Radiolytic preparation of radical cations of nitroalkanes and related compounds

Abstract

Exposure of dilute solutions of a range of organic nitro derivatives in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K gave radical cations thought to be formed from the expected primary species by some form of relaxation. These cations have e.s.r. spectra similar to that of nitrogen dioxide, and may be the isomeric nitrite cations, RONO⁺. However, the form and magnitude of the ¹³C hyperfine coupling for cations derived from ¹³CH₃NO₂ suggests that the alternative σ-structure (R˙NO₂)⁺ may be a better description.

Cations derived from CH₃CH₂NO₂, CH₃CH₂CH₂NO₂, (CH₃)₂CHNO₂, (CH₃)₃CHO₂, (CH₃)₂C(Cl)NO₂, (CH₃)₂C(Br)NO₂, (CH₃)₂C(CO₃Et)NO₂ and [(CH₃)₂CNO₂]₂ all had very similar e.s.r. spectra, with indications of extra hyperfine splitting from halogen nuclei for the chloro and bromo derivatives. The ethyl derivative was unique in giving a second species in comparable yields which exhibited very large hyperfine coupling and considerable g-value variation. This is tentatively identified as CH₃CH₂O˙ radicals complexed with NO⁺ cations. The ethyl ester derivative gave a second species thought to be the ester cation.

Several nitro aromatic compounds gave the normal π-cations initially, but all gave evidence for the rearranged NO₂-like cations on annealing. A derivative ArC(Me)₂NO₂ with a p-nitro group gave high yields of this NO₂-like cation even at 77 K, with no sign of the π-aromatic cation.

One vinyl derivative, 1-nitrocyclohexene, was also studied. This gave a complex spectrum which is assigned to a cation with its SOMO primarily confined to the alkene group. However, conversion into the rearranged cation was again detected on annealing.