Electronic Structure and Vibrational Spectroscopy in Hydrogen Bonded Clusters: Application to Water

Abstract

Hydrogen bonding is undoubtedly one of the most important interactions that alter molecular structure, charge distribution, energetics, and vibrational spectra. However, achieving theoretical descriptions that are simultaneously accurate, chemically interpretable, and computationally efficient remains challenging. This thesis addresses these issues in hydrogen-bonded clusters, with particular emphasis on water as a prototypical system whose vibrational behavior is highly sensitive to its local environment. The first part of this thesis explores how the vibrational properties of a water molecule change as it becomes part of increasingly large hydrogen-bonded networks with larger solvation shells (first, second, and third), simulating environments that mimic 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 the 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. The reliability of quantum mechanical methodologies is essential for modeling hydrogen bonding interactions. The valence complete active space self-consistent field (vCAS) method is evaluated for hydrogen-bonded dimers (H₂O)₂, (HF)₂, and HF–H₂O. The vCAS method predicts donor X–H bond contractions and corresponding vibrational blue shifts relative to the isolated monomers, which contradict both experimental results and higher-level methods. Analysis of the active orbitals indicates that the failure of the vCAS method results from an incorrect assignment of orbitals to the active space, with orbitals associated with the oxygen lone pairs being included in the active space rather than orbitals associated with the OH bonds. This leaves the donor bond without the left–right correlation required to describe bond elongation upon hydrogen-bond formation. This has significant implications for subsequent treatments of dynamical correlation based on the vCAS wavefunction. In contrast, MP2 and CCSD(T) provide a physically consistent description of hydrogen bonding for these systems, correctly reproducing donor X–H bond elongation and the associated vibrational red shifts.