Tautomeric Equilibria Studies by Mass Spectrometry

Tautomerism in organic chemistry has been extensively studied in condensed phase by spectrometric methods, mainly by IR and NMR techniques. Mass spectrometry studies start 40 years ago but just recently it has been recognized the importance of the mass spectral data for the study of tautomerism in the gas phase.
Mass spectrometry can provide valuable information in regard to tautomeric equilibria when studying mass spectra among the members of different families of organic compounds.
The relevance of the mass spectral data resides on several facts but there are two that are of key importance:
1-	Mass spectral fragmentation assignments should be tautomer specific since the corresponding abundances ratios are supposed to be correlated to the keto/enol contents.
2-	Ionization in the ion source is supposed to have no effect on the position of the equilibrium so that the results reflect the tautomers content in the gas phase previous to ionization.
Some of the carbonylic compounds do not exhibit noticeable tautomerism so the fragment abundances assigned to the enol form is very low or not measurable. Since enolization is more noticeable in the case of thio-derivatives (which correlates adequately with the oxygenated analogues), the study of their mass spectra is an interesting choice to reach some degree of generalization. 
In addition, experimental findings are supported by semiempirical theoretical calculations, which probed to be adequate not only for supporting tendency correlations among the members of a compound family but also to calculate heats of tautomerization in gas phase.
Reports using mass spectrometry for tautomerism are becoming less common. One of the reasons is that now it would appear that the interpretation of MS results is not as straightforward as it was once believed, even though in a recent review it was written that: “Mass spectrometry is the most informative and practical method for studying and identifying tautomers in the gas phase” [1]. 
In fact, mass spectrometry seems to be very informative for studying and identifying tautomers, because in this case external factors like solvents, intermolecular interactions, etc., can be excluded by transferring the tautomeric system into gas phase, where the process becomes truly unimolecular [1].
This review covers the study of Tautomerism by Mass Spectrometry in the last four decades. 
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The adsorption structure of furan on Pd(1 1 1)

The structure of molecular furan, C4H4O, on Pd(1 1 1) has been investigated by O K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s scanned-energy mode photoelectron diffraction (PhD). NEXAFS shows the molecule to be adsorbed with the molecular plane close to parallel to the surface, a conclusion confirmed by the PhD analysis. Chemical-state specific C 1s PhD data were obtained for the two inequivalent C atoms in the furan, the α-C atoms adjacent to the O atom, and the β-C atoms bonded only to C atoms, but only the PhD modulations for the α-C emitters were of sufficiently large amplitude for detailed evaluation using multiple scattering calculations. This analysis shows the α-C atoms to be located approximately 0.6 Å off-atop surface Pd atoms with an associated C–Pd bondlength of 2.13 ± 0.03 Å. Two alternative local geometries consistent with the data place the O atom in off-atop or near-hollow locations, and for each of these local structures there are two equally-possible registries relative to the fcc and hcp hollow sites. The results are in good agreement with earlier density functional theory calculations which indicate that the fcc and hcp registries are equally probable, but the PhD results fail to distinguish the two distinct local bonding geometries

A structural study of a C3H3 species coadsorbed with CO on Pd(1 1 1)

The combination of chemical-state-specific C 1s scanned-energy mode photoelectron diffraction (PhD) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) has been used to determine the local adsorption geometry of the coadsorbed C3H3 and CO species formed on Pd(1 1 1) by dissociation of molecular furan. CO is found to adopt the same geometry as in the Pd(1 1 1)c(4 × 2)-CO phase, occupying the two inequivalent three-fold coordinated hollow sites with the C–O axis perpendicular to the surface. C3H3 is found to lie with its molecular plane almost parallel to the surface, most probably with the two ‘outer’ C atoms in equivalent off-atop sites, although the PhD analysis formally fails to distinguish between two distinct local adsorption sites

Photoelectron diffraction: from phenomenological demonstration to practical tool

The potential of photoelectron diffraction—exploiting the coherent interference of directly-emitted and elastically scattered components of the photoelectron wavefield emitted from a core level of a surface atom to obtain structural information—was first appreciated in the 1970s. The first demonstrations of the effect were published towards the end of that decade, but the method has now entered the mainstream armoury of surface structure determination. This short review has two objectives: First, to outline the way that the idea emerged and the way this evolved in my own collaboration with Neville Smith and his colleagues at Bell Labs in the early years: Second, to provide some insight into the current state-of-the art in application of (scanned-energy mode) photoelectron diffraction to address two key issue in quantitative surface structure determination, namely, complexity and precision. In this regard a particularly powerful aspect of photoelectron diffraction is its elemental and chemical-state specificity

Validation of the inverted adsorption structure for free-base tetraphenyl porphyrin on Cu(111)

Utilising normal incidence X-ray standing waves we rigourously scrutinise the “inverted model” as the adsorption structure of free-base tetraphenyl porphyrin on Cu(111). We demonstrate that the iminic N atoms are anchored at near-bridge adsorption sites on the surface displaced laterally by 1.1 ± 0.2 Å in excellent agreement with previously published calculations

Local adsorption structure and bonding of porphine on Cu(111) before and after self-metalation

We have experimentally determined the lateral registry and geometric structure of free-base porphine (2H-P) and copper-metalated porphine (Cu-P) adsorbed on Cu(111), by means of energy-scanned photoelectron diffraction (PhD), and compared the experimental results to density functional theory (DFT) calculations that included van der Waals corrections within the Tkatchenko-Scheffler approach. Both 2H-P and Cu-P adsorb with their center above a surface bridge site. Consistency is obtained between the experimental and DFT-predicted structural models, with a characteristic change in the corrugation of the four N atoms of the molecule's macrocycle following metalation. Interestingly, comparison with previously published data for cobalt porphine adsorbed on the same surface evidences a distinct increase in the average height of the N atoms above the surface through the series 2H-P, Cu-P, cobalt porphine. Such an increase strikingly anti-correlates the DFT-predicted adsorption strength, with 2H-P having the smallest adsorption height despite the weakest calculated adsorption energy. In addition, our findings suggest that for these macrocyclic compounds, substrate-to-molecule charge transfer and adsorption strength may not be univocally correlated

Carrying capacity and goat botanical diet composition in an arid ecosystem, Lavalle, Argentina

8 págs, 1 tabla.-- Comunicación presentada a la XLV Reunión Científica de la Sociedad Española para el Estudio de los Pastos: "Producciones agroganaderas: gestión eficiente y conservación del medio natural" (Gijón, 28 de mayo al 3 de junio de 2005).[ES] El trabajo se llevó a cabo en el departamento de Lavalle, Mendoza, Argentina. Las unidades de pastos, más importantes desde el punto de vista forrajero son: Algarrobal de Prosopis flexuosa; Matorral con Atriplex lampa (zampal); Matorral con Tricomaria usillo (usillar); Matorral degradado con Larrea cuneifolia (jarillal) y Medanal. Los objetivos del estudio fueron estimar la capacidad sustentadora de las diferentes unidades y la composición botánica de la ingesta de los caprinos. La estimación de dicha capacidad, expresada en hectáreas por Unidades Ganadera Caprinas (ha UGC–1), se realizó mediante el método de Point Quadrat modificado para el Monte. Y por medio del análisis microhistológico de heces se determinó la composición estacional de la ingesta.Los valores medios anuales de capacidad sustentadora fueron: Algarrobal, 1,2; Zampal, 1,7; Usillar, 3,5; Jarillal, 5,8 y Médanos, 4,3 ha UGC-1. La composición de la ingesta varió durante las estaciones, observándose una predominancia de las especies arbustivas, las gramíneas perennes aparecieron en muy baja frecuencia en todas las estaciones. El Algarrobal y el Zampal son las unidades de mayor importancia forrajera, presentando receptividades ganaderas más altas y una oferta forrajera más estable a lo largo del año, respecto del resto de las unidades de pastos analizadas.[EN] The work was carried out in the department of Lavalle, Mendoza, Argentina. The units of pastures, more important from the forage matter are: Algarrobal of Prosopis flexuosa; Shrubland with Atriplex lampa (zampal); Shrubland with Tricomaria usillo (usillar); Shrubland degraded with Larrea cuneifolia (jarillal) and Dunes. The aims of the study were to estimate the carrying capacity of the different units and the botanical composition of goat diets. The carrying capacity, expressed in hectares by Units Goat (ha UG-1), was measured by the method of Point Quadrat modified for Monte and the seasonal botanical composition of goat diets, was determined by microhistological analysis of faeces. The annual average values of carrying capacity were: Algarrobal, 1.2; Zampal, 1.7; Usillar, 3.5; Jarillal, 5.8 and Dunes, 4.3 ha UG-1. The composition of the diet varied during the seasons, the predominance of the shrubs species were observed, the perennial grass appeared in low frequency in all the seasons. The Algarrobal and the Zampal are the units of greater forage importance, where forage receptivity are higher and with more stable production throughout the year, respect to the others units of pastures analyzed.Peer reviewe

Photoelectron diffraction investigation of the structure of the clean TiO2(110)(1×1) surface

The surface relaxations of the rutile TiO2(110)(1×1) clean surface have been determined by O 1 s and Ti 2p3∕2 scanned-energy mode photoelectron diffraction. The results are in excellent agreement with recent low-energy electron diffraction (LEED) and medium energy ion scattering (MEIS) results, but in conflict with the results of some earlier investigations including one by surface x-ray diffraction. In particular, the bridging O atoms at the surface are found to relax outward, rather than inward, relative to the underlying bulk. Combined with the recent LEED and MEIS results, a consistent picture of the structure of this surface is provided. While the results of the most recent theoretical total-energy calculations are qualitatively consistent with this experimental consensus, significant quantitative differences remain

Monooctyl-α-anilinobenzylphosphonic Acid as Reagent for Extraction and Separation of Bismuth(III) and Lead(II)

Monooetyl-a-anilinobenzylphosphonic acid was used as extractive reagent for the separation of bismuth(III) and lead(II). The optimal pH value for bismuth(III) extraction was 1.75, whereas lead(II) was quantitatively extracted at pH 5. The separation factor for the two investigated metal ions at pH 1.75 was 140. On the basis of extractive experimental data, the stoichiometry of the extraction mechanisms was presumed. The described method is simple and rapid and can be proposed for the separation of bis- muth(III) from lead(II)