Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
Chemistry. 2010 Mar 22;16(12):3663-75. doi: 10.1002/chem.200902599.
All the atoms in a molecule attract each other until they reach their equilibrium positions at which point the repulsive forces between the atoms just balance the attractive forces and there are no resultant forces acting on any of the atoms in the molecule. Thus, we can consider that in the equilibrium geometry the atoms in a molecule are arranged as compactly as possible. This is the basis of the ligand close packing (LCP) model according to which three or four monatomic ligands X, such as F, Cl or O (formally =O or O(-)) pack as closely as possible around a small central atom such as a boron or carbon atom giving a truly close-packed equilateral triangular AX(3) molecule or a tetrahedral AX(4) molecules. Such monatomic ligands can, to a good approximation, be described as having a spherical shape with a single ligand radius r(X). In contrast, ligands with donor atoms with lone pairs such as the oxygen atom in an OX group have a less symmetrical electron density requiring two ligand radii, r(O(lp)) in the lone pair direction, and r(O(b)) in the bonding direction, where r(O(lp)) < r(O(b)) for an approximate description. On this basis we propose an explanation for the "methyl tilt", in methanol and many related molecules, and in conjunction with the concept of compactness, a model for explaining the relative energies of the conformations of molecules containing OH and OMe ligands, including molecules that exhibit the anomeric effect. We compare our model for the anomeric effect with the widely accepted "hyperconjugation" model. We also discuss the relationship between the concept of compactness and the concept of hardness.
在分子中，所有原子都相互吸引，直到它们到达平衡位置，此时原子之间的斥力刚好平衡引力，分子中没有任何原子受到合力的作用。因此，我们可以认为在平衡几何形状中，分子中的原子尽可能紧凑地排列。这就是配体紧密堆积（LCP）模型的基础，根据该模型，三个或四个单原子配体 X，如 F、Cl 或 O（形式上为 =O 或 O(-)），尽可能紧密地围绕一个小的中心原子如硼或碳原子排列，形成真正的紧密堆积等边三角形 AX(3)分子或四面体 AX(4)分子。这样的单原子配体可以很好地近似为具有单一配体半径 r(X)的球形。相比之下，具有孤对电子的配体原子，如 OX 基团中的氧原子，具有不太对称的电子密度，需要两个配体半径 r(O(lp))在孤对电子方向上和 r(O(b))在成键方向上，其中 r(O(lp)) < r(O(b))是一个近似描述。基于此，我们提出了对甲醇和许多相关分子中“甲基倾斜”现象的解释，以及结合紧凑性概念，对含有 OH 和 OMe 配体的分子构象相对能量的解释模型，包括表现出端基效应的分子。我们将我们的端基效应模型与广泛接受的“超共轭”模型进行了比较。我们还讨论了紧凑性概念和硬度概念之间的关系。
