Spin-lattice relaxation parameters in the quantitative determination of condensed aromatic compounds by carbon-13 nuclear magnetic resonance spectrometry

Carbon-13 spin-lattice relaxation times (T/sub 1/), and nuclear Overhauser enhancement (NOE) factors have been measured at 25.1 and 75.3 MHz and 32/sup 0/C for tetralin, hexahydropyrene, octahydrophenanthrene, and 1-phenyloctane. For carbon-13 nuclei with attached protons, the relaxation is dominated by dipolar effects and T/sub 1/ and NOE values are relatively unaffected by field variations. In contrast, NOE and T/sub 1/ values for nonprotonated carbon nuclei are profoundly affected by changes in field strengths. Chemical shift anisotropies of 200 to 250 ppM are adequate to account for the observed field dependency. Solutions of the preceding compounds and naphthalene, phenanthrene, pyrene, and xanthone were studied with 0.1 M chromium acetylacetonante added. By use of 0.1 M Cr/sup 3 +/ and gated-decoupling techniques, the integrated and peak intensities of all carbon-13 signals are proportional, on a quantitative basis, to the number of /sup 13/C nuclei in the compounds studied. These techniques permit recycling times of under 2.5 s.