in conformations that are s-trans or s-cis with respect to the R-O-C(O) x dihedral. A comparison of experimental TREPR spectra with simulations indicates that the s-transconformer is observed in the case of the ethoxycarbonyl radical, 3a. In the case of the trifluorethoxycarbonyl radical, 3b, however, the additional proton HFS observed shows that it is the s-cisconformer that is formed. As calculations