Spin relaxation studies of pyrene by 13C nuclear magnetic resonance spectroscopy

The effect of the repetition and contact times on the intensity of the magic-angle spinning (MAS) spectra of pyrene has been studied. Pyrene, like a great number of fused polyaromatic compounds, has a very large 13C longitudinal relaxation time (T1Z). When using the cross-polarization (CP) MAS technique, the optimum repetition time is 800 s. The simulation of the central line of the pyrene CP-MAS spectra allowed us to determine the chemical shift of each carbon in the solid state. As the T1Z (13C of the total spectrum) of pyrene was observed to be 1780 s, attempts have been made to relax it by the addition of chromium(III) acetylacetonate or by supporting it on a carrier of gamma-Al2O3: the mixing of chromium(III) acetylacetonate to pyrene by solid mixing, or solution mixing followed by evaporation of the solvent, has no effect on its relaxation time; the Haldor Topsøe TK551 catalyst, a gamma-alumina supported Ni-Mo-P catalyst, has been found to be a very efficient spin relaxation agent for the products adsorbed on it. For example, at 25 degrees C, the 10 wt.-% pyrene-TK551 catalyst is 3 x 10(4) times more relaxed than pure pyrene.