Density functional theory study of 1,2‐dioxetanone decomposition in condensed phase

The decomposition of 1,2‐dioxetanone into a CO₂ molecule and into an excited state formaldehyde molecule was studied in condensed phase, using a density functional theory approach. Singlet and triplet ground and excited states were all included in the calculations. The calculations revealed a novel mechanism for the chemiluminescence of this compound. The triplet excitation can be explained by two intersystem crossings (ISCs) with the ground state, while the singlet excitation can be accounted by an ISC with the triplet state. The experimentally verified small excitation yield can then be explained by the presence of an energy barrier present in the potential energy surface of the triplet excited state, which will govern both triplet and singlet excitation. It was also found that the triplet ground state interacts with both the triplet excited and singlet ground states. A MPWB1K/mPWKCIS approach provided results in agreement with the existent literature. © 2012 Wiley Periodicals, Inc.     

Studying the interaction between triazines and humic substances--a new approach using open tubular capillary electrochromatography

The strength of the interaction between a pesticide and the soil organic matter is a key parameter to assess the risk of it reaching to groundwater with potentially harmful effects to human health. In this work, a new approach that allows measuring such interactions in a few minutes using a purified fraction of the soil organic matter (humic substances) is detailed. The strength of sorption is assessed via the normalised difference of elution (retention factor, k′) between the chemical of interest and a neutral marker transported via electroosmotic flow through an open tubular column supporting the immobilised humic substances (open tubular capillary electrochromatography). The immobilisation was achieved by incubating a capillary, pre-coated with a monolayer of humic acid, with an acidic solution of humic substances. This induces the formation of a supramolecular structure of humic substances as it occurs in soils. This aggregate can easily be removed using alkaline solutions, and a new structure assembled using other humic substances (HS) or different incubations conditions. The whole procedure takes 2 h. This approach has been tested using five triazines and three types of humic substances. The order of the strength of sorption of the triazines as expected from relevant literature and the relative standard deviation of k′ was between 1 and 6%. Good repeatability was also observed after long period of wash, between re-coating and repeating of the full coating with a new capillary.     

Monitoring of molecular transformations in acid-base reactions by evolving factor analysis of Fourier transform infrared spectral data

An evolving factor analysis procedure with concentration constraints (gradient concentration window) was applied to the analysis of data sets of aqueous Fourier transform infrared (FT-IR) spectra of carboxylic acids (acetic, malonic and succinic acids) collected in experiments with varying pH. Besides the calculation of the number of acid-base systems, this procedure allowed the calculation of the FT-IR spectra of the acid-base species present in equilibrium as well as the corresponding pK(a) values.     

Direct electrochemical reduction of a bromo-propargyloxy ester at vitreous carbon cathodes in dimethylformamide

Cyclic voltammograms for the reduction of ethyl 2-bromo-3-(3^′,4^′-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) at glassy carbon electrodes in dimethylformamide containing tetraalkylammonium salts exhibit three prominent waves corresponding to cleavage of the carbon–bromine bond and to subsequent reduction of ethyl trans-3-(3^′,4^′-dimethoxyphenyl)-prop-2-enoate (4). Controlled-potential electrolyses of 1 at potentials corresponding to reduction of the carbon–bromine bond afford 4 as the major product with an average yield of 56%. In the presence of a proton donor (1,1,1,3,3,3-hexafluoro-2-propanol), the quantity of 4 decreases slightly, and 2-(3^′,4^′-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3) is obtained in moderate amount (26%). We propose a mechanistic scheme whereby the major products are formed via a combination of one- and two-electron processes.     

Coal Rank Increase and Aerial Oxidation by a Combination of Fourier Transform Infrared Spectroscopy with Multivariate Analysis

This work describes that the conventional analysis by Fourier transform infrared spectroscopy of a set of coals grouped by means of multivariate analysis (nonlinear mapping and hierarchical cluster analysis) obtained results that show this methodology is a good approach to summarize and classify coals by their structural relationships and the geochemical processes of their origin.     

Porous phosphate heterostructures containing CdS quantum dots: assembly, characterization and photoluminescence

The synthesis and characterization of cadmium sulphide (CdS) quantum dots, conjugated in a porous phosphate heterostructure functionalized with aminopropyl groups is described. The resulting material has fluorescence properties with maximum emission intensity at 575 nm. The fluorescent materials are not soluble in water and exhibit high stability in aqueous solution in the pH ranges from 2 to 9. Energy dispersive X-ray spectroscopy confirmed the qualitative elemental composition of the synthesized materials and X-ray photoelectron spectra showed a surface S/Cd atomic ratio of 1.09. SEM images show that the materials are amorphous, possessing porous with sizes of several tens nanometres, homogeneous and exhibit a layered morphology. The adsorption–desorption analysis by N₂ at 77 K showed the accessibility of the CdS quantum dots onto the pores of the structure. The CdS quantum dots were stabilized by mercaptopropionic acid and bounded to the host materials by amine groups.     

A Theoretical Analysis of the Potential Role of π–π Stacking Interactions in the Photoprotolytic Cycle of Firefly Luciferin

Firefly oxyluciferin is a photoacid that presents a pH‐sensitive fluorescence, which results from pH‐dependent changes on the conformation of self‐aggregated π–π stacking complexes. Luciferin is a derivative of oxyluciferin with very similar fluorescence and photoacidic properties. This similarity indicates that luciferin is also expected to be able to form π–π stacking complexes, but no pH‐sensitive fluorescence is found for this compound. Here, a theoretical approach is used to rationalize this finding. We have found that luciferin only forms π–π stacking complexes in the ground state at acidic pH. At basic pH and in the excited state, luciferin is present as a dianion. This species is not able to self‐aggregate, owing to repulsive electrostatic interactions. Thus, this emissive species is not subject to π–π stacking interactions; this explains its pH‐insensitive fluorescence.     

LC-MS and microscale NMR analysis of luciferin-related compounds from the bioluminescent earthworm Fridericia heliota

This paper presents the main results of RP-HPLC-MS and microscale NMR analysis performed on Accompanying similar to Luciferin (AsLn(x)), compounds present in extracts of the bioluminescent earthworm Fridericia heliota that display similarities with Fridericia's luciferin, the substrate of the bioluminescent reaction. Three isomers of AsLn were discovered, AsLn(1), AsLn(2) and AsLn(3), all of which present a molecular weight of 529 Da. Their UV-Vis absorption spectra show maxima at 235 nm for AsLn(1), 238 and 295 nm for AsLn(2) and 241 and 295nm for AsLn(3). MS(n) fragmentation patterns suggest the existence of carboxylic acid and hydroxyl moieties, and possibly chemical groups found in other luciferins like pterin or benzothiazole. The major isomer, AsLn(2), presents an aromatic ring and alkene and alkyl moieties. These luciferin-like compounds can be used as models that could give further insights into the structure of this newly discovered luciferin.     

Amino,ammonio and aminioethenes: a theoretical study of their structure and energetics

We have performed high level ab initio quantum mechanical calculations for aminoethene and the three isomeric 1,1‐ (Z)‐ or (E)‐1,2‐diaminoethenes as well as their singly and doubly charged cations derived by loss of electrons and/or upon protonation. Gas phase molecular structures were computed at the MP2/6‐311 + G(3df,2p) level. Standard molar enthalpies of formation in the gas phase, at T = 298.15 K, were estimated using the G3 composite method and atomization, isodesmic and homodesmotic reactions. Other energetic parameters were also calculated at the G3 level: proton affinities, basicities and adiabatic ionization enthalpies. Theoretical and experimental data are compared. The reported experimental data refer only to aminoethene wherein the standard molar enthalpy of formation has a considerable uncertainty, although the molecular structure is well established. There are no such data, neither structural nor thermochemical, for any of the three isomeric diaminoethenes. Isoelectronic comparisons are made. For example, the diprotonated diaminoethenes are isoelectronic to isobutene and (Z)‐ and (E)‐butene, while the doubly ionized diaminoethenes are likewise related to trimethylenemethane and 1,3‐butadiene. Copyright © 2013 John Wiley & Sons, Ltd.     

Study on the effects of intermolecular interactions on firefly multicolor bioluminescence

Firefly luciferase exhibits a color-tuning mechanism based on pH-induced changes in the structure of the active site. These changes increase the polarity of the active site, and thus modulate the intermolecular interactions between the light emitter and active site molecules. In this study, the effects exerted by adenosine monophosphate (AMP), water molecules, and amino acids of Luciola cruciata luciferase active site on the emission wavelength of oxyluciferin were assessed by TD-DFT calculations. The redshift results mainly from decreased interaction of oxyluciferin with AMP and increased interaction of the emitter with a water molecule and Phe249. Breaking of a hydrogen bond between the benzothiazole oxygen atom with formation of a similar bond to the thiazolone oxygen atom is also instrumental.