No abstract

No abstract

ABSTRACT:We theoretically investigated how the formation of oxygen vacancies and the addition of niobium and chromium atoms as dopants modify the varistor properties of TiO . The calculations were carried out at the HF level using a contracted charges and band gap after oxygen vacancy formation and the presence of dopants in the lattice are analyzed and discussed. It is shown that the formation of oxygen vacancies decreases the band gap while an opposite effect is found when dopants are located in the reduced surface. The theoretical results are compared with available experimental data. A plausible explanation of the varistor behavior of this system is proposed.

ABSTRACT:The presence of a heterocyclic ring containing a basic center linked via a methylene chain to a substituted guanidine or thiourea polar side chain, such as found in the H2-antagonist metiamide, which has an imidazole heterocyclic ring, has often been identified as one of the requirements for H2-antagonist activity. In ranitidine, on the other hand, the imidazole ring is substituted for a furan ring, yielding a more active biological H2 antagonist. In this work, we have used the ab initio Hartree-Fock (HF) and second-order Møller-Plesset (MP2) methods in order to investigate the open and folded ranitidine conformations, of the type observed in metiamide. Five basis sets (3-21G, 3-21+G * * , 6-31G, 6-31+G * * , and 6-31+G * * ) were used in order to obtain fully optimized geometric parameters that indicated good agreement with the experimental crystallographic data. We have also investigated in this work the effects of solvents in both ranitidine and metiamide. Monocationic ranitidine was also investigated. All our results, indicate that, as in metiamide, the folded conformation is also preferred. We have investigated Mulliken and natural bond order (NBO) charge distributions, electrostatic and hydrogen bond effects on stabilizing the conformations and discussed the interactions of ranitidine with the biological receptor.

No abstract