\[k_\text{uni} = \frac{k_1k_2[\mathrm M]}{k_{-1}[\mathrm M] + k_2}\]
含义Meaning
解释气相单分子反应：分子先与碰撞伙伴 M 碰撞被活化为 A*，A* 再分解或被去活化。高压时为一级，低压时为二级（“降落”区）。
Explains gas-phase unimolecular reactions: collisions with M activate A to A*, which either reacts or is deactivated. First order at high pressure, second order at low pressure (fall-off).
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \([\mathrm M]\) | 碰撞伙伴（总气体）浓度collision-partner concentration | mol·L⁻¹ |
| \(k_1, k_{-1}\) | 碰撞活化、去活化速率常数activation and deactivation rate constants | L·mol⁻¹·s⁻¹ |
| \(k_2\) | 活化分子分解速率常数decomposition rate constant of A* | s⁻¹ |
推导Derivation
A + M ⇌ A* + M（k₁, k₋₁），A* → P（k₂）。
A + M ⇌ A* + M, A* → P.
对 A* 稳态：\([\mathrm A^*] = k_1[\mathrm A][\mathrm M]/(k_{-1}[\mathrm M] + k_2)\)。
Steady state for A*.
\(v = k_2[\mathrm A^*] = k_\text{uni}[\mathrm A]\)。高压 \(k_\text{uni} \to k_1k_2/k_{-1}\)（一级）；低压 \(\to k_1[\mathrm M]\)（二级）。
High pressure: first order; low pressure: second order.
适用条件与局限Assumptions & limitations
简单林德曼模型预测的降落压力偏高；RRK、RRKM 理论考虑能量在各振动模式间的分布。
Simple Lindemann predicts fall-off too high; RRK/RRKM theory accounts for energy among vibrational modes.