ray spectroscopy and ab-initio modeling

Although fluid–melt partitioning of trace elements like Sr, Ba, La, and Y is known to be strongly influenced by the fluid and melt chemical composition, their speciation in silicate-saturated fluids is studied insufficiently at high temperatures and pressures. Here, high energy-resolution fluorescence detection–X-ray absorption spectroscopy (HERFD–XAS) has been applied to investigate the local environment of strontium in crystalline model compounds, silicate glasses, and aqueous solutions. Acquisition of Sr K-edge HERFD–XAS spectra of aqueous solutions of SrCl2 and Sr(OH)2, and three aqueous fluids with dissolved silicate components was done in situ at temperatures to 780°C and pressures to ∼800MPa using hydrothermal diamond-anvil cells.Experiments were complemented by theoretical spectroscopy calculations using the finite difference method near edge structure (FDMNES) code. This approach was validated for a number of crystalline model compounds. For the silicate glasses and aqueous solutions (SrCl2 and Sr(OH)2), small clusters were examined. Either symmetric or distorted SrO6 clusters were found to describe Sr complexation in peraluminous or peralkaline glasses. However, small ‘static’ clusters seem not to be fully suited to account for the dynamically changing atomic arrangements in aqueous solutions at high temperature. Therefore, ab-initio molecular dynamics simulations were performed and used as input for modeling of X-ray absorption spectra. Analyses of these simulations indicated [SrCl(H2O)6]+ and Sr(OH)2(H2O)4 as the most likely complexes in the chloride and hydroxide solutions, respectively.Analysis of the spectra of the silicate-rich fluids shows that both melt and fluid composition strongly influence Sr complexation. For the silicate-rich fluids, formation of Sr–Cl complexes occurs at low (Na+K)/Cl and (Si+Al)/(Na+K) ratios in the fluid, whereas Sr hydroxide and possibly silicate complexes (similar to those in the silicate glass) are favored at higher ratios. Our X-ray spectroscopic results offer an explanation for the dependence of fluid–melt partitioning of Sr on melt composition measured in previous ex situ studies, and highlight the importance of components other than chloride (silicate and aluminosilicate) in controlling metal speciation in fluid–melt systems at high temperatures and pressures.

Intermediate Role of Gallium in Oxidic Glasses: Solid State NMR Structural Studies of the Ga2O3–NaPO3 System

A series of (NaPO3)1–x(Ga2O3)x glasses (0 ≤ x ≤ 0.35) prepared by conventional melt-quenching methods has been structurally characterized by various complementary high resolution one-dimensional and two-dimensional (2D) solid state magic angle spinning nuclear magnetic resonance (MAS NMR) techniques, which were validated by corresponding experiments on the crystalline model compounds GaPO4 (quartz) and Ga(PO3)3. Alloying NaPO3 glass by Ga2O3 results in a marked increase in the glass transition temperature, similar to the effect observed with Al2O3. At the atomic level, multiple phosphate species QnmGa (n = 0, 1, and 2; m = 0, 1, 2, and 3) can be observed. Here n denotes the number of P–O–P and m the number of P–O–Ga linkages, and (m + n ≤ 4). For resolved resonances, the value of n can be quantified by 2D J-resolved spectroscopy, refocused INADEQUATE, and a recently developed homonuclear dipolar recoupling method termed DQ-DRENAR (double-quantum based dipolar recoupling effects nuclear alignment reduction). Ga3+ is dominantly found in six-coordination in low-Ga glasses, whereas in glasses with x > 0.15, lower-coordinated Ga environments are increasingly favored. The connectivity between P and Ga can be assessed by heteronuclear 71Ga/31P dipolar recoupling experiments using 71Ga{31P} rotational echo double resonance (REDOR) and 31P {71Ga} rotational echo adiabatic passage double resonance (READPOR) techniques. Up to x = 0.25, the limiting composition where this is possible, the second coordination sphere of all the gallium atoms is fully dominated by phosphorus atoms. Above x = 0.25, 71Ga static and MAS NMR as well as REDOR experiments give clear spectroscopic evidence of Ga–O–Ga connectivity. 31P/23Na REDOR and REAPDOR results indicate that gallium has no dispersion effect on sodium ions in these glasses. They also indicate significant differences in the strength of dipolar interactions for distinct QnmGa species, consistent with bond valence considerations. On the basis of these results, a comprehensive structural model is developed. This model explains the compositional trend of the glass transition temperatures in terms of the concentration of bridging oxygen species (P–O–P, P–O–Ga, and Ga–O–Ga) in these glasses. The results provide new insights into the role of Ga2O3 as an intermediate oxide, with features of both network modifier and network former in oxide glasses.

DQ-DRENAR: A new NMR technique to measure site-resolved magnetic dipole-dipole interactions in multispin-1/2 systems: Theory and validation on crystalline phosphates

A new solid state NMR technique is described for measuring homonuclear dipole-dipole interactions in multi-spin-1∕2 systems under magic-angle spinning conditions. Re-coupling is accomplished in the form of an effective double quantum (DQ) Hamiltonian created by a symmetry-based POST-C7 sequence consisting of two excitation blocks, attenuating the signal (intensity S'). For comparison, a reference signal S0 with the dipolar re-coupling absent is generated by shifting the phase of the second block by 90° relative to the first block. As in rotational echo double resonance, the homonuclear dipole-dipole coupling constant can then be extracted from a plot of the normalized difference signal (S0 - S')∕S0 versus dipolar mixing time. The method is given the acronym DQ-DRENAR ("Double-Quantum-based Dipolar Re-coupling effects Nuclear Alignment Reduction"). The method is analyzed mathematically, and on the basis of detailed simulations, with respect to the order and the geometry of the spin system, the dipolar truncation phenomenon, and the influence of the chemical shift anisotropy on experimental curves. Within the range of (S0 - S')∕S0 ≤0.3-0.5 such DRENAR curves can be approximated by simple parabolae, yielding effective squared dipole-dipole coupling constants summed over all the pairwise interactions present. The method has been successfully validated for (31)P-(31)P distance determinations of numerous crystalline model compounds representing a wide range of dipolar coupling strengths.

Conformational disorder of membrane peptides investigated from solid-state NMR line widths and line shapes.

A challenge in the application of solid-state NMR spectroscopy to membrane peptides and proteins is the relatively broad line widths compared to those for solution NMR spectra. To understand the linewidth contributions to membrane protein NMR spectra, we have measured the inhomogeneous and homogeneous line widths of several well-studied membrane peptides under immobilized conditions. (13)C T(2) relaxation times of uniformly (13)C-labeled residues show that the homogeneous line widths of the peptides are comparable to those of crystalline model compounds under identical (1)H decoupling and magic angle spinning conditions, indicating that the homogeneous line widths are determined by conformation-independent factors, including residual dipolar coupling, J-coupling, and intrinsic T(2) relaxation. However, the membrane peptides exhibit larger apparent line widths than the crystalline compounds, indicating conformational disorder. A cationic cell-penetrating peptide, the human immunodeficiency virus TAT, exhibits the largest apparent line widths, which are about five-fold larger than the homogeneous line widths, while the transmembrane helix of the influenza M2 peptide and the β-hairpin antimicrobial peptide PG-1 show moderately larger apparent line widths than the crystalline compounds. These results are consistent with the random coil nature of the TAT peptide, which contrasts with the intramolecularly hydrogen bonded M2 and PG-1. Cross peak line shapes of 2D double-quantum correlation spectra show that the conformational disorder can occur at the residue level and can result from three origins, lipid-peptide interaction, intrinsic conformational disorder encoded in the amino acid sequence, and side-chain rotameric averaging. A particularly important lipid-peptide interaction for cationic membrane peptides is guanidinium-phosphate ion pair interaction. Thus, NMR line widths and line shapes are useful for understanding the conformational disorder of membrane peptides and proteins.

Constraining 17O and 27Al NMR spectra of high-pressure crystals and glasses: New data for jadeite, pyrope, grossular, and mullite

The 17 O NMR spectra of glasses quenched from melts at high pressure are often difficult to interpret due to overlapping peaks and lack of crystalline model compounds. High-pressure aluminosilicate glasses often contain significant amounts of [5] Al and [6] Al, thus these high-pressure glasses must contain oxygen bonded to high-coordinated aluminum. The 17 O NMR parameters for the minerals jadeite, pyrope, grossular, and mullite are presented to assist interpretation of glass spectra and to help test quantum chemical calculations. The 17 O NMR parameters for jadeite and grossular support previous peak assignments of oxygen bonded to Si and high-coordinated Al in high-pressure glasses as well as quantum chemical calculations. The oxygen tricluster in mullite is very similar to the previously observed tricluster in grossite (CaAl 4 O 7 ) and suspected triclusters in glasses. We also present 27 Al NMR spectra for pyrope, grossular, and mullite.

Oxygen Sites and Network Coordination in Sodium Germanate Glasses and Crystals: High-Resolution Oxygen-17 and Sodium-23 NMR

Sodium germanate glasses are well-studied materials in which, unlike silicates but analogous to borates, the major structural consequence of alkali addition is generally thought to involve a coordination number increase of the network-forming Ge cations. However, the nature of this change, in particular quantifying fractions of nonbridging oxygens and of five- and/or six-coordinated Ge, has remained unresolved. We present here high-resolution 17O results, including triple-quantum MAS NMR (3QMAS), on a series of crystalline model compounds that allow the definition of ranges of chemical shifts corresponding to oxygens bonded to various coordinations of Ge. These include quartz- and rutile-structured GeO2, Na4Ge9O20, Na2Ge4O9, and Na2GeO3 (germanium dioxide, sodium enneagermanate, sodium tetragermanate, and sodium metagermanate). 3QMAS spectra of Na-germanate glasses ranging from 0% to 27% Na2O clearly show the development of partially resolved peaks as alkali is added, corresponding to signals from nonbridging oxygens (in the highest Na glasses) and to oxygen bridging between one four-coordinated and one higher coordinated Ge. As in conventional models of this system, nonbridging oxygen contents are much lower than in corresponding silicates. Although we do not directly distinguish between five- and six-coordinated Ge, modeling of bridging oxygen populations and comparison with measured speciation suggest that substantial proportions of both species are likely to be present. High-field 23Na MAS NMR shows systematic decreases in mean Na-O bond distance and/or coordination number with increasing alkali content that can be compared with published results for high-temperature liquids. These results, as well as comparison of molar volumes of glasses and high-temperature liquids, suggest the possibility of significant temperature effects on liquid structure.

Determination of the Oligomeric Number and Intermolecular Distances of Membrane Protein Assemblies by Anisotropic 1H-Driven Spin Diffusion NMR Spectroscopy

Determination of the high-resolution quaternary structure of oligomeric membrane proteins requires knowledge of both the oligomeric number and intermolecular distances. The centerband-only detection of exchange (CODEX) technique has been shown to enable the extraction of the oligomeric number through the equilibrium exchange intensity at long mixing times. To obtain quantitative distances, we now provide an analysis of the mixing-time-dependent CODEX intensities using the 1H-driven spin diffusion theory. The exchange curve is fit to a rate equation, where the rate constants are proportional to the square of the dipolar coupling and the spectral overlap integral between the exchanging spins. Using a number of 13C- and 19F-labeled crystalline model compounds with known intermolecular distances, we empirically determined the overlap integrals of 13C and 19F CODEX for specific spinning speeds and chemical shift anisotropies. These consensus overlap integral values can be applied to structurally unknown systems to determine distances. Applying the 19F CODEX experiment and analysis, we studied the transmembrane peptide of the M2 protein (M2TMP) of influenza A virus bound to 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine bilayers. The experiment proved for the first time that M2TMP associates as tetramers in lipid bilayers, similar to its oligomeric state in detergent micelles. Moreover, the nearest-neighbor interhelical F-F distance between (4-19F)Phe30 is 7.9-9.5 angstroms. This distance constrains the orientation and the packing of the helices in the tetrameric bundle and supports the structural model derived from previous solid-state NMR 15N orientational data. Thus, the CODEX technique presents a general method for determining the oligomeric number and intermolecular distances in the approximately 10 angstroms range in membrane proteins and other complex biological assemblies.

Effective dipolar couplings determined by dipolar dephasing of double-quantum coherences

It is shown how homonuclear distances and homonuclear dipolar lattice sums between spin-1/2 nuclei can be measured by a pulsed solid-state NMR experiment under magic-angle spinning conditions. The presented technique is based on double-quantum coherence filtering. Instead of measuring a build-up of double-quantum coherence the pulse sequence is designed to dephase double-quantum coherence. This is achieved by exciting double-quantum coherence either with the help of the through-space dipolar coupling or the through-bond dipolar coupling while the dephasing relies on the through-space dipolar coupling as selected by a γ-encoded pulse sequence from the C/ R symmetry class. Since dephasing curves can be normalized on zero dephasing, it is possible to analyze the initial dephasing regime and hence determine dipolar lattice sums (effective dipolar couplings) in multiple-spin systems. A formula for the effective dipolar coupling is derived theoretically and validated by numerical calculations and experiments on crystalline model compounds for 13C and 31P spin systems. The double-quantum dephasing experiment can be combined with constant-time data sampling to compensate for relaxation effects, consequently only two experimental data points are necessary for a single distance measurement. The phase cycling overhead for the constant-time experiment is minimal because a short cogwheel phase cycle exists. A 2D implementation is demonstrated on [ 13C 3]alanine.

Determination of Peptide Oligomerization in Lipid Bilayers Using 19F Spin Diffusion NMR

Aggregation or oligomerization is important for the function of many membrane peptides such as ion channels and antimicrobial peptides. However, direct proof of aggregation and the determination of the number of molecules in the aggregate have been difficult due to the lack of suitable high-resolution methods for membrane peptides. We propose a 19F spin diffusion magic-angle-spinning NMR technique to determine the oligomeric state of peptides bound to the lipid bilayer. Magnetization transfer between chemically equivalent but orientationally different 19F spins on different molecules reduces the 19F magnetization in an exchange experiment. At long mixing times, the equilibrium 19F magnetization is 1/M, where M is the number of orientationally different molecules in the aggregate. The use of the 19F spin increases the homonuclear dipolar coupling and thus the distance reach. We demonstrate this technique on crystalline model compounds with known numbers of molecules in the asymmetric unit cell, and show that 19F spin diffusion is more efficient than that of 13C by a factor of approximately 500. Application to a beta-hairpin antimicrobial peptide, protegrin-1, shows that the peptide is almost completely dimerized in POPC bilayers at a concentration of 7.4 mol %. Decreasing the peptide concentration reduced the dimer fraction. Using a monomer-dimer equilibrium model, we estimate the DeltaG for dimer formation to be -10.2 +/- 2.3 kJ/mol. This is in good agreement with the previously measured free energy reduction for partitioning and aggregating beta-sheet peptides into phospholipid membranes. This 19F spin diffusion technique opens the possibility of determining the oligomeric structures of membrane peptides.

The NMR response of boroxol rings: a density functional theory study

Cluster models of boron oxide glasses are studied computationally using density functional theory. It is shown that the isotropic chemical shielding of boron in boroxol rings is about 5 ppm less than for boron in non-ring BO 3 / 2 units, and that the quadrupole coupling in ring sites is about 0.1 MHz larger than in non-ring sites, confirming assignments made in glasses and crystalline model compounds. The chemical shielding anisotropy of these sites is computed and shown to be in agreement with recent experimental measurements. Furthermore, it is shown that the reason for the different responses is not the co-planarity of BO 3 / 2 groups bound in rings, but rather the contraction in the B–O–B bond angle from about 134 ∘ in relaxed structures to 120 ∘ as found in rings.

Investigation of Hydrothermal Routes to Mixed‐Metal Cerium Titanium Oxides and Metal Oxidation State Assignment Using XANES.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Investigation of hydrothermal routes to mixed-metal cerium titanium oxides and metal oxidation state assignment using XANES.

The reaction between TiF(3) or TiO(2) and Ce(3+) in sodium hydroxide solutions yields highly crystalline NaCeTi(2)O(6) at room temperature and under mild hydrothermal conditions (T < or = 240 degrees C). There is no evidence for the formation of ternary Ce-Ti-O materials by this method, and the use of bases other than NaOH always produces poorly crystalline materials. The material NaCeTi(2)O(6) has a distorted perovskite structure with sodium and cerium ions randomly occupying the A sites: Pnma, a = 5.4517(8) A, b = 7.7292(6) A, c =5.4573(3) A. XANES spectroscopy at the Ti K edge and Ce L(III) edge, with reference to crystalline model compounds, reveals that cerium is found solely as Ce(III) and titanium as Ti(IV) in NaCeTi(2)O(6). Isomorphous substitution of Ce(3+) by Nd(3+) or Ti(4+) by V(4+) is found to be very facile under hydrothermal conditions (at a temperature of 240 degrees C), by addition of appropriate amounts of metal salts to the hydrothermal reaction mixtures. The series NaCe(1-x)Nd(x)Ti2O6 (0 < or = x < or = 1) and NaCeTi(2-x)V(x)O6 (0 <or = x < or = 1.8) can be formed directly in one step, and all materials were characterized by powder X-ray diffraction. The oxidation states of the substituent elements were determined using XANES spectroscopy, which further demonstrates that isomorphous substitution has taken place, with all vanadium present as V(IV) when the B site is substituted. Scanning electron microscopy analysis of the solids shows them to be made up of irregularly shaped particles made up of clusters of submicron-sized crystallites but that on firing in excess of 1000 degrees C the particle size is increased to approximately 10 microm and the particles have distinct, sharp edges.

NaCu5(C2)3: The First Alkali Metal Transition Metal Acetylide with a Three‐dimensional Framework Structure

AbstractBy heating the red residue of the reaction of NaC2H and CuI in liquid ammonia at 463 K for 30 min in a dynamic vacuum orange polycrystalline NaCu5(C2)3 is obtained. The crystal structure was solved and refined from X‐ray powder diffraction data (orthorhombic, Pnma, a = 732.80(2) pm, b = 1099.63(4) pm, c = 726.15(2) pm, Z = 4, RB = 0.030). It consists of a three‐dimensional framework of CuI and C22— ions with small channels running parallel to [100] and [001]. The sodium ions reside at the intersections of these channels. The vibrational spectra of NaCu5(C2)3 show some similarities with the spectra of phenylethynylcopper(I), but they are different to those of Cu2C2. Therefore NaCu5(C2)3 does probably not represent a crystalline model compound for the unknown structure of amorphous Cu2C2.