A THEORETICAL-STUDY OF CHLORINE ATOM AND METHYL RADICAL-ADDITION TO NITROGEN BASES - WHY DO CL ATOMS FORM 2-CENTER-3-ELECTRON BONDS WHEREASCH3 RADICALS FORM 2-CENTER-2-ELECTRON BONDS

Ab initio molecular orbital calculations have been carried out on a se ries of adducts between chlorine atom and NH3, NMe(3), NCl3, HN=CH2, a nd pyridine, and between methyl radical and HN=CH2 and pyridine. A two -center-three-electron (2c-3e) bond is predicted for all the chlorine adducts, whereas the CH3 adducts with the unsaturated systems form two -center-two-electron (2c-2e) bonds following promotion of one of the n itrogen lone pair electrons into a pi orbital. For chlorine adducts, the greater strength of the 2c-2e N-Cl bond compared with the 2c-3e N- Cl bond is not sufficient to compensate for the required promotion ene rgy in both the saturated and unsaturated amines. On the other hand, f or CH3 adducts of the unsaturated nitrogen bases, HN=CH2 and pyridine, the C-N and C-C bond energies are sufficiently high and the promotion energy is sufficiently low that adducts to both N and C with 2c-2e bo nds can be formed. Adducts between CH3 and saturated nitrogen centers are less stable than the separated species because of the inability of CH3 to form effective 2c-3e bonds in neutral systems (due to its low electron affinity), and because of the high excitation energy required to promote an electron from the nitrogen lone pair (due to the absenc e of suitable low-lying empty orbitals in these systems).