Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling

Abstract

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH₃-NO₂ structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO₂⁺ and CH₃⁺, rearrangement prior to dissociation accounts for fragmentation products including NO⁺, CH₂OH⁺, and CH₂NO⁺. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH₂⁺, H⁺, and NO₂²⁺. On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.