Search results for "computational"

The Synergetic Interplay of Weak Interactions in the Ion-Pair Recognition of Quaternary and Diquaternary Ammonium Salts by Halogenated Resorcinarenes

2013

The influence of halogens on the noncovalent interactions of different upper-rim-substituted hexylresorcinarenes with quaternary and diquaternary ammonium iodide salts was investigated in the gas phase by electrospray ionization Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry and in solution by 1H NMR titration studies. The electronic nature of the substituents on the upper rim of the resorcinarene was directly reflected in the order of binding strengths of the hosts towards quaternary and diquaternary ammonium cations in the gas phase. In solution, the opposite trend was observed, with generally higher binding constants for the diquaternary over the quaternary salts…

Understanding the domino reactions of alkyne-tethered N-tosylhydrazones yielding fused polycyclic pyrazoles. An MEDT study

2019

Abstract The domino reactions of alkyne-tethered N-mesylhydrazones yielding fused polycyclic pyrazoles have been studied within the Molecular Electron Density Theory (MEDT). Analysis of the Gibbs free energies indicates that the more favourable reactive path is the one in which the elimination of mesylate anion takes place before the intramolecular [3 + 2] cycloaddition (IM32CA) reaction, which corresponds with the rate-determining step of these domino processes. ELF topological analysis of the bond formation along the IM32CA reaction indicates that in spite of the high activation energy associated to this intramolecular reaction, it shows a pmr-type mechanism characterised by the presence …

Competing salt effects on phase behavior of protein solutions: tailoring of protein interaction by the binding of multivalent ions and charge screeni…

2014

The phase behavior of protein solutions is affected by additives such as crowder molecules or salts. In particular, upon addition of multivalent counterions, a reentrant condensation can occur; i.e., protein solutions are stable for low and high multivalent ion concentrations but aggregating at intermediate salt concentrations. The addition of monovalent ions shifts the phase boundaries to higher multivalent ion concentrations. This effect is found to be reflected in the protein interactions, as accessed via small-angle X-ray scattering. Two simulation schemes (a Monte Carlo sampling of the counterion binding configurations using the detailed protein structure and an analytical coarse-grain…

Multinuclear NMR study of 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one (fenchone) and its six monochlorinated derivatives

1990

1H, 13C and 17O NMR spectra of fenchone and six monochlorofenchones have been recorded. The second-order 1H NMR spectra were analysed by an iterative computer program. The 1H and 13C chlorine-induced substituent chemical shifts (SCS) were calculated. The vicinal coupling constants proved to be essential in the assignment of the chemical shifts of the geminal methyls. No clear correlation was observed between the chemical shifts of the carbonyl oxygen and the carbonyl carbon. All monochlorofenchones were previously unknown compounds and were synthesized for this work.

Understanding the regio- and chemoselective polar [3+2] cycloaddition of the Padwa carbonyl ylides with α-methylene ketones. A DFT study

2009

The regio- and chemoselective polar [3+2] cycloaddition (32CA) of the Padwa carbonyl ylide (CY) with α-methylene ketone (αMK) to yield the oxa-bridged spirocycloadduct has been studied using the DFT method at the B3LYP/6-31G(d) computational level. Six reactive channels associated to the stereo-, regio-, and chemoselective approach modes of the CY to the CC and CO reactive sites of the αMK have been analyzed. DFT calculations for this cycloaddition are in complete agreement with the experimental outcome, explaining the reactivity and selectivity of the formation of the [3+2] cycloadduct. Analysis of the global and local electrophilicity and nucleophilicity indices allows an explanation abou…

GLASS TRANSITION IN THIN POLYMER FILMS: A MOLECULAR DYNAMICS STUDY

2002

A melt of nonentangled polymer chains confined between two smooth and purely repulsive walls is studied for various film thicknesses D and temperatures. The dynamics of the supercooled films is qualitatively identical to that of the bulk, but the walls lead to faster relaxation. To quantify this observation we analyze the data by the mode-coupling theory (MCT) of the glass transition. We find that the critical temperature of MCT, Tc(D), decreases with D and that T - Tc(D) is a relevant temperature scale. The static structure factor and dynamic correlation functions at intermediate times coincide with bulk behavior when compared to the same T - Tc(D).

Pearl-necklace structures of molecular brushes with rigid backbone under poor solvent conditions: A simulation study

2010

Bottle-brush polymers, where flexible side chains containing N=20 to 50 effective monomers are grafted to a rigid backbone, are studied by molecular dynamics simulations, varying the grafting density σ and the solvent quality. Whereas for poor solvents and large enough σ the molecular brush is a cylindrical object, homogeneous in axial direction, for intermediate values of σ an axially inhomogeneous structure of "pearl-necklace" type is formed. The "pearls," however, have a strongly nonspherical ellipsoidal shape, due to the fact that several side chains cluster together in one pearl, qualitatively consistent with predictions of Sheiko et al. [Eur. Phys. J. E 13, 125 (2004)] We analyze the …

Simulation of Dense Polymer Systems in Two and Three Dimensions

1991

Dense polymer systems are modeled by self- and mutually avoiding walks on lattices. Both simple models where the step length is one lattice spacing and more complicated models where the step length is distinctly longer and may fluctuate (“bond fluctuation model”) are discussed, and it is shown that the computer simulation of such models gives useful insight to understand the thermodynamic phase behavior and the relaxational dynamics of dense polymer solutions and polymer melts. The huge demands in computing power needed for a successful simulation of such systems can be covered by parallel computers such as the multitransputer facility of the University of Mainz.

Monte Carlo simulation of polymer mixtures: recent progress

2000

Calculation of low bandgap homopolymers: Comparison of TD-DFT methods with experimental oligomer series

2016

Made available in DSpace on 2018-12-11T17:26:45Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-02-01 The performance of different DFT functionals (B3LYP, BHLYP, CAM-B3LYP, M06HF) on the prediction of vertical transition energies Evert of low bandgap homopolymers is tested against the experimentally available oligomer series (thienopyrazines and thienothiophenes). This allows for a detailed and accurate comparison on the consistency of DFT methods for chainlength evolution and polymer limit prediction, and for an understanding of geometry and time-dependent contributions to Evert by combinatorial analysis. Together with former studies on wide/medium bandgap polymers and low bandgap c…