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Rosenau T.,University of Natural Resources and Life Sciences, Vienna | Potthast A.,University of Natural Resources and Life Sciences, Vienna | Krainz K.,University of Natural Resources and Life Sciences, Vienna | Yoneda Y.,University of Natural Resources and Life Sciences, Vienna | And 2 more authors.
Cellulose | Year: 2011

In the present work, aged cotton linters have been analyzed for their chromophore content according to the CRI ("chromophore release & identification") method. Despite the very low contents in the ppb range, nine chromophores have been unambiguously identified, which makes this account the first one on defined chromophoric structures isolated from cotton. A common feature of the chromophores are 2-hydroxy-[1,4]benzoquinone, 2-hydroxyacetophenone and 5,8-dihydroxynaphthoquinone moieties, which resemble chromophoric structures found in other cellulosic substrates, such as bleached pulps or fibers. The finding of these compounds in lignin-free cotton linters confirms the previous hypothesis that those chromophores are formed from (oxidized) carbohydrate structures rather than from lignin fragments. © 2011 Springer Science+Business Media B.V. Source


Murray J.S.,CleveTheoComp LLC | Concha M.C.,University of New Orleans | Politzer P.,CleveTheoComp LLC
Journal of Molecular Modeling | Year: 2011

We have demonstrated that the variation in the experimentally-determined Si-O-N angles in XYZSi-O-N(CH 3) 2 molecules, which depends upon the positions and natures of the substituents X, Y and Z, can be explained in terms of computed electrostatic potentials on the molecular surfaces of the corresponding XYZSi-H molecules. The latter framework has been used as a model for what the nitrogen lone pair in the XYZSi-O-N(CH 3) 2 molecules sees. Both optimized geometries and electrostatic potentials of our model XYZSi-H systems have been obtained at the B3PW91/6-31G(d,p) level. We propose that the driving force for the observed Si-O-N angle contraction in XYZSi-O-N(CH 3) 2 molecules is largely the electrostatic attraction between a positive σ-hole on the silicon and the lone pair of the nitrogen. Negative regions that may be near the silicon σ-hole, arising from substituents with negative potentials, also play an important role, as they impede the approach of the nitrogen lone pair. These two factors work in synergy and attest to the electrostatically-driven nature of the Si - -N intramolecular interactions, highlighting their tunability. © 2010 Springer-Verlag. Source


Yepes D.,Andres Bello University | Murray J.S.,CleveTheoComp LLC | Perez P.,Andres Bello University | Domingo L.R.,University of Valencia | And 2 more authors.
Physical Chemistry Chemical Physics | Year: 2014

We have computationally compared three Diels-Alder cycloadditions involving cyclopentadiene and substituted ethylenes; one of the reactions is synchronous, while the others are slightly or highly asynchronous. Synchronicity and weak asynchronicity are characterized by the reaction force constant κ(ξ) having just a single minimum in the transition region along the intrinsic reaction coordinate ξ, while for high asynchronicity κ(ξ) has a negative maximum with minima on both sides. The electron localization function (ELF) shows that the features of κ(ξ) can be directly related to the formation of the new C-C bonds between the diene and the dienophile. There is thus a striking complementarity between κ(ξ) and ELF; κ(ξ) identifies the key points along ξ and ELF describes what is happening at those points. © 2014 the Partner Organisations. Source


Yepes D.,Andres Bello University | Murray J.S.,CleveTheoComp LLC | Santos J.C.,Andres Bello University | Toro-Labbe A.,University of Santiago de Chile | And 2 more authors.
Journal of Molecular Modeling | Year: 2013

We have analyzed the variation of the reaction force F(ξ) and the reaction force constant κ(ξ) along the intrinsic reaction coordinates ξ of the water-assisted proton transfer reactions of HX-N = Y (X,Y = O,S). The profile of the force constant of the vibration associated with the reactive mode, k ξ (ξ), was also determined. We compare our results to the corresponding intramolecular proton transfers in the absence of a water molecule. The presence of water promotes the proton transfers, decreasing the energy barriers by about 12 - 15 kcal mol-1. This is due in part to much smaller bond angle changes being needed than when water is absent. The κ(ξ) profiles along the intrinsic reaction coordinates for the water-assisted processes show striking and intriguing differences in the transition regions. For the HS-N = S and HO-N = S systems, two κ(ξ) minima are obtained, whereas for HO-N = O only one minimum is found. The k ξ (ξ) show similar behavior in the transition regions. We propose that this fine structure reflects the degree of synchronicity of the two proton migrations in each case. [Figure not available: see fulltext.] © 2012 Springer-Verlag. Source


Yepes D.,Andres Bello University | Murray J.S.,CleveTheoComp LLC | Politzer P.,CleveTheoComp LLC | Jaque P.,Andres Bello University
Physical Chemistry Chemical Physics | Year: 2012

Earlier work, both experimental and computational, has drawn attention to the transition region in a chemical reaction, which includes the traditional transition state but extends along the intrinsic reaction coordinate ξ from perturbed forms of the reactants to perturbed forms of the products. The boundaries of this region are defined by the reaction force F(ξ), which is the negative gradient of the potential energy V(ξ) of the system along ξ. The reaction force constant κ(ξ), the second derivative of V(ξ), is negative throughout the transition region. We have now demonstrated, for a series of twelve double proton transfer processes, that the profile of κ(ξ) in the transition region is an indicator of the synchronicity of the two proton migrations in each case. When they are fully or nearly fully synchronous, κ(ξ) has a single minimum in the transition region. When the migrations are considerably nonsynchronous, κ(ξ) has two minima separated by a local maximum. Such an assessment of the degree of synchronicity cannot readily be made from an examination of the transition state alone, nor it is easily detected in the profiles of V(ξ) and F(ξ). © 2012 the Owner Societies. Source

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