Search results for "solvent effect"
showing 10 items of 166 documents
Searching for new borondifluoride β -diketonate complexes with enhanced absorption/emission properties using ab initio tools
2018
International audience; The rational design of fluorophores with enhanced absorption/emission properties increasingly relies on theoretical chemistry, as new ab initio methods suited for electronically excited-states reduce the gap between calculated and experimental results. In this framework, Time-Dependent Density Functional Theory (TD-DFT) emerges as an attractive option as it often provides accurate results at a moderate computational cost. Here, we perform a TD-DFT-SOS-CIS(D) study of a panel of 18 borondifluoride β-diketonate complexes that can be classified as: curcuminoids, hemicurcuminoids, their ethynylene analogues, and 2′-hydroxy-chalcones. First, we reproduce the experimental …
Triggering dissymmetry in achiral dye molecules by chiral solvents: Circular dichroism experiments and DFT calculations
2011
The electronic circular dichroism spectra of achiral product “Lumogen F Red” (ROT-300) in four different chiral solvents are recorded at different temperatures. DFT calculations allow to identify two enantiomeric conformers for ROT-300. In vacuo they are equally populated; in chiral solvents one enantiomer prevails. Thermodynamic quantities involved in the chiral preference are derived. Chirality, 2011. © 2011 Wiley-Liss, Inc.
Reversibility and Diffusion in Mandelythiamin Decarboxylation. Searching Dynamical Effects in Decarboxylation Reactions
2012
Decarboxylation of mandelylthiamin in aqueous solution is analyzed by means of quantum mechanics/molecular mechanics simulations including solvent effects. The free energy profile for the decarboxylation reaction was traced, assuming equilibrium solvation, while reaction trajectories allowed us to incorporate nonequilibrium effects due to the solvent degrees of freedom as well as to evaluate the rate of the diffusion process in competition with the backward reaction. Our calculations that reproduce the experimental rate constant show that decarboxylation takes place with a non-negligible free energy barrier for the backward reaction and that diffusion of carbon dioxide is very fast compared…
A computationally feasible quantum chemical model for 13C NMR chemical shifts of PCB-derived carboxylic acids.
2004
Two quantum chemical models have been derived for the prediction of 13C NMR chemical shifts of novel PCB acids obtained from PCBs by catalytic carbonylation. 13C isotropic shielding constants were calculated employing the GIAO (gauge-independent atomic orbital) method with density functional theory (DFT). The best results were obtained by cluster calculations, which took the solvent effects into account properly. In this approach, a solvent molecule (acetone) was attached by a hydrogen bond to every hydrogen atom present in a PCB acid, and the geometry of the molecular cluster was optimized employing the AM1 method. For 158 chemical shifts, the cross-validated standard error was 2.8 ppm and…
Fluorescent substituted amidines of benzanthrone: synthesis, spectroscopy and quantum chemical calculations.
2012
Abstract Several new substituted amidine derivatives of benzanthrone were synthesized by a condensation reaction from 3-aminobenzo[de]anthracen-7-one and appropriate aromatic and aliphatic amides. The obtained derivatives have a bright yellow or orange fluorescence in organic solvents and in solid state. The novel benzanthrone derivatives were characterized by TLC analysis, 1H NMR, IR, MS, UV/vis, and fluorescence spectroscopy. The solvent effect on photophysical behaviors of these dyes was investigated, and the results showed that the Stoke’s shift increased, whereas quantum yield decreased with the growth of the solvent polarity. The structure of some dyes was confirmed by the X-ray singl…
Electronic structure of the ground and excited states of beta-carboline.
2008
Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of beta-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.
Experimental and Computational Studies of Hydrogen Bonding and Proton Transfer to [Cp*Fe(dppe)H]
2005
The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed…
Solvent effects on the conformational preferences of model peptoids. MP2 study.
2014
The influence of aqueous environment on the main-chain conformation (ω0, ϕ, and ψ dihedral angles) of two model peptoids: N-acetyl-N-methylglycine N’-methylamide (Ac-N(Me)-Gly-NHMe) (1) and N-acetyl-N-methylglycine N’,N’-dimethylamide (Ac-N(Me)-Gly-NMe2) (2) was investigated by MP2/6-311++G(d,p) method. The Ramachandran maps of both studied molecules with cis and trans configuration of the N-terminal amide bond in the gas phase and in water environment were obtained and all energy minima localized. The polarizable continuum model was applied to estimate the solvation effect on conformation. Energy minima of the Ac-N(Me)-Gly-NHMe and Ac-N(Me)-Gly-NMe2 have been analyzed in terms of the possi…
β-turn tendency in N-methylated peptides with dehydrophenylalanine residue: DFT study.
2010
The tendency to adopt β-turn conformation by model dipeptides with α,β-dehydrophenylalanine (ΔPhe) residue in the gas phase and in solution is investigated by theoretical methods. We pay special attention to a dependence of conformational properties on the side-chain configuration of dehydro residue and the influence of N-methylation on β-turn stability. An extensive computational study of the conformational preferences of Z and E isomers of dipeptides Ac-Gly-(E/Z)-ΔPhe-NHMe (1a / 1b) and Ac-Gly-(E/Z)-ΔPhe-NMe(2) (2a/2b) by B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) methods is reported. It is shown that, in agreement with experimental data, Ac-Gly-(Z)-ΔPhe-NHMe has a great tendency to adopt …
Theoretical Study of the Hydroxyl Radical Addition to Uracil and Photochemistry of the Formed U6OH• Adduct
2014
Hydroxyl radical ((•)OH) is produced in biological systems by external or endogenous agents. It can damage DNA/RNA by attacking pyrimidine nucleobases through the addition to the C5═C6 double bond. The adduct resulting from the attachment at the C5 position prevails in the experimental measurements, although the reasons for this preference remain unclear. The first aim of this work is therefore to shed light on the comprehension of this important process. Thus, the thermal (•)OH addition to the C5═C6 double bond of uracil has been studied theoretically by using DFT, MP2, and the multiconfigurational CASPT2//CASSCF methodologies. The in-vacuo results obtained with the latter protocol plus th…