Complex sentence: Quantum tunnelling becomes more probable the smaller the mass of the cation is, and the proton is the lightest possible stable cation, thus there is a minor effect from quantum tunnelling also, although it dominates at low temperatures only.
Split into simpler sentences: Quantum tunnelling becomes more probable the smaller the mass of the cation is, and the proton is the lightest possible stable cation. Thus there is a minor effect from quantum tunnelling also, although it dominates at low temperatures only.