Benchmarking Semiempirical QM Methods for Calculating the Dipole Moment of Organic Molecules

The dipole moment is a simple descriptor of the charge distribution and polarity and is important for understanding and predicting various molecular properties. Semiempirical (SE) methods offer a cost-effective way to calculate dipole moment that can be used in high-throughput screening applications although the accuracy of the methods is still in question. Therefore, we have evaluated AM1, GFN0-xTB, GFN1-xTB, GFN2-xTB, PM3, PM6, PM7, B97-3c, HF-3c, and PBEh-3c SE methods, which cover a variety of SE approximations, to directly assess the performance of SE methods in predicting molecular dipole moments and their directions using 7211 organic molecules contained in the QM7b database. We find that B97-3c and PBEh-3c perform best against coupled-cluster reference values yielding dipole moments with a mean absolute error (MAE) of 0.10 and 0.11 D, respectively, which is similar to the MAE of density functional theory (DFT) methods (∼0.1 D) reported earlier. Analysis of the atomic composition shows that all SE methods show good performance for hydrocarbons for which the spread of error was within 1 D of the reference data, while the worst performances are for sulfur-containing compounds for which only B97-3c and PBEh-3c show acceptable performance. We also evaluate the effect of SE optimized geometry, instead of the benchmark DFT geometry, that shows a dramatic drop in the performance of AM1 and PM3 for which the range of error tripled. Based on our overall findings, we highlight that there is an optimal compromise between accuracy and computational cost using GFN2-xTB (MAE: 0.25 D) that is 3 orders of magnitude faster than B97-3c and PBEh-3c. Thus, we recommend using GFN2-xTB for cost-efficient calculation of the dipole moment of organic molecules containing C, H, O, and N atoms, whereas, for sulfur-containing organic molecules, we suggest PBEh-3c.

Theoretical studies on the structure and property of alkylated dipenylamine antioxidants

Diarylamines ( Ar 2 NH ) are generally used as antioxidants to inhibit or retard the auto-oxidation degradation of lubricating oil by trapping ROO• radicals. In the present study, 20 kinds of 4,4′-disubstituted diphenylamine compounds were investigated through density functional theory (DFT) calculations. The results indicate that the N – H bond dissociation enthalpy (BDE) linearly correlates its one-electron oxidation potential, the difference in Mulliken atomic charge on the two atoms of N – H bond, the reaction rate constant of hydrogen transfer from Ar 2 NH to peroxy radical, and the chemical hardness of the resulted Ar 2 N • radical, respectively. The substitution of alkyl groups (electron-donating groups) decreases the N – H BDE, one-electron oxidation potential and the reaction rate constant, while that of significant electron-withdrawing groups such as - NO 2 and - COOCH 3 increases these three parameters. The electron-donating groups such as alkyls could improve the antioxidation performance of 4,4′-disubstitued diphenylamines whereas electron-withdrawing groups have the contrary effect. In addition, the frontier molecular orbital of Ar 2 NH has been also analyzed.