Science. 1973 Feb 9;179(4073):527-32. doi: 10.1126/science.179.4073.527.
The internal rotation barrier in ethane appears susceptible to "explanation" at a qualitative, intuitively useful, reasonably correct level. At this level, localized and delocalized MO's each produce a description. For a rigid rotation, the delocalized description is basically the "orbital control" type familiar from Woodward-Hoffmann rules (30), Walsh's rules (31), and the formation of diatomic molecules (32). This description indicates that long-range eclipsed H---H antibonding is responsible for the barrier. The equivalent localized-bond description invokes destructive interference or repulsion between eclipsed C-H bonds. The extension of this kind of description to other molecules requires care. Subsequent to rigid rotation, the relaxation of ethane into its optimum eclipsed geometry produces almost no energy change but produces an unknown degree of energy "redistribution." I have emphasized that the theoretical distinction between "orbital control" and "steric interaction" is not precise and that a clarification in terminology may be desirable in this connection. The discussion of barriers given here applies only to a limited class of molecules, exemplified by ethane. No doubt, additional factors enter into a proper description of barriers in molecules having lower symmetry or more polar bonds, or both (33). Finally, I should point out that approaches other than those in categories 1 to 3 have been made in efforts to rationalize or predict barriers to internal rotation (34). The Hellmann-Feynman theorem has been applied to find the torque on ethane at conformations between staggered and eclipsed (35). The integral Hellmann-Feynman theorem has served as a basis for discussing barriers in terms of transition densities (36) and has led to an electrostatic model for barriers which has given some remarkably successful barrier predictions (37). These approaches are mathematically valid, and it is perfectly legitimate to try to extract physical explanations from them also. Thus, several valid explanations for the barrier to internal rotation in ethane are possible. However, I feel that the explanations proposed above in terms of delocalized or localized orbitals are preferable at present inasmuch as they are couched in terms and concepts currently in the mainstream of chemical thinking.
乙烷的内旋转势垒似乎可以在定性的、直观有用的、相当正确的水平上得到“解释”。在这个水平上，定域和离域 MO 都产生了一种描述。对于刚性旋转，离域描述基本上是熟悉的伍尔兹-霍夫曼规则（30）、沃尔什规则（31）和双原子分子形成（32）中的“轨道控制”类型。这种描述表明，远程重叠的 H---H 反键合是势垒的原因。等效的定域键描述则涉及到重叠的 C-H 键之间的破坏性干涉或排斥。将这种描述扩展到其他分子需要谨慎。在刚性旋转之后，乙烷松弛到其最佳重叠几何形状几乎不会产生能量变化，但会产生未知程度的能量“重新分布”。我强调了“轨道控制”和“空间相互作用”之间的理论区别并不精确，在这方面术语的澄清可能是可取的。这里给出的关于势垒的讨论仅适用于乙烷等有限类别的分子。毫无疑问，在具有较低对称性或更极性键的分子中，或者在两者都有的情况下，会有其他因素进入到对势垒的适当描述中（33）。最后，我应该指出，除了类别 1 到 3 中的方法之外，还已经有其他方法被用于合理化或预测内旋转势垒（34）。赫尔曼-费曼定理已被应用于寻找交错和重叠构象之间乙烷的扭矩（35）。积分赫尔曼-费曼定理已被用作根据过渡密度讨论势垒的基础（36），并导致了一个静电模型，该模型对势垒进行了一些非常成功的预测（37）。这些方法在数学上是有效的，从它们中提取物理解释也是完全合理的。因此，对于乙烷的内旋转势垒，有几种可能的有效解释。然而，我认为，目前基于离域或定域轨道的解释更为可取，因为它们是用当前化学思维主流的术语和概念来表达的。
