ELECTRON-SPIN RESONANCE AND CIDNP STUDIES ON 1,3-CYCLOPENTADIYLS - LOCALIZED 1,3 CARBON BIRADICAL SYSTEM WITH A TRIPLET GROUND-STATE - TUNNELING IN CARBON-CARBON BOND FORMATION

When matrix isolated 2, 3-diazabicycio[2.2.1]heptene-2 is photolyzed at 5.5 K a strong ESR signal can be observed describable by a triplet state spin Hamiltonian with zero-field splitting parameters D/hc = 0.084 and E/hc = 0.0020 cm-1.The spectrum is attributed to the diradical 1, 3-cyclopcntadiyl. Several methyl-substituted derivatives also showed ESR spectra. The decay kinetics of the signals are temperature independent from 1.3 to 20 K suggesting a tunneling process. From the kinetics the barrier height for the formation of bicyclopentane is estimated as 2.3 ± 0.2 kcal/mol. The triplet ground state of the diradical is inferred from the temperature dependence of the ESR spectra as well as from CIDNP experiments at room temperature. © 1979, American Chemical Society. All rights reserved.