Mechanistic and kinetic aspects of chemical ionization mass spectrometry of polynuclear aromatic hydrocarbons and their halogen-substituted analogues using oxidizing reagents: A gas chromatographic-mass spectrometric and Fourier transform mass spectrometric study

Abstract

The mass spectral behavior of some substituted polynuclear aromatic hydrocarbons (PAHs) was investigated by using low-pressure chemical ionization conditions with a Fourier transform ion cyclotron resonance spectrometer (FT-MS) and by using gas chromatography-mass spectrometry (GC-MS) under higher pressure chemical ionization conditions. For the reactions of O −· with substituted PAHs, several reaction pathways are observed, including hydrogen atom transfer, hydrogen atom displacement, nucleophilic aromatic substitution, ring cleavage, and abstraction of H + 2 to generate (M - 2H) −. Bimolecular rate constants and product branching ratios for the reactions of O −· with benzene and a variety of substituted naphthalenes were obtained using FT-MS. The rate constants vary from (2.1 ± 0.4) × 10 −10 cm 3 molecule −1 s −1 for naphthalene to (2.1 ± 0.4) × 10 −9 cm 3 molecule −1 s −1 for 1 − chloronaphthalene, corresponding to reaction efficiencies of 8% and 60%, respectively. In contrast to previous pulsed radiolysis studies on the reactions of O −· with aromatic compounds in solution, O −· is observed to be a powerful gas-phase nucleophile. Nucleophilic substitution reactions are observed even in the absence of activating groups. Competition between this and other reaction pathways is related to the leaving-group abilities and inductive effects of the various substituents. Observation of reactions with the chlorinated naphthalenes implies limits on the heats of formation for the corresponding carbanions of Δ H f(1-ClC 10H − 6) < 46 ± 5 kcal mol −1 and Δ H f(2-ClC 10H − 6) < 49 ± 5 kcal mol −1 (1 kcal=4.184 kJ). Although the isomeric PAHs often give very similar mass spectra and their chromatographic peaks are not well resolved, differences in their GC-MS behavior can be used to distinguish anthracene and phenanthrene. For the unsubstituted PAHs reaction pathways are observed in agreement with recent studies and mass-selected FT-MS suggests which reaction pathways are due to ion-molecule reactions. Low ion intensities for many of the oxidation products relative to products formed by electron-transfer reactions of the PAHs suggest oxidation may occur by wall-catalyzed processes, in agreement with recent studies by Stemmler and Buchanan. The positional isomers of chloronaphthalene give clearly different mass spectra under both N 2O and N 2OO 2 chemical ionization conditions whereas the chloroanthracenes are not readily distinguished using GC-MS.