RESUMO
An improved atomistic understanding of the W-based two-dimensional transition-metal dichalcogenides (2D TMDs) is crucial for technological applications of 2D materials, since the presence of tungsten endows these materials with distinctive properties. However, our atomistic knowledge on the evolution of the structural, electronic, and energetic properties and on the nanoflake stability of such materials is not properly addressed hitherto. Thus, we present a density functional theory (DFT) study of stoichiometric (WQ2)n nanoflakes, with Q = S, Se, Te, and n = 1, ,16, 36, 66, and 105. We obtained the configurations with n = 1, ,16 through the tree growth algorithm whereas the nanoflakes with n = 36, 66, and 105 were generated from fragments of 2D TMDs with an abundant diversity of shapes and edge configurations. We found that all the most stable nanoflakes present the same Q-terminated edge configuration. Furthermore, in isomers with n = 1, ,16 sizes, nanoflakes with triangular shapes and their derivatives, such as the rhombus geometry, define magic numbers, whereas for n > 16, triangular shapes were also found for the most stable structures, because they preserve the edge configuration. A strong modulation of the Hirshfeld charges, depending on chalcogen species and core or edge position, is also observed. The modulation of the Hirshfeld charge due to the nature of the W metal atoms makes the energetic 1D â 1T' transition of (WQ2)n differ in nanoflake size in relation to (MoQ2)n nanoflakes. Our analysis shows the interplay between edge configuration, coordination environment, and shape that determines the stability of nanoflakes, and allows us to describe design principles for stable 1T' stoichiometric nanoflakes of various sizes.
RESUMO
The properties of the free nitroxyl molecule and the nitroxyl ligand in Ru(ii) tetraammines (trans-[Ru(NH3)4(nitroxyl)(n)(L)](2+n) (n = nitroxyl charge; L = NH3, py, P(OEt)3, H2O, Cl(-) and Br(-))) were studied using density functional theory. According to the calculated conformational energies, HNO complexes are more stable than their deprotonated analogues, and the singlet configuration (trans-(1)[Ru(NH3)4(L)HNO](2+)) is lower in energy than the corresponding triplet (trans-(3)[Ru(NH3)4(L)HNO](2+)). An evaluation of the σ and π components of the L-Ru-HNO bond suggest that the increased stability of these orbitals and the enhanced contributions from the HNO orbitals correlate to shorter Ru-N(H)O distances and higher νRu-HNO stretching frequencies. The stability of the Ru-HNO bond was also evaluated through a theoretical kinetic study of HNO dissociation from trans-(1)[Ru(NH3)4(L)HNO](2+). The order of the Ru-HNO bonding stability in trans-(1)[Ru(NH3)4(L)HNO](2+) as a function of L was found to be as follows: H2O > Cl(-)â¼ Br(-) > NH3 > py > P(OEt)3. This order parallels the order of the trans-effect and trans-influence series experimentally measured for L in octahedral complexes. The same trend was also observed using an explicit solvent model, considering the presence of both HNO and H2O molecules in the transition state. For this series, the calculated bond dissociation enthalpies for the Ru-HNO bonds are in the range 23.8 to 45.7 kcal mol(-1). A good agreement was observed between the calculated ΔG() values for the displacement of HNO by H2O in trans-(1)[Ru(NH3)4(P(OEt)3HNO](2+) (23.4 kcal mol(-1)) and the available experimental data for the substitution reactions of trans[Ru(NH3)4(POEt)3(Lx)](2+) (19.4 to 24.0 kcal mol(-1) for Lx = isn and P(OET)3, respectively).
RESUMO
The thermodynamic and kinetic aspects of ethyl carbamate (EC) formation through the reaction between cyanate and ethanol were investigated. The rate constant values for cyanate ion decay and EC formation are (8.0 ± 0.4) × 10(-5) and (8.9 ± 0.4) × 10(-5) s(-1), respectively, at 25 °C in 48% aqueous ethanolic solution at pH 4.5. Under the investigated experimental conditions, the rate constants are independent of the ethanol and cyanate concentrations but increase as the temperature increases (ΔH1(⧧) = 19.4 ± 1 kcal/mol, ΔS1(⧧) = −12.1 ± 1 cal/K, and ΔG1(⧧) = 23.0 ± 1 kcal/mol) and decrease as the solution pH increases. According to molecular modeling (DFT) that was performed to analyze the reaction mechanism, the isocyanic acid (HNCO) is the active EC precursor. The calculated ΔG1(⧧), ΔH1(⧧), and ΔS1(⧧) values are in very good agreement with the experimental ones.
Assuntos
Bebidas Alcoólicas , Cianatos/química , Saccharum/química , Uretana/química , Cianatos/metabolismo , Etanol/química , Concentração de Íons de Hidrogênio , Cinética , Termodinâmica , Uretana/metabolismoRESUMO
Despite the resistance developed by the Mycobacterium tuberculosis (MTb) strains, isoniazid (INH) has been recognized as one of the best drug for treatment of Tuberculosis (Tb). The coordination of INH to ruthenium metal centers was investigated as a strategy to enhance the activity of this drug against the sensitive and resistant strains of MTb. The complexes trans-[Ru(NH3)4(L)(INH)](2+) (L=SO2 or NH3) were isolated and their chemical and antituberculosis properties studied. The minimal inhibitory concentration (MIC) data show that [Ru(NH3)5(INH)](2+) was active in both resistant and sensitive strains, whereas free INH (non-coordinated) showed to be active only against the sensitive strain. The coordination of INH to the metal center in both [Ru(NH3)5(INH)](2+) and trans-[Ru(NH3)4(SO2)(INH)](2+) complexes led to a shift in the INH oxidation potential to less positive values compared to free INH. Despite, the ease of oxidation of INH did not lead to an increase in the in vitro INH activity against MTb, it might have provided sensitivity toward resistant strains. Furthermore, ruthenium complexes with chemical structures analogous to those described above were synthesized using the oxidation products of INH as ligands (namely, isonicotinic acid and isonicotinamide). These last compounds were not active against any strains of MTb. Moreover, according to DFT calculations the formation of the acyl radical, a proposed intermediate in the INH oxidation, is favored in the [Ru(NH3)5(INH)](2+) complex by 50.7kcalmol(-1) with respect to the free INH. This result suggests that the stabilization of the acyl radical promoted by the metal center would be a more important feature than the oxidation potential of the INH for the antituberculosis activity against resistant strains.
Assuntos
Antituberculosos/farmacologia , Isoniazida/farmacologia , Mycobacterium tuberculosis/efeitos dos fármacos , Rutênio/farmacologia , Animais , Antituberculosos/uso terapêutico , Chlorocebus aethiops , Isoniazida/uso terapêutico , Testes de Sensibilidade Microbiana/métodos , Mycobacterium tuberculosis/fisiologia , Rutênio/uso terapêutico , Tuberculose/tratamento farmacológico , Células VeroRESUMO
The reaction between trans-[Ru(II)(NO(+))(NH3)4(L)](3+), L = ImN, IsN, Nic, P(OMe)3, P(OEt)3, and P(OH)(OEt)2, and the Fe(III) species [Fe(III)(TPPS)], metmyoglobin, and hemoglobin was monitored by UV-vis, EPR, and electrochemical techniques (DPV, CV). No reaction was observed when L = ImN, IsN, Nic, and P(OH)(OEt)2. However, when L = P(OMe)3 and P(OEt)3, the reaction was quantitative and the products were trans-[Ru(III)(H2O)(NH3)4(P(OR)3)](3+) and [Fe(II)(NO(+))] species. Reaction kinetics data and DFT calculations suggest a two-step reaction mechanism with the initial formation of a bridged [Ru-(µNO)-Fe] intermediate, which was confirmed through electrochemical techniques (E(0)' = -0.47 V vs NHE). The calculated specific rate constant values for the reaction were in the ranges k1 = 1.1 to 7.7 L mol(-1) s(-1) and k2 = 2.4 × 10(-3) to 11.4 × 10(-3) s(-1) for L = P(OMe)3 and P(OEt)3. The oxidation of the ruthenium center (Ru(II) to Ru(III)) containing the nitrosonium ligand suggests that NO can act as an electron transfer bridge between the two metal centers.