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26 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

Indeed, according to Bennett, Brand and Williams [43] the fully symmetrical

structure of NO 2

+

should be accepted on the basis of the Raman spectrum. They also pointed out

the similarity between CO 2 and NO 2 +, the only difference being that NO 2 + ion has

one extra unit charge on the central nucleus as compared with the CO 2 molecule.

Therefore it is to be expected that the frequency of fundamental vibration of NO 2 +

should be somewhat higher than for that of CO 2 (ca. 1320 cm-1). These consider-

ations are consistent with the observed value 1400 cm-1 for NO 2 +. Moreover the

degree of depolarization (ca. 0.20 according to Chédin) confirms that the line

must arise from a totally symmetrical vibration.

This structure was confirmed in 1948 by Cox, Jeffrey and Truter [65] by X-ray

examination of nitronium perchlorate. The distance N-O was found to be 1.1 Å

(Fig. 10). Grison, Eriks and Vries [65a] gave the figure 1.154±0.01 Å.

FIG. 10. Structure and bond distances in NO 2 + cation.

Infra-red absorption spectra. For a long time the experimental difficulty of

working with nitric acid in the infra-red region due to corrosion of the cells by

nitric acid hindered examination of the infra-red absorption spectrum of nitric

acid. Originally therefore, only the near infra-red region was studied.

Thus Badger and Bauer [7] examined the spectrum of 100% nitric acid

vapour alone and in dioxane solution in the region near 1 μ. Dalmon [8,49], Dalmon

and Freymann [9], and Freymann [66] have studied the spectra of nitric acid and

its solutions in dioxane, carbon tetrachloride and water.

By using glass and polyethylene cells with silver chloride windows Ingold and

his co-workers [40] succeeded in examining the infra-red spectra of nitric and

deuteronitric acid vapour, Fréjacques [67] examined the spectra of liquid and

gaseous nitric acid, and Bethel1 and Sheppard [68] investigated the infra-red

spectra of fuming nitric acid.

Fréjacques failed to detect the NO 2 + bands in the spectra and this cast some

doubt on the existence of the NO 2 + ion in concentrated nitric acid. However, Marcus

and Fresco [69] in their extensive investigation of the infra-red spectra of nitric

acid discovered that the absorptivity of the NO 2

+

ion is very low. The NO 2

+

bands

can be observed only when a thick layer (18.4 μ) of nitric acid is used. The NO 2 +
frequencies are: 2360 and 3745 cm-1. (The Raman frequency 1400 cm-1 cannot

be present in the infra-red spectrum as it is produced by fully symmetrical vibra-

tions of NO 2 +).

The intensity of the band 2360 cm

-1
is enhanced by the addition of nitronium
fluoroborates, sulphuric acid, phosphorous pentoxide, or small amounts of acetic