Effect of quantum nuclear motion on hydrogen bonding

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Hydrogen bonds (X-H⋯Y) are crucial in chemistry and biology.
• Proton delocalization and quantum effects are key to understanding hydrogen bond properties.
• Previous models often simplify or neglect the dynamic quantum nature of the shared proton.

Purpose of the Study:

• To investigate the impact of shared proton quantum motion on hydrogen bonded complexes.
• To quantitatively describe the proton's position, vibrational frequency, and isotope effects.
• To analyze the influence of bending modes on hydrogen bond characteristics.

Main Methods:

• Utilized a two-diabatic state model Hamiltonian for symmetric hydrogen bonds.
• Employed a parametrization specific to O-H⋯O complexes.
• Analyzed vibrational energy levels of the 1D ground state adiabatic potential.
• Extended the model to include secondary geometric isotope effects and bending modes.

Main Results:

• Successfully described proton position, X-H bond length, and longitudinal vibrational frequencies.
• Quantitatively captured isotope effects for proton motion.
• Demonstrated that bending modes compete with stretching modes for moderate to weak hydrogen bonds.
• Achieved good agreement with condensed phase data across a range of hydrogen bond strengths.

Conclusions:

• The quantum motion of the shared proton is a defining feature of hydrogen bonds.
• The simplified two-state model provides valuable insights into hydrogen bond properties.
• The model semi-quantitatively captures several observed trends in hydrogen bonding.
• Further refinement including bending modes improves agreement with experimental data.