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Conformational isomerism
About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.
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TL;DR: In this article, all the theoretically possible keto and enol conformations of hexafluoro-acetylacetone (HFAA) as well as those of 2-trifluoro (2TFAA) and 4-trifi-noromethyl group(s) have been investigated at ab initio level by using the 6-31G∗∗ basis.
Abstract: All the theoretically possible keto and enol conformations of hexafluoro-acetylacetone (HFAA) as well as those of 2-trifluoro- (2TFAA) and 4-trifluoro-acetylacetone (4TFAA), have been investigated at ab initio level by using the 6-31G∗∗ basis. The correlation energy was evaluated by means of the Becke functional following the Density Functional Theory. It was found that the most stable conformers are always those stabilized by hydrogen bridges and characterized by Cs symmetry, as the parent acetylacetone. 4TFAA is about 4 kJ/mol favored with respect to 2TFAA. The withdrawing effect of the trifluoromethyl group(s) produces a decrease in the hydrogen bond strength (EHB) with respect to that of acetylacetone; in any case such strength is enhanced when 3-substituent groups are able to increase the conjugation of the system or to cause strong steric effects. In several open conformations the formation of a F⋯H–O bridge is observed, whose energy was estimated as 10 kJ/mol, at the most.
49 citations
TL;DR: Water increases this rate by 10(3), apparently by catalyzing hydrogen bond exchange, and thereby presenting functional properties analogous to that of a foldase, on the interconversion of a parallel gramicidin dimer into an antiparallel dimer.
Abstract: Polypeptide conformer interconversion in a low dielectric environment is shown to be highly dependent on water concentration. Water increases this rate by 103, apparently by catalyzing hydrogen bond exchange, and thereby presenting functional properties analogous to that of a foldase. This catalytic effect is demonstrated on the interconversion of a parallel gramicidin dimer into an antiparallel dimer. A Hill coefficient of 6.5 is observed, illustrating the highly cooperative nature of the process. Protein folding in nonpolar environs, such as the hydrophobic core of a protein or the hydrophobic domain of a lipid bilayer, may be contingent on and rate-limited by the scarcity of water.
49 citations
TL;DR: In this article, a single unified strategy for the stereocontrolled synthesis of α,α-, α, β-, and β,β-C-trehaloses (1-3) was developed.
Abstract: A single, unified strategy for the stereocontrolled synthesis of α,α-, α,β-, and β,β-C-trehaloses (1-3) was developed. The solution conformations of C-trehaloses 1-3, as well as their permethyl derivatives, were determined on the basis of vicinal coupling constants observed in the 1 H NMR spectra. The preferred conformations for α,α- and β,β-C-trehaloses (1 and 3), shown in Figure 1, were predicted and experimentally proven. A diamond-lattice analysis of α,β-C-trehalose (2), shown in Figure 2, revealed the relative stability of the three staggered conformers to be 2A> 2B>2C, and the experimental data were found to be consistent with this trend
49 citations
TL;DR: The energy differences between anti and syn conformers as well as the energy barrier for the rotation around the aryl-aryl bond of a number of 2,2'-bipyridine molecules were examined by quantum-chemical methods, indicating the importance of such weak interactions for the structure and energetics of supramolecular systems.
Abstract: What a difference! The energy differences between anti and syn conformers as well as the energy barrier for the rotation around the aryl-aryl bond of a number of 2,2'-bipyridine molecules were examined by quantum-chemical methods. The energy differences were found to be governed by the substituents directly attached to the bipyridine and their ability to form intramolecular hydrogen bonds.Quantum-chemical calculations at the BP86/TZVP level of theory were performed to determine the energy differences between the syn and the anti conformers, as well as the energy barrier for the rotation of the aryl-aryl bond of 2,2'-bipyridine molecules and a number of disubstituted derivatives. Substitutents with hydrogen-bond donor (or electron acceptor) functions or hydrogen-bond acceptors (or electron donors) are generally found to have large effects on the difference and the barrier. Substitution with a hydrogen-bond donor (or an electron acceptor) at position 6 and 6' leads to a decrease owing to a charge transfer from the pyridine nitrogen lone pair to the donor, which is caused by the formation of weak intramolecular hydrogen bonds and/or dipolar interactions, respectively. Conversely, substitution at position 4 and 4' causes an increase in the energy barrier. Substitution with a hydrogen-bond acceptor (or an electron donor) shows the opposite behavior, which can be explained by the weak intramolecular interactions. Interestingly, even very weak CH hydrogen-bond donors (electron acceptors) such as methyl groups have a significant influence. This indicates the importance of such weak interactions for the structure and energetics of supramolecular systems. The energy differences are mainly governed by the substituents directly attached to the bipyridine core as the introduction of sterically demanding groups in the periphery hardly influences the barriers or energy differences of the conformers. These findings are important for the design of heterotropic positive cooperative allosteric receptors with 2,2'-bipyridines as the allosteric centre.
49 citations
TL;DR: In this paper, a bis-solvated diisopropylamine solvate of cyclohexanone phenylimine was found to have bis-solved structure in the solid state.
Abstract: N, 6Li, and I3C NMR spectroscopic studies of lithiated cyclohexanone phenylimine (1) in tetrahydrofuran/hydrocarbon solutions detected three different species. 6Li and I5N NMR spectroscopic studies of (6Li,15N)-1 showed the two species observed at low THF concentrations to be a pair of stereoisomeric dimers and the single species appearing at high THF concentrations to be a contact ion paired monomer. Colligative measurements demonstrated the dimers to be bis-solvated at 0 OC. Measurements of the dimer-monomer equilibrium at -94 "C provided best fit to an equilibrium between tris-solvated monomers and bis or higher solvated dimers. We recently isolated a crystalline diisopropylamine solvate of lithiated cyclohexanone phenylimine that was shown to possess a bis-solved dimer structure in the solid state (Figure l).) Spectroscopic analyses showed the existence of a 2: 1 mixture of two rapidly interconverting species in hydrocarbon solution. Although previous studies of lithiated imines had uncovered similar resonance duplications," explanations hinging upon various spatial orientations of the imine carbon skeleton (Figure 2) failed to account for the specific case in question. For example, discrete rotamers around the N-Ci+, bond of lithiated isopropylimines described by Fraser2 seemed highly unlikely for an N-phenyl- substituted imine anion. It seemed equally improbable that both syn and anti isomers would be observable since e~perimental~~ and theoretical evidence5 supported a very large (4-5 kcal/mol) stabilization of the syn orientation. E/Z isomer mixtures observed for acyclic lithiated N-alkyl- and N-phenylimines would be geo- metrically impossible for the corresponding endocyclic imine anion^.^ To further understand the solution behavior of such a poorly precedented diisopropylamine solvate,6 we turned to a strategy for determining both aggregation and solvation states that has been little used in organolithium chemistry since its introduction7~* ~~
49 citations