Reactivity of radical cations in the radiolysis of amides. An ESR study and quantum chemical calculation

The electronic structures and reactivities of the radical cations of dimethylformamide, dimethylacetamide, tetramethylurea and their distonic isomers were analyzed using the MNDO-UHF semiempirical method. The calculations were carried out for "frozen-in" and optimized geometries. The results showed maximum spin population at the nitrogen atoms in the radical cations under study. The oxygen atoms and adjacent methyl groups are characterized by a smaller spin population. The positive charge is mainly concentrated on the Ct atom and methyl groups. The C-H bonds corresponding to methyl group protons in the radical cations are substantially weakened in comparison with those in neutral molecules. The C-N bonds exhibit the largest weakening. The weakened bonds are characterized by the high probability of a rupture. This prediction is confirmed by the observed directions of deprotonation resulting in amide radicals and pathways of alpha- and beta-cleavage reactions known from the mass spectrometric studies of amides. It was shown that the distonic radical cations resulting from hydrogen transfer to the oxygen atom were considerably more stable than the corresponding classical isomers. The calculation results are supported by the ESR data for amides irradiated in freonic matrices at 77 K and in liquid state in the presence of spin traps.