Dynamics of strained ring systems as studied by ESR and saturation transfer sensitive spectroscopic techniques

Effective spin-lattice relaxation rates for the cyclopentyl-1-carboxylic acid radical in X-irradiated 1,1-cyclopentanedicarboxylic acid measured by saturation-recovery and electron spin echo techniques are found to be determined by van Vleck and Raman mechanisms. Analysis of ESR, ELDOR, ENDOR, and passage spectra permitted characterization of the temperature dependence of the ring inversion frequency as τ_c^?1 = 3.35 × 10¹¹ exp(?1540/T), implying a barrier to ring inversion of 3 kcal. The magnetic resonance and double resonance spectra at low temperatures are characterized by nearly isotropic beta hyperfine couplings of 56 and 112 MHz which are averaged by the dynamic ring puckering to yield a value of 84 MHz at ambient temperatures. The gamma couplings are not resolved in the ENDOR spectra but from the linewidth of the ENDOR line at the free proton frequency a value of 3 MHz is established as the upper limit for these couplings.