Acyl radical addition to pyridine: multiorbital interactions

The addition of the acetyl radical at the various positions in both pyridine and the pyridinium ion has been investigated using DFT calculations. Additions at the 2-, 3- and 4-positions in these systems are associated with simultaneous SOMO→π* and π→SOMO interactions, with the former interaction dominating in the case of pyridine, and that latter in the case of pyridinium. Simultaneous SOMO→π*, LPN→SOMO and LPN→π*cdouble bond; length as m-dasho interactions are predicted for the addition at the nitrogen atom in pyridine. The energy barrier for attack at the nitrogen atom in pyridine is calculated to be 54 kJ mol−1 at the BHandHLYP/6-311G(d,p) level of theory, some 6 kJ mol−1 lower than for the analogous attack at any other atom in pyridine, or at any position in the pyridinium ion. Multiorbital interactions are responsible this preference, resulting in an unusual motion vector in the transition state for attack at the nitrogen atom in pyridine.