Photochemistry of HCl(H₂O)₄: Cluster model of the photodetachment of the chloride anion in water

The covalent, zwitterionic, and biradicalic forms of the HCl(H₂O)₄ cluster have been investigated with density functional theory (DFT) and time-dependent DFT (TDDFT). The equilibrium geometries and force fields of the lowest electronic states have been determined with DFT; vertical electronic excitation energies have been calculated with TDDFT. It is shown that the excited states of the H₃O⁺(H₂O)₃Cl^- zwitterion are of the charge-transfer-to-solvent (CTTS) type. The molecular and electronic structures of the H₃O(H₂O)₃Cl biradical have been characterized for the first time. The lowest electronic states of the biradical are significantly lower in energy than the CTTS excited states of the zwitterion and therefore are photochemically accessible from the latter. The electronic and vibrational absorption spectra of the biradical are essentially identical to those of the hydrated hydronium radical, H₃O(H₂O)₃, and exhibit striking similarities with the spectral signatures of the solvated electron in the liquid phase. It is argued that the photochemistry of the H₃O⁺(H₂O)₃Cl^- zwitterion represents a finite-size molecular model of the formation process of the solvated electron via the photodetachment of the chloride anion in water.