This paper extends the account given by Chddin and others of the two Raman lines, 1400 and 1050 cm.-l, which are present, but weak, in the spectrum of nitric acid, axe intensified strongly, a t the expense of molecular nitric acid frequencies, on the addition of sulphuric acid, or dinitrogen pentoxide, or phosphoric oxide, are repressed again, with restoration of nitric acid frequencies, by the subsequent addition of water, and constitute the sole observed spectrum of solid dinitrogen pentoxide. Hitherto the frequency 1400 cm.+ has always been accompanied by 1050 cm.-l, the two being comparable in intensity over a great range of intensities. The frequency 1400 cm." has now been developed without its companion by the addition t o nitric acid of either perchloric acid or selenic acid. This shows t h a t the two lines do not come from a common molecular source, and a detailed study of the spectra proves that the source of the frequency 1400 cm.-l. has no other line in its spectrum. The source is thus unambiguously identified as the nitronium ion : no other structure, which could be composed from the elements in nitric acid, would yield the observed spectral characters.The source of the companion frequency 1050 crn.-l. as produced by the use of sulphuric acid, has several other lines in its Raman spectrum : they are shown to belong to, and t o identify, the hydrogen sulphate ion. When the frequency 1400 cm.-l is developed by the use of perchloric acid, other Raman lines appear : they belong to, and they identify, the perchlorate ion. When the frequency 1400 cm.-1 is developed with the aid of selenic acid, still other lines appear : they belong to, and identify, the hydrogen selenate ion. Generally, every spectral appearance, which could be expected from the following assumed modes of ionisation of nitric acid, is observed :HNO, + 2H,S04 = NO,+ + H,O+ + 2HS04-HNO, + 2HC10, = NO,+ + H,O+ + 2C104-HNO, + 2H,SeO, = NO,+ + H,O+ + 2HSe0,-I n the example of ionisation in sulphuric acid, the reversibility of the process is demonstrated, again in extension of Chedin's work. He showed that nitric acid fines, which had been weakened by the addition of sulphuric acid, could be restored by adding water ; and it is now shown that they can also be restored by adding potassium hydrogen sulphate. The signiiicance of the other known appearances or intensifications of the frequencies 1400 and 1050 cm.-l is treated in Part V (J, 1950, paper no. 510).(1) Objects and Methods. (1.1) Object of this Series of Papers.-The aim of the work to be described in the papers of this series is the spectroscopic study of the ionic forms assumed by nitric acid, by nitrous acid, and by the related oxides of nitrogen, not only in solution in various types of solvents, but also in the crystalline lattices of salt-like compounds.The manner in which this problem originally presented itself in connexion with an investigation of the kinetics and mechanism of aromatic nitration is described in an accompanying paper by Hughes, Ingold, and Reed ( J . , 1950, 2400)...
