CF3 Rotation in 3-(Trifluoromethyl)phenanthrene: Solid State 19F and 1H NMR relaxation and Bloch-Wangsness-Redfield theory.
We have observed and modeled the 1H and 19F 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 omega/2pi = 22.5 and 53.0 MHz where CF3 rotation, characterized by a mean time tau between hops, is the only motion on the NMR time scale. All motional time scales (omegatau << 1, omegatau approximately 1, and omegatau >> 1) are observed. The 1H spins are immobile on the NMR time scale but are coupled to the 19F 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 CF3 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].
Beckmann
Peter A
PA
Department of Physics, Bryn Mawr College, 101 North Merion Avenue, Bryn Mawr, Pennsylvania 19010-2899, USA. pbeckmann@brynmawr.edu
Rosenberg
Jessie
J
Nordstrom
Kerstin
K
Mallory
Clelia W
CW
Mallory
Frank B
FB
D018360:Crystallography, X-Ray; D005461:Fluorine / Q000737:chemistry; D006845:Hydrocarbons, Fluorinated / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010616:Phenanthrenes / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D012399:Rotation; D012680:Sensitivity and Specificity; D013696:Temperature
C521700:3-(trifluoromethyl)phenanthrene; D006845:Hydrocarbons, Fluorinated; D010616:Phenanthrenes; D011522:Protons; D005461:Fluorine