Environmental effects in the vibrational shifts of water: From the monomer to models of the bulk

This study explores how the vibrational properties of a water molecule change as it becomes part of increasingly larger hydrogen-bonded networks of increasing solvation shells (first, second, and third), simulating environments mimicking liquid water. Using advanced ab initio methods [MP2 and CCSD(T)] and comparing them with classical interaction potentials, the analysis focuses on both the harmonic and anharmonic frequencies of the bending and stretching vibrations. The H-O-H bending mode quickly shifts to higher frequencies (blue shift) with solvation, rising from 1596 cm-1 in the monomer to 1653-1664 cm-1 in the first solvation shell, and stays within the observed infrared range for liquid water as more shells are added. In contrast, the O-H stretching frequencies experience significant red shifts, which depend strongly on the local hydrogen bond structure. Second-shell tetrahedral networks show large red shifts due to cooperative hydrogen bonding, while third-shell networks produce shifts typical of bulk water. The bending vibration is mainly influenced by immediate (nearest neighbor) hydrogen bond geometry and stabilizes quickly in tetrahedral environments, whereas the stretching modes reflect the broader distribution of hydrogen bond strengths that emerge in larger (beyond nearest neighbor) solvation shells. Overall, these findings deliver a systematic microscopic picture of how water's vibrational spectrum transitions from an isolated molecule to a bulk-like environment, highlighting the importance of including an environment-dependent 1-body term in classical many-body polarizable potentials for water.