Fig. 4: Calculated potential energy levels (eV) of ionized pure and hydrated THF systems.

a–c The energetics of THF⁺ (a), H₂O·THF⁺ (b), and THF·THF⁺ (c) relative to the neutral ground state. Upon ionization of the HOMO, H₂O·THF⁺ and THF·THF⁺ can relax to the highest transition state (TS), i.e., TS-1, and then rearrange significantly the molecular structures involving particularly proton transfer (PT) and finally dissociate into H₂O·C₂H₄O⁺ and C₄H₈O·C₂H₄O⁺ ion fragments, respectively, and a C₂H₄ neutral part. For THF⁺, the activation energy (E_a) from the vertical ionization point to the TS-1 state is much higher compared with the dimer systems. Thereby, the subsequent dissociation processes are not likely for the isolated THF⁺ cation. FC refers to the Franck–Condon region, which is marked by the yellow bar. All energies include zero-point vibrational energy corrections. For comparisons between THF monomer and THF·THF dimer, the energy levels in THF monomer are recalculated using the same quantum-chemistry method as for the THF·THF dimer which is shown in Supplementary Fig. 4. The white, gray, and red balls represent hydrogen, carbon, and oxygen atoms, respectively. The PT process is shown by the circled hydrogen and arrow.