Hydrogen-bonded molecules stretching/bending vibrations

Protic solvents always have more complex infrared spectra because of the presence of hydrogen bonding in the liquid state. In methanol, this involves interaction of the acidic proton on the OH group in one molecule with the oxygen atom in an adjacent molecule (fig. 5.15). The infrared spectrum shows a wide band centered at 3346 cm which is due to the -OH stretch. When methanol is dissolved as a dilute solute in carbon tetrachloride, this band is sharp and appears at 3644 cm . An -OH bending mode appears at 1449 cm. Another broad band due to -OH out-of-plane deformation is centered at 663 cm. The other features of the methanol spectrum are due to the vibrational modes of the CH3- group or to skeletal vibrations [27]. 

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... 

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