Hydrogen Bonds Of Moderate Strength Research Articles

Interaction of Phospholipid, Cholesterol, Beta-Carotene, and Vitamin C Molecules in Liposome-Based Drug Delivery Systems: An In Silico Study.

This paper investigates the interaction within a liposome-based drug delivery system in silico. Results confirmed that phospholipids, cholesterol, beta-carotene, and vitamin C in the liposome structures interact noncovalently. The formation of noncovalent interactions indicates that the liposomal structures from phospholipid molecules will not result in chemical changes to the drug or any molecules encapsulated within. Noncovalent interactions formed include (i) moderate-strength hydrogen bonds with interaction energies ranging from -73.6434 kJ·mol-1 to -45.6734 kJ·mol-1 and bond lengths ranging from 1.731 Å to 1.827 Å and (ii) van der Waals interactions (induced dipole-induced dipole and induced dipole-dipole interactions) with interaction energies ranging from -4.4735 kJ·mol-1 to -1.5840 kJ·mol-1 and bond lengths ranging from 3.192 Å to 3.742 Å. The studies for several phospholipids with short hydrocarbon chains show that changes in chain length have almost no effect on interaction energy, bond length, and partial atomic charge.

Isostructural Crystals of Bis(Guanidinium) Trioxofluoro-Phosphate/Phosphite in the Ratio 1/0, 0.716/0.284, 0.501/0.499, 0.268/0.732, 0/1—Crystal Structures, Vibrational Spectra and Second Harmonic Generation

The title structures of bis(guanidinium) trioxofluorophosphate, bis(guanidinium) trioxofluorophosphate-phosphite (0.716/0.284), bis(guanidinium) trioxofluorophosphate-phosphite (0.501/0.499), bis(guanidinium) trioxofluorophosphate-phosphite (0.268/0.732), and bis(guanidinium) phosphite are crystal-chemically isotypic. Their structures correspond to the structure of bis(guanidinium) trioxofluorophosphate which was determined by Prescott, Troyanov, Feist & Kemnitz (Z. Anorg. Allg. Chem. 2002, 628, 1749–1755). The P and O atoms of the substituted trioxofluorophosphate and phosphite anions share the same positions while the P-F and P-Hhydrido are almost parallel and oriented in the same direction. Two symmetry-independent anions and two of three symmetry-independent cations are situated on the crystallographic mirror planes. The ions are interconnected by N-H⋯O hydrogen bonds of moderate strength. The most frequent graph set motif is R22(8), which involves interactions between the primary amine groups and the trioxofluorophosphate or phosphite O atoms. Fluorine, as well as the hydrido hydrogen, avoids inclusion into the hydrogen-bond network. The Hirshfeld surface analysis was also performed for the comparison of intermolecular interactions in the title structures of bis(guanidinium trioxofluorophosphate and bis(guanidinium) phosphite. The title crystals were also characterized by vibrational spectroscopy methods (FTIR and Raman) and the second harmonic generation (SHG). The relative SHG efficiency considerably decreases from bis(guanidinium) trioxofluorophosphate to bis(guanidinium) phosphite for the fundamental 1064 nm laser line.

Crystal structures of catena-poly[[μ-aqua-di-aqua(μ3-2-methyl-propano-ato-κ4 O:O,O':O')calcium] 2-methyl-propano-ate dihydrate], catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O:O,O':O')strontium] 2-methyl-propano-ate dihydrate] and catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O:O,O':O')(calcium/strontium)] 2-methyl-propano-ate dihydrate

The crystal structures of catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O:O,O':O')calcium] 2-methyl-propano-ate dihydrate], {[Ca(C4H7O2)(H2O)3](C4H7O2)·2H2O} n , (I), catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O:O,O':O')strontium] 2-methyl-propano-ate dihydrate], {[Sr(C4H7O2)(H2O)3](C4H7O2)·2H2O} n , (II), and catena-poly[[μ-aqua-di-aqua-(μ3-2-methyl-propano-ato-κ4 O:O,O':O')(calcium/strontium)] 2-methyl-propano-ate dihydrate], {[(Ca,Sr)(C4H7O2)(H2O)3](C4H7O2)·2H2O} n , (III), are related. (III) can be considered as an Sr-containing solid solution of (I), with Ca2+ and Sr2+ occupationally disordered in the ratio 0.7936 (16):0.2064 (16). (I)/(III) and (II) are homeotypic with different space groups of Pbca and Cmce, respectively. All the title crystal structures are composed of hydro-philic sheets containing the cations, carboxyl-ate groups as well as water mol-ecules. The hydro-phobic layers, which consist of 2-methyl-propano-ate chains, surround the hydro-philic sheets from both sides, thus forming a sandwich-like structure extending parallel to (001). The cohesion forces within these sheets are the cation-oxygen bonds and O-H⋯O hydrogen bonds of moderate strength. Stacking of these sandwiches along [001] is consolidated by van der Waals forces. The structures contain columns defined by the cation-oxygen inter-actions in which just one symmetry-independent 2-methyl-propano-ate anion is included, together with three water mol-ecules. These mol-ecules participate in an irregular coordination polyhedron composed of eight O atoms around the cation. Additional water mol-ecules as well as the second 2-methyl-propano-ate anion are not part of the coordination sphere. These mol-ecules are connected to the above-mentioned columns by O-H⋯O hydrogen bonds of moderate strength. In (II), the Sr2+ cation, two of the coordinating water mol-ecules and both anions are situated on a mirror plane with a concomitant positional disorder of the 2-methyl-propyl groups; the non-coordinating water mol-ecule also shows positional disorder of its hydrogen atom.

Hydrogen Bond-Mediated Conjugates Involving Lanthanide Diphthalocyanines and Trifluoroacetic Acid (Lnpc2@TFA): Structure, Photoactivity, and Stability.

The interaction between lanthanide diphthalocyanine complexes, LnPc2 (Ln = Nd, Sm, Eu, Gd, Yb, Lu; Pc = C32H16N8, phthalocyanine ligand) and trifluoroacetic acid (TFA) was investigated in benzene, and the stability of the resulting molecular system was assessed based on spectral (UV-Vis) and kinetic measurements. Structural Density Functional Theory (DFT) calculations provided interesting data regarding the nature of the bonding and allowed estimating the interaction energy between the LnPc2 and TFA species. Conjugates are created between the LnPc2 and TFA molecules via hydrogen bonds of moderate strength (>N∙∙H··) at the meso- -bridges of the Pc moieties, which renders the sandwich system to flatten. Attachment of TFA is followed by rearrangement of electronic density within the chromophore system of the macrocycles manifested in considerable changes in their UV-Vis spectra and consequently the color of the studied solutions (from green to orange). The LnPc2@TFA conjugates including Nd, Sm, Eu, and Gd appeared evidently less photostable when exposed to UV radiation than the related mother compounds, whereas in the case of Yb and Lu derivatives some TFA-prompted stabilizing effect was noticed. The conjugates displayed the capacity for singlet oxygen generation in contrast to the LnPc2s itself. Photon upconversion through sensitized triplet–triplet annihilation was demonstrated by the TFA conjugates of Nd, Sm, Eu, and Gd.

Caesium propano-ate monohydrate.

Caesium propano-ate monohydrate, Cs+·C3H5O2 -·H2O, is composed of two symmetry-independent Cs+ cations, which are situated on the special position 4e of space group P 21 m, one symmetry-independent propano-ate mol-ecule in a general position and a pair of water mol-ecules also situated on special position 4e. Two pairs of these symmetry-independent cations, four propano-ate mol-ecules and two pairs of symmetry-independent water mol-ecules form a repeat unit. These units form columns that are directed along the c axis and possess symmetry mm2. There are four such columns passing through each unit cell. Each column is inter-connected to its neighbours by four bifurcated three-centred Ow-H⋯Op (w = water, p = propano-ate) hydrogen bonds of moderate strength. There are also four intra-molecular Ow-H⋯Op hydrogen bonds of moderate strength within each column. One Cs+ cation is coordinated by six oxygen atoms (two water and four carboxyl-ate) in a trigonal-prismatic geometry, while the other Cs+ cation is coordinated by four water and four carboxyl-ate O atoms in a tetra-gonal-prismatic arrangement.

Nuclear dynamics in BaZr0.7Ce0.2Y0.1O3−δ proton conductor as observed by neutron diffraction and Compton scattering

Concurrent neutron Compton scattering (NCS) and neutron diffraction experiments at temperatures between 70 K and 300 K have been performed on proton-conducting hydrated BaZr0.7Ce0.2Y0.1O3−δ (BZCY72) fabricated by spark plasma sintering. A combined neutron data analysis, augmented with density functional theory modelling of lattice dynamics, has enabled, for the first time, a mass-selective appraisal of the combined thermal and nuclear quantum effect on nuclear dynamics and thermodynamic stability of this technologically important proton conducting perovskite oxide. The analysis suggests that the nuclear dynamics in hydrated BZCY72 is a result of a subtle interplay of harmonic, anharmonic and thermal effects, with the increased anharmonic character of the lattice dynamics above the orthorhombic to rhombohedral phase transition at 85 K. The anharmonic effect seems to be most pronounced in the case of oxygen and cerium. The analysis of the proton momentum distribution reveals that the concentration of the hydrogen in the BZCY72 lattice is constant across the orthorhombic to rhombohedral phase transition and further down to the room temperature. Moreover, the average hydrogen concentration obtained from our analysis of the mass-resolved neutron Compton scattering data seems to be commensurate with the total vacancy concentration in the BZCY72 framework. The calculation of the vibrational enthalpy of both phases allows obtaining the value of the enthalpy of the orthorhombic to the rhombohedral phase transition of −3.1 ± 1 kJ mol−1. Finally, our analysis of the nuclear kinetic energy of the proton obtained from NCS and the oxygen-oxygen distance distributions obtained from ND allows to conclude that BZCY72 in both the orthorhombic and rhombohedral phase at 70 K and 100 K respectively falls into the category of the KDP-type crystals where proton is probably under the influence of a double-well potential and forms hydrogen bonds of moderate strength. The obtained results have important ramifications for this technological important material.

Poly[[tetra-deca-kis-(μ-propionato)hepta-barium] propionic acid monosolvate tetra-hydrate

The title compound, {[Ba7(C3H5O2)14]·0.946C3H6O2·4H2O} n , is represented by a metal-organic framework structure that is held together by Ba-O-Ba bonds, as well as by O-H⋯O hydrogen bonds of moderate strength. The structure comprises of four independent Ba2+ cations (one of which is situated on a twofold rotation axis), seven independent propionate and two independent water mol-ecules. The bond-valence sums of all the cations indicate a slight overbonding. There is also an occupationally, as well as a positionally disordered propionic acid mol-ecule present in the structure. Its occupation is slightly lower than the full occupation while the disordered mol-ecules occupy two positions related by a rotation about a twofold rotation axis. In addition, the methyl group in the symmetry-independent propionic acid mol-ecule is also disordered, and occupies two positions. Each propionic acid mol-ecule coordinates to just one cation from a pair of symmetry-equivalent Ba2+ sites and is simultaneously bonded by an O-H⋯Opropionate hydrogen bond. This means that on a microscopic scale, the coordination number of the corresponding Ba2+ site is either 9 or 10. The methyl as well as hy-droxy hydrogen atoms of the disordered propionic acid mol-ecule were not determined.

Crystal structure of strontium thio-sulfate monohydrate.

SrS2O3·H2O was obtained from an aqueous solution of Na2S2O3 and Sr(NO3)2 and crystallizes in space group P with all atoms at general positions. The Sr2+ ion exhibits an [8+1] coordination defined by two terminal S and six O atoms of thio-sulfate ions, one of the latter at a longer distance, and by one O atom of a water mol-ecule. Two thio-sulfate anions act as bidentate, four as monodentate ligands. The structure consists of mainly ionically inter-acting layers lying parallel to the crystallographic ab plane. The layers are connected by O-H⋯S and O-H⋯O hydrogen bonds of moderate strength.

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study.

The uranyl(vi) 'Pacman' complex [(UO2)(py)(H2L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp2Ti(η2-Me3SiC[triple bond, length as m-dash]CSiMe3) and [Cp2TiCl]2 to oxo-titanated uranyl(v) complexes [(py)(Cp2TiIIIOUO)(py)(H2L)] 1 and [(ClCp2TiIVOUO)(py)(H2L)] 2. Combination of ZrII and ZrIV synthons with A yields the first ZrIV-uranyl(v) complex, [(ClCp2ZrOUO)(py)(H2L)] 3. Similarly, combinations of Ae0 and AeII synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)2(ClMgOUO)(py)(H2L)] 4, [(py)2(thf)2(ICaOUO)(py) (H2L)] 5; the zinc complexes [(py)2(XZnOUO)(py)(H2L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)3(RbOUO)(py)(H2L)] 8 and hexameric [(MOUO)(py)(H2L)]6 (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N-H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M4 edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the UV-O and O-M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The Oexo-Zn bonds have a larger covalent contribution compared to the Mg-Oexo/Ca-Oexo bonds, and more covalency is found in the U-Oexo bond in 7 (ZnI), in agreement with the calculations.

Pyridine-3-carboxamide-telluric acid (1/1).

In the title structure, C6H6N2O·H6O6Te, the pyridine-3-carboxamide and telluric acid mol-ecules are inter-connected by conventional O-H⋯N, N-H⋯O and O-H⋯O hydrogen bonds of moderate strength as well as by π-π inter-actions between the pyridine rings. The strongest hydrogen bond in the structure is formed between a hydroxyl group of the H6TeO6 mol-ecule and the N-pyrimidine N atom. The structure is unusual because of presence of the alternating sheets, which contain H6TeO6 and pyridine-3-carboxamide mol-ecules, respectively. These sheets are aligned parallel to (001).

Migrating hydrogen in 2,4,6-triaminopyrimidinium(1+) x hydrogen trioxofluorophosphate(−) x monohydrate/2,4,6-triaminopyrimidinium(2+)1–x trioxofluorophosphate(2–)1–x monohydrate (0.0 < x < 0.73) with changing temperature

The data of the title structure of 2,4,6-triaminopyrimidinium(1+) x hydrogen trioxofluorophosphate(1−) x monohydrate/2,4,6-triaminopyrimidinium(2+)1−x trioxofluorophosphate(2−)1−x monohydrate (0.0 < x < 0.73), with the sum formula C4H11FN5O4P, were collected in 10 K intervals between 290 and 150 K, as well as at 100 K. The most interesting feature is the presence of a moderate though still strong O—H...N/O...H—N hydrogen bond. Its bridging H atom was found to be disordered over two positions in the temperature interval 290–220 K, with the larger occupation towards the O atom. The occupation in the latter position decreased with decreasing temperature, though not monotonously. At 210 K and below, the difference electron-density maps showed just one maximum which was closer to the N atom. Though difference scanning calorimetry (DSC) revealed no anomaly in the interval 303–93 K, the dependence of the length of the unit-cell axes b and c on temperature showed a kink in the slope at about 190 K. The N...O distance of the mentioned hydrogen bond or the occupational parameter of the electron density of the disordered H atom indicate less pronounced anomalies for the temperatures 270 and 230 K. The cation and the anion, as well as the water molecules, form layers which are parallel to (10\overline{1}) and which are interconnected by O—H...N (in the interval 290–220 K), N—H...O and O—H...O hydrogen bonds of moderate strength. The layers are arranged into centrosymmetric double layers in which the F atoms are directed outwards. The layers are interconnected by water–acid O—H...O hydrogen bonds, weak N—H...F interactions and π-electron ring...π-electron ring interactions. The dependence of P—F bond lengths on bonding properties of the trioxofluorophosphate or hydrogen trioxofluorophosphate anions is discussed. It has been recognized that a carbon-bonded F atom avoids participation in a hydrogen-bond pattern; however, this property seems to be also common to fluorine bonded to P, As and S, especially if an oxygen ligand is also present in a molecule. The deposited material contains an overview of the determined structures with trioxofluorophosphate or hydrogen trioxofluorophosphate anions.

Benzylammonium heptanoate

The title 1:1 stoichiometric salt, C7H10N+·C7H13O2 −, is formed by proton transfer between hepta­noic acid and benzyl­amine. This combination contrasts to the recently published 2:1 acid–amine adduct of cation, anion and neutral acid molecule from the same components [Wood & Clarke (2013 ▶). Acta Cryst. E69, o346–o347]. There are N—H⋯O hydrogen bonds of moderate strength in the structure [the most important graph-set motifs are R 2 4(8) and R 4 4(12)], as well as weak C—H⋯O inter­actions.

2,6-Dichloroaniline–4-(2,6-dichloroanilino)pent-3-en-2-one (1/2)

The asymmetric unit of the title compound, C6H5Cl2N·2C11H11Cl2NO, is composed of one mol­ecule of an enamino–ketone [i.e. –(2,6-dichloro­phenyl­amino)­pent-3-en-2-one] and half a mol­ecule of 2,6-dichloro­aniline, the whole mol­ecule of the latter component being generated by twofold rotational symmetry. In this latter mol­ecule, there are two intra­molecular N—H⋯Cl contacts. In the enamino–ketone mol­ecule, there is an N—H⋯O hydrogen bond of moderate strength, and the dihedral angle between the benzene ring and penta­none fragment [C—C(—N)=C—C(=O)—C; planar within 0.005 (1) Å] is 81.85 (7)°. In the crystal, two mol­ecules of the enamino–ketone are bridged by a mol­ecule of 2,6-dichloro­aniline via N—H⋯O hydrogen bonds of moderate strength. There are also π–π inter­actions present, involving the benzene rings of inversion-related enamino–ketone mol­ecules [centroid–centroid distance = 3.724 (4) Å].

Bis(tetraethylammonium) oxalate dihydrate

The title compound, 2C8H20N+·C2O4 2−·2H2O, synthesized by neutralizing H2C2O4·2H2O with (C2H5)4NOH in a 1:2 molar ratio, is a deliquescent solid. The oxalate ion is nonplanar, with a dihedral angle between carboxyl­ate groups of 64.37 (2)°. O—H⋯O hydrogen bonds of moderate strength link the O atoms of the water mol­ecules and the oxalate ions into rings parallel to the c axis. The rings exhibit the graph-set motif R 4 4(12). In addition, there are weak C—H⋯O inter­actions in the crystal structure.

Dimethyl hydrazine-1,2-dicarboxylate–triphenylphosphine oxide (1/1)

In the crystal structure of the title compound, C4H8N2O4·C18H15OP, two triphenyl­phosphine oxide mol­ecules and two dimethyl hydrazine-1,2-dicarboxyl­ate mol­ecules are connected via N—H⋯O hydrogen bonds of moderate strength and are related via a twofold rotational axis. Weak Car—H⋯ O contacts strengthen the crystal structure.

Engineering Three-Dimensional Chains of Porous Nanoballs from a 1,2,4-Triazole-carboxylate Supramolecular Synthon

Glycine ethyl ester was recruited in an amine exchange process based on a transamination to afford ethyl 4H-,2,4-triazol-4-yl-acetate (L1). The acid hydrolysis of this molecule leads to quantitative isolation of 4H-1,2,4-triazol-4-yl acetic acid (L2). This versatile synthon crystallizes in a noncentrosymmetric orthorhombic space group (Fdd2) with Z = 16. This crystal structure is the first one for a 1,2,4-triazole ligand constructed from an amino acid derivative. The strong intermolecular hydrogen bonding O-H center dot center dot center dot N (2.570(3) angstrom) connects molecules into infinite one-dimensional chains running parallel to the b axis, and the structure is further extended by numerous but moderate strength hydrogen bonds (C-H center dot center dot center dot O). Prominent features of L2 are the presence of diverse potential coordinating groups such as carboxylic acid and triazole on the same framework as well as the inherent flexibility of the ligand backbone. Reaction of L1 or L2 with aq. Cu(BF4)(2) in aq. DMF gives dark blue crystals which crystallize in a noncentrosymmetric, cubic space group (1 (4) over bar 3m) and which were formulated as [Cu-3(mu(3)-O)(L2)(3). (H2O)(3)]BF4 center dot H2O (C1). The self-assembly of C3-symmetric, mu(3)-oxo bridged triangular tricopper secondary building blocks (SBB) formed an unique architecture which encompasses voluminous nanoball voids of 1 nm. The total solvent accessible volume is 4477.5 angstrom(3) which accounts for 48% of the cell volume. The crystal network stability was studied by thermogravimetric analysis (TGA) differential thermal analysis (DTA) and scanning electron microscopy (SEM) analyses. Sorption properties and gas storage capacities were measured by BET. CI shows no preference for N-2(g), but a reversible H-2(g) uptake of 21 cm(3)/g was observed, Morphology analysis by SEM on single crystals of CI shows ultrawells of square shape irregularly located on the surface, whose origin is due to desolvation or crystal defects. Mercury porosimetry measurements reveal pore size distribution with a diameter ranging from 350 nm to 2.3 mu m.

3-(1H-Benzimidazol-2-ylsulfanyl)propan-1-ol

Molecules of the title compound, C10H12N2OS, are interconnected by N—H...O hydrogen bonds of moderate strength into infinite chains. There are also intramolecular O—H...N bonds of moderate length. Weaker interactions are represented by C—H...N bonds as well as by π–π interactions between the imidazole and benzene rings [distance between centroids = 3.6865 (8) Å].

Electron Charge Density Distribution from X-ray Diffraction Study of the M-Nitrophenol Compound in the Monoclinic Form

At room temperature, the m-Nitrophenol (m-NPH) appears in two polymorphicstructures: orthorhombic and monoclinic forms. In the present work, we shall focus on themonoclinic form of this compound which has a centrosymmetric structure with the spacegroup P21/n. The molecular dipole moment has been estimated experimentally. Highresolution single crystal diffraction experiment was performed at low temperature withMoKα radiation. The crystal structure was refined using the multipolar model of Hansen andCoppens (1978). The molecular electron charge density distribution is described accurately.The study reveals the nature of inter-molecular interactions including charge transfer andhydrogen bonds. In this crystal, hydrogen bonds of moderate strength occur between thehydroxyl group and the O atom in the nitro one.

N-(2-Cyanopropan-2-yl)isobutyramide

The title compound, C8H14N2O, is stabilized in the solid state as a keto tautomer. The asymmetric unit contains two mol­ecules with different conformations. In the crystal structure, inter­molecular N—H⋯O hydrogen bonds of moderate strength link the mol­ecules into chains along the c axis.

Nature of Urea−Fluoride Interaction: Incipient and Definitive Proton Transfer

1,3-bis(4-nitrophenyl)urea (1) interacts through hydrogen bonding with a variety of oxoanions in an MeCN solution to give bright yellow 1:1 complexes, whose stability decreases with the decreasing basicity of the anion (CH3COO- > C6H5COO- > H2PO4- > NO2- > HSO4- > NO3-). The [Bu4N][1.CH3COO] complex salt has been isolated as a crystalline solid and its molecular structure determined, showing the formation of a discrete adduct held together by two N-H...O hydrogen bonds of moderate strength. On the other hand, the F- ion first establishes a hydrogen-bonding interaction with 1 to give the most stable 1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation of HF2-. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu4N][1.HCO3], whose crystal and molecular structures have been determined.