Department of Chemistry, The University of Tennessee , Knoxville, Tennessee 37996-1600, United States.
J Phys Chem A. 2014 Feb 13;118(6):1150-4. doi: 10.1021/jp412588f. Epub 2014 Jan 30.
Kinetic diameters are often invoked in discussing gas adsorption and permeation in porous and polymeric materials. However, how these empirical kinetic diameters relate to the size and shape of the molecules as manifested by their "electron cloud" is unclear. In this paper, we obtain the quantum mechanical (QM) diameters of several common gaseous molecules by determining the cross-sectional sizes of their iso-electronic density surfaces at a predetermined small value. We show that the QM diameters are in good agreement with the kinetic diameters. For example, the trends for important gas pairs such as O2 versus N2 and CO2 versus N2 are consistent between the QM diameters and the most often quoted kinetic diameters. Hence, our work now provides a quantum mechanical basis for the empirical kinetic diameters and will be useful for designing separation media for small gaseous molecules according to their sizes.
动力学直径在讨论多孔和聚合材料中的气体吸附和渗透时经常被提及。然而，这些经验动力学直径与分子的大小和形状（如“电子云”所示）的关系尚不清楚。在本文中，我们通过确定其等电子密度表面在预定小值下的横截面积来获得几种常见气态分子的量子力学（QM）直径。我们表明，QM 直径与动力学直径吻合良好。例如，对于 O2 与 N2 和 CO2 与 N2 等重要气体对，QM 直径与最常引用的动力学直径之间的趋势是一致的。因此，我们的工作现在为经验动力学直径提供了量子力学基础，并将有助于根据分子大小设计用于小气体分子的分离介质。
