Cyclobutanediyls: A new class of localized biradicals. synthesis and epr spectroscopy

Nine triplet cyclobutanediyls (1) have been synthesized and observed by matrix isolation EPR spectroscopy. The zero-field splitting (zfs) parameters provide detailed information on the spin densities in these structures. The observed zfs values can be quantitatively modeled by using a straightforward semiempirical scheme, as long as one explicitly incorporates the spin polarization effects known to be important in radicals such as allyl and benzyl. In addition, interpretable hyperfine coupling (hfc) has been observed in many cyclobutanediyls. Spectral simulation produces the hfc constants, which provide further information on spin distribution and indicate that the four-membered ring in 1 is planar. Several new procedures have been developed for the synthesis of the azoalkane precursors, 3, which are described in detail.