Elimination of methane from protonated acetaldehyde: The Ab initio transition state

The transition state (TS) for loss of CH₄ from protonated acetaldehyde has been located at the second-order Moller-Plesset (MP2)/6-31G(d,p) level of theory. The activation energy is predicted to be 263.9 kJ/mol starting from the more stable form (methyl and hydrogen E) and 261.6 kJ/mol starting from the less stable form (methyl and hydrogen Z) that is required for reaction. The products (methane and the formyl ion) are predicted to lie 136.6 kJ/mol below the TS for their formation. MP2 methods underestimate the heats of formation of both the TS and the reaction products by about 40 kJ/mol when compared with experiment. Restricted Hartree-Fock (RHF) calculations give much more accurate relative energies. The MP2 TS leads directly to fragmentation and is described as a protonation of the methyl group by the acidic proton on oxygen. Under RHF theory the reaction is stepwise. An RHF TS similar to the MP2 TS leads to a nonclassical intermediate (which is stable at this level of theory) that has one of the C---H bonds protonated. This mechanism (protonation of an alkyl group) appears to be a general one for high energy 1,2 eliminations from organic cations. (J Am Soc Mass Spectrom 1994, 5, 1102-1106)     

Artificial neural system modeling of Monte Carlo simulations of polymers

In this work, a neural network was used to learn features in potential energy surfaces and relate those features to conformational properties of a series of polymers. Specifically, we modeled Monte Carlo simulations of 20 polymers in which we calculated the characteristic ratio and the temperature coefficient of the characteristic ratio for each polymer. We first created 20 rotational potential energy surfaces using MNDO procedures and then used these energy surfaces to produce 10000 chains, each chain 100 bonds long. From these results we calculated the mean-square end-to-end distance, the characteristic ratio and its corresponding temperature coefficient. A neural network was then used to model the results of these Monte Carlo calculations. We found that artificial neural network simulations were highly accurate in predicting the outcome of the Monte Carlo calculations for polymers for which it was not trained. The overall average error for prediction of the characteristic ratio was 4,82%, and the overall average error for prediction of the temperature coefficient was 0,89%.     

A NOVEL EOR POLYMER (Ⅱ)——INVESTIGATION ON IN-SITU GELATION OF SMRF SYSTEM IN BEREA CORE

In-situ gelation of aqueous sulfomethylated resorcinol formaldehyde (SMRF) system inBerea core has been investigated. Two sets of displacement experiments were conducted with thissystem (containing 5% NaCl, 0. 036% CaCl_2. 2H_2O). The brine permeabilities of the coreswere reduced significantly from about 600 to 0.1 md. The in-situ gelation in Berea core occurreda little bit earlier than gelation anticipated from bulk test in the experiments. The gel time waseasier to control at initial pH between 6 and 8. During injection of SMRF system, the apparentviscosity was less than 1 mPa·s at 41℃.     

Structure characterizations and protein resistance of chitosan membranes selectively crosslinked by poly(ethylene glycol) dimethacrylate

Chitosan (CS) is a fragile material with a high modulus of elasticity. Improving its flexibility as well as membrane permeability are the key aspects that need to be addressed for using CS as a biomaterial. Poly(ethylene glycol) (PEG) has several unique properties such as protein resistance, low toxicity, immunogenicity, and good solubility in both water and organic solvents. In this study, a vinyl compound was grafted to the C-6 position of CS by protection-grafting-deprotection. The vinyl CS was then crosslinked with PEG dimethacrylate (PEGDMA) selectively at its C-6 position to form CS-g-PEG copolymer membranes. Analyses from spectra of Fourier-transform infrared and nuclear magnetic resonance confirmed the chemical structure of the crosslinking CS-g-PEG copolymer membranes. Thermal and mechanical properties of the prepared CS-g-PEG membranes were measured and well-correlated to their structures. The incorporation of PEGDMA into the CS increased the material’s flexibility and thermal resistance. Finally, the CS-g-PEG membranes were found to have good protein resistance and blood compatibility; therefore, it has potential application as the biomedical material especially for hemodialysis.     

Three New Chiral Aminoethers from Tartaric Acid for Improved Asymmetric Syntheses with Organolithium Reactions. Preliminary communication

The chiral aminoethers 1–4 , accessible from tartaric acid, are tested as complexing ligands in organolithium reactions. The tetramine 4 turns out to cause highest inductions [see Table 2 and Equations (1)–(4)].     

Reactions of siliranes with elemental sulfur and with t-butyl mercaptan. Preparation of the 2,3-dithia-1-silacyclopentane ring system

The reaction of 1,1-dimethyl-trans-2,3,-bis(2′,2′-dimethylcyclopropylidene)-1-silirane with S₈ gives a mixture of four isomeric products derived from incorporation of one sulfur atom and a cyclopropylcarbinyl-to-butenyl type rearrangement. This silirane reacts with t-butyl mercaptan to give a product of mercaptan addition in which a cyclopropylcarbinyl-to-butenyl rearrangement also has occurred. Hexamethylsilirane reacts with S₈ to give 1,1,4,4,5,5-hexamethyl-2,3-dithia-1-silacyclopentane in high yield. These reactions are discussed in terms of free radical mechanisms.