Hydrogen Positions Research Articles

The effect of hydrogen injection parameters on the quality of hydrogen–air mixture formation for a PFI hydrogen internal combustion engine

To research the quality of the hydrogen–air mixture formation and the combustion characteristics of the hydrogen fueled engine under different hydrogen injection timings, nozzle hole positions and nozzle hole diameter, a three-dimensional simulation model for a PFI hydrogen internal combustion engine with the inlet, outlet, valves and cylinder was established using AVL Fire software. In the maximum torque condition, research focused on the variation law of the total hydrogen mass in the cylinder and inlet and the space distribution characteristics and variation law of velocity field, concentration field and turbulent kinetic energy under different hydrogen injection parameters (injection timings, nozzle hole positions and nozzle hole area) in order to reveal the influence of these parameters on hydrogen–air mixture formation process. Then the formation quality of hydrogen–air mixture was comprehensively evaluated according to the mixture uniformity coefficient, the remnant hydrogen percentage in the inlet and restraining abnormal combustion (such as preignition and backfire). The results showed that the three hydrogen injection parameters have important influence on the forming quality of hydrogen–air mixture and combustion state. The reasonable choice of the nozzle hole position of hydrogen, nozzle hole diameter and the hydrogen injection time can improve the uniformity of the hydrogen–air mixing in the cylinder of the hydrogen internal combustion engine, and the combustion heat release reaction is more reasonable. At the end of the compression stroke, the equivalence ratio uniform coefficient increased at first and then decreased with the beginning of the hydrogen injection. When hydrogen injection starting point was with 410–430°CA, equivalence ratio uniform coefficient was larger, and ignition delay period was shorter so that the combustion performance index was also good. And remnant hydrogen percentage in the inlet was less, high concentration of mixed gas in the vicinity of the inlet valve also gathered less, thus suppressing the preignition and backfire. With the increase of the distance between the nozzle and the inlet valve, the selection of the hydrogen injection period is narrowed, and the optimum hydrogen injection time was also ahead of time. The results also showed that it was favorable for the formation of uniform mixing gas when the nozzle hole diameter was 4 mm.

Predicting the Position of the Hydrogen Atom in the Short Intramolecular Hydrogen Bond of the Hydrogen Maleate Anion from Geometric Correlations

The position of the hydrogen atom inside the strong and short intramolecular hydrogen bond of the hydrogen maleate anion strongly varies depending on the crystalline environment. Therefore, it has not been possible in the past to accurately determine it using X-ray diffraction data although there are 292 hydrogen maleate crystal structures with different cations in the literature. In this study, a geometric correlation for the accurate prediction of the hydrogen position in the short intramolecular hydrogen bond is presented. The results used to derive the correlation are obtained from low-temperature neutron-diffraction studies on nine different hydrogen maleate salts that span the whole range from perfectly symmetric to highly asymmetric intramolecular hydrogen bonds. Since the only variable in the correlation as derived from the neutron data is the O···O distance, the hydrogen atom position in question can subsequently be predicted using information that is accurately available from routine X-ray data....

Preliminary neutron diffraction analysis of challenging human manganese superoxide dismutase crystals.

Superoxide dismutases (SODs) are enzymes that protect against oxidative stress by dismutation of superoxide into oxygen and hydrogen peroxide through cyclic reduction and oxidation of the active-site metal. The complete enzymatic mechanisms of SODs are unknown since data on the positions of hydrogen are limited. Here, methods are presented for large crystal growth and neutron data collection of human manganese SOD (MnSOD) using perdeuteration and the MaNDi beamline at Oak Ridge National Laboratory. The crystal from which the human MnSOD data set was obtained is the crystal with the largest unit-cell edge (240 Å) from which data have been collected via neutron diffraction to sufficient resolution (2.30 Å) where hydrogen positions can be observed.

Hydrogen Atomic Positions of O-H···O Hydrogen Bonds in Solution and in the Solid State: The Synergy of Quantum Chemical Calculations with ¹H-NMR Chemical Shifts and X-ray Diffraction Methods.

The exact knowledge of hydrogen atomic positions of O–H···O hydrogen bonds in solution and in the solid state has been a major challenge in structural and physical organic chemistry. The objective of this review article is to summarize recent developments in the refinement of labile hydrogen positions with the use of: (i) density functional theory (DFT) calculations after a structure has been determined by X-ray from single crystals or from powders; (ii) 1H-NMR chemical shifts as constraints in DFT calculations, and (iii) use of root-mean-square deviation between experimentally determined and DFT calculated 1H-NMR chemical shifts considering the great sensitivity of 1H-NMR shielding to hydrogen bonding properties.

Quest for hydrogen environments in hydrogen-bonded systems

We performed proton high-resolution nuclear magnetic resonance (NMR) measurements on KH2PO4 ferroelectrics. The 1H high-resolution NMR measurements were performed using the magic-angle spinning technique, which removes the proton-proton dipolar coupling and the chemical shift anisotropy in solid-state materials. High-resolution magic-angle spinning 1H NMR spectroscopy provides information on the chemical structure in the KH2PO4 system. From the measured chemical shift data, we determined the oxygen separation distance of the O-H···O hydrogen bond. The localized hydrogen position of the environments near heavy atoms can be depicted using Morse potentials, which provide the wave function of the eigenstate and the probability distribution of hydrogen in the local structure based on the solutions of the Schr¨odinger equation.

1H CSA parameters by ultrafast MAS NMR: Measurement and applications to structure refinement

A 1H anisotropic-isotropic chemical shift correlation experiment which employs symmetry-based recoupling sequences to reintroduce the chemical shift anisotropy in ν1 and ultrafast MAS to resolve 1H sites in ν2 is described. This experiment is used to measure 1H shift parameters for L-ascorbic acid, a compound with a relatively complex hydrogen-bonding network in the solid. The 1H CSAs of hydrogen-bonded sites with resolved isotropic shifts can be extracted directly from the recoupled lineshapes. In combination with DFT calculations, hydrogen positions in crystal structures obtained from X-ray and neutron diffraction are refined by comparison with simulations of the full two-dimensional NMR spectrum. The improved resolution afforded by the second dimension allows even unresolved hydrogen-bonded sites 1H to be assigned and their shift parameters to be obtained.

Structure, thermal expansion and incompressibility of MgSO4·9H2O, its relationship to meridianiite (MgSO4·11H2O) and possible natural occurrences

Since being discovered initially in mixed-cation systems, a method of forming end-member MgSO4·9H2O has been found. We have obtained powder diffraction data from protonated analogues (using X-rays) and deuterated analogues (using neutrons) of this compound over a range of temperatures and pressures. From these data we have determined the crystal structure, including all hydrogen positions, the thermal expansion over the range 9–260 K at ambient pressure, the incompressibility over the range 0–1.1 GPa at 240 K and studied the transitions to other stable and metastable phases. MgSO4·9D2O is monoclinic, space group P21/c, Z = 4, with unit-cell parameters at 9 K, a = 6.72764 (6), b = 11.91154 (9), c = 14.6424 (1) Å, β = 95.2046 (7)° and V = 1168.55 (1) Å3. The structure consists of two symmetry-inequivalent Mg(D2O)6 octahedra on sites of \bar 1 symmetry. These are directly joined by a water–water hydrogen bond to form chains of octahedra parallel with the b axis at a = 0. Three interstitial water molecules bridge the Mg(D2O)6 octahedra to the SO4 2− tetrahedral oxyanion. These tetrahedra sit at a ≃ 0.5 and are linked by two of the three interstitial water molecules in a pentagonal motif to form ribbons parallel with b. The temperature dependences of the lattice parameters from 9 to 260 K have been fitted with a modified Einstein oscillator model, which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion coefficient, αV, is substantially larger than that of either MgSO4·7D2O (epsomite) or MgSO4·11D2O (meridianiite), being ∼ 110 × 10−6 K−1 at 240 K. Fitting to a Murnaghan integrated linear equation of state gave a zero-pressure bulk modulus for MgSO4·9D2O at 240 K, K 0 = 19.5 (3) GPa, with the first pressure derivative of the bulk modulus, K′ = 3.8 (4). The bulk modulus is virtually identical to meridianiite and only ∼ 14% smaller than that of epsomite. Above ∼ 1 GPa at 240 K the bulk modulus begins to decrease with pressure; this elastic softening may indicate a phase transition at a pressure above ∼ 2 GPa. Synthesis of MgSO4·9H2O from cation-pure aqueous solutions requires quench-freezing of small droplets, a situation that may be relevant to spraying of MgSO4-rich cryomagmas into the surface environments of icy satellites in the outer solar system. However, serendipitously, we obtained a mixture of MgSO4·9H2O, mirabilite (Na2SO4·10H2O) and ice by simply leaving a bottle of mid-winter brine from Spotted Lake (Mg/Na ratio = 3), British Columbia, in a domestic freezer for a few hours. This suggests that MgSO4·9H2O can occur naturally – albeit on a transient basis – in certain terrestrial and extraterrestrial environments.

Hydrogen positions in single nanocrystals revealed by electron diffraction.

The localization of hydrogen atoms is an essential part of crystal structure analysis, but it is difficult because of their small scattering power. We report the direct localization of hydrogen atoms in nanocrystalline materials, achieved using the recently developed approach of dynamical refinement of precession electron diffraction tomography data. We used this method to locate hydrogen atoms in both an organic (paracetamol) and an inorganic (framework cobalt aluminophosphate) material. The results demonstrate that the technique can reliably reveal fine structural details, including the positions of hydrogen atoms in single crystals with micro- to nanosized dimensions.

Refinement of labile hydrogen positions based on DFT calculations of 1H NMR chemical shifts: comparison with X-ray and neutron diffraction methods.

Numerous gas phase electron diffraction, ultra-fast electron diffraction, X-ray and neutron diffraction experiments on β-dicarbonyl compounds exhibiting enol-enol tautomeric equilibrium, with emphasis on acetylacetone and dibenzoylmethane, have so far been reported with conflicting results on the structural details of the O-HO intramolecular hydrogen bond and resulted in alternative hypotheses on the intramolecular hydrogen bond potential function either a double minimum potential corresponding to two tautomeric forms in equilibrium or a single symmetrical one. We demonstrate herein, firstly, that the DFT calculated OH 1H NMR chemical shifts of acetylacetone and dibenzoylmethane exhibit a strong linear dependence on the computed OO hydrogen bond length of ∼-50 ppm Å-1 and as a function of the O-HO bond angle of ∼1 ppm per degree, upon the transfer of the hydrogen atom from the ground state toward the transition state. Secondly, the refinement of labile hydrogen atomic positions in intramolecular hydrogen bonds based on the root-mean-square deviation between experimentally determined and DFT calculated 1H NMR chemical shifts in solution can provide high resolution structures of O-H and O(H)O bond lengths and O-HO bond angles with an accuracy of ∼10-2 Å and ∼0.5°, respectively. Thirdly, the calculated 1H NMR chemical shifts in solution of the two ground state tautomers in equilibrium of acetylacetone and dibenzoylmethane are in excellent agreement with the experimental value, even for moderate basis sets for energy minimization. In contrast, the single symmetrical structure in a strongly delocalized system is a transition state with calculated 1H NMR chemical shifts which strongly deviate from the experimental value. Fourth, the DFT calculated ground state O-H bond lengths of acetylacetone and dibenzoylmethane are in quantitative agreement with the literature data which take into account the effect of quantum nuclear motion. The DFT structural results are critically discussed with respect to the state-of-the-art variable temperature X-ray and neutron diffraction methods.

Crystal structure of cronstedtite-6T 2, a non-MDO polytype

The crystal structure of cronstedtite-6 T 2 was refined using a crystal from the new occurrence in Pohled, Czech Republic. The space group is P 3 1 , lattice parameters a = 5.4976(3), c = 42.601(1) A, Z = 6, composition (Fe 2+ 2.515 Fe 3+ 0.485 ) (Si 1.515 Fe 3+ 0.485 ) O 5 (OH) 4 , R obs = 4.13% for 3244 independent reflections. The separate R obs values 2.50% and 6.96% were obtained for 1033 subfamily and 2196 characteristic reflections, respectively. The structure is built of edge-sharing octahedral ( Oc ), and corner-sharing tetrahedra l ( Tet ) sheets forming the 1:1 layers by sharing apical corners of Tet sheet. There are two independent 1:1 layers, where the odd one is shifted with respect of the even one by −( a 1 + a 2 )/3 and raised by c /6 of the hexagonal cell. The sextuple multiplicity is achieved by mapping this pair of layers by 3 1 axis repeatedly to two other equivalent positions raised by c /3, 2 c /3. The polytype belongs to the subfamily (Bailey’s group) A. There are two tetrahedral and three octahedral sites per 1:1 layer ( T 1, T 2, M 1, M 2, M 3 in even layers, T 11, T 12, M 11, M 12, M 13 in odd layers), all in general positions. The M 3, M 13 octahedra are smaller than M 1, M 2, M 11, M 12, thus Oc sheets in both layers are meso-octahedral . In even layers, however, the M 2 octahedron is somewhat smaller than M 1, so the Oc sheet is “transitional” to a hetero-octahedral character. The occupancies of Si:Fe in T positions were refined to: T 1, 0.96:0.04(1); T 2, 0.63:0.37(1); T 11, 0.55:0.45(1); T 12, 0.89:0.11(1). Both rings of tetrahedra are strongly ditrigonalized, α values are +11.4(5)°, and +10.9(5)°, in even and odd layers, respectively. The Franzini type is A. Four hydrogen positions were localized in each layer (eight altogether). Three of them per layer (six altogether) are involved in hydrogen bonds linking the OH groups (donors) of the Oc sheet with basal oxygen atoms of the Tet sheet of the adjacent 1:1 layer. The structure is an example of OD structure of more than one kind of layers with a very low degree of desymmetrization. Cronstedtite-6 T 2 is a non-MDO polytype, because it contains more than one kind of layer triplets. Comparison of stacking sequences with other polytypes of the A subfamily is presented.

Hydrogen behavior in SAF 2205 duplex stainless steel

This paper describes austenitic-ferritic duplex stainless steels, SAF 2205, in the presence of hydrogen. The duplex stainless steels (DSS) properties include excellent resistance to stress corrosion cracking, high strength and good weldability. Those steels are preferably used in industries combining hydrogen and loads. Hydrogen location in addition to hydrogen binding energy with the steel's defects are of great importance for the analysis of hydrogen embrittlement model in that steel. It is known from previously published works that the susceptibility to hydrogen embrittlement will depend on the competition between reversible and irreversible traps; meaning a direct relation to the hydrogen's state and position in the steel. In this work, we examine the local hydrogen concentration, trapping and distribution by two modern and advanced techniques: thermal desorption spectrometry (TDS) and we support it by time of flight-secondary ion mass spectrometer (ToF-SIMS). In this paper, we support and give for the first time new insights and better understanding to the hydrogen embrittlement mechanism in SAF 2205.The trapping energies levels were calculated using TDS and Lee and Lee's model. This model revealed reversible in addition to irreversible trapping sites. Also the trapping controlling mechanism was found to be a combination of detrapping controlled mechanism and diffusion controlled mechanism. The use of ToF-SIMS for local imaging the distribution of hydrogen species supports the discussion of the different hydrogen traps in this type of steel. The hydrogen embrittlemet phenomenon in SAF 2205 will be discussed in details in that paper.

Use of Quantum Theory of Atoms in Molecules in the Search for Appropriate Hydrogen Atom Locations in X-ray Diffraction Based Studies

A novel approach for the determination of hydrogen positions in studies based on X-ray diffraction is presented. The comparison of proton positions with their corresponding electron density maxima has been performed by means of the Quantum Theory of Atoms in Molecules. The observed differences mainly depend on the type and electronegativity of the atom covalently bonded to the hydrogen, changing in the range of Δr = 0.004 ÷ 0.032 A, while being simultaneously independent from the involvement of atom in the hydrogen bonding. It is postulated that the Δr correction should be taken into account in electron density based studies. In theoretical single point calculations (sp) based on X-ray diffraction analysis, an elongation of hydrogen covalent bonds with Δr corrections instead of neutronographic values is proposed. This procedure utilized in this work for the analysis of intramolecular O–H···O hydrogen bonding properties leads to good agreement between quantum theoretical results for the optimized structure...

1H ΝΜR chemical shift assignment, structure and conformational elucidation of hypericin with the use of DFT calculations – The challenge of accurate positions of labile hydrogens

Hypericin is one of the principal active constituents of Hypericium (Saint John's wort). We present in this paper: (i) chemical shift assignment of the neutral and ionic form of hypericin based on DFT 1H NMR calculations, and conflicting literature experimental OH 1H NMR chemical shifts are critically evaluated; (ii) investigation of the cooperative nature of intra- and intermolecular hydrogen bonding of the bay OH groups in the neutral form of hypericin in acetone and DMSO; (iii) demonstration of a very strong intramolecular hydrogen bonding of the deprotonated bay OH…O- moiety in DMSO and (iv) excellent linear correlation between experimental and computed 1H chemicals shifts for a variety of basis sets. Comparison of the literature X-ray structures with those obtained with DFT calculations clearly demonstrates the advantages of computation in investigating resonance assignments, ionization state, accurate labile hydrogen positions, and structural and conformational properties of natural products at an atomic level.

Understanding and modeling the hydrogen-abstraction from dimethyl ether by the methyl radical with torsional anharmonicity

Hybrid density functionals M06-2X and BMK in combination with the MG3S basis set, along with double-hybrid density functional B2PLYP with the TZVP basis set, were employed to better characterize the hydrogen atom transferring from dimethyl ether (DME) to the methyl radical. The distinct orientations of hydrogen atoms (in-plane and out-of-plane ones) in the DME molecule were taken into account. Density functional energetics was validated by comparison with the high-level CBS-QB3, G4, and G4MP2 calculations. Kinetic calculations were performed by means of the conventional transition-state theory (TST), canonical variational TST (CVT), and improved CVT (ICVT) over a wide temperature range 200–2600K to clarify the available experimental measurements, and tunneling effect and anharmonic torsion were also included. Two transition structures are located with hydrogen-abstraction occurring at the in-plane and out-of-plane hydrogen positions, which are in fact related via the relative motion of the CH3⋯H⋯CH2 and OCH3 moieties about the CO bond. Based on a single reaction pathway, the M06-2X/MG3SICVT rate constants with multidimensional small-curvature tunneling correction and proper treatment of anharmonic torsions compare well with most of the available experimental data. Variational effects on the computed rate constants are found to be negligibly small. Activation energies for the CH3+DME reaction increase substantially with temperature, and exhibit a nonlinear dependence on the temperature. Therefore, rate constants are fitted to the four-parameter expression k(T)=8.59×10-16(T/300)5.11exp[-19.65(T+140.90)/R(T2+140.902)] cm3molecule−1s−1 over the broad temperature range.

Combined effects of obstacle position and equivalence ratio on overpressure of premixed hydrogen–air explosion

To investigate the combined effects of obstacle position and equivalence ratio on overpressure of premixed hydrogen–air explosion, an experimental study was performed in a 5 L duct with premixed hydrogen–air mixtures over a wide range of equivalence ratios, and along with the variation of a single obstacle position. In this paper, the equivalence ratios of the hydrogen–air mixtures were varied from 0.6 to 1.4, the obstacle was centrally located and was respectively 100, 200, 300 mm from the bottom end of the duct, the experiment without an obstacle was designed as a control experiment. For brevity, four configurations were defined according to the variation of the single obstacle position. The results indicated that the overpressure of premixed hydrogen–air explosion was closely related to the flame structure. The rise of dp/dt for hydrogen–air mixtures occurred when the flame started to feel the presence of the obstacle. The maximum overpressure could be observed at the moment that the two flames, generated from the gap between the obstacle and the inwall of the duct, just started to merge in the downstream region of the obstacle. It was also found that the venting overpressure might be barely observed under the combined effects of obstacle position and equivalence ratio. The rise time of dp/dt in a given configuration was gradually shortened with increasing equivalence ratio. Additionally, it was not simply a synergistic effect of obstacle position and equivalence ratio on peak overpressure. The peak overpressure increased with increasing obstacle position for lean hydrogen–air mixtures, and the maximum peak overpressure occurred in the downstream region of the farthest obstacle position. Interestingly, for the stoichiometric and rich mixtures, the peak overpressure reached the maximal when the obstacle was at the middle position. The occurrence of the maximum peak overpressure mainly depended on the maximum flame surface area within the duct.

Electronic structure of interstitial hydrogen in lutetium oxide fromDFT+Ucalculations and comparison study withμSRspectroscopy

The electronic structure of hydrogen impurity in ${\mathrm{Lu}}_{2}{\mathrm{O}}_{3}$ was studied by first-principles calculations and muonium spectroscopy. The computational scheme was based on two methods which are well suited to treat defect calculations in $f$-electron systems: first, a semilocal functional of conventional density-functional theory (DFT) and secondly a $\text{DFT}+U$ approach which accounts for the on-site correlation of the $4f$ electrons via an effective Hubbard-type interaction. Three different types of stable configurations were found for hydrogen depending upon its charge state. In its negatively charged and neutral states, hydrogen favors interstitial configurations residing either at the unoccupied sites of the oxygen sublattice or at the empty cube centers surrounded by the lanthanide ions. In contrast, the positively charged state stabilized only as a bond configuration, where hydrogen binds to oxygen ions. Overall, the results between the two methods agree in the ordering of the formation energies of the different impurity configurations, though within $\text{DFT}+U$ the charge-transition (electrical) levels are found at Fermi-level positions with higher energies. Both methods predict that hydrogen is an amphoteric defect in ${\mathrm{Lu}}_{2}{\mathrm{O}}_{3}$ if the lowest-energy configurations are used to obtain the charge-transition, thermodynamic levels. The calculations of hyperfine constants for the neutral interstitial configurations show a predominantly isotropic hyperfine interaction with two distinct values of 926 MHz and 1061 MHz for the Fermi-contact term originating from the two corresponding interstitial positions of hydrogen in the lattice. These high values are consistent with the muonium spectroscopy measurements which also reveal a strongly isotropic hyperfine signature for the neutral muonium fraction with a magnitude slightly larger (1130 MHz) from the ab initio results (after scaling with the magnetic moments of the respective nuclei).

Partially ordered state of ice XV.

Most ice polymorphs have order–disorder “pairs” in terms of hydrogen positions, which contributes to the rich variety of ice polymorphs; in fact, three recently discovered polymorphs— ices XIII, XIV, and XV—are ordered counter forms to already identified disordered phases. Despite the considerable effort to understand order–disorder transition in ice crystals, there is an inconsistency among the various experiments and calculations for ice XV, the ordered counter form of ice VI, i.e., neutron diffraction observations suggest antiferroelectrically ordered structures, which disagree with dielectric measurement and theoretical studies, implying ferroelectrically ordered structures. Here we investigate in-situ neutron diffraction measurements and density functional theory calculations to revisit the structure and stability of ice XV. We find that none of the completely ordered configurations are particular favored; instead, partially ordered states are established as a mixture of ordered domains in disordered ice VI. This scenario in which several kinds of ordered configuration coexist dispels the contradictions in previous studies. It means that the order–disorder pairs in ice polymorphs are not one-to-one correspondent pairs but rather have one-to-n correspondence, where there are n possible configurations at finite temperature.

On the Crystal-Chemistry of Bjarebyite, BaMn2+2Al2(PO4)3(OH)3, From the Palermo #1 Pegmatite, Grafton County, New Hampshire, Usa

Abstract Bjarebyite, BaMn 2+ 2 Al 2 (PO 4 ) 3 (OH) 3 , is a rare secondary phosphate mineral in granitic pegmatites. From the crystallographic point of view it is exhaustively described in the monoclinic unit cell with a 9.0671(2) A, b 12.1500(4) A, c 4.9403(1) A, β 100.399(2)°, V 535.31(2) A 3 , and space group P 2 1 / m . Single crystal X-ray diffraction data were collected with Mo K α radiation. The hydrogen positions were located by difference Fourier map and refined in the final steps of the crystal structure anisotropic refinement, leading to an R index of 1.99%. A very good agreement was found with microprobe chemical analyses and bond valence calculations which show expected values for all the atomic sites. These features were confirmed by comparing the Raman spectra of bjarebyite and kulanite.

Solution (31)P NMR Study of the Acid-Catalyzed Formation of a Highly Charged {U24Pp12} Nanocluster, [(UO2)24(O2)24(P2O7)12](48-), and Its Structural Characterization in the Solid State Using Single-Crystal Neutron Diffraction.

The first neutron diffraction study of a single crystal containing uranyl peroxide nanoclusters is reported for pyrophosphate-functionalized Na44K6[(UO2)24(O2)24(P2O7)12][IO3]2·140H2O (1). Relative to earlier X-ray studies, neutron diffraction provides superior information concerning the positions of H atoms and lighter counterions. Hydrogen positions have been assigned and reveal an extensive network of H-bonds; notably, most O atoms present in the anionic cluster accept H-bonds from surrounding H2O molecules, and none of the surface-bound O atoms are protonated. The D4h symmetry of the cage is consistent with the presence of six encapsulated K cations, which appear to stabilize the lower symmetry variant of this cluster. (31)P NMR measurements demonstrate retention of this symmetry in solution, while in situ (31)P NMR studies suggest an acid-catalyzed mechanism for the assembly of 1 across a wide range of pH values.

Ammonothermal Synthesis, Crystal Structure, and Properties of the Ytterbium(II) and Ytterbium(III) Amides and the First Two Rare-Earth-Metal Guanidinates, YbC(NH)3 and Yb(CN3H4)3.

We report the oxidation-controlled synthesis of the ytterbium amides Yb(NH2)2 and Yb(NH2)3 and the first rare-earth-metal guanidinates YbC(NH)3 and Yb(CN3H4)3 from liquid ammonia. For Yb(NH2)2, we present experimental atomic displacement parameters from powder X-ray diffraction (PXRD) and density functional theory (DFT)-derived hydrogen positions for the first time. For Yb(NH2)3, the indexing proposal based on PXRD arrives at R3̅, a = 6.2477(2) Å, c = 17.132(1) Å, V = 579.15(4) Å(3), and Z = 6. The oxidation-controlled synthesis was also applied to make the first rare-earth guanidinates, namely, the doubly deprotonated YbC(NH)3 and the singly deprotonated Yb(CN3H4)3. YbC(NH)3 is isostructural with SrC(NH)3, as derived from PXRD (P63/m, a = 5.2596(2) Å, c = 6.6704(2) Å, V = 159.81(1) Å(3), and Z = 2). Yb(CN3H4)3 crystallizes in a structure derived from the [ReO3] type, as studied by powder neutron diffraction (Pn3̅, a = 13.5307(3) Å, V = 2477.22(8) Å(3), and Z = 8 at 10 K). Electrostatic and hydrogen-bonding interactions cooperate to stabilize the structure with wide and empty channels. The IR spectra of the guanidinates are compared with DFT-calculated phonon spectra to identify the vibrational modes. SQUID magnetometry shows that Yb(CN3H4)3 is a paramagnet with isolated Yb(3+) (4f(13)) ions. A CONDON 2.0 fit was used to extract all relevant parameters.