Free radicals can be defined as chemical species which have a single unpaired electron. In the important case (for organic chemistry) of the methyl radical, the radical center is trivalent and trigonally hybridized (Scheme 1). The sp2 hybridized carbon atom and the three hydrogens are coplanar and the unpaired (odd) electron occupies a 2p carbon atomic orbital (AO), here arbitrarily designated as 2pz.