Robust Output Tracking Control of an Uncertain Linear System via a Modified Optimal Linear-Quadratic Method

This study investigates the robust output tracking problem for a class of uncertain linear systems. The uncertainties are assumed to be time invariant and to satisfy the matching conditions. According to the selected nominal parameters, an optimal solution with a prescribed degree of stability is determined. Then, an auxiliary input via the use of an adapting factor, connected to the nominal optimal control, is introduced to guarantee the robustness and prescribed degree of stability for the output tracking control of the uncertain linear systems. This method is very simple and effective and can reject bounded uncertainties imposed on the states. A maglev vehicle model example is given to show its effectiveness.     

Vapor‐Phase Reaction of Citronellal over Mesoporous Molecular Sieves MCM‐41 and Zeolites

The vapor‐phase reaction of citronellal (CTN) at 220 °C and atmospheric pressure has been studied using mesoporous molecular sieves and zeolites in a fixed‐bed reactor. The primary products included isopulegol (IPG), menthone, and pulegol with subsequent reactions to form cyclic hydrocarbons. The CTN conversion and the product selectivity depend on the acidity and the textural property of catalysts. Lewis and/or Brönsted acid sites are essential for catalyzing this reaction. An increase of SiO₂/Al₂O₃ mol ratio diminishes the acid amount of all catalysts and enhances both the surface area and the structural order of MCM‐41. The catalytic activity follows the order of MCM‐41 > HZSM‐5 > Hβ > USY, in accordance with the relative total acid amount except that of MCM‐41. Despite its low acidity, Si‐MCM‐41 exhibits the best catalytic performance due to its uniform mesopores, large surface area and good stability; the CTN conversion and the IPG yield attain 91.9% and 58.6%, respectively, after at least 25 h time‐on‐stream.     

Theoretical study of addition reactions of heavy carbenes to carbon and boron nitride nanotubes

In an effort to gain insight into the activation energies and reaction enthalpies of the chemical functionalization of carbon and boron nitride nanotubes, calculations using density functional theory have been carried out for the cycloaddition of a heavy carbene to a single-walled carbon (SWCNT; C(130)H(20)) and a boron nitride (SWBNNT; B(65)N(65)H(20)) nanotube. The (CH(3))(2)X + SWCNT and (CH(3))(2)X + SWBNNT (X = C, Si, Ge, Sn, and Pb) reactions are the subject of the present study. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. The major conclusions that can be drawn from this work are as follows: (i) Considering both the activation barrier and reaction enthalpy based on the model calculations presented here, it is found that the order of (CH(3))(2)X reactivity is X = C > Si > Ge > Sn > Pb, irrespective of whether cycloaddition is to a SWCNT or a SWBNNT sidewall. That is to say, (CH(3))(2)C and (CH(3))(2)Si can readily add to the sidewalls of SWCNT and SWBNNT, whereas (CH(3))(2)Ge, (CH(3))(2)Sn, and (CH(3))(2)Pb are unreactive. (ii) Since the chemical reactivities of SWCNT and SWBNNT sidewalls closely resemble those of the small C(16)H(10) and B(8)N(8)H(10) molecules, at least in a qualitative sense, the use of the above small molecules as models is sufficient to provide qualitatively correct results. (iii) Our theoretical observations indicate that all the (5,5) SWCNT and SWBNNT cycloadducts favor opened rather than closed three-membered ring structures. (iv) The theoretical investigations demonstrate that the singlet-triplet splitting of the carbene species (R(2)X) as well as that of the small model molecules can be used as a diagnostic tool to predict the addition reactivities of carbene analogues and sidewalls of various nanotubes, respectively. Moreover, the results obtained in this work allow a number of predictions to be made.     

Nucleophilic catalysis with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) for the esterification of carboxylic acids with dimethyl carbonate

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) is an effective nucleophilic catalyst for carboxylic acid esterification with dimethyl carbonate (DMC). The reaction pathway of this new class of nucleophilic catalysis has been studied. A plausible, multistep mechanism is proposed, which involves an initial N-acylation of DBU with DMC to form a carbamate intermediate. Subsequent O-alkylation of the carboxylate with this intermediate generates the corresponding methyl ester in excellent yield. In the absence of DBU or in the presence of other bases, such as ammonium hydroxide or N-methylmorpholine, the same reaction affords no desired product. This method is particularly valuable for the synthesis of methyl esters that contain acid-sensitive functionality.     

An efficient synthesis of the novel triazoloquinazoline adenosine antagonist,CGS 15943

An efficient synthesis of the novel triazoloquinazoline adenosine antagonist, CGS 15943, is reported in five steps in approximately 50% overall yield. A key reaction in the synthetic sequence is the double cyclization of an N-(substituted-2-cyanophenyl)carbamate with a carboxylic acid hydrazide to afford a [1,2,4]triazolo-[1,5-c]quinazolin-5(6H)-one in high yield without either a Dimroth or “translocative” rearrangement occurring. Another key reaction is the condensation of a 2-(1H-1,2,4-triazol-5-yl)benzenamine with cyanamide under acidic conditions to prepare a guanidine.     

Mechanism of abstraction reactions of dimetallenes (R2X=XR2; X = C, Si, Ge, Sn, Pb) with halocarbons: a theoretical study

Potential energy surfaces for the abstraction reactions of dimetallenes with halocarbons have been studied using density functional theory (B3LYP). Five dimetallene species, (SiH(3))(2)X=X(SiH(3))(2), where X = C, Si, Ge, Sn, and Pb, have been chosen in this work as model reactants. The present theoretical investigations suggest that the relative dimetallenic reactivity increases in the order C=C < Si=Si < Ge=Ge < Sn=Sn < Pb=Pb. That is to say, for halocarbon abstractions there is a very clear trend toward lower activation barriers and more exothermic reactions on going from C to Pb. Moreover, for a given dimetallene, the overall barrier heights are determined to be in the order CF(4) > CCl(4) > CBr(4) > CI(4). That is, the heavier the halogen atom (Y), the more facile its abstraction from CY(4). Halogen abstraction is always predicted to be much faster than the abstraction of a CY(3) group irrespective of the dimetallene or halocarbon involved. Our model conclusions are consistent with some available experimental findings. Furthermore, both a configuration mixing model based on the work of Pross and Shaik and bonding dissociation energies can be used to rationalize the computational results. These results allow a number of predictions to be made.     

Theoretical study of halophilic reactions of stable silylenes with chloro- and bromocarbons

A theoretical study of the mechanism of the reaction of stable silylenes with halocarbons has been carried out using the B3LYP density functional method. The main findings are as follows: (1) Lewis acid-base complexes formed between silylenes and halocarbons do not play a role in silylene insertion chemistry into halocarbons; therefore, the acid-base complex mechanism proposed by West et al. (J. Am. Chem. Soc. 2002, 124, 4186) is not appropriate to describe the disilane formation reaction. (2) The disilane formation reactions follow the energetically favorable general reaction pathway (X = halogen): (i) Y2Si: + HCX3 --> TS1 --> Y2XSi-CHX2. (ii) Y2Si: + Y2XSi-CHX2 --> TS2 --> Y2XSi-SiY2CHX2. (3) The observed preference of stable silylenes to undergo C-X bond insertion rather than C-H bond has been investigated. The theoretical findings suggest that this preference is a result of the thermodynamic factor. (4) Stable silylenes prefer to insert into a C-Br rather than a C-Cl bond because the energy barrier to insertion is lower, and the reaction is more exothermic.     

Halogenated ketenes. 39. A new synthesis for substituted coumarins

A new general synthesis of substituted coumarins is described. The in situ cycloaddition of chloroketenes with α-methoxymethylenecyclohexanones yields (4 + 2) cycloaddition products, 3,4-dihydro-2-pyranones. The chlorine atom is reductively removed and methanol is spontaneously eliminated to yield the 5,6,7,8-tetra-hydrocoumarins. Dehydrogenation of these compounds results in good yields of the substituted coumarins.     

Carbonylchromium derivatives of bismuth: new syntheses and relevance to cbond;o activation

We have discovered a series of novel pentacarbonylchromium derivatives of bismuth from the reactions of NaBiO(3) with [Cr(CO)(6)] in KOH/MeOH solutions. When the reaction was carried out at room temperature, the highly charged [Bi[Cr(CO)(5)](4)](3-) (1) was obtained, whose structure was shown by X-ray analysis to possess a central bismuth atom tetrahedrally coordinated to four [Cr(CO)(5)] groups. As the reaction was heated at 80 degrees C, the methyl-substituted complex [MeBi[Cr(CO)(5)](3)](2-)(2) was obtained, presumably via the CbondO activation of MeOH. Further reactions of 1 with CH(2)Cl(2) or CHtbondCCH(2)Br form the halo-substituted complexes [XBi[Cr(CO)(5)](3)](2-)(X=Cl, 3; Br, 4), respectively. On the other hand, the reactions of 1 with RI (R=Me, Et) led to the formation of the alkyl-substituted complexes [RBi[Cr(CO)(5)](3)](2-)(R=Me, 2; Et). The formation of complexes 1-4 is discussed, presumably via the intermediate bismuthinidene [Bi[Cr(CO)(5)](3)](-) or the trianion [Bi[Cr(CO)(5)](3)](3-).