Quantum effect on the internal proton transfer and structural fluctuation in the H+ 5 cluster

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Computational Chemistry

Background:

• Understanding the behavior of small protonated hydrogen clusters is crucial for various chemical and physical processes.
• Investigating the quantum mechanical effects on molecular structures at finite temperatures provides fundamental insights.

Purpose of the Study:

• To investigate the thermal equilibrium state of the protonated hydrogen cluster, H+(5).
• To explore the influence of temperature on the structure, dynamics, and electronic properties of H+(5).

Main Methods:

• Utilized ab initio path integral molecular dynamics (PIMD) to quantize nuclear and electronic degrees of freedom.
• Employed the second-order Møller-Plesset perturbation theory for the force field calculations.
• Analyzed structural fluctuations and their impact on molecular orbital energies.

Main Results:

• At 5-200 K, H+(5) exhibits a stable structure with a central proton surrounded by two H(2) units.
• Quantum tunneling of the proton is enhanced at lower temperatures (5 K) with reduced inter-H(2) distance.
• Proton delocalization increases with temperature (up to 200 K), weakening its correlation with inter-H(2) distance.
• H(2) units exhibit free rotation around the C(2) axis, with quantum effects diminishing at higher temperatures.
• The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap decreases with increasing temperature due to cluster expansion.

Conclusions:

• The study elucidates the temperature-dependent structural and dynamic behavior of H+(5).
• Quantum effects, such as proton tunneling and H(2) rotation, are significant at low temperatures but diminish with increasing thermal energy.
• Temperature-induced structural changes, particularly cluster expansion, play a critical role in modulating the electronic properties of H+(5).