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176 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

α− Halogen substitution has also a marked effect upon nitro group frequencies.

According to Brown and Haszeldine the frequencies increase under tie influence of

the α− chlorine atom.

They are: 1575±5 cm-1 and 1348±6 cm-1, while under the influence of two

α− chlorine atoms they are displaced:
1587±10 cm-1 and 1332±5 cm-1
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 :
for asymmetric vibrations by 12-24 cm-1
for symmetric vibrations by 30-42 cm-l
Aromatic nitro compounds have been the subject of investigation by numerous

authors: France1 [27], Lothrop et al. [28], Randle and Whiffen [29], and Brown

[23].

Kross and Fassel [30], on the basis of investigation of over thirty aromatic

compounds, gave the following vibration frequencies of the nitro group:
asymmetric: in non-polar solvents 1523±8 cm-1
as solids 1525±10 cm-1
symmetric: in non-polar solvents 1344±7 cm-1
as solids 1339±7 cm-1

These bands shift under the influence of several factors.

According to Brown [23], Kross and Fassel [30] and Conduit [7], the asymmetric

frequence is considerably reduced from cu. 1534 cm-1 (e.g. to 1510 cm-1) under

the influence of a powerful electron-donor such as NH 2.

Conduit classified the asymmetric frequencies according to the number of nitro

groups :
mononitro 1509-1540 cm-1
dinitro 1539-1552 cm-1
trinitro 1554-1567 cm-1
The only discrepancy in the above correlation arises when a dinitro compound

also contains strong electron-donor groups (e.g. NR 2.

Lippert [31] draws attention to the fact that the nitro group frequencies can

be shifted under the influence of a solvent. They are decreased in polar solvents

and their intensity increases. This effect is particularly strong when the nitro com-

pounds contain strong electron-repelling groups. Thus p- nitrodimethylaniline gives

bands in non-polar CCl 4 and strongly polar CCl 3 CN as shown below:

CCl 4 CCl 3 CN
asymmetric vibrations 1506 cm-1 1332 cm-1
symmetric vibrations 1496 cm-1 1320 cm-1
Lippert [31] tried to explain this phenomenon by assuming that a polar solvent
enhances the stability of the resonance structure of a solute