Core-valence correlation effects on IR calculations: the BF3 and BCl3 cases

Area of Science:

• Computational chemistry
• Molecular spectroscopy
• Quantum chemistry

Background:

• Accurate theoretical prediction of molecular properties is crucial for understanding chemical phenomena.
• Infrared (IR) spectroscopy provides valuable information on molecular vibrations, including wavenumbers and intensities.
• Core-valence correlation effects, which account for electron correlation between core and valence shells, can influence calculated molecular properties.

Purpose of the Study:

• To investigate the impact of core-valence correlation on the infrared wavenumbers and intensities of Boron Trifluoride (BF3) and Boron Trichloride (BCl3) molecules.
• To compare the significance of core-valence correlation effects with the effects of basis set augmentation.
• To reconcile discrepancies between theoretical and experimental IR intensity data for BCl3.

Main Methods:

• Coupled-Cluster Single-, Double-, and (Triple)-Excitations (CCSD(T)) level of theory was employed.
• Dunning's correlation-consistent basis sets, including double-zeta (cc-pVDZ) and triple-zeta (cc-pVTZ) with core-valence (cv) functions, were utilized.
• Calculations were performed using both core-valence correlation and frozen-core approximations for comparison.
• Results were benchmarked against experimental data and calculations with augmented basis sets (e.g., aug-cc-pVTZ).

Main Results:

• Core-valence correlation effects were found to be smaller than the impact of adding augmented diffuse functions to the basis set.
• Observed trends in infrared wavenumbers and intensities were primarily attributed to augmented functions rather than core-valence functions.
• The study confirmed significant discrepancies between theoretical and experimental intensities for the stretching mode of BCl3.

Conclusions:

• Basis set augmentation (e.g., adding diffuse functions) has a more pronounced effect on IR wavenumbers and intensities than core-valence correlation for BF3 and BCl3.
• The inclusion of core-valence correlation is less critical than employing augmented basis sets for accurate IR spectral predictions in these molecules.
• Further investigation is needed to fully understand and resolve the observed intensity deviations for BCl3.