Asymmetry, hydrogen bond

The large increase of the high-frequency bandwidth is but one challenge for the nuclear dynamics theories of hydrogen bonding, which are the subject of this chapter. Other challenges are the band asymmetry, the puzzling appearance... 

P2J2A Z = 8 Dx = 1.931 R = 0.034 for 2,321 intensities. There are two molecules in the asymmetry unit, and both exhibit the syn disposition (—84.5°, + 76.1°) for the base. The conformation of the D-ribosyl group is 3T4 (28.5°, 39.4°) in molecule A and 3T2 (359.9°, 36.2°) in molecule B. The exocyclic, C-4 -C-5 bond torsion-angle is gauche+ for both molecules (55.2°, 59.7°). The purine bases of the crystallograph-ically independent molecules are paired by N-l-H 0-6 hydrogen bonds across a pseudo-two-fold axis. The bases are stacked such that the Br atoms are tucked under the pyrimidine moiety of the adjacent... 

Other structural analyses of crystals in which the bifluoride is present are listed in Table 7. One compound, p-toluidinium fluoride [C7H,oN ][HF2 ], is worthy of further comment. The first X-ray diffraction study reported a symmetrical anion (Denne and MacKay, 1971), but a later analysis showed that the proton was not centred between the two fluorines and 7 f h values were 102.5 and 123.5 pm (Williams and Schneemeyer, 1973). This can be explained not by a double minimum potential energy well but by asymmetry due to other forces, such as secondary hydrogen bonding between one end of the bifluoride anion and the N—H group of the cation. An alternative explanation attributes the asymmetry of the bifluoride hydrogen bond to an unsymmetrical crystal field caused by the cation (Ostlund and Bellenger, 1975). 

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