Semi-empirical and ab initio quantum chemical characterisation of pyridine derivatives as HCl inhibitors of aluminium surface

Quantum chemical methods are becoming ever more prevalent for assessing surface interactions of different molecules using cluster models and semi-empirical, ab initio Hartree–Fock and density functional theory (DFT) studies considering the standard potential energy surfaces. Examination of the efficacy of some pyridine derivatives to counter aluminium corrosion in hydrochloric acid using ab initio and semi-empirical quantum chemical deductions and its comparison with the available experimental data forms the basis of this research. It is believed that the inhibition efficiency has lucid correlation with the total energy of inhibitor molecules and highest occupied molecular orbital energy levels calculated by DFT study methods and thus the adsorption energy for the pyridine on Al₁, Al₅, Al₁₄, Al₁₈ and Al₂₆ clusters are determined to assess the convergence of the results with respect to size of the cluster. Subsequently, Al₂₆ is used for the inhibitor/aluminium cluster interface investigations. Results highlight the reaction between pyridine molecules and appropriately active sites such as corners and steps or kinks and screw dislocations towards which pyridine molecules are attracted as is evident from a three times rise in adsorption energy from (−35) to (−107) kJ mol⁻¹. Therefore, inhibition mechanism is primarily associated with local properties. Interactions take place between the surface defect and the nitrogen group of the pyridine molecule however, the possibility of ion pair formation between protonated pyridine and chloride ion and its influence on the general adsorption of pyridine on aluminium is also examined. The interaction energies of pyridine and aluminium cluster with the natural bond orbital are also reported.     

Structural features of tricarbonyl(N-methyl-2-pyridinecarboxyamide)chloro-rhenium(I)-potential precursor of radiopharmaceuticals

Structural characteristics of the novel [fac-Re(CO)₃L]Cl complex, where L denotes the N-methyl-2-pyridinecarboxyamide, are presented. Molecular structure of the complex has been established by means of X-ray single crystal diffraction and compared with quantum mechanical calculations. It has been shown that pyridinethioamide is softer base than pyridineamide. As a result of different Re^I–S and Re^I–O bond covalent shortenings (reflecting their diverse ionic/covalent character ratio) difference of the bond lengths decreases from a theoretical value of 0.45 Å to the value of 0.28 Å.     

α‐Tetraphosphorus Trichalcogenide Diamides and Imides containing both Sulfur and Selenium

Reaction of a mixture of bicyclic phosphorus sulfide selenide iodides α‐P₄S_nSe_3−nI₂ (n = 0–3) with PrⁱNH₂ and Et₃N gave corresponding diamides α‐P₄S_nSe_3−n(NHPrⁱ)₂ (n = 0–3) and imides α‐P₄S_nSe_3−n(μ‐NPrⁱ) (n = 2–3), identified in solution by ³¹P NMR. In one isomer of α‐P₄S₂Se(μ‐NPrⁱ), the C₂ symmetry of imides such as α‐P₄S₃(μ‐NPrⁱ) was broken, allowing relative assignment of ²J NMR couplings to the PNP bridge and the PSP bridge opposite to it. The coupling through the sulfur bridge was found to be reduced to ca. zero, in contrast to previous assumptions for this class of compounds. Ab initio models were calculated at the MPW1PW91/svp level for the sulfide selenide imides and for a selection of bond rotamers of the diamides, and at the MPW1PW91/LanL2DZ(d) level for the sulfide selenide diiodides. Different skeletal isomers were prevalent for the mixed chalcogenide diamides than for the diiodides, showing that exchange of chalcogen between skeletal positions took place in the amination reaction even at room temperature. Similar differences to those observed were predicted by the models, suggesting that equilibrium was attained.     

Preparation and Conformation of Organo‐Bridged Bis[tris(arylchalcogenolato)tin] Compounds – An Experimental and Quantum Chemical Study

Seven organo‐bridged bis[tris(arylchalcogenolato)tin] compounds with the general formulae (R′E)₃Sn–R–Sn(ER′)₃ (R = –(CH₂)₄–, 1,4‐bis(methyl)benzene, 4,4′‐bis(methyl)biphenyl; R′ = Ph, 1‐Np, 2‐Np; E = S, Se) were synthesized and characterized by means of X‐ray diffractometry as well as NMR spectroscopy. Three different conformations of the arylchalcogenolato groups ER′ with respect to the bridging group R were rationalized and explained by means of quantum chemical investigations.     

Pd-catalyzed regioselective hydroarylation of α-(2-aminoaryl)-α,β-ynones with organoboron derivatives as a tool for the synthesis of quinolines: experimental evidence and quantum-chemical calculations

The Pd-catalyzed hydroarylation of β-(2-aminoaryl)-α,β-ynones with organoboron derivatives, leading to 2,4-diarylquinolines in good to excellent yields through sequential cycloamination, has been investigated. The reaction is catalyzed by both Pd(II) and Pd(0) precatalysts, and can be carried out even under neutral conditions. The regiochemical outcome is inverted with respect to the Pd-catalyzed hydroarylation of β-(2-aminoaryl)-α,β-ynones with aryl iodides. This aspect has been rationalized using quantum-chemical calculations, which show significant differences between the energy barriers of the regioisomeric transition states for the migratory insertion (hydropalladation) step, and are consistent with the charge density of the π complex that undergoes such insertion.     

Bidentate phosphines as ligands in the palladium-catalyzed intramolecular arylation: the intermolecular base-assisted proton abstraction mechanism

The palladium-catalyzed arylation of aryl bromides can be carried out in the presence of bidentate phosphines, such as dppm, dppe, dppf, and Xantphos under mild conditions. The experimental results and the DFT calculations fully support for this reaction a mechanism proceeding by an intermolecular proton abstraction.     

Proton Transfer Isomerization of Pyrazole in the Ground State:σν sπ Mechanism and Water Assisting Effect

The proton transfer isomerization of pyrazole and the water assisting effect by looping 1 to 4 water molecules on the singlet state potential energy surface have been investigated by using hybrid density functional theory method (B3PW91) with a 6-311++G** basis set. Two mechanisms were proposed to explain the mono- and multi-water assisting effects, respectively. The reactants and products of all groups have been characterized on their potential energy surfaces. For the isomerization of monomolecule pyrazole, the isomerization energy barrier is 46.4 kcal·mol-1. For the monohydration assisting mechanism, the reactant complex is connected to the product complex via two saddle points. The corresponding isomerization barriers are 46.7and 23.0 kcal(mol-1, respectively. As to the multihydration assisting mechanism, the isomerization barriers are 12.0, 10.9 and 13.14 kcal(mol-1 accordingly, when the number of water molecules is 2, 3 and 4, respectively. The multihydration assisting isomerization can occur in water-dominated environments, for example, in the organism, and thereby is crucial to energy transference. The deproton and dehydrogen energies of monomolecule pyrazole and various hydrated pyrazoles were calculated and then found much bigger than the isomerization barriers of their relative complexes, suggesting the impossibility of deprotonation or dehydrogenation. The isomerization of pyrazole is a proton-coupling-electron-migration process, but two different mechanisms are noticed, viz.σ- and π-type mechanisms. The π-bond of pyrazole participates in isomerization in the π-type mechanism, whereas only σelectron takes part in isomerization in the σ-type mechanism.     

Structure–property studies and orientation effects of polythiophenes containing mesogenic side chains

Liquid crystalline materials display unique properties, which can be exploited in organic light emitting diodes. Polythiophene model compounds containing phenyl groups linked with azomethine, ester, and alkoxy groups [thiophene‐3‐alkyloxy benzoyloxy aniline series (N series) and thiophene‐3‐alkoxy phenoxy amino benzoate series (R Series)] were synthesized. Molecular orbital calculations were performed and the predicted band gaps compared to understand the effects of spacer length and linkage. The experimental photoabsorption characteristics are compared with the theoretically predicted band gap. Photoabsorption and emission studies on N series and R series polymers as the function of polarizer angle suggest that polymers of both series emit polarized light in all base color ranges. The electroemission characteristics of the above‐synthesized polymers were also recorded as the function of polarizer angle. The results show that the compounds also emit polarized electroemission, and the EL polarization ratio decreases with the increase of alkoxy chain length for N and R polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1463–1477, 2008