Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations

Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H 2 O loss from T + (H 2 O) n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T + (H 2 O) n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n , consistent with the general trend of increasing metastable H 2 O loss in the experimental data. The lowest-energy conformers of T + (H 2 O) 3 and T + (H 2 O) 4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters.