Definitive evidence for the existence of isomeric chlorophenyl radicals (C6H4Cl) from charge inversion mass spectrometry and DFT calculations

Free radical species are much more reactive than stable molecules, and so usually exist only as transient intermediates in chemical reactions. Charge inversion mass spectrometry using alkali metal targets is an effective method for determining the structure and dissociation processes of radicals, and can also enable differentiation between isomeric forms of compounds whose parent ions have similar mass spectra and similar collisionally activated dissociation spectra, such as the isomers of dichlorobenzene and chlorophenol. The charge inversion process using alkali metal targets proceeds via near-resonant neutralization, followed by spontaneous dissociation of the excited neutrals, and then endothermic negative ion formation. In the normalized charge inversion spectra of ortho-, meta-, and para-dichlorobenzene (C6H4Cl2) measured in this work, the intensities of the peaks associated with chlorine anions (Cl−) are almost same for each of the isomers, whereas the intensities of the peaks associated with the chlorophenyl anions (C6H4Cl−) display a strong dependence on the isomeric structure of the parent compound. The similarities of the Cl− ion peak intensities indicate that neutralization cross-sections and branching ratios to produce Cl radicals are the same for each of the isomeric precursor C6H4Cl2+ ions. The strong isomer-dependence of the peak intensities of C6H4Cl− anions suggests that the chlorophenyl radicals (C6H4Cl) formed from C6H4Cl2 by loss of Cl do not undergo isomerization, and that the electron transfer cross-sections to form the negative ions are strongly isomer-dependent. Density functional theory (DFT) calculations on the o-, m-, and p-C6H4Cl radicals show that the barriers to isomerization are in excess of 2.8 eV, and these high isomerization barriers are believed to be the reason for the absence of isomerization among the C6H4Cl radicals during the charge inversion process. Calculated adiabatic electron affinities and vertical electron affinities both show differences for the o-, m-, and p-isomers, but the trend in the magnitude of the electron affinities has an inverse relation to the peak intensities of the isomeric C6H4Cl− ions in the charge inversion spectra. It is, therefore, assumed that the cross-sections for negative ion formation are influenced by many factors other than the electron affinity, such as the relatively large geometrical distortion in the chlorophenyl radicals predicted in this work. The good agreement between the experimental results and the theoretical predictions obtained in this work provides compelling evidence for the existence of isomeric C6H4Cl radicals that do not undergo isomerization during charge inversion mass spectrometry.