ELECTRON SPIN RESONANCE SPECTROSCOPY OF XANTHYL FREE RADICALS .2. 9-PHENYLXANTHYL - SPATIAL CONFIGURATION

A study has been made of the esr spectrum of the 9-phenylxanthyl free radical (Figure 1) in order to gain information regarding the spatial configuration of the 9-phenyl substituent. 9-Phenylxanthyl radical is produced by homolytic thermal cleavage of 9,9′-diphenyldixanthyl at temperatures as low as -70°. The esr spectrum observed for this radical has been reduced to hyperfine coupling constants. These splittings are (in gauss): |a1H| = 3.306 ± 0.010, |a2H| = 0.86 ± 0.03, a3H = 3.809 ± 0.010, |a4H| = 0.74 ± 0.03, |aoH| = 0.74 ± 0.03, |amH| = 0.612 ± 0.008, and |apH| = 0.86 ± 0.03. Assignments to ring positions are made by analogy to the xanthyl radical and by means of McLachlan SCF-MO calculations of the spin density distribution. The g value of 9-phenylxanthyl is 2.00289 ± 0.00001. The plane of the 9-phenyl group has been determined to be approximately 60° from the plane of the xanthyl moiety by a comparison of the experimental splittings on the 9-phenyl group with splittings calculated using MO spin density distributions and McConnell's relationship. Critical to this determination of the angle of twist is the relationship between the Hückel MO resonance integral parameter β* and the angle φ0, β* = β cos φ0. This relationship, which has been previously justified on intuitive grounds, is shown in this work to be approximately valid. Our result for φ0 is compared with those of previous studies which employ esr to estimate spatial configuration. The angle of twist in 9-phenylxanthyl is close to those found for structurally similar radicals such as phenyl-substituted anthracene and naphthacene radical ions.