QTAIM Atomic Charge and Polarization Parameters and Their Machine-Learning Transference among Boron-Halide Molecules

Area of Science:

• * Quantum Chemistry
• * Molecular Spectroscopy
• * Computational Chemistry

Background:

• * Atomic charges are invariant to out-of-plane distortions, making molecular vibrations valuable for electronic structure studies.
• * Boron trihalides (BX₃) possess highly polar bonds but exhibit unexpectedly weak out-of-plane bending intensities.
• * Existing atomic charge models do not fully explain these low intensities.

Purpose of the Study:

• * To investigate the out-of-plane infrared intensities of boron trihalides (BX₃, where X = H, F, Cl, Br).
• * To understand the interplay between atomic charges, charge transfer, and dipolar polarization in determining these intensities.
• * To develop a reliable method for estimating these intensities using machine learning.

Main Methods:

• * Quantum theory of atoms in molecules (QTAIM) was used to calculate atomic charges and dipoles.
• * The charge, charge transfer, dipolar polarization model was applied at the QCISD/aug-cc-pVTZ quantum level.
• * A machine-learning decision-tree algorithm was employed for parameter transference and intensity estimation.

Main Results:

• * Calculated infrared intensities for boron trihalides range from 0.6 to 106.1 km mol^-1.
• * Dipole moments from charge displacement are nearly canceled by electronic density polarization, explaining low intensities.
• * The hydride (BH₃) intensity is significantly higher than that of trichloride (BCl₃), despite BCl₃ having a more negative Cl charge.
• * Machine learning provided intensity estimates with a root-mean-square error of 2.1 km mol^-1.

Conclusions:

• * The cancellation of dipole moments by polarization is the primary reason for low out-of-plane intensities in boron trihalides.
• * Direct transference of atomic charge and dipole parameters between different boron trihalides is unreliable.
• * Machine learning offers a robust method for accurate intensity estimation in these systems.