Nitro compounds group frequencies

N—O Stretching Vibrations Nitro Compounds In the nitroalkanes, the bands occur near 1550 and 1372 cm-1. Conjugation lowers the frequency of both bands, resulting in absorption near 1550-1500 and 1360-1290 cm-1. Attachment of electronegative groups to the a carbon of a nitro compound causes an increase in the frequency of the asymmetrical N02 band and a reduction in the frequency of the symmetrical band chloropicrin, C13CN02, absorbs at 1610 and 1307 cm-1. 

Nitro groups of nitroalkanes can be identified by strong infrared bands at about 1550 cm-1 and 1375 cm-1, whereas the corresponding bands in the spectra of aromatic nitro compounds occur at slightly lower frequencies. A weak n —> n transition occurs in the electronic spectra of nitroalkanes at around 270 nm aromatic nitro compounds, such as nitrobenzene, have extended conjugation and absorb at longer wavelengths ( 330 nm). 

According to T. Urbanski [26] the bands of the nitro group in aliphatic nitro compounds under the influence of the hydrogen bond can be shifted to lower frequencies ... 

Frank, Horman and Scheibe [34] also found that asymmetric N02 vibrations could be shifted towards lower frequencies under the influence of conjugation in aromatic compounds. They examined their substances in potassium bromide. A number of aromatic nitro compounds in dilute solutions were recently examined by T. Urbanski and Dabrowska [35], They found that when a nitro group was placed in the para position with respect to another group, its symmetric stretching modes were of a lower frequency than in the meta position. This was most likely due to the strong conjugation of substituents with the ring ... 

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 N02 group in nitramines, R2NN02, is known (ref. Id, p. 241) to exhibit its nitrogen resonance at lower frequencies than that of C-nitro compounds. Some further examples (Table XXVI) are available from recent studies (119) on the isomeric systems [60], [61]. 

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

The group frequencies of aromatic compounds near 1600 cm are excellent for recognizing the presence of aromatic rings in the spectrum of an unknown compound, although some olefinic nitro, amino, and carbonyl compounds can also have bands there. 

Some aromatic nitro compounds have a band near 1575 cm which might be confused with the normal group frequency assignment for conjugated rings at 1585 cm ... 

In general, there are three regions of the infrared (IR) spectra that merit special attention. Bands in the range 2400-2100 cm can be assigned to the N-N stretching frequency of adsorbed N2 or N20, bands below 1700 cm" can be attributed to oxidized nitrogen species (nitrite, nitrate, and nitro compounds), and those around 3800-3600 cm" to hydroxyl groups.Table 4.1 summarizes the main IR bands identified for a Rh/ceria catalyst in contact with a NgO/He flow. 

Nitroaminocompounds of the type (II) were also examined in suspension in paraffin oil. They contain NH groups which give rise to bands with frequencies 3030 and 1610 cm1. The r correspond to NH groups bonded with hydrogen bonds. The frequencies of the nitro group are lowered in all nitro niino compounds with an exception of the compound (IIa) which does not contain hydroxyl groups. This may be accounted for by the weakness of bifurcated hydrogen bonds. 

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