Theoretical study of the 4-pyridylmethyl and the 2- and 5-pyrimidyl-methyl cations and radicals

Abstract

Geometry optimized SCF–MO calculations in the INDO approximation have been performed on the 4-pyridyl-methyl (1), 2-pyrimidylmethyl (3), and 5-pyrimidylmethyl (5) radicals and their corresponding cations (2), (4), and (6). The spin distribution in (1), (3), and (5) and the charge distribution in (2), (4), and (6) provide evidence that mesomeric effects in these systems are similar to those in the benzyl radical and cation. The optimized geometries all have a quinonoid distortion which is more pronounced in the cation. The π-bond orders and the ring–CH₂·(or ring-CH₂⁺) rotational barriers were obtained. The results are discussed in terms of resonance hybrid structures. The ionization potentials of the radicals were obtained and found to be in the order (3) > (1) > (5). The calculated ionization potential of (1) was 8.71 V in good agreement with the experimental value of 8.40 V obtained by electron impact measurements.