Stationary points along the potential energy surface for the H-abstraction reaction CH₃NH₂ + H^• → H₂C^•NH₂ + H₂. The upper panel shows the configuration where the incoming H atom is located beneath the -NH₂ group, weakening its interaction with the ice surface and leading to a lower activation barrier (9.9 kJ mol⁻¹). The lower panel shows a configuration in which the H atom originates from a different binding site and does not perturb the interaction of CH₃NH₂ with the surface, resulting in a higher barrier (30.2 kJ mol⁻¹). Bond distances are in Å.