Computational studies on acyclic amidyl radicals: Pi and Sigma states and conformations

Restricted open and unrestricted Becke3LYP/6-31 + G(d) calculations on Pi a nd Sigma states as well as equilibrium geometries of the formamidyl radical (1) and four of its dialkyl substituted derivatives 2-5 have been carried out. While all radicals studied are significantly twisted about the RN-C(O) bond and show a Pi-type total spin density, the calculations confirm the s pecial status of N-tert-butyl acetamidyl (4) that was found with EPR spectr oscopy. Each of the torsional double-minimum potentials of N-methyl and N-i sopropyl radicals 2, 3, and 5 shows a low barrier to interconversion for tw o equivalent conformers whereas 4 is situated in a steeper well with a larg er twist angle which explains reported EPR C-13 hyperfine splittings. (C) 1 999 Elsevier Science B.V. All rights reserved.