Liu Shubin, Govind Niranjan, Pedersen Lee G
Renaissance Computing Institute, University of North Carolina, Chapel Hill, North Carolina 27599-3455, USA.
J Chem Phys. 2008 Sep 7;129(9):094104. doi: 10.1063/1.2976767.
Continuing our recent endeavor, we systematically investigate in this work the origin of internal rotational barriers for small molecules using the new energy partition scheme proposed recently by one of the authors [S. B. Liu, J. Chem. Phys. 126, 244103 (2007)], where the total electronic energy is decomposed into three independent components, steric, electrostatic, and fermionic quantum. Specifically, we focus in this work on six carbon, nitrogen, and oxygen containing hydrides, CH(3)CH(3), CH(3)NH(2), CH(3)OH, NH(2)NH(2), NH(2)OH, and H(2)O(2), with only one rotatable dihedral angle [angle]H-X-Y-H (X,Y=C,N,O). The relative contributions of the different energy components to the total energy difference as a function of the internal dihedral rotation will be considered. Both optimized-geometry (adiabatic) and fixed-geometry (vertical) differences are examined, as are the results from the conventional energy partition and natural bond orbital analysis. A wealth of strong linear relationships among the total energy difference and energy component differences for different systems have been observed but no universal relationship applicable to all systems for both cases has been discovered, indicating that even for simple systems such as these, there exists no omnipresent, unique interpretation on the nature and origin of the internal rotation barrier. Different energy components can be employed for different systems in the rationalization of the barrier height. Confirming that the two differences, adiabatic and vertical, are disparate in nature, we find that for the vertical case there is a unique linear relationship applicable to all the six molecules between the total energy difference and the sum of the kinetic and electrostatic energy differences. For the adiabatic case, it is the total potential energy difference that has been found to correlate well with the total energy difference except for ethane whose rotation barrier is dominated by the quantum effect.
延续我们近期的工作，在本研究中，我们运用其中一位作者（刘尚斌，《化学物理杂志》126卷，244103页，2007年）最近提出的新能量划分方案，系统地探究小分子内部旋转势垒的起源。在该方案中，总电子能量被分解为三个独立的组分：空间能、静电能和费米子量子能。具体而言，本研究聚焦于六种含碳、氮、氧的氢化物，即CH(3)CH(3)、CH(3)NH(2)、CH(3)OH、NH(2)NH(2)、NH(2)OH和H(2)O(2)，它们仅含有一个可旋转的二面角[角]H-X-Y-H（X、Y = C、N、O）。我们将考虑不同能量组分对总能量差的相对贡献随内部二面角旋转的变化情况。同时考察了优化几何结构（绝热）和固定几何结构（垂直）的能量差，以及传统能量划分和自然键轨道分析的结果。我们观察到不同体系的总能量差与能量组分差之间存在大量强线性关系，但尚未发现适用于两种情况的所有体系的通用关系，这表明即使对于像这样的简单体系，对于内部旋转势垒的本质和起源也不存在普遍适用、独一无二的解释。在解释势垒高度时，不同的能量组分可用于不同的体系。我们证实绝热和垂直这两种能量差在本质上是不同的，发现对于垂直情况，总能量差与动能和静电能差之和之间存在适用于所有六个分子的独特线性关系。对于绝热情况，除了乙烷的旋转势垒主要由量子效应主导外，发现总势能差与总能量差有很好的相关性。
