Characterizing Infrared Spectra of OH ^– ·(H ₂ O) ₂ and OH ^– ·(H ₂ O) ₃ with Constrained Nuclear-Electronic Orbital Molecular Dynamics

The vibrational spectra of OH^-·(H₂O)_<i>n</i> clusters for small <i>n</i> have been well established experimentally, with fundamental modes largely assigned. However, clear assignment of highly anharmonic modes and combination bands associated with strong hydrogen bonds, which often manifest as broad spectral features, remains challenging. In this work, we employ constrained nuclear-electronic orbital molecular dynamics (CNEO-MD) to provide detailed peak assignments and plausible physical interpretations for the vibrational spectra of OH^-·(H₂O)_n clusters with <i>n</i> = 2 and 3. The CNEO framework incorporates nuclear quantum effects, particularly nuclear quantum delocalization, through the underlying effective potential energy surfaces. When combined with classical molecular dynamics, CNEO-MD further captures coupling effects between vibrational modes. Leveraging machine-learned potentials, we perform a series of temperature-dependent CNEO-MD simulations and use the resulting spectra to facilitate peak assignment. Our results largely confirm the experimental assignments reported by Johnson and coworkers [<i>J. Chem. Phys.</i> <b>2016</b>, 145, 134304], while also providing direct, physically grounded interpretations of previously unassigned features.