The" 0 NMR spectra were obtained with a Bruker WP 200 SY multinuclear spectrometer operating at 27.1 MHz for 170. No field/frequency locking system was used, and the spectra were obtained at about 30" C. Chemical shifts were measured relative to acetone as external reference. The acetone chemical shift (+ 572 ppm from water) was previously determined in a separate experiment.