Molecular dynamics simulation of the electron ionization mass spectrum of tabun

Abstract

Tabun (ethyl N,N-dimethylphosphoramidocyanidate), or GA, is a chemical warfare nerve agent produced during the World War II. The synthesis of its analogs is rather simple; thus, it is a significant threat. Furthermore, experiments with tabun and other nerve agents are greatly limited by the involved life risks and the severe restrictions imposed by the Chemical Weapons Convention. For these reasons, accurate theoretical assignment of fragmentation pathways can be especially important. In this work, we employ the Quantum Chemistry Electron Ionization Mass Spectra method, which combines molecular dynamics, quantum chemistry methods, and stochastic approaches, to accurately investigate the electron ionization/mass spectrometry (EI/MS) fragmentation spectrum and pathways of the tabun molecule. We found that different rearrangement reactions occur including a McLafferty involving the nitrile group. An essential and characteristic pathway for identification of tabun and analogs, a two-step fragmentation producing the m/z 70 ion, was confirmed. The present results will be also useful to predict EI/MS spectrum and fragmentation pathways of other members of the tabun family, namely, the O-alkyl/cycloalkyl N,N-dialkyl (methyl, ethyl, isopropyl, or propyl) phosphoramidocyanidates. K E Y W O R D Schemical defense, chemical warfare nerve agents, ethyl N,Ndimethylphosphoramidocyanidate, fragmentation pathways, GA, mass spectrometry, molecular dynamics, tabun

Simulation of the electron ionization mass spectra of the Novichok nerve agent

“…Peak intensities are estimated by compiling the proportions of positive fragments originated from each trajectory of the ensemble, thus producing the EI mass spectrum 30 . Figure 1 illustrates these steps; more details of the method can be found in the original papers 30,49,51 and in our recent work on the EI/MS of Tabun 29 …”

“…32,59 Given that the QCEIMS method simulates every fragmentation trajectory, each reaction pathway can be identified and examined directly on the atomic level, which is not feasible experimentally. 29,49 Furthermore, by avoiding the need for synthesis and characterization of a dangerous controlled nerve agent, this approach allows the investigation of EI/MS fragmentation pathways safely.…”

“…Our simulations' scores ranged from 403 to 460, values comparable with other theoretical EI/MS studies. 29,48,49 For Cpd 3, it was 455.…”

“…Nevertheless, the study of EI/MS fragmentation pathways is useful as a predictive tool to identify compounds, mainly when EI/MS standards are not available 28 . Considering this scenario in the case of nerve agents, we published recently a comprehensive theoretical investigation of the EI/MS of Tabun, another dangerous nerve agent 29 …”

“…28 Considering this scenario in the case of nerve agents, we published recently a comprehensive theoretical investigation of the EI/MS of Tabun, another dangerous nerve agent. 29 In our previously published investigation of the EI/MS of Tabun, we employed the Quantum Chemistry Electron Ionization Mass Spectrometry (QCEIMS) 30 method, based on Born-Oppenheimer Molecular Dynamics (BOMD). 31 Several dissociation mechanisms such as McLafferty rearrangements 32 and two-step fragmentations were found and discussed.…”