Analysis of nuclear quantum effects on hydrogen bonding

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Computational Chemistry

Background:

• Hydrogen bonding is fundamental in chemistry and biology.
• Nuclear quantum effects (NQEs) significantly influence molecular properties, especially for light atoms like hydrogen.
• Understanding NQEs is crucial for accurately modeling chemical systems.

Purpose of the Study:

• To investigate the impact of NQEs on hydrogen bonding in hydrogen fluoride (HF)n clusters and a partially solvated fluoride anion, F-(H2O).
• To elucidate how NQEs affect structural and electronic properties, including bond distances and vibrational frequencies.
• To explore the role of cluster size and deuteration on these quantum effects.

Main Methods:

• Path integral Car-Parrinello molecular dynamics (PICPMD) to include NQEs.
• Second-order vibrational perturbation theory (VPT2) for vibrational analysis.
• Analysis of structural parameters (e.g., F-F and F-O distances) and electronic properties (e.g., 19F shielding constants).

Main Results:

• A directional change in NQEs' impact on hydrogen-bonding strength in (HF)n clusters was observed with increasing cluster size.
• NQEs increase F-F distances in smaller clusters ((HF)n=2-4) but decrease them in larger clusters ((HF)n>4).
• For F-(H2O), NQEs decrease the F-O distance, strengthen hydrogen bonding, and lower the 19F shielding constant, indicating reduced electron density on fluorine due to hydrogen delocalization. Deuteration reverses these effects.

Conclusions:

• NQEs play a critical role in determining hydrogen bond characteristics in both neutral clusters and ionic systems.
• The balance between electrostatic interactions and zero-point energy effects governs the NQEs' impact on hydrogen bond strength.
• Deuteration significantly alters the manifestation of NQEs, highlighting the importance of isotopic effects in condensed-phase systems.