Denitration of nitroaromatic compounds by arylnitrile radical cations

Substituted nitrobenzenes react with substituted benzonitrile radical catio ns in an ion trap mass spectrometer by a novel ion/molecule reaction involv ing NO2 elimination. Formation of an arylated nitrile, Ar1+N=CAr2 (where Ar -1, Ar-2 = aryl), is indicated by collision induced dissociation and compar ison 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 experimen tal observation that the reaction rate decreases as the collision energy is increased. Electron withdrawing and donating substituents on either the io nic or the neutral reagent have little effect on the relative amount of pro duct observed, pointing to a radical mechanism. Related denitration reactio ns were found to occur, between nitrobenzene and its radical cation and bet ween phenylisonitrile and ionized nitrobenzene. These reactions are suggest ed to yield Ar1+N(=O)OAr2 and Ar2+N=CAr1 respectively. The denitration reac tion was applied to trinitrotoluene (TNT) as a possible diagnostic reaction for the presence of nitroaromatic explosives. (J Am Soc Mass Spectrom 2000 , 11, 33-39) (C) 2000 American Society for Mass Spectrometry.