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

Labile nature of water clusters (H2O)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 OH 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 Cn axis of symmetry remain ‘robust’ exhibiting no major structural changes up to their own electric-field breakdown thresholds, where dominant OH 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 OH bonds for the deformable species is appraised through ‘Atoms in Molecule’ approach.