Electric Field Pushes the Water Dimer Radical Cation toward the Hemibonded Structure

The water dimer radical cation, (H₂O)₂^+•, is a key intermediate in ionized water relevant to radiation, atmospheric, and energy-conversion chemistry. It exists mainly as a hydrogen-bonded cation-radical ([H₃O⁺···OH^•]) and a symmetric hemibonded form ([H₂O···OH₂]^+•), the former being energetically preferred in the absence of external perturbations. Motivated by recent experimental evidence of hemibonded species in microdroplets and by reports of the existence of extremely strong interfacial electric fields (EFs), we investigate how EFs affect the stability and interconversion of these two conformations. Accurate quantum-chemical [CCSD(T)] and multireference CASPT2 calculations show that the hydrogen-bonded and hemibonded forms exhibit qualitatively different responses to positive and negative EFs in terms of energetics, structure, and vibrational signatures. A positive EF stabilizes the hydrogen-bonded form and enhances H₃O⁺–OH^• separation, whereas a negative EF destabilizes it and drives proton transfer, which completes at approximately −0.6 V Å^–1. In contrast, the hemibonded form and the transition state are stabilized by both EF polarities, with a symmetric energy response around zero field. Consequently, negative EFs strongly reduce the barrier and energy gap between the two conformations, rendering the conversion effectively barrierless at ≈−2.0 V Å^–1. Beyond representing, to the best of our knowledge, the first application of CASPT2 to open-shell water radical systems under external EFs, these results demonstrate that intense EFs can promote hemibond stabilization, providing mechanistic insight into water cation chemistry in microdroplets and EF-assisted catalysis.