All structural optimizations were performed using density functional theory (DFT) with the Q-Chem 5.4 software package. The clusters were optimized using the ωB97X-V functional with an aug-cc-pVDZ basis set and the obtained structures confirmed as minimum points on the PES via frequency calculations. For more accurate energies, we performed single-point calculations of all optimal structures using ωB97X-V functional with an aug-cc-pVTZ basis set. The binding energies (BE) of the protonated structures were calculated according to the following equation (E represents energy, N represents number of molecules)