ABSTRACT
The detailed kinetic mechanisms for the reactions of hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) with the methyl radical (CH3) are discussed. These are important reactions in combustion and Titan's atmosphere chemistry and were investigated at the CCSD(T)/cc-pVQZ//M06-2X/6-311++G(2df,2p) level of theory. The multiwell and multichannel potential energy surface (PES) was constructed. The rate constants were determined by using variational transition state theory (VTST) and Master Equation/Rice-Ramsperger-Kassell-Marcus (ME/RRKM) method over a temperature range of 300-2000 K and a pressure range of 1-10 000 torr. Corrections of the Eckart tunneling effect were included and the calculated results were in good agreement with the literature. A clear dependence of the reaction mechanism on temperature and pressure was revealed via detailed kinetic and species analysis. For the HCN reaction, the channel of C-addition forms an intermediate that is dominant at low temperatures and high pressures, leading to the total rate constant exhibiting a pressure dependence, but this dependence disappears at high temperatures. The H-abstraction channel is more competitive with increasing temperatures, but it is still not dominant. For the HNC reaction, the C-addition channel is dominant, and CH3CN and H constitute almost all the products. The proposed temperature and pressure-dependent rate constants can be used in the combustion and atmospheric model development for related systems.
ABSTRACT
The suppression effect of hydrophobic nano SiO2 of different concentrations as flow-enhancing additives synergizing with CaCO3 to inhibit gas explosions was systematically studied in a self-built LabVIEW-based explosion test platform. The results showed that the addition of hydrophobic powder can reduce the angle of rest and enhance the flowability of mixed powders, and improve the powder diffusion effect and storability. Meanwhile, changing the proportion and concentration of the mixed powders had a significant impact on the combustion reaction, so that the flame propagation velocity and explosion overpressure decreased significantly. However, excessive powder concentration will promote the combustion reaction at the initial stage of the explosion, and the synergistic inhibition effect of the two powders on explosions is better than that of a single powder. Based on the above results, the optimum suppression concentration and proportion were determined, the mechanism of suppressing gas explosion by a powder was analyzed, and the coupling relationship between flame velocity and pressure was summarized.
ABSTRACT
Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthase (NOS). In the cardiovascular system, endothelial NOS (eNOS) has a major role in maintaining vascular tone and endothelial function, as well as in mediating many other vascular protective properties. Evidence from humans and animals have demonstrated that decreased BH4 bioavailability, with subsequent uncoupling of eNOS, has significant effects on the pathogenesis of endothelial dysfunction, which is a hallmark of vascular injury in cardiovascular disorders, including hypertension, hyperlipidemia, and diabetes. In this review, we discuss the synthesis of BH4, its molecular mechanisms regulating eNOS coupling, the pathophysiologic roles of decreased BH4 bioavailability in cardiovascular diseases, and the potential therapeutic application of BH4 in clinics.