The N-H - X Hydrogen Bonding

The electronegativity of halogen atoms is aU obviously larger than that of the H atom. However, due to its low polarizability and tightly contracted lone pairs, the F atom is unable to compete with the O or N atom, which is the stronger H-bond 

A -(2-Chloro)phenyl amides also readily form five-membered N-H - Cl hydrogen bonding (HB-105-112) [69], and the amide unit also forms intermolecular N-H 0=C hydrogen bonding. For all the compounds, the Cl or Br atoms at the a-position of the acetyl group do not involve intramolecular or intermolecular interactions. 

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N—H.. X system. (X = C1-, Br, 0, N). NH stretching and out-ofplane bending frequencies can also be simply related, as shown in Fig. 9 b. The points (Table 4) correspond to frequencies of imidazole and triazole molecules in different environments, i. e., physical state and in complexes with various metal salts. The range of the vNH (3550—2700 cm-1) and yNH(515—940 cm-1) frequencies is smaller than that of the O—H.. 0 system, the N—H.. X hydrogen bonds being weaker. The dyNH frequency shift is less than half that of the drNH shift and the dy/yo relative shift for imidazole crystal amounts to 83%. As shown by deuteration studies of these molecules (26,131) the NH out-of-plane mode of imidazole and triazole does not appear to be mixed with other out-of-plane vibrations. Yet the yNH(j 2i) vibration of pyrrole is strongly... 

The remarkable protonation features of 21 led to the formulation of the diproto-nated species as the water cryptate, [H20 cr 21, 2H+] 25, in which the water molecule accepts two +N-H---0 bonds from the protonated nitrogens and donates two 0-H---N bonds to the unprotonated ones [2.17, 2.96], The second protonation of 21 is facilitated by the substrate it represents a positive cooperativity effect, mediated by H20, in which the first proton and the effector molecule water set the stage both structurally and energetically for the fixation of a second proton. When 21 is tetraprotonated it forms the chloride cryptate cryptate [Cl c 21,4H+] 26, in which the included anion is bound by four +N-H---X- hydrogen bonds [2.97] (see also Chapt. 3). 

Figure 35 Schematic representation of single layer halometallate perovskites [ MX4 ]2" with (a) monoammonium (RNI I.J) and (b) diammonium (11[3NRNI1]) organic cations as spacer units linked to inorganic layers via N-H X hydrogen bonds. Reprinted from D. B. Mitzi, J. Chem. Soc. Dalton Trans., 1-12 (2001). Reproduced by permission of the Royal Society of Chemistry.

NH stretching frequency. Both systems yield a simple relation between the isotopic frequency ratio and NH stretching frequency (Fig. 11c). However, unlike the O—H.. 0 system, the vNH/vND ratio is at least 1.30 for imidazole and 1.21 for the protonated bases. This indicates that NH.. Y and N—H+.. X hydrogen bonds are not very strong and that the proton remains attached to the nitrogen even for the strongest interaction. 

Crystals of several amino-acids and of a few simple peptides and other compounds related to proteins have been analyzed recently by three-dimensional methods. These and other accurate determinations of structure constitute the best experimental sources of information about the dimensions and configuration of the polypeptide chain. The present paper comprises a critical summary of this information— in particular, of the principal structural features of the amide group and the N—H---0 hydrogen bonds, as derived from X-ray diffraction analyses of crystals of amino-acids, peptides, and other organic compounds. 

The foregoing conclusions are further supported by a refined X-ray analysis of pyrid-2-one, which indicated that the mobile hydrogen atom is attached to the nitrogen atom in the solid state and that individual molecules are bound into helices by N—H- -0 hydrogen bonds. An oxo structure is also indicated by the molar refractivity of pyrid-2-one. The dipole moment of 4-methoxypyridine is ca. 3.0 debyes in dioxane, whereas the values for pyrid-4-one and its 1-methyl derivative are much higher, ca. 6.0 debyes indicating the... 

The crystal structures of raer-[lr(en)(enl I )C13]C1 1120 and mer-[Ir(en)(en )Cl3] show that the coordination geometry of Ir is almost identical in the two complexes, with the only difference being in the conformation of the unidentate en and enH+ groups.122 Density functional theory and ab initio calculations have been performed on the two complexes and the calculated confirmations agree well with the X-ray diffraction values.123 The enH+ ligand is stabilized via intramolecular N—H - - Cl hydrogen bonds. 

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