B3LYP Calculation of Deuterium Quadrupole Coupling Constants in Molecules

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy

Background:

• Nuclear quadrupole coupling constants (nqcc's) are sensitive probes of the electronic environment in molecules.
• Accurate theoretical prediction of nqcc's is crucial for interpreting experimental data and understanding molecular structure.
• Previous high-level calculations, such as those using Møller-Plesset perturbation theory (MP4), provided accurate nqcc's but were computationally expensive.

Purpose of the Study:

• To evaluate the accuracy of the B3LYP/6-31G(df,3p) model for calculating deuterium nqcc's.
• To compare the performance of B3LYP with higher-level theoretical methods.
• To investigate potential reassignments of experimental nqcc values for specific molecules.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP hybrid functional with the 6-31G(df,3p) basis set.
• Comparison of calculated nqcc's with experimental data for a diverse set of 25 molecules.
• Analysis of specific cases, including benzene, borane carbonyl, and nitric acid, with adjustments for hydrogen bond lengths and experimental assignments.

Main Results:

• The B3LYP/6-31G(df,3p) model achieved accuracy comparable to MP4 calculations for deuterium nqcc's.
• A root-mean-square (rms) deviation of 3.2 kHz (2.7%) was observed between calculated and experimental nqcc's for 25 molecules.
• Calculations suggest a reassignment of experimental inertial axes for benzene's nqcc's and improved agreement for borane carbonyl and nitric acid using optimized hydrogen bond lengths.

Conclusions:

• The B3LYP/6-31G(df,3p) model offers a computationally efficient and accurate method for determining deuterium nqcc's.
• The study highlights the importance of accurate structural parameters, particularly hydrogen bond lengths, in theoretical nqcc calculations.
• Potential errors in existing experimental assignments were identified, prompting further investigation.