EPR studies of pyrazoline radical ions that are potential precursors to non-Kekulé polyene radical ions

Abstract

EPR spectra are reported for radical anion and (for the first time) radical cation derivatives of pyrazolines. The radical anion spectra are consistent with the unpaired electron being localized principally in the NN π*-orbital and this agrees both with theoretical calculations and literature precedents. For the radical canons of 4-alkylidenepyrazolines, the near degeneracy of the NN n^–-and CC π-orbitals makes the distribution of the unpaired electron difficult to predict. The spectra obtained indicate that the unpaired electron is chiefly associated with the NN orbital. There is also significant density in the CC bond. This is underestimated by UHF-INDO calculations. In principle, an elimination of (neutral) nitrogen from these radical ions would generate non-Kekulé polyene radical ions but so far this has not been achieved.