Beckmann Peter A, Rheingold Arnold L
Department of Physics, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010-2899, USA.
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Dr., La Jolla, California 92093-0358, USA.
J Chem Phys. 2016 Apr 21;144(15):154308. doi: 10.1063/1.4944981.
The dynamics of methyl (CH3) and fluoromethyl (CF3) groups in organic molecular (van der Waals) solids can be exploited to survey their local environments. We report solid state (1)H and (19)F spin-lattice relaxationexperiments in polycrystalline 3-trifluoromethoxycinnamic acid, along with an X-ray diffraction determination of the molecular and crystal structure, to investigate the intramolecular and intermolecular interactions that determine the properties that characterize the CF3 reorientation. The molecule is of no particular interest; it simply provides a motionless backbone (on the nuclear magnetic resonance(NMR) time scale) to investigate CF3 reorientation occurring on the NMR time scale. The effects of (19)F-(19)F and (19)F-(1)H spin-spin dipolar interactions on the complicated nonexponential NMRrelaxation provide independent inputs into determining a model for CF3 reorientation. As such, these experiments provide much more information than when only one spin species (usually (1)H) is present. In Sec. IV, which can be read immediately after the Introduction without reading the rest of the paper, we compare the barrier to CH3 and CF3 reorientation in seven organic solids and separate this barrier into intramolecular and intermolecular components.
有机分子（范德华）固体中甲基（CH₃）和氟甲基（CF₃）基团的动力学可用于研究其局部环境。我们报道了多晶3 - 三氟甲氧基肉桂酸的固态¹H和¹⁹F自旋 - 晶格弛豫实验，以及分子和晶体结构的X射线衍射测定，以研究决定CF₃重取向特性的分子内和分子间相互作用。该分子本身并无特别之处；它只是提供了一个在核磁共振（NMR）时间尺度上静止的骨架，用于研究在NMR时间尺度上发生的CF₃重取向。¹⁹F - ¹⁹F和¹⁹F - ¹H自旋 - 自旋偶极相互作用对复杂的非指数NMR弛豫的影响，为确定CF₃重取向模型提供了独立的信息。因此，这些实验提供的信息比仅存在一种自旋种类（通常是¹H）时要多得多。在第四节中（可在读完引言后直接阅读，无需阅读本文其余部分），我们比较了七种有机固体中CH₃和CF₃重取向的势垒，并将此势垒分为分子内和分子间成分。
