定义Definition
三角锥形是中心原子与三个原子相连、并带一对孤对电子时的分子构型（VSEPR 记作 AX₃E）。三个原子构成锥底，中心原子在锥顶，如 NH₃、PCl₃、H₃O⁺。
Trigonal pyramidal is the shape of a molecule whose central atom is bonded to three atoms and carries one lone pair (AX₃E in VSEPR notation). The three atoms form the base of a pyramid with the central atom at the apex, as in NH₃, PCl₃ and H₃O⁺.
成因与键角Origin and angles
中心原子周围共有四个电子对，呈四面体排布；其中一个是孤对电子，只看原子位置便是三角锥形。孤对电子对成键电子对排斥更强，使键角小于 109.5°：NH₃ 为 107°，PCl₃ 约 100°，PH₃ 约 93.5°。
The central atom has four electron pairs in a tetrahedral arrangement; one is a lone pair, so the atoms alone form a pyramid. The lone pair repels the bonding pairs more strongly, closing the angles below 109.5°: 107° in NH₃, about 100° in PCl₃ and about 93.5° in PH₃.
性质Properties
三角锥形分子属于 C₃ᵥ 点群，三个键偶极不能抵消，因此是极性分子。NH₃ 的孤对电子使它能作为路易斯碱和配体，接受质子生成正四面体形的 NH₄⁺。NH₃ 分子中的 N 原子能快速穿过三个 H 构成的平面发生翻转（伞形翻转）。
Trigonal pyramidal molecules belong to point group C₃ᵥ and their three bond dipoles do not cancel, so they are polar. The lone pair of NH₃ lets it act as a Lewis base and ligand, accepting a proton to form tetrahedral NH₄⁺. The nitrogen in NH₃ rapidly passes through the plane of the hydrogens (umbrella inversion).
年表Timeline
- 1940
西奇威克与鲍威尔用电子对数目解释分子形状
Sidgwick and Powell relate molecular shape to the number of electron pairs
- 1957
吉莱斯皮与尼霍姆完善 VSEPR 理论
Gillespie and Nyholm develop VSEPR theory