CF_3 Rotation in 3-(Trifluoromethyl)phenanthrene: Solid State ^(19)F and ^1H NMR Relaxation and Bloch−Wangsness−Redfield Theory

Abstract

We have observed and modeled the ^1H and ^(19)F solid-state nuclear spin relaxation process in polycrystalline 3-(trifluoromethyl)phenanthrene. The relaxation rates for the two spin species were observed from 85 to 300 K at the low NMR frequencies of ω/2π = 22.5 and 53.0 MHz where CF3 rotation, characterized by a mean time τ between hops, is the only motion on the NMR time scale. All motional time scales (ωτ ≪ 1, ωτ ≈ 1, and ωτ ≫ 1) are observed. The ^1H spins are immobile on the NMR time scale but are coupled to the ^(19)F spins via the unlike-spin dipole−dipole interaction. The temperature dependence of the observed relaxation rates (the relaxation is biexponential) shows considerable structure and a thorough analysis of Bloch−Wangsness−Redfield theory for this coupled spin system is provided. The activation energy for CF_3 rotation is 11.5 ± 0.7 kJ/mol, in excellent agreement with the calculation in a 13-molecule cluster provided in the companion paper where the crystal structure is reported and detailed ab initio electronic structure calculations are performed [Wang, X.; Mallory F. B.; Mallory, C. W; Beckmann, P. A.; Rheingold, A. L.; Francl, M. M J. Phys. Chem. A 2006, 110, 3954].