O-H- -N Hydrogen Bonds

Unlike the 0-H 0 bonds, there are no examples of strong N-H 0 hydrogen bonds. The strong and almost symmetrical N H 0 configuration which might be expected to favor the amino imino tautomerism 

There have been four statistical analyses of N-H 0 hydrogen-bond geometries, summarized in Thbles 7.15 to 7.18  

The data in Thble 7.15 are based on the neutron diffraction analyses of the amino acids [60]. They are mainly concerned with -N(H2)H 02C-bonds, since these molecules occur as zwitterions in the crystals. 

The data in Thble 7.16 reflect a broader-based analysis using mainly normalized values from the more reliable X-ray crystal structures of the purines and pyrimidines [61]. 

The data in Thble 7.17 are the same type of analysis as in Thble 7.16, using only the nucleoside and nucleotide crystal structures [62]. 

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These chains are coiled in a precise way which is determined to a large extent by N-H O hydrogen bonds of length 279 ... 

Figure 6. Side view of energetically most favorable complex formed between L-sorbose and hexamethylentetramine (HMTA) auxiliary. Complex is stabilized by N-H-O hydrogen bond interaction.

C13HuC1N307 2,2 -Anhydro-[5-chloro-l-(3,5-di-0-acetyl-/ -D-arabino-furanosyl)-6-oxocytosine] (ACAFCC)166 I4t Z — 8 D = 1.56 R = 0.066 for 818 intensities. The glycosyl disposition of the anhydronucleoside is constrained to the syn (— 68.8°) orientation. The conformation of the D-arabinofuranosyl group is a flattened 4E (232.6°, 18.0°), and the exocyclic, C-4 -C-5 bond torsion-angle is g auche+ (50.2°). The adjacent bases are connected by N-H O hydrogen-bonds between the N-4 atom of one molecule and the carbonyl oxygen atom (0-4) of another. The twofold-symmetry-related bases are stacked, with an interbase separation of 329 pm. 

The chitobiose unit has been treated as a rigid body, and by using the full-matrix, least-squares, rigid-body, refinement procedure, the structure was refined to an R factor of 40.7%. Visually estimated intensities were used. The structure was found to be free from short contacts, and to be stabilized by an intrachain OH-3—0-5 hydrogen-bond and one interchain N-H—O hydrogen-bond. 

Fig. 2. Examples of the chair (1) and the distorted boat (2) conformation as well as pairs of trimetaphosphimate ions connected by N-H O hydrogen bonds (3). 

Fig. 3. Examples for conformations in (P02NH)f ions (1, saddle 2, chair 3, distorted boat 4, crown) as well as some structural aspects (formation of strings (5) and of columns (6) by N-H O hydrogen bonding) of tetrametaphosphimates.

To facilitate comparison of CAHB complexes, we tabulate some properties of the N—H- O hydrogen bond of the cation-assisted NH4+- OH2 complex (Fig. 5.9(b)) for direct comparison with those of analogous neutral and... 

Table 5.14. Comparisons of N—H- O hydrogen bonds in cation-assisted, neutral, and anion-assisted complexes...

In the course of the synthetic efforts to isolate 59, a co-crystal containing the starting aminotriazole compound 60 could be obtained depending on the mode of purification (chromatography or crystallization) <2004AXC733>. Crystallographic study of these products showed that the molecules 59 are linked into simple chains, whereas in 60 the components are linked by combination of two-center N-H- N and N-H- O hydrogen bonds. 

Fig. 6. One dimeric unit [Na2ATP2] in the structure of disodium adenosine triphosphate trihydrate (57). The broken line indicates a N—H...O hydrogen bond...

D-a-hydroxyvaleric add, L-valine, L-lactic acid] 3, having a 36-atom ring with twelve carbonyl oxygen atoms. From i.r. (65) and n.m.r. (66) spectroscopy it was established that the cationic complex has three-fold symmetry with the peptide carbonyl oxygen atoms involved in N—H... O hydrogen bonding. According to optical rotatory dispersion (ORD) and... 

A, respectively. There are also intralayer N-H- O hydrogen bonds between NH2 and tri date groups (Table 3.3). 

The structure determination of bis(5,5 -diethylbarbiturato)bis(picoline)zinc shows zinc tetra-hedrally bonded to the deprotonated nitrogen atoms of the barbital anions (Zn—N = 1.99, 2.01 A) and to the nitrogen atoms of the picoline ligands (Zn—N = 2.07, 2.10 A). The molecules are linked by N—H O hydrogen bonds.577... 

Figure 5. N—H bond lengths in N—H O hydrogen bonding systems. The phenomenon of N—H bond stretching is only weakly represented (if at all), probably because there are only few nearly matched systems that have been characterized by neutron diffraction. Neutron diffraction data are from the CSD. From top to bottom all N—ammonium-carboxylate, ApKa 5 neutral heterocycle-...

The methylsulfinyl group, which adopts a trails conformation, links the pyridine and benzimidazole rings in an almost coplanar orientation. Thus the molecule, as a whole, adopts a nearly extended form. Two centrosymmetrically related molecules form a cyclic dimer by intermolec-ular N-H- O hydrogen bonding, and the dimers are held together by van der Waals contacts between the neighboring aromatic rings in the crystal structure. 

During the course of a study of the salts formed by saccharin with quinine, haloperidol, mirtazapine, pseudoephedrine, lamivudine, risperidone, sertraline, venlafaxine, zolpidem, and amlodipine, a 1 1 cocrystal of saccharin and piroxicam was detected [68]. In the crystal structure, the asymmetric unit was found to consist of one saccharin molecule and one zwitterionic piroxicam molecule that were linked by two sets of N—H O hydrogen bonds. The piroxicam-saccharin synthons were in turn linked through bridging C—H O hydrogen bonds. Interestingly, the drug substance solubility out of the cocrystal was found to be comparable to that of the marketed piroxicam product. 

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