Hydrogen-bonded systems imidazole crystal

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 reconstruction of the bandshape of the imidazole crystal was also performed using Car-Parrinello molecular dynamics (CPMD) simulation [73] of the unit cell of the crystal the results reproduce both the frequencies and intensities of the experimental IR spectrum of bands reasonably well, which we attribute to the application of dipole moment dynamics. The results are presented in Fig. 8 [70]. These and other recent CPMD calculations, on 2-hydroxy-5-nitrobenzamide crystal [71], oxalic acid dihydrate [72], and other systems [64-69], show that the CPMD method is adequate for spectroscopic investigations of complex systems with hydrogen bonds since it takes into account most of mechanisms determining the hydrogen bond dynamics (anharmonicity, couplings between vibrational modes, and intermolecular interactions in crystals). 

The best known of the potential mercaptoimidazoles are the imidazoline-2- and benz-imidazoline-2-thiones, which resemble imidazolin-2-ones in that the tautomeric form (53 X = S) is the preferred form. The crystal structure and the HNMR spectrum of 1,3-dimethyl-3H-imidazoline-2-thione have been interpreted as showing partial double bond character in the N—C—N system, but no aromaticity (70CC56). However, the preference for a betaine structure (56) rather than (57) or (58) should be accepted with caution since it is really only a resonance structure similar to others which undoubtedly contribute to the overall structures of oxo-, thio- and amino-imidazoles. Measurement of the piSTa values for a series of imidazoline-2-thiones substituted variously on C-4, C-5, N-1 and N-3 by hydrogen, phenyl or methyl shows that all of the values are similar. Approximate Kr values calculated show that these compounds exist even more in the thione forms (53, X = S 58) than do the corresponding thiazoline-2-thiones and oxazoline-2-thiones. The UV spectra in aqueous solution support thione structures, as do dipole moment and X-ray studies (76AHC(S1)280, p. 400). 

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