Strong symmetric hydrogen bonds

This type of hydrogen bonding is termed symmetric in contrast to the more common unsymmetric form. Symmetrical hydrogen bonds form in only the strongest bonded systems. These are frequently anionic like FHF and the carboxylales mentioned above. An example of a strong, symmetric hydrogen bond in a cation is the bis(A -nitrosopyrrolidine)hydrogen cation (Fig. 8.5). >2... 

Thus the inclusion of the polariton part into the absorption coefficient of a solid with strong symmetric hydrogen bonds predicts that instead of the narrow... 

The mechanism of the polariton absorption by strong symmetric hydrogen bonds has successfully been applied for the description of anomalous of the infrared spectra of the a-KICVHIC crystal. A complex investigation has been done at the disclosing of the molecular mechanism of the bacteriorhodopsin functioning. 

Although the vibrational spectra of polywater appear to be completely unique and different from those of any known substance (Lippincott et al., 1969), these spectra have several features that are quite similar to those of hydrogen-bonded systems which have very strong symmetric hydrogen bonds such as those occurring in KHF2 and... 

Vibrational spectroscopy is useful in identification of protonic entities and their configuration. It can determine the type of association for instance, whether M-OH entities form infinite chains, cyclic or open dimers, and the position of a proton in strong, symmetric hydrogen bonds where it can distinguish a truly centred proton from a statistically symmetrical case . 

The position of the rOH and vOD bands in naphthazarin seem to be shifted by about 600 cm-1 with respect to the free phenolic OH groups. This is a shift which corresponds to strong hydrogen bonding but it is not as much as might be expected for a symmetric hydrogen bond [11]. 

The X-ray crystal structure of [Pv TPP(OH)2 ]OH 2H2 O has been determined.32 The porphyrin skeleton is distorted S4 symmetrically to attain short P—distance ( 1.90 A). The hydroxy groups are strongly intermolecularly hydrogen bonded, and the phosphorus atom is displaced from the N4 plane by 0.096 A. The antimony(V) ion is centred in the porphyrin plane in [Sb(OEP)(OH)2]+ with a somewhat large Sb—distance of 2.065(6) A.31 The structure of [Bi(OEP)]NO, is of interest, since the four-coordinate BiUI ion protrudes from the porphyrin N4 plane by 1.09 A with a very long Bi—Npor distance of 2.32 A.31... 

As seen from Table 3, the tetrazolate ring is a symmetric structure with nearly uniform bond lengths. The strong intermolecular hydrogen bonds inherent to these compounds can presumably considerably affect the geometry of the tetrazolate ring. 

With stronger bases, the formation of even stronger symmetrical hydrogen bonding causes the appearance of an almost continuous absorption in the overall spectrum, with three maxima and two minima, the so-called ABC contour. The minima correspond to the first overtones of the MOH deformation modes, the in-plane deformation 50H, and the out-of-plane deformation, 7OH. The overall ABC contour is due to the Fermi resonance of the vOH mode (strongly shifted dovmwards and broadened) with these overtones, both strongly shifted upwards... 

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