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211.
Insights into unusual stability of 5‐membered‐ring endocyclic benzyl carbocations in aqueous solution 下载免费PDF全文
The acyclic o‐oxygen benzyl carbocation 1 , the 6‐membered‐ring endocyclic o‐oxygen benzyl carbocation 2 , and the 5‐membered‐ring endocyclic o‐oxygen benzyl carbocation 3 were used as model compounds to get insights into the general phenomenon for the unusual stability of the 5‐membered‐ring endocyclic benzyl carbocations in aqueous solution. The hydride‐ion affinities of 1 , 2 , and 3 in gas phase, acetonitrile, and DMSO were calculated and compared by the density functional theory method, and 3 isodesmic reactions were designed to confirm their thermodynamic stability. What we found is that the 5‐membered‐ring endocyclic o‐oxygen stabilizes the benzyl carbocation 3 less than the acyclic o‐oxygen stabilizes the benzyl carbocation 1 in gas phase because of ring strain and through‐bond induction. However, the high solvation energies of the 5‐membered‐ring endocyclic o‐oxygen benzyl carbocation 3 not only offset the destabilizing effects of ring strain and through‐bond induction but also make it even more stable than the acyclic o‐oxygen benzyl carbocation 1 in polar solvents like acetonitrile, DMSO, and water. 相似文献
212.
Javier Gotta Sonia Keunchkarian Cecilia Castells Mario Reta 《Journal of separation science》2012,35(20):2699-2709
The prediction capability of the solvation parameter model in reverse‐phase liquid chromatography at different methanol‐water mobile phase compositions and temperatures was investigated. By using a carefully selected set of solutes, the training set, linear relationships were established through regression equations between the logarithm of the solute retention factor, logk, and different solute parameters. The coefficients obtained in the regressions were used to create a general retention model able to predict retention in an octadecylsilica stationary phase at any temperature and methanol‐water composition. The validity of the model was evaluated by using a different set (the test set) of 30 solutes of very diverse chemical nature. Predictions of logk values were obtained at two different combinations of temperature and mobile phase composition by using two different procedures: (i) by calculating the coefficients through a mathematical linear relationship in which the mobile phase composition and temperature are involved; (ii) by using a general equation, obtained by considering the previous results, in which only the experimental values of temperature and mobile phase composition are required. Predicted logk values were critically compared with the experimental values. Excellent results were obtained considering the diversity of the test set. 相似文献
213.
Jiulin Du 《Physica A》2012,391(4):1718-1728
Transition state theory (TST) is generalized to nonequilibrium systems with power-law distributions. The stochastic dynamics that gives rise to the power-law distributions for the reaction coordinate and momentum is modeled by Langevin equations and corresponding Fokker-Planck equations. It is considered that a system far away from equilibrium does not have to relax to a thermal equilibrium state with Boltzmann-Gibbs distribution, but asymptotically approaches a nonequilibrium stationary state with a power-law distribution. Thus, we obtain a possible generalization of TST rates to nonequilibrium systems with power-law distributions. Furthermore, we derive the generalized TST rate constants for one-dimensional and n-dimensional Hamiltonian systems away from equilibrium, and obtain a generalized Arrhenius rate for systems with power-law distributions. 相似文献
214.
《Physics and Chemistry of Liquids》2012,50(6):733-745
ABSTRACTAbraham model correlations are derived for describing gas-to-ionic liquid and water-to-ionic liquid partition coefficients from published experimental data for solutes dissolved in both N-triethyl(octyl)ammonium bis(fluorosulfonyl)imide and 1-butyl-3-methyl-pyrrolidinium bis(fluorosulfonyl)imide. Derived Abraham model correlations describe the observed partition coefficient data to within 0.13 log units. As part of the current study the existing equation coefficients for the N-triethyl(octyl)ammonium cation were updated and reported for the first time were equation coefficients for the bis(trifluorosulfonyl)imide anion. 相似文献
215.
Reza Izadi Najafabadi Mohammad Reza Housaindokht Mohammad Sadegh Sadeghi Googheri Mohsen Sargolzaei Mohammad Izadyar 《International journal of quantum chemistry》2012,112(14):2675-2680
In this study we have calculated the acidity constant (pKa) of imidazole ring in Histidine‐Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results ΔGS values are located between ?69.38 and ?18.82 kcal mol?1 which are related to His+–Gly and His forms, respectively. pKa of the dipeptides in the aqueous phase was obtained from the calculated gas‐phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self‐consistent reaction field formalism involving polarized continuum models. According to our calculations pKa values are between 5.50 and 8.19 that are belong to His+–ILe and His+–Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
216.
R. Pastor-Satorras A. Vespignani 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(4):583-587
We describe the construction of a conserved reaction-diffusion system that exhibits self-organized critical (avalanche-like)
behavior under the action of a slow addition of particles. The model provides an illustration of the general mechanism to
generate self-organized criticality in conserving systems. Extensive simulations in d = 2 and 3 reveal critical exponents compatible with the universality class of the stochastic Manna sandpile model.
Received 16 November 2000 相似文献
217.
218.
Nicolas Budin Nicolas Majeux Catherine TenetteSouaille Amedeo Caflisch 《Journal of computational chemistry》2001,22(16):1956-1970
Program to engineer peptides (PEP) is a build‐up approach for ligand docking and design with implicit solvation. It requires the knowledge of a seed from which it iteratively grows polymeric ligands consisting of any type of amino acid, i.e., natural and/or nonnatural from a user‐defined library. At every growing step, a genetic algorithm is used for conformational optimization of the last added monomer in the rigid binding site. Pruning is performed at every growing step by selecting sequences according to binding energy with electrostatic solvation. PEP is applied to three members of the caspase family of cysteine proteases using Asp at P1 as seed. The optimal P4–P2 peptide recognition motifs and variants thereof are docked correctly in the active site (backbone root‐mean‐square deviation < 0.9 Å). Moreover, for each caspase, the P4–P2 sequences of potent aldehyde inhibitors are ranked among the 15 hits with the most favorable PEP energy. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1956–1970, 2001 相似文献
219.
We present a molecular dynamics study of the solvation properties of large spherical ions S+ and S− of same size, in water, chloroform and acetonitrile solutions, and at a water–chloroform interface. According to the “extrathermodynamic” TATB hypothesis, such ions have identical free energies of transfer from water to any solvent. We find that this is not the case, because S− interacts better than S+ with water (by about 20 kcal mol−1), while S+ is better solvated by acetonitrile (by about 2 kcal mol−1) and chloroform (about 8 kcal mol−1) solvents. The importance of “long-range” electrostatic interactions on the charge discrimination by solvent is demonstrated by the comparison of standard vs corrected methods to calculate: (i) the electrostatic potential at the centre of the solute; (ii) the interaction energies between the ions and the solvents; and (iii) the free energies of charging the neutral sphere S0 to S+ and S−, respectively. These conclusions are obtained with several solvent models and simulation conditions. The question of ion pairing for the S+S−, S+Cl− and S−Na+ pairs is also examined in the three solvents. Finally, simulations at a liquid–liquid water–chloroform interface represented explicitly, show that S+ and S− are highly surface active, although they do not possess, like classical surfactants, an amphiphilic topology. Adsorption at the interface is found with different methodologies and at different ion concentrations. These results are important in the context of the “TATB hypothesis”, and for our understanding of solvation of large hydrophobic ions in pure liquids or in heterogeneous liquid environments. 相似文献
220.