dl‐Arginine monohydrate at 100 K

In the title compound, C₆H₁₄N₄O₂·H₂O, the α‐amino group is neutral. The molecular side chain including the guanidinium group is not fully extended, having a near gauche–gauche conformation [χ³ = 59.0 (1)°; χ⁴ = 72.8 (1)°]. The network of hydrogen bonds stabilizing the crystal lattice includes those formed between the deprotonated and negatively charged α‐carboxyl­ate groups and the positively charged amino groups of the guanidinium group of neighbouring mol­ecules. N—H?O=C and water‐mediated N—H?O hydrogen bonds link individual mol­ecules to produce pairs of spiral motifs laterally connected by N—H?O and C—H?O hydrogen bonds.     

2,4,5,7‐Tetra­methyl­phenanthrene at 150 K

The title compound, C₁₈H₁₈, crystallized in the centrosymmetric space group P2₁/c with one mol­ecule as the asymmetric unit. The methyl‐group H atoms at the 4 and 5 positions are ordered, while those at the 2 and 7 positions are disordered. The torsion of the bay region of the core is notably similar to that of other 4,5‐di­methyl­phenanthrenes. No substantial C—H?π interaction occurs in this structure.     

Acetone‐4‐methyl­thio­semicarbazone at 220 K

In the title compound, C₅H₁₁N₃S, the trans conformation is stabilized by a weak intramolecular N—H?N hydrogen bond. Unusually, one N—H bond is not involved in any hydrogen‐bond interactions and instead the mol­ecules form a one‐dimensional polymer via N—H?S intermolecular hydrogen bonds.     

Quinolin‐6‐ol at 100 K

The title compound, C₉H₇NO, has two symmetry‐independent molecules in the asymmetric unit, which have different conformations of the hydroxy group with respect to the quinoline ring. One of the molecules adopts a cis conformation, while the other shows a trans conformation. Each type of independent molecule links into a separate infinite O—H...N hydrogen‐bonded chain with the graph‐set notation C(7). These chains are perpendicular in the unit cell, one extended in the a‐axis direction and the other in the b‐axis direction. There is also a weak C—H...O hydrogen bond with graph‐set notation D(2), which runs in the c‐axis direction and joins the two separate O—H...N chains. The significance of this study lies in the comparison drawn between the experimental and calculated data of the crystal structure of the title compound and the data of several other derivatives possessing the hydroxy group or the quinoline ring. The correlation between the IR spectrum of this compound and the hydrogen‐bond energy is also discussed.     

l‐Alanylglycyl‐l‐alanine monohydrate at 20 K

The X‐ray crystal structure of the title compound, C₈H₁₅N₃O₄·H₂O, at 20 K (space group P2₁) reveals that the mol­ecular conformation of the tripeptide is remarkably different from the water‐free form (space group P2₁2₁2₁) reported previously [Padiyar & Seshadri (1996), Acta Cryst. C 52 , 1693–1695].     

Adiponitrile at 100 K

Adiponitrile, C₆H₈N₂, is a key intermediate in the synthesis of the polyamide Nylon 66 and is produced industrially on a large scale. We have determined the crystal and molecular structure of adiponitrile by single‐crystal X‐ray analysis at 100 K, a suitable crystal (m.p. 275 K) having been grown from the melt at low temperature. The compound crystallizes in the monoclinic space group P 2₁/c with Z = 2. In the crystal structure, the molecule adopts an exact C_i‐symmetric gauche –anti –gauche conformation of the C—C—C—C skeleton about an inversion centre. The molecules are densely packed, with short intermolecular contacts between the α‐H and nitrile N atoms.     

Lithium phospho­enolpyruvate monohydrate at 85 K

The crystal structure of the title compound, lithium (1‐carboxy­ethenyl­oxy)­phospho­nate monohydrate, Li⁺·C₃H₄O₆P^?·­H₂O, is governed by lithium–oxy­gen interactions and hydrogen bonds. The Li⁺ cation is tetrahedrally coordinated by phosphate and water O atoms. The phospho­enolpyruvate monoanions form carboxyl‐to‐carboxyl and phosphate‐to‐water hydrogen bonds.     

l‐Methioninium chloride and l‐seleno­methioninium chloride at 103 K

The crystal structures of the title compounds, (S)‐1‐carboxy‐3‐(methyl­sulfanyl)­propanaminium chloride, C₅H₁₂NO₂S⁺·Cl⁻, and (S)‐1‐carboxy‐3‐(methyl­selanyl)­propanaminium chloride, C₅H₁₂NO₂Se⁺·Cl⁻, are isomorphous. The proton­ated l ‐methionine and l ‐seleno­methionine mol­ecules have almost identical conformations and create very similar contacts with the Cl⁻ anions in the crystal structures of both compounds. The amino acid cations and the Cl⁻ anions are linked viaN—H⋯Cl⁻ and O—H⋯Cl⁻ hydrogen bonds.     

Potassium hydrogen trans‐glutaconate monohydrate at 295, 245 and 40 K,and its rubidium analogue at 298 K

A centrosymmetric and short O—H?O hydrogen bond was found in isomorphic crystals of potassium hydrogen trans‐glutaconate monohydrate (potassium hydrogen trans‐pent‐2‐ene‐1,5‐dioate, K⁺·C₅H₅O₄^?·H₂O), (I), and rubidium hydrogen trans‐glutaconate monohydrate (rubidium hydrogen trans‐pent‐2‐ene‐1,5‐dioate, Rb⁺·C₅H₅O₄^?·H₂O), (II). The O?O distance at room temperature is 2.444 (3) Å in (I), and 2.417 (4) Å in (II). The O?O distance for (I) showed no significant decrease at low temperatures.     

β‐Y2Si2O7, a new thortveitite‐type compound,determined at 100 and 280 K

A new form of Y₂Si₂O₇ (diyttrium heptaoxodisilicate) has been synthesized which is isotypic with thortveitite, Sc₂Si₂O₇, and crystallizes in the centrosymmetric space group C2/m, both at 100 and 280 K. The Y³⁺ cation occupies a distorted octahedral site, with Y—O bond lengths in the range 2.239 (2)–2.309 (2) Å. The SiO₄ tetrahedron is remarkably regular, with Si—O bond lengths in the range 1.619 (2)–1.630 (2) Å. The bridging O atom of the Si₂O₇ pyrosilicate group shows a large anisotropic displacement perpendicular to the Si—O bond. Changes in lattice and structural parameters upon cooling are small with, however, a distinct decrease of the anisotropic displacement of the briding O atom. Structure solution and refinement in the non‐centrosymmetric space group C2 are possible but do not yield a significantly different structure model. The Si—O—Si bond angle of the isolated Si₂O₇ groups is 179.2 (1)° at 280 K in C2 and 180° per symmetry in C2/m. The C2/m structure model is favoured.     

Two polymorphs of anhydrous 4‐pyridone at 100 K

Two polymorphs of the title compound, C₅H₅NO, (I), have been obtained from ethanol. One polymorph crystallizes in the monoclinic space group C2/c [henceforth (I)‐M], while the other crystallizes in the orthorhombic space group Pbca [henceforth (I)‐O]. In the two forms, the lattice parameters, cell volume and packing motifs are very similar. There are also two independent molecules of 4‐pyridone in each asymmetric unit. The molecules are linked by N—H...O hydrogen bonds into one‐dimensional zigzag chains extending along the b axis in the (I)‐M polymorph and along the a axis in the (I)‐O polymorph, with the graph set C₂²(12). The structures are stabilized by weak C—H...O hydrogen bonds linking adjacent chains, thus forming a ring with the graph set R₆⁵(28). The significance of this study lies in the analysis of the hydrogen‐bond interactions occurring in these structures. Analyses of the crystal structures of the two polymorphs of 4‐pyridone are helpful in elucidating the mechanism of the generation of spectroscopic effects observed in the IR spectra of these polymorphs in the frequency range of the N—H stretching vibration band.     

Benzyl 5‐carboxy‐4‐ethyl‐3‐methyl­pyrrole‐2‐carboxyl­ate

In the title compound, C₁₆H₁₇NO₄, the benzyl­oxy­carbonyl group is anti to the pyrrolic N atom. The mol­ecules are joined into head‐to‐head dimers by hydrogen bonds involving the carboxyl­ic acid groups. There is orientational disorder of these groups over two positions with approximately equal occupancy. A weaker hydrogen bond between the pyrrolic N atom and the carbonyl O atom of the benzyl­oxy­carbonyl group joins the dimers into chains running parallel to the [110] direction.     

N‐Acetyl‐l‐tyrosine methyl ester monohydrate at 293 and 123 K

The crystal structure of a protected l ‐tyrosine, namely N‐acetyl‐l ‐tyrosine methyl ester monohydrate, C₁₂H₁₅NO₄·H₂O, was determined at both 293 (2) and 123 (2) K. The structure exhibits a network of O—H...O and N—H...O hydrogen bonds, in which the water molecule plays a crucial role as an acceptor of one and a donor of two hydrogen bonds. Molecules of water and of the protected l ‐tyrosine form hydrogen‐bonded layers perpendicular to [001]. C—H...π interactions are observed in the hydrophobic regions of the structure. The structure is similar to that of N‐acetyl‐l ‐tyrosine ethyl ester monohydrate [Soriano‐García (1993). Acta Cryst. C 49 , 96–97].     

2,6‐Bis[(2‐hydroxy‐3‐methoxybenzylidene)hydrazinocarbonyl]pyridine monohydrate

In the title compound, C₂₃H₁₉N₅O₆·H₂O, the two components are linked into complex chains by a combination of two independent O—H...O and two independent N—H...O hydrogen bonds. The complex chains are linked into a two‐dimensional sheet network viaπ–π stacking interactions and C—H...O hydrogen bonds.     

2‐(4‐Acetyl‐3,5‐di­hydroxy‐2‐methyl­phenoxy)‐4,6‐di­methoxy‐3‐methyl­benzoic acid

In the title compound, C₁₉H₂₀O₈, the benzene rings are nearly perpendicular to each other [dihedral angle 80.2 (2)°]. The carboxy group is twisted out while both the methoxy and acetyl groups are almost coplanar with their attached benzene rings. The hydroxy group is involved in an intramolecular O—H?O hydrogen bond with the acetyl O atom and the compound is connected through an intermolecular O—H?O contact to form a dimer. The crystal structure is stabilized by intermolecular O—H?O hydrogen bonds.     

Methyl 2‐hydroxy‐2‐[5′‐hydroxy‐5′‐(1‐hydroxy‐1‐methyl­ethyl)‐5,2′‐di­methyl­per­hydro‐2,2′‐bi­furan­yl‐5‐yl]­acetate

The relative configuration was determined for the title compound, C₁₆H₂₈O₇, which was obtained as a mixture of epimers. There are both intra‐ and intermolecular hydrogen bonds, the latter forming dimers around the crystallographic centres of inversion.     

trans‐4‐Bromo‐ONN‐azoxy­benzene at 100 K

The crystal structure of the α isomer of trans‐4‐bromo­azoxy­benzene [systematic name: trans‐1‐(bromophenyl)‐2‐phenyl­diazene 2‐oxide], C₁₂H₉BrN₂O, has been determined by X‐ray dif­frac­tion. The geometries of the two mol­ecules in the asymmetric unit are slightly different and are within ∼0.02 Å for bond lengths, ∼2° for angles and ∼3° for torsion angles. The azoxy bridges in both mol­ecules have the typical geometry observed for trans‐azoxy­benzenes. The crystal network contains two types of planar mol­ecules arranged in columns. The torsion angles along the Ar—N bonds are only 7 (2)°, on either side of the azoxy group.     

7‐Vinyl‐8‐aza‐7‐de­aza‐2′‐deoxy­adenosine monohydrate

In the title compound, 4‐amino‐1‐(2‐deoxy‐β‐d ‐eythro‐pento­furan­osyl)‐3‐vinyl‐1H‐pyrazolo­[3,4‐d]­pyrimidine monohydrate, C₁₂H₁₅N₅O₃·H₂O, the conformation of the gly­cosyl bond is anti. The furan­ose moiety is in an S conformation with an unsymmetrical twist, and the conformation at the exocyclic C—C(OH) bond is +sc (gauche, gauche). The vinyl side chain is bent out of the heterocyclic ring plane by 147.5 (5)°. The three‐dimensional packing is stabilized by O—H·O, O—H·N and N—H·O hydrogen bonds.     

1,3,4‐Triphenyl‐7‐trifluoromethyl‐1H‐pyrazolo[3,4‐b]quinoline at 293 and 100 K

In the structure of the title compound, C₂₉H₁₈F₃N₃, belonging to the space group P6₅ (or P6₁), three symmetry‐independent molecules are arranged in two chains, with two molecules alternating along the 3₂ axes, whereas the remaining molecule forms a chain along [0001] due to the 6₅ screw axis. The conformation of each of the molecules is stabilized by an intramolecular C—H...N hydrogen bond, with C...N distances in the range 2.964 (6)–3.069 (5) Å at room temperature (293 K) and 2.943 (4)–3.084 (4) Å at low temperature (100 K). One molecule has its –CF₃ group ordered even at 293 K, which can be explained only by considering its involvement in two weak intermolecular C—H...F interactions, with C...F distances in the range 3.084 (6)–3.302 (5) Å at 293 K and 3.070 (3)–3.196 (3) Å at 100 K, and also a C—F...N interaction, with a C...N distance of 3.823 (5) Å at 293 K and 3.722 (4) Å at 100 K. The trifluoromethyl groups in the two remaining molecules are disordered at 293 K, whereas at 100 K the continuous (dynamic) positional disorder of one of the –CF₃ groups (of the molecule forming the chain along [0001]) is totally eliminated while the –CF₃ group disorder remains for the third molecule.     

Benz[cd]indol‐2(1H)‐one at 298 and 100 K

Weakly diffracting crystals of benz[cd]indol‐2(1H)‐one (naphtholactam), C₁₁H₇NO, were unsuitable for data collection by early photographic methods. However, a diffractometer data set collected at room temperature in 1989 was solved and refined. The peak scans were broad, and the results indicated disorder or a satellite crystal. Recent data collection (on another crystal from the same sample) with an area detector at 100 K revealed the same disorder, and made it possible to refine two different, more complete, disorder models. Both models assume an occasional 180° rotation of the nearly planar centrosymmetric cis‐lactam dimer. The refinements differ, especially in the anisotropic displacement parameters for the –C(=O)—NH– portion of the molecule. Both models at 100 K give a C—N (`amide') bond distance of 1.38 Å, about 0.04 Å longer than the average distance in saturated γ‐lactams in the Cambridge Structural Database. Cohesive packing interactions between molecules include opposing‐dipole dimers; the packing may explain the 10:1 ratio favoring the major‐occupancy molecule.