The Unimolecular Chemistry of Protonated and Deprotonated 2,2-Dinitroethene-1,1-Diamine (FOX-7) Studied by Tandem Mass Spectrometry and Computational Chemistry

Abstract

2,2-Dinitroethene-1,1-diamine (FOX-7) was studied by means of electrospray ionization (ESI) and chemical ionization (CI) mass spectrometry in both positive and negative ion mode. Detailed mechanisms of unimolecular fragmentations of protonated and deprotonated FOX-7 were investigated using high- and low-energy collision-induced dissociation (CID) mass spectrometry, neutral fragment reionization mass spectrometry and quantum chemistry calculations. In deprotonated FOX-7, elimination of the carbodiimide molecule was identified as the energetically most favored fragmentation channel, closely resembling the base hydrolysis of FOX-7. The dinitromethanide ion is formed during this fragmentation as revealed by comparison with CID mass spectra of an isobaric ion prepared by the ESI of authentic sodium dinitromethanide. The proton affinity of FOX-7 was estimated as 855 kJ mol−1 by high-accuracy quantum chemistry calculations. This value corresponds to protonation at the C-2 position, though the oxygen-protonated tautomer was found to be nearly isoenergetic in the gas phase. In acetonitrile, the nitro group-protonated FOX-7 was found to be significantly less stable then its C-2 tautomer. These theoretical findings are clearly reflected in differences in fragmentations of ESI- and CI-generated [M + H]+ ions. Interestingly, the consecutive losses of OH• and NO2• radicals instead of a whole HNO3 molecule were found to account for the most abundant fragment ion in the positive ESI CID mass spectra. In the CI-generated [M + H]+ and [M+D]+ ions, substantial internal energy effects upon the CID were observed.