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

Beckmann, Peter A. and Rosenberg, Jessie and Nordstrom, Kerstin and Mallory, Clelia W. and Mallory, Frank B. (2006) CF_3 Rotation in 3-(Trifluoromethyl)phenanthrene: Solid State ^(19)F and ^1H NMR Relaxation and Bloch−Wangsness−Redfield Theory. Journal of Physical Chemistry A, 110 (11). pp. 3947-3953. ISSN 1089-5639. doi:10.1021/jp056643o.

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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].

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Additional Information:© 2006 American Chemical Society. Received 16 November 2005. Published online 24 February 2006. Published in print 1 March 2006.
Issue or Number:11
Record Number:CaltechAUTHORS:20170606-123544330
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Official Citation:CF3 Rotation in 3-(Trifluoromethyl)phenanthrene:  Solid State 19F and 1H NMR Relaxation and Bloch−Wangsness−Redfield Theory Peter A. Beckmann, Jessie Rosenberg, Kerstin Nordstrom, Clelia W. Mallory, and Frank B. Mallory The Journal of Physical Chemistry A 2006 110 (11), 3947-3953 DOI: 10.1021/jp056643o
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77969
Deposited By: Ruth Sustaita
Deposited On:06 Jun 2017 21:09
Last Modified:15 Nov 2021 17:35

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