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High Barriers (high + barrier)
Selected AbstractsKeto-enol tautomerism in linear and cyclic ,-diketones: A DFT study in vacuo and in solutionINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2008Giuliano Alagona Abstract DFT geometry optimizations have been performed at the B3LYP/6-31G* level in the gas phase and at the IEF-PCM/B3LYP/6-31G* level in tetrahydrofuran (THF) and aqueous solutions using scaled radii for the diketo and ketoenol forms of acetylacetone and cyclohexanedione. To evaluate basis set effects, starting from the aforementioned minima, the 6-311++G** optimized structures have been obtained. A number of complexes of both systems including one explicit water molecule have been considered up to the B3LYP/6-311++G** level, for cyclohexanedione taking into account the B3LYP/6-31G* basis set superposition errors as well. The diketo,ketoenol interconversion mechanisms have been investigated at the B3LYP/6-31G* level in vacuo. Interestingly, the geometric constraint due to the presence of the ring facilitates the description of the reaction mechanism in cyclohexanedione. Despite the very different flexibility of the two systems that in the case of acetylacetone prevents a straightforward interconversion of the diketo to the most stable of its ketoenol forms, both reactions occur with a very high barrier (about 62,63 kcal/mol), unaffected by continuum solvents, that decreases by 2.5,3.5 kcal/mol after the inclusion of thermal corrections. The barriers are almost halved, becoming ,31,35 kcal/mol, for the addition of a single water molecule according to various model reaction paths. Thermal corrections are limited (0.8,1.6 kcal/mol) for those adducts. The formation of a 1,1-diol, explored in the case of acetylacetone, might facilitate the obtainment of the most stable diketo conformation, featuring the carbonyl groups in distinct orientations. Inclusion of dispersion and basis set effects via the G2MP2 protocol does not alter the relative stability of both system tautomers. In contrast, the G2MP2 interconversion barriers for the isolated systems in vacuo are close to the B3LYP ones, whereas they turn out to be somewhat higher than the free energy-based B3LYP barriers in the presence of a catalytic water molecule. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] An improved OPLS,AA force field for carbohydratesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2002D. Kony Abstract This work describes an improved version of the original OPLS,all atom (OPLS,AA) force field for carbohydrates (Damm et al., J Comp Chem 1997, 18, 1955). The improvement is achieved by applying additional scaling factors for the electrostatic interactions between 1,5- and 1,6-interactions. This new model is tested first for improving the conformational energetics of 1,2-ethanediol, the smallest polyol. With a 1,5-scaling factor of 1.25 the force field calculated relative energies are in excellent agreement with the ab initio -derived data. Applying the new 1,5-scaling makes it also necessary to use a 1,6-scaling factor for the interactions between the C4 and C6 atoms in hexopyranoses. After torsional parameter fitting, this improves the conformational energetics in comparison to the OPLS,AA force field. The set of hexopyranoses included in the torsional parameter derivation consists of the two anomers of D -glucose, D -mannose, and D -galactose, as well as of the methyl-pyranosides of D -glucose, D -mannose. Rotational profiles for the rotation of the exocyclic group and of different hydroxyl groups are also compared for the two force fields and at the ab initio level of theory. The new force field reduces the overly high barriers calculated using the OPLS,AA force field. This leads to better sampling, which was shown to produce more realistic conformational behavior for hexopyranoses in liquid simulation. From 10-ns molecular dynamics (MD) simulations of ,- D -glucose and ,- D -galactose the ratios for the three different conformations of the hydroxymethylene group and the average 3JH,H coupling constants are derived and compared to experimental values. The results obtained for OPLS,AA,SEI force field are in good agreement with experiment whereas the properties derived for the OPLS,AA force field suffer from sampling problems. The undertaken investigations show that the newly derived OPLS,AA,SEI force field will allow simulating larger carbohydrates or polysaccharides with improved sampling of the hydroxyl groups. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1416,1429, 2002 [source] Democracy, Diversification and Growth Reversals,THE ECONOMIC JOURNAL, Issue 540 2009David Cuberes There is much evidence that less democratic countries experience more high-frequency growth volatility. In this article we report a similar finding about volatility in the medium term: we find evidence that reversals of trend-growth are sharper and more frequent in non-democracies. Motivated by this evidence, we construct a model in which non-democracies have high barriers to entry for new firms. This leads to less sectoral diversification and so, in an uncertain environment, to larger growth swings in less democratic countries. We present empirical evidence that confirms the positive relation between democracy and industrial diversification. [source] Charge-Shift Bonding,A Class of Electron-Pair Bonds That Emerges from Valence Bond Theory and Is Supported by the Electron Localization Function ApproachCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2005Sason Shaik Prof. Abstract This paper deals with a central paradigm of chemistry, the electron-pair bond. Valence bond (VB) theory and electron-localization function (ELF) calculations of 21 single bonds demonstrate that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the fluctuation of the electron pair density plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RECS, and in ELF by a depleted basin population with large variances (fluctuations). CS bonding is shown to be a fundamental mechanism that is necessary to satisfy the equilibrium condition, namely the virial ratio of the kinetic and potential energy contributions to the bond energy. The paper defines the atomic propensity and territory for CS bonding: Atoms (fragments) that are prone to CS bonding are compact electronegative and/or lone-pair-rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and involves: a) homopolar bonds of heteroatoms with zero static ionicity, b) heteropolar , and , bonds of the electronegative and/or electron-pair-rich elements among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc), c) all hypercoordinate molecules. Several experimental manifestations of charge-shift bonding are discussed, such as depleted bonding density, the rarity of ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reactions as compared to hydrogen-transfers. [source] | ||||||||||