Browsing by Subject "Elements"

Now showing 1 - 5 of 5
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    Accurate ring strain energies of unsaturated three-membered heterocycles with one group 13–16 element
    (American Chemical Society, 2022-05-02) Rey Planells, Alicia; Espinosa Ferao, Arturo; Química Orgánica
    High-quality ring strain energy (RSE) data for 1H-unsaturated (CH)2X parent rings, where X is a group 13–16 element, are reported in addition to the 2H-isomers of the pnictogenirene rings. RSE data are obtained from appropriate homosdesmotic reactions and calculated at the DLPNO-CCSD(T)/def2-TZVPP//B3LYP-D3/def2-TZVP(ecp) level. 1H-Tallirene and 1H-plumbirene have unique donor–acceptor structures between an acetylene π(CC) orbital and an empty p orbital of a metallylene subunit (a Dewar–Chatt–Duncanson description) and therefore cannot be described as proper rings but as pseudocyclic structures. Also, 1H-indirene and 1H-oxirene lack ring critical points and constitute borderline cases of pseudorings. 1H-Unsaturated rings exhibit enhanced RSE compared to their saturated homologues. The mechanism of ring strain relaxation by increasing the s character in the lone pair (LP) of group 15–16 elements is remarkable and increases on descending the groups. Furthermore, RSE is affected by the aromatic character of group 13 rings and certain aromatic or antiaromatic character in group 14 or 15–16 rings, respectively, which tend to vanish on descending the group as shown by NICS(1) values. 2H-Unsaturated rings were found only for group 15 elements (although only 2H-azirine shows a proper cyclic structure) and displayed lower RSE (higher stability) than the corresponding 1H-isomers.
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    Accurate ring strain energy calculations on saturated three-membered heterocycles with one group 13–16 element
    (American Chemical Society, 2020-08-17) Rey Planells, Alicia; Espinosa Ferao, Arturo; Química Orgánica
    Accurate ring strain energy (RSE) data for parent (CH2)2X rings are reported, where X are group 13–16 elements (El) in their lowest oxidation state, from the second to the sixth row, with their covalence completed by bonds to H. They are obtained from appropriate homodesmotic and hyper-homodesmotic reactions at different levels up to the CCSD(T) level, thus providing a benchmark of high-quality reference RSE values, as well as acceptably accurate faster lower-level options. Derivatives of indium, thallium, and lead cannot be properly described by a three-member ring connectivity, because they display a unique donor–acceptor structure from an ethylene π(C═C) orbital to an empty p orbital of a metallylene subunit. The RSE of groups 13 and 14 heterocycles increases on descending in the group (except for Ga and Ge), while it decreases for groups 15 and 16. The latter is presumably due to a strain-releasing mechanism favored by the increase of p-character at the sp3-type atomic orbital used by El in the endocyclic El–C bonds, %p(El)El-C, originated by the tendency of the El lone pairs in groups 15–16 to increase their s-character. This strain-releasing mechanism does not exist in heavier tetrels, which keep almost unchanged the p-character in the endocyclic bonds at El, whereas for triels the p-character is still lower owing to their sp2-like hybridization. Remarkable linear correlations were found between the RSE and either the above-mentioned %p(El)El-C, the distal C–C bond distance or the relaxed force constants for the endocyclic bond angles.
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    Pour une nouvelle approche en phonologie diachronique
    (Universidad de Murcia, Servicio de Publicaciones., 2016-02-11) Bretos Bórnez, Jesús; Tejedor de Felipe, Didier
    Resumen: Toda descripción lingüística necesita un modelo. El análisis de los procesos fonológicos que producen los "cambios" en una lengua no es una excepción. Lo que aquí proponemos es un nuevo enfoque que permite abordar los cambios que se producen en francés desde sus orígenes hasta el francés actual. Para ello, nos situamos en el marco de la Fonología Declarativa, siguiendo los trabajos en sincronía llevados a cabo fundamentalmente por Bird (1991, 1995) y Angoujard (1997, 2006). Adoptar esta perspectiva en diacronía nos lleva a refutar algunos puntos de vista comúnmente admitidos. Así, en un plano estrictamente "evolutivo", cuando un número n de "cambios" tiene lugar en el sistema fonológico de una lengua l, se suele decir que tal o cual estado de una lengua ha "evolucionado" pero, la realidad es que cada uno de estos "cambios" es el resultado de una acumulación de variaciones paramétrica con respecto a un estado de lengua anterior.
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    Ring strain energies of three-membered homoatomic inorganic rings El3 and diheterotetreliranes El2Tt (Tt = C, Si, Ge): accurate versus additive approaches
    (American Chemical Society, 2022-09-05) Rey Planells, Alicia; Espinosa Ferao, Arturo; Química Orgánica
    Accurate ring strain energies (RSEs) for three-membered symmetric inorganic rings El3 and organic dihetero-monocycles El2C and their silicon El2Si and germanium El2Ge analogues have been computed for group 14–16 “El” heteroatoms using appropriate homodesmotic reactions and calculated at the DLPNO-CCSD-(T)/def2-TZVPP//B3LYP-D4/def2-TZVP(ecp) level. Rings containing triels and Sn/Pb heteroatoms are studied as exceptions to the RSE calculation as they either do not constitute genuine rings or cannot use the general homodesmotic reaction scheme due to uncompensated interactions. Some remarkable concepts already related to the RSE such as aromaticity or strain relaxation by increasing the s-character in the lone pair (LP) of the group 15–16 elements are analyzed extensively. An appealing alternative procedure for the rapid estimation of RSEs using additive rules, based on contributions of ring atoms or endocyclic bonds, is disclosed.
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    Toxic and essential element concentrations in Iberian Ibex (Capra pyrenaica) from the Sierra Nevada Natural Park (Spain): reference intervals in whole blood
    (Springer, 2015-12-21) Ráez-Bravo, Arián; Granados, José Enrique; Cano-Manuel, Francisco Javier; Soriguer, Ramón C.; Fandos, Paulino; Pérez, Jesús M.; Pavlov, Igor Y.; Romero García, Diego; Ciencias Sociosanitarias
    Iberian ibex (Capra pyrenaica) blood samples from the Sierra Nevada Natural Park (Spain) were analyzed to establish concentrations of toxic and essential elements. Samples (whole blood from 32 males and 34 females) were taken from wild animals and the concentrations of inorganic elements considered as (1) non-essential and toxic (Pb, Cd and As), (2) essential but potentially toxic (Cu, Zn and Mn) and (3) occasionally beneficial (B, Cr, Al and Ni), as well as (4) essential minerals (Ca, Na, K, P, Mg, S, Co and Fe), were analyzed. The low concentration of Pb and Cd indicated that there is no heavy metal contamination in this geographical area for these elements. The concentration of elements in this ibex population was defined for different genders and ages. Significant differences between genders were only found for Mg and Cu, while significant differences in concentrations of Ca, Cr, Fe, Mn, P, S and Zn were found between ages.