Phenylnitrene Radical Cation Rearrangements.

The electronic structure and the rearrangements of the phenylnitrene radical cation CHN 2 have been investigated at DFT and CASPT2(7,9) levels of theory. The B state has the lowest energy of five identified electronic states, and it can undergo ring expansion to the 1-azacycloheptetetraene radical cation 4 with an activation energy of ca. 28 kcal/mol. Ring opening and recyclization provide a route to 5-cyanocyclopentadiene radical cation 8, which may undergo facile 1,5-hydrogen shifts. The 2-, 3-, and 4-pyridylcarbene radical cations 31, 35 , and 39 interconvert with the phenylnitrene radical cation via azacycloheptatetraenes with activation barriers <35 kcal/mol. The carbene-carbene and carbene-nitrene rearrangements, ring expansions, ring contractions, ring openings (e.g., to cyanopentadienylidene 28), and cyclizations taking place in all these radical cations are completely analogous to the thermal and photochemical rearrangements.