定义Definition
重叠式是沿 C–C 键看去，前后两个碳上的键互相重合（二面角 0°）的构象；交叉式是前后键互相错开 60° 的构象。交叉式能量更低。
In an eclipsed conformation the bonds on the two carbons line up when viewed along the C–C bond (dihedral 0°); in a staggered conformation they are offset by 60°. Staggered is lower in energy.
乙烷Ethane
乙烷重叠式比交叉式高约 12 kJ/mol（每对 H/H 重叠约 4 kJ/mol），室温下每秒互变约 10¹¹ 次。能垒主要来自扭转张力：重叠时成键电子对排斥增大，交叉时 σ(C–H) → σ*(C–H) 超共轭更有利。
Eclipsed ethane is ~12 kJ/mol above staggered (~4 kJ/mol per H/H eclipse) and interconverts ~10¹¹ times per second at room temperature. The barrier is torsional: bonding-pair repulsion when eclipsed and better σ(C–H) → σ*(C–H) hyperconjugation when staggered.
丁烷Butane
丁烷有四种典型构象：对位交叉（anti，最稳定）、邻位交叉（gauche，+3.8 kJ/mol）、部分重叠（+14 kJ/mol）、全重叠（两甲基重叠，约 +19 kJ/mol）。
Butane has anti (most stable), gauche (+3.8 kJ/mol), partially eclipsed (+14 kJ/mol) and fully eclipsed (~+19 kJ/mol) conformations.
邻位交叉效应Gauche effect
1,2-二氟乙烷等含电负性取代基的分子反而倾向邻位交叉式，称邻位交叉效应，可用超共轭解释。
Molecules with electronegative substituents such as 1,2-difluoroethane prefer gauche — the gauche effect, explained by hyperconjugation.
年表Timeline
- 1936
肯普与皮策由熵数据推断乙烷能垒
Kemp and Pitzer infer ethane’s barrier from entropy data