13C Spectral assignment and spin-lattice relaxation in medium-sized molecules

The individual ¹³C spin-lattice relaxation times in three molecules of different molecular size - codeine, brucine and reserpine - have been determined. In all three molecules, T₁ for methine carbons, being part of the isotropically reorientating skeleton, ranges from 0.15 to 0.23 sec corresponding to reorientational correlation times of 3 × 10⁻¹⁰ and 2 × 10⁻¹⁰ sec. All carbons, including the quaternaries, ar shown to be relaxed through dipole-dipole interaction. The relaxation rate of non-proton-bearing carbons is found to be governed by geminal and vicinal protons thus allowing reliable assignments to be made that are unavailable from other techniques. The peak heights in completely relaxed ¹³C Fourier transform spectra of medium-sized molecules are shown to reflect spin-spin relaxation, thus permitting a qualitative estimate of this parameter. As an example, the spectrum of 3-cholestanone is discussed.