Danila Ruiz

The evidence for the occurrence of tautomeric structures for selected aldehydes and thioaldehydes

Mass spectra of selected aldehydes and thioaldehydes have been analyzed and specific fragmentation assignments have been done to keto and enol tautomers, although many peaks can be assigned to both forms (i.e. mass spectra are superimposed on one another). The enolization rate for aldehydes is generally favored by the increase in the steric effect caused by α substitution to the carbonyl group. The analysis of the corresponding mass spectra has allowed to establishing an acceptable correlation between selected ion abundances ratios and approximate enolization equilibrium constants (carried out by means of DFT calculations). The influence of temperature on the enol/keto selected fragments abundance ratios (for different aldehydes and thioaldehydes and for different pair of ions of the same compound whenever possible) is studied in order to estimate the enthalpy difference for the tautomeric equilibria. The results indicate that the thioketo-thioenol equilibrium can be studied by mass spectrometry and the ionization in the ion source have negligible effect on the position of that equilibrium.

Spectrometric studies and theoretical calculations of some β-ketonitriles

The tautomerism of some beta-ketonitriles is investigated by the analysis of their mass spectra and theoretical calculations performed at the MP2/6-31G(d,p) level. The mass spectra of some beta-ketonitriles can provide valuable information regarding the keto-enol and nitrile-ketenimine equilibria taking place in the gas phase. The predictive value of this methodology is supported by the influence of the nature and size of substituents on tautomeric equilibria and the rather good correlation existing between the abundance ratios of selected fragments. Results show that the tautomeric equilibria of these bifunctional compounds can be evaluated by mass spectrometry.

Spectrometric Study of the Nitrile-Ketenimine Tautomerism

Mass spectrometry is used to evaluate the occurrence of the nitrile-ketenimine tautomerism. Mass spectra of two differently substituted nitriles, ethyl-4,4-dicyano-3-methyl-3-butenoate and diethyl-2-cyano-3-methyl-2-pentenodiate are examined looking for common mass spectral behaviors. Ion fragmentation assignments for specific tautomers allow to predict the presence of the corresponding structures. Additionally, the mass spectrum and nuclear magnetic resonance spectra of ethyl-4,4-dicyano-2,2-diethyl-3-methyl-3-butenoate and that of the corresponding amination product support the occurrence of the ketenimine tautomer in the equilibrium.

Experimental Study of Tautomeric Equilibria of 2-Cyanobenzoic Acids in Gas, Solution and Solid Phase

2-cyanobenzoic acids are very important compounds in several sciences, and it is because of this that studying their tautomeric equilibria is of great interest. Ring-chain tautomeric equilibria of 2-, 3- and 4- cyanobenzoic, 3-benzyl-2-cyanobenzoic and 1-cyano-2-naphthoic acids were studied in gas phase by means of mass spectrometry (MS). Furthermore, evidences of ring-chain tautomerism of 2-cyanobenzoic acid were found in solution using 1H and 13C nuclear magnetic resonance and in solid phase by infrared spectroscopy.

Adsorption and removal of phenoxy acetic herbicides from water by using commercial activated carbons: experimental and computational studies

In this study, commercial activated carbons (GAB and CBP) were successfully used for the removal of two phenoxy acetic class-herbicides, 4-chloro-2-methyl phenoxy acetic acid and 2.4-dichlorophenoxy acetic acid (MCPA and 2.4-D) from aqueous solution. The adsorbent materials were characterized, and their equilibrium adsorption capacity was evaluated. The results suggest that the microporous properties of GAB activated carbon enhanced the adsorption capacity, in comparison to CBP carbon. Thus, the increasing in the ionic strength favored the adsorption removal of both pesticides, indicating that electrostatic interactions between the pollutant and the adsorbate surface are governing the adsorption mechanism, but increasing pH values decreased adsorption capacity. Experimental data for equilibrium was analyzed by two models: Langmuir and Freundlich. Finally, computational simulation studies were used to explore both the geometry and energy of the pesticides adsorption.

Tautomerism of uracil and related compounds: A mass spectrometry study:

It has been demonstrated that uracil has a preponderant tautomeric form, but it is also known that different tautomers co-exist in this equilibrium. In this work, mass spectrometry is used as a helpful tool to analyse the equilibria, using derivative compounds to forbid the presence of some tautomers and ion trap mass spectrometry to follow relevant fragmentation pathways. Theoretical calculations were performed to confirm tautomers abundance by energy minimization in gas phase. Analysis of mass spectra of uracil, three methyl-substituted uracils, 2-thiouracil and three benzouracils suggest that uracil exists mainly as three tautomers in gas phase: one major structure that corresponds to the classical structure of uracil (pyrimidine-2,4(1H,3H)-dione) bearing two carbonyls and two NH moieties, and two minor enolic forms (4-hydroxypyrimidin-2(1H)-one and 2-hydroxypyrimidin-4(1H)-one). Such tautomeric distribution is supported by theoretical calculations, which show that they are the three most stable tautomers.

Spectrometric Studies and Semi-Empirical Calculations of Some Thiomorpholides

Mass spectra of selected thiomorpholides have been analysed and specific fragmentation assignments have been done to characterised and weigh co-existing keto and enol tautomers. The predictive value of this methodology in regard to keto-enol equilibria occurring in gas phase is not only supported by the influence of substitution nature and size of these but also by the good correlation found between the selected fragments abundances ratio, semi-empirical calcula- tion (AM1) of the corresponding heats of tautomerization and nuclear magnetic resonance data. The results show that the thioketo-thioenol equilibrium can be studied by mass spectrometry where ionization in the ion source has a negligible ef- fect on the position of that equilibrium and by GC/MS where solvent separation in the analytical column seems to pre- clude shifts from the equilibrium reached at the injection port of the gas chromatograph.