Nitronium salts characterization

There are more than 15 crystalline nitronium salts that have been isolated and characterized with a variety of counterions. The most important salts are of BI , , ... 

On the other hand, some of these species are stable enough to be isolated as salts. Nitronium cation N02 exists in equilibrium with nitric acid at ambient temperature however, more than 15 crystalline nitronium salts with a variety of counter ions have been isolated and characterized [ 1 ]. The most widely used salt N02+BF4- is made by the reaction of HN03 and BF3 in anhydrous hydrogen fluoride [41] ... 

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... 

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

The regioselectivity of nitration of toluene with nitronium salts has been successfully altered by their prior complexation with crown ethers. Complexation of NO2 BFJ by 18-C-6 crown ether substantially altered the selectivity in nitration of toluene and benzene as reported by Elsenbaumet and Wasserraan [128]. Similar effect was observed with polyethylene oxides. Savoie al. reported isolation of the IS-C-b-NOa BFJ complex and its characterization [129], Masci carried out the yet most detailed study on the effect of crown ethers on the selectivity of electrophilic aromatic nitration [130]. 

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. 

In addition to the mechanisms of electrophile formation, another critical consideration relates to electrophile strength. There has been a vast amount of work done to characterize electrophile strengths [19]. Although much of the work relates to chemistry with n-type nucleophiles and nonaromatic tc-nucleophiles, some studies have sought to estimate electrophile strengths in SgAr reactions. Relative electrophile strengths became apparent as the synthetic S Ar reactions were developed. While the nitronium ion (NO ) salts react with benzene under mild conditions, carboxonium ions such as protonated formaldehyde (CH2=OH+) are weaker electrophiles and consequently do not react with benzene. 

It is possible to prepare and characterize solid salts containing the nitronium ion, for example, N02" C104. ... 

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