urbaf2

GENERAL INFORMATION ON NITRO COMPOUNDS 177

An increase in the number of molecules of this polar structure results in increasing

the negative charge of the acceptor group, i.e. NO 2 , hence in a decrease of frequency

and a rise of intensity of absorption.

A number of authors have found that conjugation of double bonds lowers

the frequency of NO 2 bands. Shechter and Shepherd [25], Fieser et al. [32], Eck-
stein, Kraczkiewicz, T. Urbanski and Wojnowska [33] have all reported that this
occurs with nitro-olefins.
A similar phenomenon was noticed in the case of aromatic compounds. Thus,
Bellamy [24] has pointed out that in polynitro compounds when one nitro group
remains coplanar whilst another is twisted out of the plane of the ring under the
influence of steric hindrance, the degree of aromatic conjugation is reduced and
a new higher frequency band appears.
Frank, Hörman and Scheibe [34] also found that asymmetric NO 2 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:

The nitro group in the ortho position also gave a higher frequency. Thus for

instance in the case of o-, m- and p- nitroanisoles the frequencies were 1352, 1347

and 1341 cm-1 respectively. In the case of m- and p- nitrophenol the frequencies

were 1349 and 1339 cm-1 respectively.

The frequency of the nitro group in o- nitrophenol was markedly reduced, to 1320

cm

-1

, as the result of hydrogen bonding between the nitro and phenolic group:

Bellamy [34] pointed out that in polyaromatic compounds multiple peaks can

occur if hydrogen bonds are attached to the nitro group.

Steric effects of bulky substituents in the ortho position to the nitro group (such

as I) can reduce the frequency of the p- nitro group vibrations, as shown by Perret

and Holleck [36] (Table 24).