A. Fonseca et al., SPIN RELAXATION STUDIES OF PYRENE BY C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY, Solid state nuclear magnetic resonance, 4(4), 1995, pp. 203-216
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 larg
e C-13 longitudinal relaxation time (T-1Z). When using the cross-polar
ization (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 T-1Z (C-13 of the total spectrum) of pyrene was observed to be 17
80 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 eff
ect on its relaxation time; the Haldor Topsoe TK551 catalyst, a gamma-
alumina supported Ni-Mo-P catalyst, has been found to be a very effici
ent spin relaxation agent for the products adsorbed on it. For example
, at 25 degrees C, the 10 wt.-% pyrene-TK551 catalyst is 3 . 10(4) tim
es more relaxed than pure pyrene.