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) w ere studied by neutralization-reionization mass spectrometry (NRMS) and a c ombination of ab initio PMP2/6-311G(2d,p) and density functional theory B3L YP/6-311G(2d,p) calculations, The experiment and theory agreed on the radic als being stable species in the gas phase. The order of 0 K relative enthal pies was established as 2 (most stable) < 4 (+17 kJ mol(-1)) < 3 (+22 kJ mo l(-1)). This differed from the order of cation enthalpies which was 2(+) (m ost stable) < 3(+) (+90 kJ mol(-1)) < 4(+) (+105 kJ mol(-1)). Metastable-io n spectra of 2(+), 3(+) and 4(+) showed losses of hydrogen cyanide as the d ominating 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 clea vage of the N-C-2 bond and elimination of hydrogen cyanide to form the 3-bu ten-1-yn-3-yl radical (6), which required 272 kJ mol(-1) at the thermochemi cal threshold at 0 K, The lowest-energy dissociation in 4 proceeded by clea vage of the C-2-C-3 bond and elimination of hydrogen cyanide to form 6, whi ch required 273 kJ mol(-1) at 0 K, The dissociations of pyridyl radicals ob served upon collisional neutralization were, in general, consistent with th e mechanisms of pyridine pyrolysis proposed earlier by Kiefer, Kern and cow orkers and by Here and Russell. The different energetics and dissociation m echanisms accounted for the difference in the NRMS spectra of 2(+)-4(+) whi ch allowed partial isomer differentiation.