ELECTRON SPIN RESONANCE SPECTROSCOPY OF XANTHYL FREE RADICALS .I. XANTHYL RADICAL - A PLANAR DIPHENYLMETHYL

The xanthyl radical (Figure 1, R = H) is investigated because of its structural similarity to diphenylmethyl. This radical is produced by homolytic thermal cleavage of dixanthyl in n-tridecane solvent in the temperature range 180-260°. The esr spectrum has been reduced to hyperfine-coupling constants. The splittings are |a1H| = 3.425 ± 0.008 G, a2H = 0.988 ± 0.005 G, |a3H| = 4.047 ± 0.010 G, |a4H| = 0.894 ± 0.005 G, and |a9H| = 12.729 ± 0.008 G. Esr results obtained for 3-deuterioxanthyl and 2-chloroxanthyl radicals and theoretical calculations of the spin density distribution of xanthyl radical are employed in the assignment of the above splittings to ring positions. The proton splittings of xanthyl radical have been calculated by means of the McConnell relation and Hückel MO and McLachlan SCF-MO computations of spin densities. Xanthyl is assumed to be planar in these calculations. The McLachlan theory spin densities lead to fairly good agreement with experiment. It is shown that the effect of the oxygen heteroatom on the spin density distribution is small and, consequently, the xanthyl radical can be considered as essentially a planar diphenylmethyl.