Denitration of nitroaromatic compounds by arylnitrile radical cations

Abstract

Substituted nitrobenzenes react with substituted benzonitrile radical cations in an ion trap mass spectrometer by a novel ion/molecule reaction involving NO 2 elimination. Formation of an arylated nitrile, Ar 1 +NCAr 2 (where Ar 1, Ar 2 = aryl), is indicated by collision induced dissociation and comparison with the behavior of the authentic ion. Ab initio calculations (MP2/6-31G∗//HF/6-31G∗) show the reaction of the unsubstituted compounds (Ar 1, Ar 2 = phenyl) to be exothermic by 48 kcal/mol, consistent with the experimental observation that the reaction rate decreases as the collision energy is increased. Electron withdrawing and donating substituents on either the ionic or the neutral reagent have little effect on the relative amount of product observed, pointing to a radical mechanism. Related denitration reactions were found to occur, between nitrobenzene and its radical cation and between phenylisonitrile and ionized nitrobenzene. These reactions are suggested to yield Ar 1 +N(O)OAr 2 and Ar 2 + NCAr 1, respectively. The denitration reaction was applied to trinitrotoluene (TNT) as a possible diagnostic reaction for the presence of nitroaromatic explosives.