Short H-bond

The dihydrate of the tetramer is particularly stable and is, in fact, the bishydroxonium salt of tetrametaphosphimic acid [H30]J[(NH)4P4-06(0H)2] the anion of which has a boat configuration and is linked by short H bonds (246 pm) into a two-dimensional sheet (Fig. 12.29). The related salts M4[NHP02)4].- H20 show considerable variation in conformation of the tetrametaphosphimate anion, as do the 8-membered heterocyclic tetraphosphazenes (NPX2)4 (p. 537), e.g. 

The erbium ion is coordinated to eight carboxyhc oxygens [2.362—2.415 A)] from four oxalate (acid oxalate) moieties which forms a square antiprism around Er(III). A water molecule forms the cap (Er—OH2 =2.441 A) above the large square face of the antiprism. The acid oxalates (HOOCCOO) and oxalate ions occupy the crystallographic sites at random. The statistically averaged oxalate groups are centrosymmetric and planar. A very short H-bond (2.43 A) has been observed between two water molecules in two equivalent molecules, but the physical significance is difficult to assess because these waters are disordered in the molecule. 

A curve is constructed, in which the theoretical doublet separation is plotted against von. However, it must be pointed out that in the case of very short H-bonds, the circumstances are very complicated and the curve is no more valid. 

Finally the structure of the tosylate ciyptate of R3Bp demonstrates the size exclusion expected of the large cation (Fig. 11). Even so, it is not devoid of interaction with the cationic host, in the lattice at least, as each anion exhibits one moderately short H-bond contact to one of the NH1 functions of the cryptate. These direct H-bond contacts are often supported by indirect water-mediated links of shorter dimensions, acting as part of branched hydrate chains which run through the less hydrophobic section of the lattice. 

Here erf, and erf are the Pauli matrices, that act on the proton (deuteron) wavefunctions potential wells of the proton (deuteron) potential energy profile of the th H-bond (in accordance with the available diffraction data the short H-bonds O-H- O with two off-center sites of a proton are considered). The parameters of Hamiltonian (1), i.e., the tunneling parameter Cl and the Ising parameters J,p describe the motion of a proton (deuteron) along H-bond and the effective pair interactions of these particles, respectively. When Cl is substantially smaller than Jy the static approximation (Cl = 0) becomes valid. Instead of equation (1) one has in this case... 

In low dielectric organic solvents and enzyme active sites a number of hydrogen bonds between groups with similar pKa exhibit highly deshielded 1H NMR peaks (>16 ppm), low isotopic fraction factors and relatively short H-bonds (data on neutron and x-ray diffraction analysis (Gerlt and Gassman, 1992 Zundel, 2000 Cleland and Northrop, 1999). 

One more assumption which determines the coefficient ft relates the molecular dipole moment to the contribution of the ionic structure 6. The relation presumes that ionic character does not change when the H bond forms. Even within this complex of assumptions we can proceed to calculate the importance of the coveilent H bond term, only after the proton position in the H bond is specified. For the particular case of both A and B taken to be oxygen atoms, Ck>ulson and Danielsson assumed a constant A—H bond length of 1.0 A (448). This leads to their conclusion that the long bond [2.8 A] is essentially electrostatic, the covalent contribution. . . amounting only to a few percent. Even so, they add, With the short H bond [2.5 A]. . . covalency is beginning to be appreciable. ... 

Rundle (1488) and later by Welsh (2154). The solid curve in Fig. 9-1, drawn according to a visual best fit, has a standard error of estimate of r equal to 0.02 A. In addition. Fig. 9-1 displays the asymptotic behaviors. For extremely long H bonds the asymptote is tziken to be the 0.96 A bond length of gaseous water (monomer). For short H bonds the asymptote must be the dotted line representing... 

Banks. J. Chem. Phys. 19, 1205-6 (1951). x-ray, IR nickel dimethylglyoximc, a short H bond. 

Short H-Bonds for Organic Nanotubes and the Solvent Effect... 

Figure 11 (see color section). Longitudinal H-bond relay comprised of CHQs and water, (a) Tubular polymer structure of a single nanotube obtained with X-ray analysis for the heavy atoms and with ab initio calculations for the H-orientations (top and side views), (b) One of the four pillar frames of short H-bonds represents a 1-D H-bond relay composed of a series of consecutive OH groups [hydroxyl groups (—OH) in CHQs and the OHs in water molecules]. Reproduced by permission of American Chemical Society Ref. [54]. 

Figure 12 cant d (see color section). Short H-bonds in CHQ nanotubes in the top figures are three types of H-bonding involved with 1-D H-bonds of CHQ nanotubes [(a) > a, (.b) a >... 

The idea of the covalent nature of short H-bonds is analyzed later in this chapter in detail. This is not a new idea, originally claimed by Pauling [20] who stated the covalent nature of the H-bond for the (FHF) ion. The covalency of H-bonds was also discussed early on by Coulson and Danielsson [21] and this topic was analyzed in more recent studies [22]. Gilli et al. [23] introduced the so-called electrostatic-covalent H-bond model (ECHBM) where the following features were summarized ... 

Fig. 34.12. CaUx[4]hydroquinone nanotubes structure, longitudinal one-dimensional H-bond relay vi. intertubular tt-tt stacking, the HREM image of a single nanotube, and the water-accessible surface of the tubes. Each tube has four pillar frames of short H-bonds, and the pore size is 8 X 8 A. The unit cell is drawn by the dashed lines. (Reproduced with permission from American Chemical Society [126,127].)...

Within the quoted range of O-H-0 distances above, most are >2.50 A. The remaining short H bonds are often placed in a special class. These bonds are stronger and... 

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