Pressure-Dependent Rate Constant Caused by Tunneling Effects: OH + HNO₃ as an Example

Tunneling effects on chemical reactions are well-known and have been unambiguously demonstrated by processes that involve the motion of hydrogen atoms at low temperature. However, the process by which tunneling effects cause a falloff curve (i.e., how reaction rate constants depend on pressure) has apparently not been previously documented. This work points out that falloff curves can indeed be caused by tunneling and explains the effect in simple terms. This is an interesting feature of quantum tunneling, which can appear in low temperature chemistry (such as in atmospheric or interstellar environments). In this Letter, we use high-level coupled-cluster calculations in combination with master-equation methods on the well-studied reaction of OH with HNO₃, which plays an important role in the upper troposphere and lower stratosphere. Our results in combination with available experimental data clearly demonstrate that the tunneling correction depends on not just temperature, but also pressure.