Assessing Constrained Nuclear-Electronic Orbital Theory for the Floppy CH5+ Ion: Geometry-Dependent Zero-Point...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Molecular Dynamics

Background:

• Protonated methane (CH5+) is a fluxional ion with a complex potential energy surface.
• Understanding its dynamics requires incorporating nuclear quantum effects like delocalization and zero-point energy.

Purpose of the Study:

• To apply constrained nuclear-electronic orbital (CNEO) methods to CH5+ as a test case for quantum-corrected effective potentials.
• To examine the structural features, hydrogen rearrangement dynamics, and IR spectra of CH5+ using CNEO.

Main Methods:

• Constrained nuclear-electronic orbital (CNEO) theory applied to CH5+.
• Ab initio molecular dynamics simulations on the CNEO effective potential energy surface.
• Fourier-filtered analysis and Gaussian mixture modeling of C-H bond lengths.

Main Results:

• CNEO predicts a more symmetric C2v minimum energy structure for CH5+ compared to conventional methods.
• CNEO molecular dynamics shows enhanced hydrogen rearrangement starting at 50 K.
• Three distinct structural/dynamical motifs (equilibrium, fluxional, transition-state hovering) were identified.

Conclusions:

• The CNEO framework efficiently captures qualitative structural, dynamical, and spectroscopic trends in fluxional CH5+.
• Challenges remain in fully capturing quantum nuclear effects beyond zero-point energy with classical trajectory methods.