Nitro groups, characteristic frequencies

In the infrared spectra (IR) of nitroazoles characteristic bands correspond to asymmetric (vj and symmetric (vs) stretching vibrations of the nitro group. It is known that the position of viis band is more subject to the substituent influence in comparison with the position of vs band of the complicated form. This appears to be related to some vibrations of the cycle. Thus, variation of the substituents is reflected in vibrations of the heterocycle, which, in turn, results in shifting the nitro group vs frequency, even in cases when there are no changes of force constants or electron distribution in the N02 group. Therefore, the frequencies vary rather randomly. 

The conclusion of the existence of —0-—H 0—N— bonds is also based on the observation that the bands at 1567 cirr1 and 1361-1340 cm 1, which are characteristic of the nitro group in nitroparaffins, are shifted to lower frequencies, i.e. to 1555-1543 cm 1 and 1319-1310 cm 1 respectively and are weakened and widened in the presence of two or three hydroxyl groups. 

Characteristic stretching vibrations of the nitro group of 5(6)-nitrobenzimidazoles lie in frequency ranges of 1540-1510 (vj and 1350-1330 cm-1 (vs) [709, 715, 1085-1090], The N02 stretching vibration frequencies of 5(6)-nitrobenzimidazoles and 5- and 6-nitroindazole have been measured (Table 3.62). 

In tlie aliphatic nitro derivatives, Raman frequencies characteristic for the nitro group occur at 1,383 cm and 1,555 cm These values are between those observed in compounds containing a single N—O bond (approximately 1,000 cm ) and those containing a double N=0 bond (approximately 1,640 cm ). In nitrobenzene these characteristic frequencies are lower, being 1,345 cm i and 1,523 cm respectively, which is most probably due to molecular resonance with the benzene ring and the contribution of the structure ... 

The C=0 absorption bands of 2-carbonyl substituted thiophenes such as 2-thiopheneaIdehyde and 2-acetylthiophene, always occur at lower frequencies than those of the corresponding 3-isomers, because of the greater single-bond character of the C=0 bond in 2-substituted compounds. This follows from the increased conjugation of the substituent in the 2-position over that in the 3-position as is evident from the resonance structures in Section II,A.. The absorption shifts in 5-substituted 2-acetylthiophenes are in linear relation with Hammett s a-values for the substituents. The difference in the absorption frequencies of 2,5-dimethyl-3-acetylthiophene (1672 cm ) and 2,5-di-f-butyl-3-acetylthiophene (1684 cm ) may be ascribed to steric inhibition of resonance. The Raman spectrum of 2-thiophene-aldehyde shows two CJ=0 lines at 1658 and 1675 cm . The characteristic absorption frequencies of the nitro group in the six isomeric bromonitrothiophenes occur at 6.52-6.63 and at 7.45-7.58 The IR spectrum of thiophene in the region between 0.87-1.2 ix has been obtained. ... 

Figure 26. Characteristic group frequencies of nitro compounds...

---