Search results for "Quantum"
showing 10 items of 9714 documents
Citric Acid Based Carbon Dots with Amine Type Stabilizers: pH-Specific Luminescence and Quantum Yield Characteristics
2020
We report the synthesis and spectroscopic characteristics of two different sets of carbon dots (CDs) formed by hydrothermal reaction between citric acid and polyethylenimine (PEI) or 2,3-diaminopyridine (DAP). Although the formation of amide-based species and the presence of citrazinic acid type derivates assumed to be responsible for a blue emission is confirmed for both CDs by elemental analysis, infrared spectroscopy, and mass spectrometry, a higher abundance of sp2-hybridized nitrogen is observed for DAP-based CDs, which causes a red-shift of the n-π* absorption band relative to the one of PEI-based CDs. These CD systems possess high photoluminescence quantum yields (QY) of ∼40% and ∼48…
Unprecedented blue intrinsic photoluminescence from hyperbranched and linear polyethylenimines: Polymer architectures and pH-effects
2007
The intrinsic photoluminescence properties of hyperbranched polyethylenimines (PEIs) and their linear counterpart (LPEIs) have been studied in absence of any classical fluorescent probes. The comparison of the inherent fluorescence emission between hyperbranched polyethylenimines and their linear analogues demonstrates that linear polyamines are capable of producing strong intrinsic photoluminescence species having long excited lifetimes without need of having a tridimensional-branched structure. The creation of inherently fluorescent polymeric centers from hyperbranched and linear polyethyleimines can be modulated by specific chemical modification and oxidation of amine groups as well as b…
Stimuli-responsive brushes with active minority components: Monte Carlo study and analytical theory
2015
Using a combination of analytical theory, Monte Carlo simulations, and three dimensional self-consistent field calculations, we study the equilibrium properties and the switching behavior of adsorption-active polymer chains included in a homopolymer brush. The switching transition is driven by a conformational change of a small fraction of minority chains, which are attracted by the substrate. Depending on the strength of the attractive interaction, the minority chains assume one of two states: An exposed state characterized by a stem-crown-like conformation, and an adsorbed state characterized by a flat two-dimensional structure. Comparing the Monte Carlo simulations, which use an Edwards-…
Entanglement in polystyrene detected by translational diffusion of photo-labeled molecules
1988
Mesure des coefficients de diffusion de polystyrenes marques et comparaison avec une equation phenomenologique reliant la masse moleculaire critique entre les enchevetrements Mc a ces masses moleculaires respectives de la chaine diffusant et de la matrice. Comparaison avec les theories recentes
Enzyme molecular mechanism as a starting point to design new inhibitors: a theoretical study of O-GlcNAcase.
2011
O-Glycoprotein 2-acetamino-2-deoxy-β-d-glucopyranosidase (O-GlcNAcase) hydrolyzes O-linked 2-acetamido-2-deoxy-β-d-glucopyranoside (O-GlcNAc) residues from post-translationally modified serine/threonine residues of nucleocytoplasmic protein. The chemical process involves substrate-assisted catalysis, where two aspartate residues have been identified as the two key catalytic residues of O-GlcNAcase. In this report, the first step of the catalytic mechanism used by O-GlcNAcase involving substrate-assisted catalysis has been studied using a hybrid quantum mechanical/molecular mechanical (QM/MM) Molecular Dynamics (MD) calculations. The free energy profile shows that the formation of the oxazol…
Competition between liquid-crystalline ordering and glassy freezing in melts of semiflexible polymers: A monte carlo simulation
1999
We present results of a Monte Carlo simulation of dense melts of semiflexible polymers using the bond-fluctuation model. The chosen Hamiltonian increases the chain stiffness upon cooling which in turn leads to glass-transition like freezing of the polymer mobility. Employing an efficient simulation algorithm, which is able to equilibrate the simulated systems to lower temperature than the Rouse-type algorithm showing the glassy freezing, we are able to observe an isotropic-nematic phase transition. This transition lies above the glass transition temperature one would extrapolate from the observed freezing behavior.
Monte Carlo simulations of the polymer glass transition: From the test of theories to material modeling
1997
We present results on the glass transition in polymer melts using Monte Carlo simulations of the bond fluctuation lattice model. There are two questions we address in this work. What is the temperature dependence of the entropy density in such a model polymer melt and how well is it described by theories like the Gibbs-DiMarzio theory of the glass transition? And to what degree is one able to map the Hamiltonian of such an abstract lattice model onto a specific polymer material and use it to model the large scale and long time properties of a realistic polymer melt?
Electronic structure of polysilanes: influence of substitution and conformation
1993
Abstract The valence effective Hamiltonian (VEH) quantum-chemical approach is used to investigate the electronic properties of polysilane. The valence band structure calculated for this fully saturated polymer is analyzed in terms of orbital contributions and compared to that of the closely related carbon polymer, polyethylene. The effects of alkyl substitution and silicon backbone conformation are studied by elucidating the modifications that these structural changes induce on the electronic valence band structure of all-trans unsubstituted polysilane. The VEH results predict a decrease of the band gap upon alkyl substitution and on going from helical to all-trans conformations.
Theoretical study of the OH addition to the endocyclic and exocyclic double bonds of the d-limonene
2005
Abstract The initial step of the d-limonene + OH gas-phase reaction mechanism was investigated by means of ab initio calculations. We have considered eight different possibilities for the OH addition, corresponding to the two C–C double bonds, the two C atoms of each double bond, and the syn or anti orientation, with respect to the isopropenyl group (endocyclic attack) or the ring cycle (exocyclic attack). Activation energies calculated at the QCISD(T)/6-31G(d)//UMP2/6-31G(d) level, show that there are preferred orientations for the OH addition under atmospheric conditions of temperature and pressure.
Experimental and theoretical investigations for the regio and stereoselective transformation of trans 1,2,3-trisubstituted aziridines into trans oxaz…
2003
The regio and stereoselective transformation of trans 1,2,3-trisubstituted aziridines into trans oxazolidin-2-ones takes place in good yield. However, the cis configuration at C2 and C3 in monocyclic aziridines is a limiting factor for this transformation. Ab initio calculations show that while the ring-opening process assisted by iodide is regioselective, the subsequent ring-closure is responsible for the retention of the configuration at the trans oxazolidin-2-ones. The larger energy found for the ring-closure process for the cis aziridines accounts for the non-formation of the cis oxazolidin-2-ones.