Electron Paramagnetic Resonance as a Probe of Conformational Flexibility in Cyclic Alkanes

Abstract

Electron paramagnetic resonance (EPR) spectra of a homologous series of nitroxyl radicals derived from cyclic alkanones with 6 to 15 carbons in the ring have been obtained as a function of temperature. Long-range hyperfine coupling (0.7 to 2.2 G) was observed in the low-temperature spectra for rings with 6, 7, and 9 to 15 carbons. Deuteration of the 11- to 14-membered rings indicated that the long-range coupling was to one of the protons on the carbons adjacent to the point of attachment of the nitroxyl radical. For the 9- to 15-membered rings, there was a temperature region in which the hyperfine coupling decreased rapidly. This decrease is attributed to rapid conformational averaging on the EPR time scale. For the 12-, 13-, and 15-membered rings, the high temperature spectra exhibited resolved hyperfine coupling which can be computer-simulated, assuming four equivalent protons. The activation energies estimated from the coalescence temperatures in the EPR spectra fall in the range estimated by nuclear magnetic resonance and show similar influences of ring size on rigidity.