Base Catalysis of Sodium Salts of [Ta6−xNbxO19]8− Mixed-Oxide Clusters

The solid base catalysis of sodium salts of Lindqvist-type metal oxide clusters was investigated using a Knoevenagel condensation reaction. We successfully synthesized the sodium salts of Ta and Nb mixed-oxide clusters Na8−nHn[(Ta6−xNbx)O19]·15H2O (Na-Ta6−xNbx, n = 0, 1, x = 0–6) and found them to exhibit activity for proton abstraction from nitrile substrates with a pKa value of 23.8, which is comparable to that of the conventional solid base MgO. The Ta-rich Na-Ta6 and Na-Ta4Nb2 exhibited high activity among Ta and Nb mixed-oxide clusters. Synchrotron X-ray diffraction (SXRD) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray absorption spectroscopy (XAS) revealed the structure of Na-Ta6−xNbx: (1) The crystal structure changed from Na7H[M6O19]·15H2O to Na8[M6O19]·15H2O (M = Ta or Nb) by the anisotropic expansion of the unit cell with an increase in Ta content; (2) Highly symmetrical Lindqvist [Ta6−xNbxO19]8− was generated in Na-Ta4Nb2 and Na-Ta6 because of the symmetrical association of Na+ ions with [Ta6−xNbxO19]8− in the structure. DFT calculation revealed that the Lindqvist structures with high symmetry have large NBO charges on surface oxygen species, which are strongly related to base catalytic activity, whereas the composition hardly affects the NBO charges. The above results showed that the Brønsted base catalysis was sensitive to the symmetry of the Lindqvist [Ta6−xNbxO19]8− structure. These findings contribute to the design of solid base catalysts composed of anionic metal oxide clusters with alkaline-metal cations.

Quantum-chemical simulation of the interaction of 2-methyl-5,7-dinitrobenzo[d]oxazole with tetrahydride borate ion by DFT method

The aim of the work was quantum chemical modeling by the theory of the functional density functional of the interaction of 2-methyl-5,7-dinitrobenzo[d]oxazole with tetrahydride borate ion. For this, geometric optimization and calculation of the total energies of the 2-methyl-5,7-dinitrobenzo[d]oxazole molecule in the gas phase and water were carried out using the DFT/B3LYP/aug-cc-pVDZ method. To determine the likely reaction centers for a nucleophile attack, atomic charges according to Mulliken and NBO charges in the molecule of the initial substrate were established. The largest positive charges according to Mulliken in the studied molecule, both in the gas phase and in water, are concentrated on the carbon atoms C4 and C6, whereas in the case of NBO analysis, such is the carbon atom C2. Analysis of nucleophilic atomic frontal electron densities on the atoms of the substrate showed that from the point of view of orbital control, the attachment of a nucleophile is most likely to the carbon atom in the 4 position. The data obtained are consistent with previous experimental studies in which a rigid base – methoxide ion is attached to a carbon atom C2, which is a rigid acid reaction center, and a reaction with a soft base – tetrahydride borate ion proceeds along softer acid centers – carbon atoms C4 and C6 of the benzene ring. The calculation of the total energies of the proposed -adducts allowed us to establish that the most thermodynamically stable structure when one hydride ion is attached to the 2-methyl-5,7-dinitrobenzo[d]oxazole molecule is the product of nucleophile addition to the C4 carbon atom, and in the case of two hydride ions – at positions 4 and 6 of the annelated benzene ring activated by nitro groups. Thus, the calculation of the Mulliken charges, as well as the values of the nucleophilic atomic frontal electron densities at the atoms of the substrate, best reflects the course of the reaction under conditions of orbital control with a tetrahydroborate ion, and NBO charges are better suited to describe the course of the reaction under conditions of charge control.

What can pKa and NBO charges of the ligands tell us about the water and thermal stability of metal organic frameworks?

The pKa1 values and NBO charges of 32 carboxylate ligands and 31 N-heterocyclic ligands were calculated at 298.15 K with the B3LYP/6-31+G(d,p) level of calculations, which are useful to understand the water and thermal stability of Metal–Organic Frameworks (MOFs).

Predicting Heats of Explosion of Nitroaromatic Compounds through NBO Charges and 15N NMR Chemical Shifts of Nitro Groups

This work presents a new quantitative model to predict the heat of explosion of nitroaromatic compounds using the natural bond orbital (NBO) charge and 15N NMR chemical shifts of the nitro groups (15NNitro) as structural parameters. The values of the heat of explosion predicted for 21 nitroaromatic compounds using the model described here were compared with experimental data. The prediction ability of the model was assessed by the leave-one-out cross-validation method. The cross-validation results show that the model is significant and stable and that the predicted accuracy is within 0.146 MJ kg−1, with an overall root mean squared error of prediction (RMSEP) below 0.183 MJ kg−1. Strong correlations were observed between the heat of explosion and the charges (R2 = 0.9533) and 15N NMR chemical shifts (R2 = 0.9531) of the studied compounds. In addition, the dependence of the heat of explosion on the presence of activating or deactivating groups of nitroaromatic explosives was analyzed. All calculations, including optimizations, NBO charges, and 15NNitro NMR chemical shifts analyses, were performed using density functional theory (DFT) and a 6-311+G(2d,p) basis set. Based on these results, this practical quantitative model can be used as a tool in the design and development of highly energetic materials (HEM) based on nitroaromatic compounds.

Theoretical Investigation of Solvation Effects on the Tautomerism of Maleic Hydrazide

A DFT study is used to calculate structural data of tautomers of maleic hydrazide (MH) in the gas phase and selected solvents such as benzene (non-polar solvent), tetrahydrofuran (polar aprotic solvent) and methanol (protic solvent), dimethyl sulfoxide (polar aprotic solvent) and water (protic solvent) using PCM model. All tautomers are optimized at the B3LYP/6−31++G(d,p). The results show that the tautomer MH2except in methanol is more stable than the other tautomers but in methanol MH5(Diol) is more stable. In addition, stability of the tautomers in deferent solvents shows interesting results. Variation of dipole moments and NBO charges on atoms in the solvents were studied.