Water clusters (H2O)n [n = 9–20] in external electric fields: Exotic OH stretching frequencies near breakdown

Abstract Labile nature of water clusters (H 2 O) n [ n  = 9–20] in external electric fields is explored through their vibrational IR spectra under the M06-2X density functional prescription. Field-deformable clusters on the verge of breakdown exhibit particularly low-frequency, isolated, exotic O H stretches in the range of 2591–3021 cm −1 , markedly below those for their field-free counterparts, near 3200–3700 cm −1 . On the other hand, a few clusters among those endowed with a C n axis of symmetry remain ‘robust’ exhibiting no major structural changes up to their own electric-field breakdown thresholds, where dominant O H stretching frequencies manifest in the form of collective modes that appear only beyond 3200 cm −1 ; at frequencies greater than those of their field-free counterparts. Weakening of the covalent O H bonds for the deformable species is appraised through ‘Atoms in Molecule’ approach.