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NITRATION AND NITRATING AGENTS 7

on the electron diffraction of nitric acid vapour and measurements of the Raman

spectrum of anhydrous nitric acid by Chedin [6].

Nitric acid molecules are linked by hydrogen bonds. This assumption was

based originally on the results of studies of the nitric acid absorption spectrum

in the very near infra-red (ca. 1μ) by Badger and Bauer [7], Dalmon [8], Dalmon

and Freymann [9] and also on the X-ray investigation by Luzzati [10] (Fig. 16).

FIG. 1. (a) Bond distances and bond angles of the molecule of nitric acid (Maxwell

and Mosley [5]); (b) Crystalline nitric acid: molecules of nitric acid associated

through hydrogen bonds (Luzzati [10]).

More recent infra-red investigations have confirmed the existence of hydro-

gen bonds. According to Ingold, Cohn and Poole [40] they exist even in the va-

pour phase.

The association of the molecules is presented by a chain (I) or a ring (II) formula.

In both cases hydrogen bonds are present between the OH and NO 2 groups.

Gillespie and Millen [11] suggested another form of presentation of the associa-

tion of nitric acid molecules by means of hydrogen bonds. They assumed that

oxygen atoms linked by hydrogen bonds are tetrahedrally coordinated as in water

molecules (III) :