The barrier $\Delta\varepsilon_0^{\ddagger}$ is the energy difference between the transition-state reference level and the separated reactant reference levels. Consistent <zero-point energies> of stable transverse modes belong in this reference-level difference. It is an energy per reacting pair; a molar activation energy uses $R$ instead of $k$ in the exponential.

Solution

Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 2 / A8 / b / iii / Solution

 Solution (source code)