CleveTheoComp LLC

Cleveland, OH, United States

CleveTheoComp LLC

Cleveland, OH, United States

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Yepes D.,Andrés Bello University | Murray J.S.,CleveTheoComp LLC | Perez P.,Andrés 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.


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.


Politzer P.,CleVeTheoComp LLC | Peralta-Inga Shields Z.,CleVeTheoComp LLC | Bulat F.A.,Fable Theory and Computation LLC | Murray J.S.,CleVeTheoComp LLC
Journal of Chemical Theory and Computation | Year: 2011

Historically, two important approaches to the concept of electronegativity have been in terms of: (a) an atom in a molecule (e.g., Pauling) and (b) the chemical potential. An approximate form of the latter is now widely used for this purpose, although it includes a number of deviations from chemical experience. More recently, Allen introduced an atomic electronegativity scale based upon the spectroscopic average ionization energies of the valence electrons. This has gained considerable acceptance. However it does not take into account the interpenetration of valence and low-lying subshells, and it also involves some ambiguity in enumerating d valence electrons. In this paper, we analyze and characterize a formulation of relative atomic electronegativities that is conceptually the same as Allen's but avoids the aforementioned problems. It involves the property known as the average local ionization energy, Ī(r), defined as Ī(r)=Σρi(r)|εi|ρ(r), where ρi(r) is the electronic density of the ith orbital, having energy εi, and ρ(r) is the total electronic density. Ī(r) is interpreted as the average energy required to remove an electron at the point r. When Ī(r) is averaged over the outer surfaces of atoms, taken to be the 0.001 au contours of their electronic densities, a chemically meaningful scale of relative atomic electronegativities is obtained. Since the summation giving Ī(r) is over all occupied orbitals, the issues of subshell interpenetration and enumeration of valence electrons do not arise. The procedure is purely computational, and all of the atoms are treated in the same straightforward manner. The results of several different Hartree-Fock and density functional methods are compared and evaluated; those produced by the Perdew-Burke-Ernzerhof functional are chemically the most realistic. © 2011 American Chemical Society.


Murray J.S.,CleveTheoComp LLC | Riley K.E.,University of Puerto Rico at San Juan | Politzer P.,CleveTheoComp LLC | Clark T.,The Interdisciplinary Center | Clark T.,University of Portsmouth
Australian Journal of Chemistry | Year: 2010

The prototypical directional weak interactions, hydrogen bonding and σ-hole bonding (including the special case of halogen bonding) are reviewed in a united picture that depends on the anisotropic nature of the molecular electrostatic potential around the donor atom. Qualitative descriptions of the effects that lead to these anisotropic distributions are given and examples of the importance of σ-hole bonding in crystal engineering and biological systems are discussed. © CSIRO 2010.


Yepes D.,Andrés Bello University | Donoso-Tauda O.,Andrés Bello University | Perez P.,Andrés Bello University | Murray J.S.,CleveTheoComp LLC | And 4 more authors.
Physical Chemistry Chemical Physics | Year: 2013

A variety of experimental and computational analyses support the concept that a chemical reaction has a transition region, in which the system changes from distorted states of the reactants to distorted states of the products. The boundaries of this region along the intrinsic reaction coordinate ξ, which includes the traditional transition state, are defined unambiguously by the minimum and maximum of the reaction force F(ξ), which is the negative gradient of the potential energy V(ξ). The transition region is characterized by the reaction force constant κ(ξ), the second derivative of V(ξ), being negative throughout. It has recently been demonstrated that the profile of κ(ξ) in the transition region is a sensitive indicator of the degree of synchronicity of a concerted reaction: a single κ(ξ) minimum is associated with full or nearly full synchronicity, while a κ(ξ) maximum (negative) between two minima is a sign of considerable nonsynchronicity, i.e. a two-stage concerted process. We have now applied reaction force analysis to the Diels-Alder cycloadditions of the various cyanoethylenes to cyclopentadiene. We examine the relative energy requirements of the structurally- and electronically-intensive phases of the activation processes. We demonstrate that the variation of κ(ξ) in the transition region is again indicative of the level of synchronicity. The fully synchronous cycloadditions are those in which the cyanoethylenes are symmetrically substituted. Unsymmetrical substitution leads to minor nonsynchronicity for monocyanoethylene but much more-i.e. two stages-for 1,1-dicyano- and 1,1,2-tricyanoethylene. We also show that the κ(ξ) tend to become less negative as the activation energies decrease. © 2013 the Owner Societies.


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.


Politzer P.,CleveTheoComp LLC | Reimers J.R.,University of Sydney | Murray J.S.,CleveTheoComp LLC | Toro-Labbe A.,University of Santiago de Chile
Journal of Physical Chemistry Letters | Year: 2010

The reaction force F(R) and the reaction force constant κ(R) provide a rigorously based approach to characterizing a chemical process. The energies associated with its different stages are presented and discussed for a number of examples. Analysis of these suggests an alternative, modified expression for the Hammond-Leffler postulate. We show that diabatic analysis leads to a description of the process that is qualitatively very similar to that coming from F(R) and κ(R), quantitatively so for proton-transfer reactions. Reaction force analysis provides a unifying framework that can encompass a variety of independent concepts relating to chemical processes. © 2010 American Chemical Society.


Yepes D.,Andrés Bello University | Murray J.S.,CleveTheoComp LLC | Politzer P.,CleveTheoComp LLC | Jaque P.,Andrés 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.


Yepes D.,Andrés Bello University | Murray J.S.,CleveTheoComp LLC | Santos J.C.,Andrés 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.


Shields Z.P.,CleveTheoComp LLC | Murray J.S.,CleveTheoComp LLC | Politzer P.,CleveTheoComp LLC
International Journal of Quantum Chemistry | Year: 2010

Halogen bonding, RAX⋯B, and hydrogen bonding, RAH⋯B, are electrostatically-driven noncovalent interactions of covalently-bonded halogens RAX and hydrogens RAH with negative sites B. A significant difference between halogen and hydrogen bonding is that the former is typically near-linear (the R-X-B angles are close to 180°), whereas the latter is more likely to be nonlinear (R-H-B angles sometimes considerably less than 180°). In this work, we have looked at the properties of several RABr⋯B and RAH⋯B complexes as functions of these angles. The differences in the directional tendencies in these interactions can be attributed to the presence of nonbonding valence electrons on the bromines, and their absence on hydrogen. We also found that for a given negative site, the halogen and hydrogen bonding interaction energies correlate very well with the positive electrostatic potentials created at it by RAX and RAH. This attests to the electrostatically-driven nature of these interactions. Overall, this study provides support for regarding both halogen and hydrogen bonding as subsets of α-hole interactions. © 2010 Wiley Periodicals, Inc.

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