urbaf2

GENERAL INFORMATION ON NITRO COMPOUNDS 179

in benzoic acid). According to Perret and Holleck [36], this is due to the out-of-
plane deformation vibrations of the carboxylic group which occur under the

influence of the nitro groups.

An extensive study of the infra-red absorption spectra of nitro compounds

was reported recently by Pristera, Halik, Castelli and Fredericks [37a]. The fol-

lowing are bands which are valuable in the structural investigation of unknown
nitro compounds and their mixtures (Table 25).

Recently a number of authors (Holder and Klein [38], Ogg Jr. and Ray [39]

have examined certain nitro compounds, particularly in comparison with the nitrite

ion (NO 2

• ).

NUCLEAR MAGNETIC RESONANCE AND X-RAY EXAMINATION

OF NITRO COMPOUNDS

The major application of nuclear magnetic resonance spectroscopy to organic

chemistry involves the study of proton shifts (the “chemical shift” of proton),

The aromatic proton frequencies of substituted benzenes fall between 2.0 and

3.5 according to the number and nature of the substituents. Corio and Dailey

[39a] have examined a number of monosubstituted benzenes.

The proton frequency of benzene was found to be 2.73.

In nitrobenzene the frequencies of the meta and para protons are lowered by

the figures of the same order (-0.30 and -0.42 respectively), whereas the ortho

protons are of much lower frequency (-0.97).

A similar phenomenon was found with COOH, COCl, COOCH 3 and CCl 3 ,

whereas CN lowers the frequency by 0.30 equally in all three positions: o-, m- and p-.

Schmidt, Brown and D. Williams [40] examined the nuclear magnetic resonance

of

14

N in three aromatic nitro compounds: nitrobenzene, nitrobenzoic acid, di-

nitrobenzoyl chloride.

In all three compounds the^14 N peak was observed. In each case the peak was

not so wide as in NO 2 - ion and considerably wider than that in the NO 3 - ion.

The broadness occurring with nitro groups (and even greater broadness in the

NO 2 - ion) was indicative of large electric asymmetries at^14 N in these groups.

The shifts measured ranged from 165 milligauss in an ether solution of

(NO 2 ) 2 C 6 H 3 COCl to 230 milligauss in C 6 H 5 NO 2 , in a field of 10,700 gauss

(NH 4 + as a standard).

X-ray investigations indicate that the nitro group has a planar or near-planar

structure, and that the atom distances and the bond angles for the nitro group

of dinitrobenzene are: