Raman nitronium salt

The Raman spectrum of nitric acid shows two weak bands at 1050 and 1400 cm. By comparison with the spectra of isolated nitronium salts ( 2.3.1), these bonds were attributed to the nitrate and nitronium ion respectively. Solutions of dinitrogen pentoxide in nitric acid show these bands , but not those characteristic of the covalent anhydride , indicating that the self-dehydration of nitric acid does not lead to molecular dinitrogen pentoxide. Later work on the Raman spectrum indicates that at —15 °C the concentrations of nitrate and nitronium ion are 0-37 mol 1 and 0 34 mol 1 , respectively. The infra-red spectrum of nitric acid shows absorption bands characteristic of the nitronium ion. The equivalence of the concentrations of nitronium and nitrate ions argues against the importance of the following equilibrium ... 

Infrared and Raman are also rapid spectroscopic techniques that have been useful in the characterization of electrophiles in the condensed phase. Many superelectrophiles are expected to possess characteristic or new vibrational modes. The harmonic vibrational frequencies and infrared intensities for the nitronium ion (N02+) and protonitronium ion (HNO22"1") have been estimated using ab initio molecular orbital calculations (Table 5).37 Although the vibrational modes for the superelectrophile (HN022+) clearly differ from that of the monocation, data were so far not reported for the superelectrophile using infrared and Raman spectroscopy. When nitronium salts were dissolved in excess HF-SbFs, no apparent... 

Examination of Raman and infra-red absorption spectra (which will be discussed later) confirmed the nitronium salt structure of all these compounds. 

Mixtures of nitric and sulphuric acids contain nitronium sulphates which have been described by Ingold and associates, Woolf and Emeleus(Vo). I,p. 19). Reval-licr and co-workers [10] have found by Raman spectroscopy and vapour pressure measurements, that compounds made by acting with SOj on nitric acid are salts of nitronium ion (NO ) and sulphate anions. Vitse [11] established the structure of the compound N2 0s,4S03 as nitronium ion salt by X-ray crystallography. The salts of pyrosulphuric acid (Vol. 1, p. 12) can be present only in a mixture of nitric acid with oleum or SO3. They are described in the paragraph on nitronium salts (p. 27). The presence of NO in various solutions was discussed in Vol. I, pp. 14- 49. 

Among other compounds nitric acid-boron trifluoride HN03.2BF3 complex (m.p. 53 0 obtained by Revallier and associates [37] proved by Raman spectroscopy to possess the structure of nitronium salt NO " (BF3)20M [38]. 

The preliminary experiments were carried out using both nitronium tetrafluorophosphate in sulpholane and nitronium hexa-fluorophosphate in nitromethane. However, the products from these experiments had unexpectedly high concentrations of nitroso compounds this was traced to significant amounts of nitrosonium salts in the supposedly pure nitronium salts. A method was devised for the purification of nitronium hexafluorophosphate (see Experimental) which, from the raman spectra, was completely effective in removing this impurity. The same method did not work well for nitronium tetrafluoroborate and so all subsequent work was carried out using the hexafluorophosphate salt. 

The purity of these samples was estimated from the Raman spectra and from the yields of 2, 4-dinitrotoluene obtained from the salts. p-Nitrotoluene reacts readily with nitronium salts to give a quantitative yield of 2, 4-dinitrotoluene, but does not react with nitrosonium salts at room temperature. The purified nitronium salt (C, 0. 254 g) was dissolved in nitromethane (3 ml) and added dropwise to a solution of -nitrotoluene (0.402 g) in 2 ml of the same solvent. After fifteen minutes the reaction... 

The nitronium salts have been well characterized by infrared and Raman spectroscopy, NMR, and by x-ray crystallography. All spectros-... 

The IV-nitropyridinium and A -nitroquinolinium salts are stable (but moisture sensitive) crystalline reagents, well characterized by spectroscopic methods (NMR, IR, Raman). They are prepared in essentially quantitative yield by the slow addition of the corresponding pyridine to an equivalent amount of the nitronium salt in acetonitrile, nitromethane. or sulfolane solution. It is important to add the pyridine to the solution of the nitronium ion, because excess pyridine present during the reaction can lead to opening of the pyridinium ring. The iV-nitropyridinium salts can be used as isolated compounds or they can be generated in situ. 

Such solutions also exhibit a Raman absorption band at 1400 cm. A similar band is observed for solutions of nitric acid in other strong acids, such as perchloric acid. The same band is present, but much weaker, in concentrated nitric acid. It is possible to prepare solid nitronium salts such as N02 C104 , and here too this band is found. This strongly supports assignment of the band to the NOa ion. 

The presence of N02, the nitronium ion, both in this solution and in a number of salts (some of which, e.g. NOz C104e, have actually been isolated) has been confirmed spectroscopically there is a line in the Raman spectrum of each of them at 1400 cm"1 which can only originate from a species that is both linear and triatomic. Nitric acid itself is converted in concentrated sulphuric acid virtually entirely into N02, and there can be little doubt left that this is the effective electrophile in nitration under these conditions. If the purpose of the sulphuric acid is merely to function as a highly acid medium in which NOz can be released from HO—N02, it would be expected that other strong acids, e.g. HC104, would also promote nitration. This is indeed found to be the case, and HF plus BF3 are also effective. The poor performance of nitric acid by itself in the nitration of benzene is thus explained for it contains but little N02 the small amount that is present is obtained by the two-stage process... 

The evidence for the existence of the nitronium ion is compelling. Salts such as nitronium perchlorate, N02 C10/, and nitronium tetrafluoroborate, N02 Bp4, have been isolated and successfully used as nitrating agents. A line in the Raman spectrum at 1400 cm that originates from a linear triatomic species, which N02 is, provides spectroscopic confirmation. 

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