Distinction of Isomeric Pyridyl Cations and Radicals by Neutralization-Reionization Mass Spectrometry,ab Initio and Density Functional Theory Calculations

Isomeric pyridyl radicals, 2-pyridyl (2), 3-pyridyl (3) and 4-pyridyl (4) were studied by neutralization-reionization mass spectrometry (NRMS) and a combination of ab initio PMP2/6-311G(2d,p) and density functional theory B3LYP/6-311G(2d,p) calculations. The experiment and theory agreed on the radicals being stable species in the gas phase. The order of 0 K relative enthalpies was established as 2 (most stable) < 4 (+17 kJ mol⁻¹) < 3 (+22 kJ mol⁻¹). This differed from the order of cation enthalpies which was 2⁺ (most stable) < 3⁺ (+90 kJ mol⁻¹) < 4⁺ (+105 kJ mol⁻¹). Metastable-ion spectra of 2⁺, 3⁺ and 4⁺ showed losses of hydrogen cyanide as the dominating dissociations, which were 273, 184 and 168 kJ mol⁻¹ endothermic, respectively. Radical 2 underwent competitive dissociations by losses of acetylene and hydrogen cyanide for which comparable threshold energies, 292 and 290 kJ mol⁻¹, respectively, were obtained computationally. Radicals 3 and 4 cannot eliminate acetylene via low-energy paths or intermediates as investigated by computations. The lowest-energy dissociation in 3 was cleavage of the N–C-2 bond and elimination of hydrogen cyanide to form the 3-buten-1-yn-3-yl radical (6), which required 272 kJ mol⁻¹ at the thermochemical threshold at 0 K. The lowest-energy dissociation in 4 proceeded by cleavage of the C-2-C-3 bond and elimination of hydrogen cyanide to form 6, which required 273 kJ mol⁻¹ at 0 K. The dissociations of pyridyl radicals observed upon collisional neutralization were, in general, consistent with the mechanisms of pyridine pyrolysis proposed earlier by Kiefer, Kern and coworkers and by Hore and Russell. The different energetics and dissociation mechanisms accounted for the difference in the NRMS spectra of 2⁺–4⁺ which allowed partial isomer differentiation.