A Statistically Supported Antioxidant Activity DFT Benchmark—The Effects of Hartree–Fock Exchange and Basis Set Selection on Accuracy and Resources Uptake

Polyphenolic compounds are now widely studied using computational chemistry approaches, the most popular of which is Density Functional Theory. To ease this process, it is critical to identify the optimal level of theory in terms of both accuracy and resource usage—a challenge we tackle in this study. Eleven DFT functionals with varied Hartree–Fock exchange values, both global and range-separated hybrids, were combined with 14 differently augmented basis sets to calculate the reactivity indices of caffeic acid, a phenolic acid representative, and compare them to experimental data or a high-level of theory outcome. Aside from the main course, a validation of the widely used Janak’s theorem in the establishment of vertical ionization potential and vertical electron affinity was evaluated. To investigate what influences the values of the properties under consideration, linear regression models were developed and thoroughly discussed. The results were utilized to compute the scores, which let us determine the best and worst combinations and make broad suggestions on the final option. The study demonstrates that M06–2X/6–311G(d,p) is the best fit for such research, and, curiously, it is not necessarily essential to include a diffuse function to produce satisfactory results.

A simple computational approach for pKa calculation of organosulfur compounds

The present work is related to predicting the pKa values of organosulfur compounds through Density Functional Theory (DFT). In this study 22 organo-sulfur compounds have been considered to calculate theoretical pKa values. Main emphasis has been given on the substitution of different groups on the Sulfur atom. The computations were performed in the presence of Dimethyl sulfoxide (DMSO) as solvent. Experimentally the order of increase of acidity is; Sulfides < < Sulfoxides < Sulfones. Our computed pKa values also follow the same order. The theoretical pKa values are computed using the DFT method B3LYP, with the basis sets 6-31G(d), 6-31+G(d,p) and IEFPCM bulk solvation model. The majority of computed pKa values are in excellent agreement with the experimental ones through the diffuse function basis set. Hence this computational approach, B3LYP/6-31+G(d,p)/IEFPCM, could be utilized to predict the pKa values of these types of organosulfur compounds.

Ab initio Calculations and Qualitative Valence Bond Considerations for H2N-NO2 (Nitramide) and H2N-NO (Nitrosamine)

The structures, energies and vibrational frequencies of H2N-NO2 and H2N-NO have been calculated at HF, MP2(FC), MP2(FULL) and CISD levels of theory using a polarized triple-zeta 6-311+G(d,p) basis augmented with a diffuse function. In addition, the structures and energies were also computed at the QCISD(T)/6-311+G(d,p) level of theory. The structure and bonding in H2N-NO2 and H2N-NO is discussed on the basis of a Natural Bond Orbital Analysis (NBO analysis) and in addition based on qualitative valence bond (VB) considerations.

Rearrangement of the Fulminate Anion (CNO-) to the Cyanate Anion (OCN-). Possible Intermediacy of the Oxazirinyl Anion

The rearrangement of the fulminate anion (CNO-) to the cyanate anion (OCN-) has been examined by using high-level ab initio molecular orbital calculations which include a diffuse-function-augmented polarization basis set and electron correlation incorporated at the full fourth-order Moller-Plesset level (MP4). The reaction is predicted to be exothermic by 275 kJ mol-1. Our best calculations indicate theinvolvement of a metastable cyclic oxazirinyl anion intermediate. However, this lies in an extremely shallow potential well and, in contrast to the predictions of semiempirical calculations, is unlikely to have more than a fleeting existence. The fulminate and cyanate anions are calculated to be stable with respect to electron loss and stable with respect to intramolecular rearrangement; accordingly, both should be observable gas-phase species.