含义Meaning
同一类反应中活化能与反应焓线性相关：放热越多，能垒越低。\(\alpha\) 介于 0 与 1 之间，反映过渡态位置（哈蒙德假说的定量形式）。
Within a reaction family, activation energy is linear in reaction enthalpy: more exothermic means a lower barrier; α (0–1) locates the TS — a quantitative Hammond postulate.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(E_a\) | 活化能activation energy | kJ·mol⁻¹ |
| \(E_0\) | 该类反应的固有能垒intrinsic barrier of the family | kJ·mol⁻¹ |
| \(\alpha\) | 过渡态位置参数transition-state position parameter | 1 |
推导Derivation
用两条交叉的势能曲线（反应物、产物）近似：产物曲线下移 \(\Delta H\) 时，交点（过渡态）下移约 \(\alpha\Delta H\)。
Model reactant and product curves as intersecting lines; lowering the product curve by ΔH lowers the crossing point by about αΔH.
直线近似给出线性关系；用抛物线则得马库斯方程，α 随 ΔH 变化。
Linear curves give a linear law; parabolas give Marcus theory with ΔH-dependent α.
适用条件与局限Assumptions & limitations
只在同一反应家族、ΔH 变化不大时成立。
Only within one family over a modest ΔH range.
例题Worked example
自由基夺氢：Cl· 夺取 C–H 放热、能垒低、选择性差；Br· 吸热、能垒高、选择性好（叔 > 仲 > 伯）。
H abstraction: Cl· is exothermic and unselective; Br· endothermic and selective (3° > 2° > 1°).