Zhou Wei, Wu Hui, Udovic Terrence J, Rush John J, Yildirim Taner
NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Dr., MS 6102, Gaithersburg, Maryland 20899, USA.
J Phys Chem A. 2008 Dec 11;112(49):12602-6. doi: 10.1021/jp807033m.
Using neutron inelastic scattering and diffraction, we have studied the quantum methyl rotation in zeolitic imidazolate framework-8 (ZIF-8: Zn(MeIM)(2), MeIM = 2-methylimidazolate). The rotational potential for the CH(3) groups in ZIF-8 is shown to be primarily 3-fold in character. The ground-state tunneling transitions at 1.4 K of 334 +/- 1 mueV for CH(3) groups in hydrogenated ZIF-8 (H-ZIF-8) and 33 +/- 1 mueV for CD(3) groups in deuterated ZIF-8 (D-ZIF-8) indicate that the barrier to internal rotation is small compared to almost all methylated compounds in the solid state and that methyl-methyl coupling is negligible. A 2.7 +/- 0.1 meV scattering peak assigned to the ground-state to first-excited-state, hindered rotational transition for H-ZIF-8, combined with a approximately 3 meV activation energy for methyl-group 3-fold jump reorientation estimated by quasi-elastic neutron scattering, suggests a very low methyl rotational barrier of approximately 7 meV. Results are compared to the CH(3) rotational amplitude at 3.5 K derived from neutron diffraction data, which are also consistent with a small 3-fold barrier and a very low energy rotational oscillation.
利用中子非弹性散射和衍射，我们研究了沸石咪唑酯骨架-8（ZIF-8：Zn(MeIM)₂，MeIM = 2-甲基咪唑酯）中的量子甲基旋转。结果表明，ZIF-8中CH₃基团的旋转势主要具有三重特征。氢化ZIF-8（H-ZIF-8）中CH₃基团在1.4 K时的基态隧穿跃迁为334±1 μeV，氘代ZIF-8（D-ZIF-8）中CD₃基团的基态隧穿跃迁为33±1 μeV，这表明与几乎所有固态甲基化化合物相比，内部旋转的势垒较小，且甲基-甲基耦合可忽略不计。一个2.7±0.1 μeV的散射峰被指定为H-ZIF-8从基态到第一激发态的受阻旋转跃迁，结合准弹性中子散射估计的甲基基团三重跳跃重取向的约3 meV活化能，表明甲基旋转势垒非常低，约为7 meV。将结果与由中子衍射数据得出的3.5 K时的CH₃旋转振幅进行比较，这也与小的三重势垒和非常低能量的旋转振荡一致。
