The Simulation of Solvent Polarizabilities and Dipolarities with Polarizable Continuum Model.

The ability of polarizable continuum models (PCM) to simulate nonspecific solvent effects (dipolarity and polarizability) was evaluated by calculating the transition energies of 1,1,10,10-tetrabutyldecanonaene (ttbp9) and 2-N,N-dimethylamino-7-nitrofluorene (DMANF), basis of Catalán's polarizability (SP) and dipolarity (SdP) solvent scales, respectively. Time-dependent density-functional theory (TD-DFT) calculations were performed at different levels of theory, employing four basis sets in 10 different solvents, covering the full range of the normalized SP and SdP scales. Transition energies were calculated using linear response (LR) and corrected linear response (cLR2) schemes. Although these methods yielded variable mean absolute errors, the LR-PCM calculations reproduced medium polarizability and dipolarity trends. While calculated ttbp9 transition energies correlated with SP and Laurence's dispersion-induced (DI) scales, the DMANF transition energies correlated poorly with SdP or Laurence's ES dipolarity scales. This result agrees with the fact that DMANF solvatochromism is "contaminated" by solvent polarizability and HB acidity. The incorporation of SP or DI contributions led to much better (r2 > 0.95) correlations with the DMANF-calculated transitions. The results offer a clearer picture of the limitations of continuum models in simulating the behavior of solvatochromic dyes in solution by pointing out their poor performance when specific solvent effects, such as hydrogen-bond interactions, play a significant role in their solvatochromism.

Alkylated Benzodithienoquinolizinium Salts as Possible Non-Fullerene Organic N-Type Semiconductors: An Experimental and Theoretical Study.

Three photobicyclized benzodithienoquinolizinium tetrafluoroborates (BPDTQBF4) were prepared and evaluated by UV-Vis and fluorescence spectral, electrochemical analysis, and by theoretical calculations as possible organic n-type semiconductors. Evaluation and comparison of their LUMO levels, HOMO-LUMO energy gaps as monomeric and π-stacked dimers with those of other materials, suggest their potential as organic n-type semiconductors. Calculations of their relative charge carrier mobilities confirmed this potential for one derivative with a long (C-14) alkyl chain appended to the polycyclic planar π-system.

Use of Nonideality Parameters for the Analysis of the Thermodynamic Properties of Binary Mixtures.

Two parameters are introduced, a nonideality index (NII) and a nonideality area (NIA), for the analysis of the variations of thermodynamic properties of binary mixtures. Their calculation is based, for the NII, on experimental plots of the variation of a given property (density, refractive index, viscosity, etc.), and for the NIA, on experimental plots of its excess value (excess enthalpies, molar volumes, viscosities, refractive indices, isentropic compressibilities, etc.) with the mixture composition. Both nonideality parameters are therefore not based on any theoretical model or on its derived parameters, being applicable with any fitting equation. The NII or NIA values for a thermodynamic property of a series of related binary mixtures reveal interesting trends, which are not evident in most analyses of such systems. Besides their predictive value, these trends provide information on the relevance of solvent characteristics of the binary mixture, thus supporting new insights or interpretations of the experimental data.

On the physical-chemical nature of solvent polarizability and dipolarity.

The positive solvatochromism of three dyes, with a spectral behavior strongly dependents on the medium dipolarity/polarizability, was studied theoretically. Both a polarizable continuum-solvent model (CSM) and explicit solvent molecules were employed to model solvent effects. The CSM approach, coupled with ten different TDDFT methods, yielded unsatisfactory results in eleven solvents. The explicit-solvation calculations, thought of much higher computational cost, yielded excellent results. As CSM schemes are known correctly model non-specific electrostatic effects, our results indicate that the traditionally considered non-specific nature of solvent dipolarity needs to be reconsidered, requiring the explicit consideration of the solute-solvent interactions for their accurate theoretical description.

The location of amphiphobic antioxidants in micellar systems: The diving-swan analogy.

A protocol for determining the location of antioxidants (AOs) in a micro-heterogeneous medium was applied to three series of AOs with increasing hydrophobicities: chromancarboxylic acid ("Trolox") esters, caffeic acid and its esters, and gallic acid and its esters. The observed paradoxical behaviour of these and other commonly encountered antioxidants was rationalized with the aid of a pictorial simile, the "diving-swan" analogy, that explains the orientation and location of an amphiphobic AO when it reacts with a radical probe in the micellar interface.

The Solvatofluorochromism of 2,4,6-Triarylpyrimidine Derivatives.

Seven new 2,4,6-triarylpyrimidines were synthesized and their solvatofluorochromism investigated in 12 solvents and in an aqueous micellar solution of reduced Triton X-100. A multiparametric analysis of their emission band showed that the solvent dipolarity and basicity were mainly responsible for their solvatofluorochromism, which arose from an internal charge-transfer from a donor fragment to the pyrimidine acceptor, confirmed by theoretical calculations. In the micellar system, quenching of their fluorescence by addition of derivatives of 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) radical was investigated and the results were consistent with the spectral changes brought about by the micro-heterogeneous system.

Cut-off effect of radical TEMPO derivatives in olive oil-in-water emulsions.

Three oil-in-water emulsions were prepared from mixtures of olive oil and Tween 20 in water. The effectiveness of a series of radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) derivatives of variable lipophilicity in reactions with antioxidant Trolox, and as pyrene-fluorescence quenchers, was compared in the three emulsions. A "cut-off" effect was observed for the pyrene quenching by the probes, but not for their reaction with Trolox. The results were rationalized in terms of the amphiphobic nature of the probes, and the different locations of probe, pyrene and Trolox in the three-phase microheterogeneous systems.

Thermohalochromism of phenolate dyes conjugated with nitro-substituted aryl groups.

The cationic halochromism and thermohalochromism of four phenolate dyes conjugated with aryl moieties substituted with one or two nitro groups were investigated in the presence of organic (tetra-n-butylammonium bromide and benzyltriethylammonium chloride) and inorganic (sodium perchlorate) salts, in hydrogen-bond donating (water, 1-propanol, 1-butanol and 2-propanol) and hydrogen-bond accepting (acetonitrile and dimethylsulfoxide) solvents. Although a positive halochromic response was observed in water for tetraalkylammonium salts, their thermohalochromic behavior was negligible. A negative halochromic behavior was observed for the dyes in all solvents, when the added cation was Na+. Plots of Δλmaxvs. c (Na+) allowed the apparent association constants for the solvated phenolate-cation pair to be estimated. In most cases, a positive thermohalochromism was observed in the range of 25-50°C, exceptions being the more sterically hindered phenolate dyes in the less polar solvents 2-propanol and acetonitrile. The observed variations were rationalized by invoking the effect of temperature on the phenolate-cation, phenolate-solvent and cation-solvent interactions.

A Monte Carlo-quantum mechanics study of a solvatochromic π* probe.

The solvation and the solvatochromic behavior of 5-(dimethylamino)-5'-nitro-2,2'-bithiophene 1, the basis of a π* scale of solvent polarities, was investigated theoretically in toluene, dichloromethane, methanol and formamide with a Monte Carlo and quantum mechanics (QM/MM) iterative approach. The calculated transition energies of the solvatochromic band of 1, obtained as averages of statistically uncorrelated configurations, including the solute and explicit solvent molecules of the first solvation layer, besides showing good agreement with the experimental transitions, reproduced very well the positive solvatochromism of this probe in various solvents.

The effect of micellization on the EPR spectra and reactivity of 2,2,4,4-tetramethylpiperidinoxyl (TEMPO) radicals.

A series of 4-alkanoyloxy-2,2,6,6-tetramethylpiperidinoxyl radicals was prepared, and their reactivity in water vis-à-vis antioxidant Trolox was compared. Spectral (electron paramagnetic resonance) and dynamic-light-scattering measurements suggested the formation of micelles for the more hydrophobic members of the series. The observed increase in reactivity for the micelle-forming radicals reflected the increased local concentration of the radical fragment on the micellar interface. Copyright © 2016 John Wiley & Sons, Ltd.

A single theoretical descriptor for the bond-dissociation energy of substituted phenols.

Relative to the corresponding value of phenol, the bond-dissociation energies (BDE) of substituted phenols correlate well with a single descriptor: the Mulliken charge on the oxygen atom of the phenoxyl radical. However, the correlation fails for phenols ortho-substituted with polar groups. Internal reaction coordinates (IRC) for the model reaction of hydrogen abstraction by the hydroperoxyl radical from various 2- and 4-substituted phenols were calculated in order to investigate the role of intra-molecular hydrogen bonds and steric effects on the process. Calculations yielded theoretical values in good agreement with experimental ΔBDE values. The hydrogen-abstraction process was further analyzed in terms of density functional theory (DFT)-based reactivity indices such as local electrophilicity, the Fukui function for nucleophilic attack, and dual descriptor values of the phenolic hydroxyl oxygen along the IRC.

In search of the thermo/halochromism of the E(T)(30) pyridinium-N-phenolate betaine dye.

The thermohalochromic behavior of Reichardt's E(T)(30) betaine - the temperature-dependent variation of its halochromic band in the presence of a cation - was investigated for the first time in NaI solutions of HBD- (methanol, ethanol, 1-propanol, 1-butanol) and non-HBD-solvents (acetonitrile, dimethylformamide) at 15 and 55 °C. The solvent-dependent thermohalochromism of the pyridinium-N-phenolate betaine dye was interpreted in terms of the effect of the temperature on the dye-cation association in solution.

Substituent electrophilicities in the NMR spectra of barbituric derivatives.

Comparison of the (1) H and (13) C NMR spectra of a series of substituted 5-benzylidene-N,N'-dimethylbarbituric acids (1) revealed chemical-shift variations of different centers that correlated with the theoretical electrophilicities or with the substituent electrophilic constant σ(ω) , in an example of the usefulness of these DFT-based indices.

The positive halochromism of phenolate dyes in hydroxylic solutions of tetraalkylammonium cations.

By contrast with the negative halochromic behaviour shown by phenolate betaines in the presence of alkaline and alkaline-earth cations, the addition of tetraalkylammonium salts to hydroxylic solutions of these dyes generate bathochromic shifts of their charge-transfer band. This positive halochromic behaviour by organic cations was examined systematically and its origin rationalized by nonspecific changes of the medium permittivity, and by specific dye-cation interactions in solution.

Electrospray ionization mass spectrometric fragmentation of hydroquinone derivatives.

The fragmentation patterns of nine di-, tri- and tetracyclic hydroquinones with potential antitumor activity were rationalized by invoking competing mechanisms that included sterically accelerated homolytic cleavage, Meerwein-type rearrangements and dehydrations through elimination or intramolecular nucleophilic substitution.

The thermochromism of the ET(30) betaine in a micro-heterogeneous medium: a spectral and dynamics simulation study.

A spectral investigation of the thermochromic behavior of Reichardt's E(T)(30) betaine in aqueous solutions of block copolymers ("poloxamers") P407, P237 and P105 was carried out as a function of temperature and concentration. The betaine microenvironment at various stages of the micellization process in these systems was mimicked with the aid of molecular dynamics simulations of model systems. These consisted of the E(T)(30) probe in boxes of water molecules, in the presence of an isolated block copolymer of formula (PEO)(11)-(PPO)(16)-(PEO)(11), and of a micelle formed of 50 of these unimers.

Binding of arachidonic acid and two flavonoid inhibitors to human 12- and 15-lipoxygenases: a steered molecular dynamics study.

The lipoxygenases (LOX) are a family of non-heme iron-containing dioxygenases which catalyze the stereospecific insertion of molecular oxygen into arachidonic acid, leading to hydroxy derivatives as end products. In this work, we docked arachidonic acid and two of its competitive inhibitors, flavonoids baicalein and quercetin, into the binding pockets of human 12- and 15-lipoxygenase. Steered molecular dynamics (SMD) simulations were employed to study the unbinding processes of the substrate and inhibitors from the two isoforms.

Heteroarylisopropylamines as MAO inhibitors.

The in vitro monoamine oxidase inhibitory (MAOI) activities of 11 heteroarylisopropylamines vis-à-vis MAO-A and MAO-B were described and interpreted in terms of possible interactions with the enzyme active site. Molecular dynamics simulations allowed a comparison between the most active MAO-A inhibitor of the series, the 1-(2-benzofuryl)-2-aminopropane, and the specific, analogous MAO-A substrate serotonin.

MAO inhibition by arylisopropylamines: the effect of oxygen substituents at the beta-position.

Twenty-nine arylisopropylamines, substituted at the beta-position of their side chain by an oxo, hydroxy, or methoxy group, were evaluated in vitro as MAO-A and MAO-B inhibitors. The oxo derivatives ('cathinones') were in general less active as MAO-A inhibitors than the corresponding arylisopropylamines, but exhibited an interesting MAO-B inhibiting activity, which was absent in the hydroxy, methoxy, and beta-unsubstituted analogues. These results suggest that selective affinity for the two MAO isoforms in this family of compounds is modulated not only by the aryl substitution pattern but also by the side-chain substituents on the arylalkylamine scaffold.

An NMR study of merocyanine-type dyes derived from barbituric acid.

The 13C NMR of two solvatochromic dyes derived from a barbituric acid acceptor and dimethylaminophenyl donor fragments, compound 1 and the related merocyanine 2, were recorded in various solvents. The observed chemical-shift variations were used to interpret their structural differences and solvatochromic behavior in solution.