Beckmann Peter A, Rosenberg Jessie, Nordstrom Kerstin, Mallory Clelia W, Mallory Frank B
Department of Physics, Bryn Mawr College, 101 North Merion Avenue, Bryn Mawr, Pennsylvania 19010-2899, USA.
J Phys Chem A. 2006 Mar 23;110(11):3947-53. doi: 10.1021/jp056643o.
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].
我们已经观察并模拟了多晶3-（三氟甲基）菲中的1H和19F固态核自旋弛豫过程。在ω/2π = 22.5和53.0 MHz的低核磁共振频率下，从85到300 K观察到了两种自旋物种的弛豫速率，其中以跳跃之间的平均时间τ为特征的CF3旋转是核磁共振时间尺度上唯一的运动。观察到了所有运动时间尺度（ωτ << 1、ωτ ≈ 1和ωτ >> 1）。1H自旋在核磁共振时间尺度上是不动的，但通过异自旋偶极-偶极相互作用与19F自旋耦合。观察到的弛豫速率的温度依赖性（弛豫是双指数的）显示出相当多的结构，并提供了对该耦合自旋系统的布洛赫-旺斯尼斯-雷德菲尔德理论的全面分析。CF3旋转的活化能为11.5±0.7 kJ/mol，与配套论文中报道晶体结构并进行详细的从头算电子结构计算的13分子簇中的计算结果非常吻合[Wang, X.; Mallory F. B.; Mallory, C. W; Beckmann, P. A.; Rheingold, A. L.; Francl, M. M J. Phys. Chem. A 2006, 110, 3954]。
