BI-EXPONENTIAL NUCLEAR-SPIN-LATTICE RELAXATION IN SOLID HEXAFLUOROACETYLACETONE

The reorientation of CF3 rotors in solid hexafluoroacetylacetone has b een studied using pulsed NMR techniques. The temperature dependence of both fluorine (F-19) and hydrogen (H-1) nuclear spin-lattice relaxati on has been measured using saturation-recovery pulse sequences. Follow ing saturation of one of the spin reservoirs, the evolution with time of the longitudinal magnetisations associated with both nuclei are obs erved to be strongly bi-exponential. A theory is presented for the spi n-lattice relaxation which is mediated by the heteronuclear dipole-dip ole interaction between F-19 and H-1 nuclei. The experimental results are in good quantitative agreement with this relaxation theory. The sp ectral density functions have been evaluated using the known molecular structure and they are consistent with the experimental data. From th is analysis we conclude that the relaxation is dominated by an intramo lecular F-19-H-1 dipolar interaction. The spectral density function J( 0)(omega(H)-omega(F)) makes an important contribution to the relaxatio n theory with the result that the motional spectrum is conveniently sa mpled at the difference frequency between H-1 and F-19 Larmor precessi ons. Two physically distinguishable CF3 rotors have been identified in this investigation, their dynamics are characterised by activation en ergies of 7.5+/-0.4 and 13.3+/-0.8 kJ mol-1, respectively.