Polar aspects of intramolecular interactions in 2‐amino‐5‐nitro‐4‐methylpyridines and 2‐amino‐3‐nitro‐4‐methylpyridines

The dipole moments of twelve 2‐N‐substituted amino‐5‐nitro‐4‐methylpyridines ( I‐XII ) and three 2‐N‐substituted amino‐3‐nitro‐4‐methylpyridines ( XIII‐XV ) were determined in benzene. The polar aspects of intramolecular charge‐transfer and intramolecular hydrogen bonding were discussed. The interaction dipole moments, μ_int, were calculated for 2‐N‐alkyl(or aryl)amino‐5‐nitro‐4‐methylpyridines. Increased alkylation of amino nitrogen brought about an intensified push‐pull interaction between the amino and nitro groups. The solvent effects on the dipole moments of 2‐N‐methylamino‐5‐nitro‐4‐methyl‐( I ), 2‐N,N‐dimethylamino‐5‐nitro‐4‐methyl‐ ( II ) and 2‐N‐methylamino‐3‐nitro‐4‐methylpyridines ( XIII ) were different. Specific hydrogen bond solute‐solvent interactions increased the charge‐transfer effect in I , but it did not disrupt the intramolecular hydrogen bond in XIII.     

(Nitro‐κN)[2‐(phenyldiazenyl‐κN2)pyridine‐κN](2,2′:6′,2′′‐terpyridine‐κ3N)ruthenium(II) tetrafluoroborate

The Ru—N bond distances in the title complex, [Ru(NO₂)(C₁₁H₉N₃)(C₁₅H₁₁N₃)]BF₄ or [Ru(NO₂)(tpy)(azpy)]BF₄, [tpy is 2,2′:6′,2′′‐ter­pyridine and azpy is 2‐(phenyl­azo)­pyridine], are Ru—N_py 2.063 (4), Ru—N_azo 2.036 (4), Ru—N_nitro 2.066 (3) Å, and Ru—N_tpy 2.082 (4), 1.982 (3) and 2.074 (4) Å. The azo N atom is trans to the nitro group. The azo N=N bond length is 1.265 (5) Å, which is the shortest found in such complexes to date. This indicates a multiple bond between Ru and the N atom of the nitro group, and π‐­backbonding [dπ(Ru) π*(azo)] is decreased.     

Hydrogen‐bonded structures of the isomeric compounds of quinoline with 2‐chloro‐5‐nitrobenzoic acid, 3‐chloro‐2‐nitrobenzoic acid, 4‐chloro‐2‐nitrobenzoic acid and 5‐chloro‐2‐nitrobenzoic acid

The structures of four isomeric compounds, all C₇H₄ClNO₄·C₉H₇N, of quinoline with chloro‐ and nitro‐substituted benzoic acid, namely, 2‐chloro‐5‐nitrobenzoic acid–quinoline (1/1), (I), 3‐chloro‐2‐nitrobenzoic acid–quinoline (1/1), (II), 4‐chloro‐2‐nitrobenzoic acid–quinoline (1/1), (III), and 5‐chloro‐2‐nitrobenzoic acid–quinoline (1/1), (IV), have been determined at 185 K. In each compound, a short hydrogen bond is observed between the pyridine N atom and a carboxyl O atom. The N...O distances are 2.6476 (13), 2.5610 (13), 2.5569 (12) and 2.5429 (12) Å for (I), (II), (III) and (IV), respectively. Although in (I) the H atom in the hydrogen bond is located at the O site, in (II), (III) and (IV) the H atom is disordered in the hydrogen bond over two positions with (N site):(O site) occupancies of 0.39 (3):0.61 (3), 0.47 (3):0.53 (3) and 0.65 (3):0.35 (3), respectively.     

N,N′‐Diethyl‐4‐nitrobenzene‐1,3‐diamine, 2,6‐bis(ethylamino)‐3‐nitrobenzonitrile and bis(4‐ethylamino‐3‐nitrophenyl) sulfone

N,N′‐Diethyl‐4‐nitrobenzene‐1,3‐diamine, C₁₀H₁₅N₃O₂, (I), crystallizes with two independent molecules in the asymmetric unit, both of which are nearly planar. The molecules differ in the conformation of the ethylamine group trans to the nitro group. Both molecules contain intramolecular N—H...O hydrogen bonds between the adjacent amine and nitro groups and are linked into one‐dimensional chains by intermolecular N—H...O hydrogen bonds. The chains are organized in layers parallel to (101) with separations of ca 3.4 Å between adjacent sheets. The packing is quite different from what was observed in isomeric 1,3‐bis(ethylamino)‐2‐nitrobenzene. 2,6‐Bis(ethylamino)‐3‐nitrobenzonitrile, C₁₁H₁₄N₄O₂, (II), differs from (I) only in the presence of the nitrile functionality between the two ethylamine groups. Compound (II) crystallizes with one unique molecule in the asymmetric unit. In contrast with (I), one of the ethylamine groups, which is disordered over two sites with occupancies of 0.75 and 0.25, is positioned so that the methyl group is directed out of the plane of the ring by approximately 85°. This ethylamine group forms an intramolecular N—H...O hydrogen bond with the adjacent nitro group. The packing in (II) is very different from that in (I). Molecules of (II) are linked by both intermolecular amine–nitro N—H...O and amine–nitrile N—H...N hydrogen bonds into a two‐dimensional network in the (10) plane. Alternating molecules are approximately orthogonal to one another, indicating that π–π interactions are not a significant factor in the packing. Bis(4‐ethylamino‐3‐nitrophenyl) sulfone, C₁₆H₁₈N₄O₆S, (III), contains the same ortho nitro/ethylamine pairing as in (I), with the position para to the nitro group occupied by the sulfone instead of a second ethylamine group. Each 4‐ethylamino‐3‐nitrobenzene moiety is nearly planar and contains the typical intramolecular N—H...O hydrogen bond. Due to the tetrahedral geometry about the S atom, the molecules of (III) adopt an overall V shape. There are no intermolecular amine–nitro hydrogen bonds. Rather, each amine H atom has a long (H...O ca 2.8 Å) interaction with one of the sulfone O atoms. Molecules of (III) are thus linked by amine–sulfone N—H...O hydrogen bonds into zigzag double chains running along [001]. Taken together, these structures demonstrate that small changes in the functionalization of ethylamine–nitroarenes cause significant differences in the intermolecular interactions and packing.     

Methyl 4‐[(1‐acetyl‐3‐tert‐butyl‐1H‐pyrazol‐5‐yl)amino]‐3‐nitrobenzoate

In the molecule of the title compound, C₁₇H₂₀N₄O₅, there are two intramolecular N—H...O hydrogen bonds having amidic and nitro‐group O atoms as the acceptors and together forming a three‐centre N—H...(O)₂ system. These interactions appear to play an important role in controlling the relative orientation of the pyrazole and aryl rings. The bond distances provide evidence for some polarization of the electronic structure. Molecules are linked into simple chains by a single C—H...O hydrogen bond.     

4‐Amino derivatives of 7‐nitro‐2,1,3‐benzoxadiazole: the effect of the amino moiety on the structure of fluorophores

The crystal structures of three 4‐amino derivatives of 7‐nitro‐2,1,3‐benzoxa­diazole with increasing substituent ring size, viz. 7‐nitro‐4‐(pyrrolidin‐1‐yl)‐2,1,3‐benzoxa­diazole, C₁₀H₁₀N₄O₃, 7‐nitro‐4‐(piperidin‐1‐yl)‐2,1,3‐benzoxa­diazole, C₁₁H₁₂N₄O₃, and 4‐(azepan‐1‐yl)‐7‐nitro‐2,1,3‐benzoxa­diazole, C₁₂H₁₄N₄O₃, have been determined in order to understand their photophysical behaviour. All three were found to crystallize in centrosymmetric space groups. There is considerable electron delocalization compared with the parent compound, although the five‐membered oxa­diazole ring apparently does not participate in this. The length of the C—N bond between the amino N atom and the 7‐nitro­benzoxa­diazole system is found to be shorter than in similar compounds, as is the C—N_nitro bond. In each structure, the nitro group lies in the plane of the benzoxa­diazole unit.     

1‐(2‐Hydroxyethyl)‐3‐nitro‐1, 2, 4‐triazole and its Complexes with Copper(II) Chloride and Copper(II) Perchlorate

1‐(2‐Hydroxyethyl)‐3‐nitro‐1, 2, 4‐triazole (hnt), prepared by alkylation of 3‐nitro‐1, 2, 4‐triazole with 2‐chloroethanol, was found to react with copper(II) chloride and copper(II) perchlorate in acetonitrile/ethanol solutions giving complexes [Cu₂(hnt)₂Cl₄(H₂O)₂] and[Cu(hnt)₂(H₂O)₃](ClO₄)₂, respectively. They are the first examples of coordination compounds with a neutral N‐substituted 3‐nitro‐1, 2, 4‐triazole ligand. 1‐(2‐Hydroxyethyl)‐3‐nitro‐1, 2, 4‐triazole and the obtained complexes were characterized by NMR and IR spectroscopy, X‐ray, and thermal analyses. [Cu₂(hnt)₂Cl₄(H₂O)₂] presents a dinuclear chlorido‐bridged complex in which hnt acts as a chelating bidentate ligand, coordinated to the metal by a nitrogen atom of the triazole ring and an oxygen atom of the nitro group, and the copper atoms are inconsiderably distorted octahedral coordination. [Cu(hnt)₂(H₂O)₃](ClO₄)₂comprises a mononuclear complex cation, in which two nitrogen atoms of two hnt ligands in trans configuration and three water oxygen atoms form a square pyramidal environment around the copper atom, which is completed to an distorted octahedron with a bifurcated vertex due to two additional elongated Cu–O bonds with two nitro groups. In both complexes, Cu–O bonds with the nitro groups may be considered as semi‐coordinated.     

Two N‐(2‐phenylethyl)nitroaniline derivatives as precursors for slow and sustained nitric oxide release agents

Notwithstanding its simple structure, the chemistry of nitric oxide (NO) is complex. As a radical, NO is highly reactive. NO also has profound effects on the cardiovascular system. In order to regulate NO levels, direct therapeutic interventions include the development of numerous NO donors. Most of these donors release NO in a single high‐concentration burst, which is deleterious. N‐Nitrosated secondary amines release NO in a slow, sustained, and rate‐tunable manner. Two new precursors to sustained NO‐releasing materials have been characterized. N‐[2‐(3,4‐Dimethoxyphenyl)ethyl]‐2,4‐dinitroaniline, C₁₆H₁₇N₃O₆, (I), crystallizes with one independent molecule in the asymmetric unit. The adjacent amine and nitro groups form an intramolecular N—H…O hydrogen bond. The anti conformation about the phenylethyl‐to‐aniline C—N bond leads to the planes of the arene and aniline rings being approximately perpendicular. Molecules are linked into dimers by weak intermolecular N—H…O hydrogen bonds such that each amine H atom participates in a three‐center interaction with two nitro O atoms. The dimers pack so that the arene rings of adjacent molecules are not parallel and π–π interactions do not appear to be favored. N‐(4‐Methylsulfonyl‐2‐nitrophenyl)‐l ‐phenylalanine, C₁₆H₁₆N₂O₆S, (II), with an optically active center, also crystallizes with one unique molecule in the asymmetric unit. The l enantiomer was established via the configuration of the starting material and was confirmed by refinement of the Flack parameter. As in (I), there is an intramolecular N—H…O hydrogen bond between adjacent amine and nitro groups. The conformation of the molecule is such that the arene rings display a dihedral angle of ca 60°. Unlike (I), molecules are not linked via intermolecular N—H…O hydrogen bonds. Rather, the carboxylic acid H atom forms a classic, approximately linear, O—H…O hydrogen bond with a sulfone O atom. Pairs of molecules related by twofold rotation axes are linked into dimers by two such interactions. The packing pattern features a zigzag arrangement of the arene rings without apparent π–π interactions. These structures are compared with reported analogues, revealing significant differences in molecular conformation, intermolecular interactions, and packing that result from modest changes in functional groups. The structures are discussed in terms of potential NO‐release capability.     

High‐ and low‐temperature phases in isostructural 4‐chloro‐3‐nitroaniline and 4‐iodo‐3‐nitroaniline

The structures of 4‐chloro‐3‐nitroaniline, C₆H₅ClN₂O₂, (I), and 4‐iodo‐3‐nitroaniline, C₆H₅IN₂O₂, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single‐crystal X‐ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high‐temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene‐ring planes at two different orientations. In the low‐temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room‐temperature cell. Each of the low‐temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low‐temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low‐temperature phases. It seems that competition between the primary amine–nitro N—H...O hydrogen bonds which form three‐centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N—H...N hydrogen bonding of moderate strength which results in the graph‐set motif C(3). This graph‐set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high‐ and the low‐temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high‐temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C 60 , o328–o330].     

Contrasting the supramolecular structures in the isomeric pair 5‐bromo‐3‐nitrosalicylaldehyde phenylhydrazone and 3‐bromo‐5‐nitrosalicylaldehyde phenylhydrazone

Isomeric 5‐bromo‐3‐nitrosalicylaldehyde phenylhydrazone and 3‐bromo‐5‐nitrosalicylaldehyde phenylhydrazone, C₁₃H₁₀BrN₃O₃, both crystallize with two molecules in the asymmetric unit. In both isomers, an intramolecular O—H...N hydrogen bond links the hydroxy group and the imine N atom. In the 5‐bromo‐3‐nitro isomer, there are two independent N—H...O hydrogen‐bonded chains, each molecule in the asymmetric unit forming its own chain. These chains are then linked to form a three‐dimensional framework by a combination of weak C—H...O, C—H...Br, C—H...π and π–π stacking interactions. In the 3‐bromo‐5‐nitro isomer, N—H...O hydrogen bonds link the independent molecules alternately into a zigzag chain, which is reinforced by a weak C—H...O interaction. Individual chains are linked by a C—H...Br interaction and a three‐dimensional framework is generated by π–π stacking interactions.     

Hydro­gen‐bonded chains in N‐(2‐nitrophenyl)phenylamine

Molecules of the title compound, C₁₂H₁₀N₂O₂, are markedly non‐planar. There is an intramolecular N—H?O hydrogen bond, and the mol­ecules are linked into zigzag chains by a single C—H?O hydrogen bond. Comparisons are made with the supramolecular aggregation in isomeric amino–nitro derivatives, and in some N‐methylnitro­anilines.     

DFT Study of the Group Interactions in 1,3,5‐Triamino‐2,4,6‐Trinitrobenzene and 1,3,5‐Triamino‐2,4,6‐Tridifluoroaminobenzene

Density functional calculations on isodesmic disproportionation reactions of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) and 1,3,5‐triamino‐2,4,6‐tridifluoroaminobenzene (TATDB) indicate that the interaction between nitro groups on meta carbons of TATB, which brings about unstability to the molecule, is surprisingly larger than that between difluroamino groups in TATDB. The electron‐withdrawing and electron‐donating groups generate large positive and very small negative values of E_{disproportion}, respectively. When both electron‐withdrawing and electron‐donating groups are attached to the benzene skeleton at the same time, large negative disproportionation energy is produced, which stabilizes the derivatives. The values of E_{disproportion} for TATB and TATDB are predicted to be ‐48.03 kJ/mol and ‐63.54 kJ/mol, respectively, indicating that the total interaction among groups with stabilization effects in TATDB is larger than that in TATB. The large difference of the E_{disproportion} values between TATB and TATDB is derived from the large difference between the interactions of the meta‐nitro group and those of meta‐difluoroamino groups. The energy barriers for the C‐N internal rotation of NO₂ group and NF₂ groups are 74.7 kJ/mol and 185.8 kJ/mol for TATB and TATDB, respectively. The large energy barrier for the rotation of the NF₂ group is caused by its stabilization interaction with neighbor amino groups, instead of steric effects. When the number of pairs of amino‐nitro or amino‐difluoroamino groups increases, there are more negative charges on the NO₂/NF₂ groups and on the O/F atoms.     

1‐Amino‐2‐fluoromonocarba‐closo‐dodecaborates: K[1‐H2N‐2‐F‐closo‐1‐CB11H10] and [Et4N][1‐H2N‐2‐F‐closo‐1‐CB11I10]

The potassium salt of the [1‐H₂N‐2‐F‐closo‐1‐CB₁₁H₁₀]^– anion ( 1 ) was obtained from an insertion reaction of Li₃[7‐H₂N‐nido‐7‐CB₁₀H₁₀] with BF₃ · OEt₂. Anion 1 was protonated to the neutral species 1‐H₃N‐2‐F‐closo‐1‐CB₁₁H₁₀ (H 1 ) and it was iodinated with ICl to the [1‐H₂N‐2‐F‐closo‐1‐CB₁₁I₁₀]^– anion ( 2 ). All species were characterized by multinuclear NMR, IR, and Raman spectroscopy as well as by elemental analysis. The structure of H 1· (CH₃)₂CO was studied by single‐crystal X‐ray diffraction and the experimentally determined bond lengths are compared to values derived from density functional calculations.     

Unexpected beauty and diversity in the structures of three homologous 4,5‐dialkoxy‐1‐ethynyl‐2‐nitrobenzenes: the subtle interplay between intermolecular C—H…O hydrogen bonds and alkyl chain length

The synthesis, ¹H and ¹³C NMR spectra, and X‐ray structures are described for three dialkoxy ethynylnitrobenzenes that differ only in the length of the alkoxy chain, namely 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene, C₁₄H₁₇NO₄, 1,2‐dibutoxy‐4‐ethynyl‐5‐nitrobenzene, C₁₆H₂₁NO₄, and 1‐ethynyl‐2‐nitro‐4,5‐dipentoxybenzene, C₁₈H₂₅NO₄. Despite the subtle changes in molecular structure, the crystal structures of the three compounds display great diversity. Thus, 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene crystallizes in the trigonal crystal system in the space group , with Z = 18, 1,2‐dibutoxy‐4‐ethynyl‐5‐nitrobenzene crystallizes in the monoclinic crystal system in the space group P 2₁/c , with Z = 4, and 1‐ethynyl‐2‐nitro‐4,5‐dipentoxybenzene crystallizes in the triclinic crystal system in the space group , with Z = 2. The crystal structure of 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene is dominated by planar hexamers formed by a bifurcated alkoxy sp‐C—H…O,O′ interaction, while the structure of the dibutoxy analogue is dominated by planar ribbons of molecules linked by a similar bifurcated alkoxy sp‐C—H…O,O′ interaction. In contrast, the dipentoxy analogue forms ribbons of molecules alternately connected by a self‐complementary sp‐C—H…O₂N interaction and a self‐complementary sp²‐C—H…O₂N interaction. Disordered solvent was included in the crystals of 1‐ethynyl‐2‐nitro‐4,5‐dipropoxybenzene and its contribution was removed during refinement.     

3‐(4‐Acetyl­phenyl)‐1‐(4‐nitro­phenyl)­triazene

The crystal structure of the title compound, C₁₄H₁₂N₄O₃, shows that the stereochemistry about the N=N double bond of the N=N—N(H) moiety is trans. The whole mol­ecule is almost planar (r.m.s. deviation = 0.0654 Å), the interplanar angle between the phenyl rings being 0.7 (1)° and the largest interplanar angle being that between the phenyl ring and the nitro group of the 4‐nitro­phenyl substituent [11.5 (2)°]. Intermolecular N—H⋯O interactions between mol­ecules related by translation give rise to chains along the [110] and [10] directions, and these chains are held together by N⋯O π–π interactions. An unequal distribution of the double‐bond character among the N atoms suggests a delocalization of π electrons over the diazo­amine group and the adjacent aryl substituents.     

Palladium(II)‐catalyzed cyclization of W‐(2′,4′‐dienyl)‐2‐alkynamides to α‐alkylidene‐γ‐butyrolactams

In the presence of CuCl₂, N‐(2′, 4′‐dienyl)‐2‐alkynamides can be converted to α‐alkylidene‐σ‐butyrolactams under the catalysis of palladium(II). In this reaction, CuCl₂ is used to oxidize Pd(0) to regenerate Pd(II), or the carbon‐palladium bond is quenched by the oxidative cleavage reaction of CuCl₂.     

Crystal Structure and Characterization of a Double‐helical‐chain Coordination Polymer Zinc Complex Constructed by Flexible Dicarboxylate Ligand 2‐Nitro‐benzene‐1,4‐di(oxyacetate) and Rigid 2,2′‐Bipyridine Ligand

A novel double‐helical‐chain coordination polymer [Zn(nbdoa)(2,2′‐bipy)(H₂O)]_n constructed by flexible 2‐nitro‐benzene‐1,4‐di(oxyacetate) ligand and rigid 2,2′‐bipyridine ligand was obtained by hydrothermal reaction. The crystal structure demonstrates that there is a double‐helical chain with an inner channel running parallel to the helix axis without any interpenetration, which is connected to network via π‐π stacking and hydrogen bond interactions. The thermal analysis shows that the infinite helical structure is stable up to 536 K. The luminescence property is investigated and the complex shows photoluminescence in the solid state at room temperature.     

2,2‐Di­methyl‐1‐(2,4,6‐tri­nitro­phenyl)­hydrazine

The title mol­ecule (DMPH‐H), C₈H₉N₅O₆, was investigated to provide comparison with 2,2‐di­phenyl‐1‐picryl­hydrazine, which unlike DMPH‐H is readily oxidizable to form a well known stable free radical (DPPH). The structure shows essential differences in the configuration of the hydrazine‐N atoms, the ortho‐nitro group orientations and the crystal packing. The bond angles of the di­methyl­amino N atom [107.90 (13), 108.96 (12) and 112.21 (13)°] are consistent with a tetrahedral N atom and sp³ hybridization.     

2‐[2‐Methyl‐5‐nitro‐4‐(phenyl­sulfonyl­methyl)­imidazol‐1‐yl]­ethanol and 2‐methyl‐5‐nitro‐4‐phenyl­sulfonyl­methyl‐1,3‐thia­zole

The title compounds, C₁₃H₁₅N₃O₅S and C₁₁H₁₀N₂O₄S₂, respectively, both contain a phenyl­sulfonyl group connected, through a methyl­ene bridge, to either a substituted nitro­imidazole or nitro‐1,3‐thia­zole ring. In the imidazole‐containing mol­ecule, the nitro and sulfonyl groups are trans relative to the sulfonyl–methyl bond, while in the thia­zole‐containing mol­ecule, these substituents are cis. The stabilizing interactions within the crystals are also different between the two compounds.     

2‐Ethyl‐1‐[5‐(4‐methylphenyl)pyrazol‐3‐yl]‐3‐(thiophene‐2‐carbonyl)isothiourea: molecular ribbons containing three types of hydrogen‐bonded ring

The title compound, C₁₈H₁₈N₄OS₂, was prepared by reaction of S,S‐diethyl 2‐thenoylimidodithiocarbonate with 5‐amino‐3‐(4‐methylphenyl)‐1H‐pyrazole using microwave irradiation under solvent‐free conditions. In the molecule, the thiophene unit is disordered over two sets of atomic sites, with occupancies of 0.814 (4) and 0.186 (4), and the bonded distances provide evidence for polarization in the acylthiourea fragment and for aromatic type delocalization in the pyrazole ring. An intramolecular N—H...O hydrogen bond is present, forming an S(6) motif, and molecules are linked by N—H...O and N—H...N hydrogen bonds to form a ribbon in which centrosymmetric R₂²(4) rings, built from N—H...O hydrogen bonds and flanked by inversion‐related pairs of S(6) rings, alternate with centrosymmetric R₂²(6) rings built from N—H...N hydrogen bonds.