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Carbon nanotube (CNT) based nanocomposites have attracted much interest, owing to the conductive properties and mechanical reinforcement CNTs may transfer to the complete material. When using CNTs as a filler material for the development of (polymer) nanocomposites, a key element is the dispersion of the CNTs. This will strongly influence at what point a 3D percolating CNT network is formed, also know as the percolation threshold. For various reasons, the percolation threshold should be as low as possible. However, due to the strong interactions between CNTs, achieving good dispersion in the matrix can be problematic. For this purpose, specialized dispersion techniques are used in the preparation of CNT nanocomposites, such as latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-moulded. A complementary approach for lowering the percolation threshold is limiting the volume of the material that is accessible to CNTs. Here a phase separated morphology is desired, with CNTs ideally only found in one of the phases, leading to volume exclusion or double percolation. In this work a polystyrene (PS) / polymethylphenylsiloxane (PMPS) blend system was studied as a matrix for CNT nanocomposites. The study of these polymer blend nanocomposites was performed using thermal analysis techniques, such as DSC, as well as surface characterization and rheology. While an excellent dispersion of CNTs by polydimethylsiloxane (PDMS) was reported before, this is to our knowledge the first study on the related PMPS, which seems to show similar CNT-dispersing properties Unlike the strongly immiscible behaviour known for PS/PDMS blends however, the PS/PMPS system showed partial miscibility. While this means that phase separated morphologies can still be attained, which can be used for volume exclusion, this also makes it possible to develop homogeneous blends where PMPS seems to act more as a CNT compatibiliser. Clear proof of mechanical percolation was found for such systems, and conductivity studies are underway.

An important application of carbon nanotubes (CNTs) is their use as a nanofiller in polymer nanocomposites. If a percolating CNT network is formed, the conducting properties of the CNTs can be transferred to the complete material. For various reasons, the amount of added filler at which such a network is formed, defined as the percolation threshold, should be as low as possible. However, due to the strong interactions between CNTs, achieving good dispersion in the matrix can be problematic, lowering the percolation threshold. For this purpose, specialized dispersion techniques are used in the preparation of CNT nanocomposites, such as latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-moulded. A complementary approach for lowering the percolation threshold is limiting the volume of the material that is accessible to CNTs. This requires a phase separated system such as a polymer blend, leading to volume exclusion or double percolation. In this work a polystyrene (PS) / polymethylphenylsiloxane (PMPS) blend system was studied as a matrix for CNT nanocomposites. The study of these polymer blend nanocomposites was performed using thermal analysis techniques, such as DSC, as well as surface characterization and rheology. While an excellent dispersion of CNTs by polydimethylsiloxane (PDMS) was reported before, this is to our knowledge the first study on the related PMPS, which seems to show similar CNT-dispersing properties Unlike the strongly immiscible behaviour known for PS/PDMS blends however, the PS/PMPS system showed partial miscibility. While this means that phase separated morphologies can still be attained, which can be used for volume exclusion, this also makes it possible to develop homogeneous blends where PMPS seems to act more as a CNT compatibiliser. Clear proof of mechanical percolation was found for such systems, and conductivity studies are underway.

The importance of anisotropy in the evaluation of dispersion interactions

The concept of bond ionicity, obtained via a valence bond analysis, is invoked in the interpretation of the catalytic activity of supported vanadium oxides, in analogy with previous work conducted within the framework of conceptual DFT. For a set of model clusters representing the vanadium oxide supported on SiO(2), Al(2)O(3), TiO(2), ZrO(2), the ionic character of the vanadium-oxygen bond, involved in the dissociative adsorption of methanol on the catalyst, wasquantified. Detailed scrutiny shows that this ionicity increases from the Al through the Zr support, in agreement with the increasing catalytic activity through this series; the case of the Si supported oxide is found to be an exception however, giving rise to the most ionic V-O bond of the different compounds studied. This finding is confirmed by calculations on smaller clusters focusing on detail in the pi back bonding. (C) 2011 Elsevier B. V. All rights reserved.

Information Carriers and Information Theory in Quantum Chemistry."

Recent examples of our works in the field of molecular engineering of chromophores for two-photon photodynamic therapy (PDT), and their vectorization into biological medium are presented and discussed. In a first section of this article, we show that a commercial amphiphilic diblock copolymer can be used as a micellar container for efficient delivery of s standard two-photon PDT sensitizer into cells. In a second section, we show how a simple molecular engineering strategy can be used to improve the performances of two-photon PDT sensitizer in the biological transparency-window.

In this work, the partitioning of higher multipole polarizabilities, such as dipolequadrupole, quadrupoledipole, and quadrupolequadrupole polarizabilities, into atomic contributions is studied. Partitioning of higher multipole polarizabilities is necessary in the study of accurate interaction energies where dispersion interactions are of importance. The fractional occupation Hirhsfeld-I (FOHI) method is used to calculate the atomic polarizabilities and is briefly explained together with the methodology for partitioning of the polarizabilities. The atomic multipole polarizabilities are calculated for different sets of molecules, linear alkanes, water clusters, and small organic molecules with different functional groups. It is found that the atomic and group contributions of the dipole and quadrupole polarizabilities are transferable as a function of the functional groups.

Despite recent advances in computing negative electron affinities using density-functional theory, it is an open issue as to whether it is appropriate to use negative electron affinities, instead of zero electron affinity, to compute the chemical hardness of atoms and molecules with metastable anions. We seek to answer this question using the accepted empirical rules linking the chemical hardness to the atomic size and the polarizability; we also propose a new correlation with the C6 London dispersion coefficient. For chemical reactivity in the gas phase, it seems to make no difference whether negative, or zero, electron affinities are used for systems with metastable anions. For reactions in solution the evidence that is presently available is insufficient to establish a preference. In addressing this issue, we noted that electron affinity data from which atomic chemical hardness values are computed are out of date; an update to Pearson's classic 1988 table [Inorg. Chem., 1988, 27, 734–740] is thus provided.

Density functional theory calculations at the B3LYP/SVP and B3LYP/6-311G(d) levels were carried out for a series of XH(3)B(80) complexes with X = {N, P, As, B, Al}. To probe the regioselectivity of B(80), the electronic Fukui function, the molecular electrostatic potential (MEP), and the natural bond orbital (NBO) were determined. These indices were shown to provide reliable guides to predict the relative reactivities of the boron buckyball sites. Thermodynamic stabilities of the complexes formed by the reaction of B(80) with nucleophiles (NH(3), PH(3), AsH(3)) and electrophiles (BH(3), AlH(3)) are in good agreement with the prediction of regioselectivity indicated on the basis of Fukui and MEP indices. The qualitative results suggest the boron buckyball to be an amphoteric and hard molecule. It has two distinct reactive sites localized on caps and frame, which act as acids and bases, respectively. Most of the complexes are stable with formation energies comparable to that of the analogous complexes of the borane molecule, BH(3)BH(3), BH(3)NH(3), and BH(3)AlH(3). The B-H-B bond characteristics of diborane are recovered in B(80)BH(3). Exohedral complexes are more stable than endohedral complexes. The most stable complexes are those with NH(3) on the caps and BH(3) on the pentagonal ring of B(80).

Valence Photoemission Spectra of Aqueous Fe(2+/3+) and [Fe(CN)6](4-/3-) and Their Interpretation by DFT Calculations.

The S E Ar reaction was scrutinized using a quantitative, fragment-based interaction energy decomposition analysis (EDA) as implemented in the ADF program. The interaction energy between monosubstituted benzene derivatives and a model electrophile at the onset of the reaction was studied and decomposed into Pauli repulsion, electrostatic interaction and orbital interaction terms for a plane parallel to the molecular plane. As such experimentally observed selectivity patterns arise in the orbital interaction, stressing again the role of both electrophile and benzene derivative in determining the regioselectivity. Using the HSAB principle and MO theory, the orientation mechanism of the SEAr is further explored.

Following the spirit of the Hohenberg-Kohn theorems of DFT, the atomic and molecular density functions are used as the key parameters to extract chemical information. The functional form of the Kullback-Leibler relative information measure permitted the construction of density functionals, which along with the choice of a good reference density, for example, noble gas elements, is shown to reveal the periodicity in Mendeleev’s table. Similarly, the specific choice of the transition state as reference, leads to valuable information on the chemically interesting reaction profiles. The investigation of atomic complexity measures is reported in terms of the non-relativistic and relativistic shape electron densities. Such an analysis leads to the interesting result that the atomic complexity derived from the relativistic electron density increases with atomic number whereas a saturation value is attained when the non-relativistic density is employed. It is shown that a relative complexity measure can be defined which reflects the diversity of electron density functions with respect to a reference atom as the sub shells are filled across the periodic table.

The Influence of F(-) Coordination on Al(3+) Hydrolysis Reactions from Density Functional Theory Caculations

Atomic denisty functions: atomic physics calculations analyzed with methods from quantum chemistry.

The applicability of the local hardness as defined by the derivative of the chemical potential with respect to the electron density is undermined by an essential ambiguity arising from this definition. Further, the local quantity defined in this way does not integrate to the (global) hardness - in contrast with the local softness, which integrates to the softness. It has also been shown recently that with the conventional formulae, the largest values of local hardness do not necessarily correspond to the hardest regions of a molecule. Here, in an attempt to fix these drawbacks, we propose a new approach to define and evaluate the local hardness. We define a local chemical potential, utilizing the fact that the chemical potential emerges as the additive constant term in the number-conserving functional derivative of the energy density functional. Then, differentiation of this local chemical potential with respect to the number of electrons leads to a local hardness that integrates to the hardness, and possesses a favourable property; namely, within any given electron system, it is in a local inverse relation with the Fukui function, which is known to be a proper indicator of local softness in the case of soft systems. Numerical tests for a few selected molecules and a detailed analysis, comparing the new definition of local hardness with the previous ones, show promising results., 30 pages (including 6 figures, 1 table)

Intramolecularly Coordinated Tin (II) Selenide and Triseleneoxostannonic Acid Anhydride

Conceptual density-functional theory (DFT) provides a mathematical framework for using changes of the electron density to understand chemical reactions and chemical reactivity. The key idea is that by studying the response of a molecule or materials to perturbations, one can decipher its reactivity preferences. If a system reacts favorably to a perturbation, then this indicates that the system will react favorably with a certain class of reagents. Differentials of the energy may thus be interpreted as reactivity indicators. Because of the key role of energy differentials, the mathematical framework of conceptual DFT is similar to classical thermodynamics, with state functions, variational principles, and Legendre transforms. In this chapter we use this thermodynamic simile to present the mathematical underpinnings of conceptual DFT. Applications to systems of interest to organic, inorganic, and biological chemists are used to demonstrate how these abstract concepts may be applied to concrete chemical problems.

Three intrinsic group properties, the group electronegativity, hardness, and softness, were obtained for 30 organic groups in a nonempirical way starting from the definitions within the framework of density functional theory and applying limited configuration interaction (CISD). The calculated group electronegativities correlatcd well with most scales introduced for this quantity. The values were compared with the field factor F and with the 13 C 1 J CC (ipso-ortho) coupling constant in monosubstituted benzenes, the most important experimental quantity known to reflect substituent electronegativities

This letter is concerned with the calculation and interpretation of the atom-condensed linear response kernel as defined within conceptual density functional theory. An easily applicable and accurate methodology following from second order perturbation theory is used to analyse a series of cyclohexane and benzene derivatives. We show that the variation of the condensed linear response matrix elements with internuclear distance provides a means to differentiate between inductive, resonance and hyperconjugation effects and conclude that this quantity measures the extent of electron delocalisation.

Monomeric Organoantimony(I) and Organobismuth(I) Compounds Stabilized by an NCN Chelating Ligand: Syntheses and Structures."

Cooperativity in Al (3+) Hydrolysis Reactions from Density Functional Theory Calculations."

It is generally known that in order to achieve conductive properties in nanocomposites, a percolating network must be formed by the conducting nanofiller particles, in this case carbon nanotubes (CNTs). For various reasons, the amount of added filler at which such a network is formed, defined as the percolation threshold, should be as low as possible. This requires a good dispersion of the CNT filler, which is often a problem due to the strong self-interaction of CNTs. For this purpose, specialized dispersion techniques are used in the preparation of these nancomposites, such as latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-molded.1,2 A complementary approach for lowering the percolation threshold is limiting the volume of the material that is accessible to CNTs. This requires a phase separated system such as a polymer blend. The study of polymer blend nanocomposites was performed using thermal analysis techniques, such as DSC and TA instruments rapid heating and cooling DSC (RHC) prototype, as well as surface characterization and rheology. The first system studied based on this approach is a polystyrene (PS) / polymethylphenylsiloxane (PMPS) blend system, where the PMPS phase shows preferential interaction with CNTs. While an excellent dispersion of CNTs by polydimethylsiloxane (PDMS) was reported before, this is to our knowledge the first study on the related PMPS.3 In the case of a co-continuous phase separated morphology, the preferential interaction of CNTs with PMPS can lead to so called "double-percolation", lowering the percolation threshold.4 A study of PS/PMPS blends was therefore undertaken, indicating partial miscibility, unlike the strongly immiscible behaviour known for PS/PDMS blends.5 In a second system, prepared by TUe, CNTs were incorporated in a polypropylene (PP) matrix with a crosslinked ethylene propylene diene monomer (EPDM) rubber phase. Here the crosslinked material is not accessible to CNTs, again making it possible to lower the percolation threshold. As PP does not have the remarkable dispersive qualities of siloxanes, CNTs are incorporated using the latex methodology, yielding a more complex system in which surfactants play a role. References 1. N. Grossiord, J. Loos, O. Regev, and C.E. Koning, Chem. Mater., 18, 1089 (2006). 2. O. Regev, P. ElKati, J. Loos, and C.E. Koning. Adv. Mater., 16, 248 (2004) 3. A. Beigbeder, M. Linares, M. Devalckenaere, P. Degee, M. Claes, D. Beljonne, R. Lazzaroni, and P. Dubois, Adv. Mater., 5, 1003 (2008) 4. A. Göldel, G. Kasaliwal, and P. Pötschke, Macromol. Rapid Commun. 30, 423 (2009) 5. M. Maric, and C. Macosko, J. Polym. Sci. Polym. Phys. 40, 346 (2002)

No-carrier added (n.c.a.) 2-[18F]fluoromethyl-l-phenylalanine was found to be very sensitive to hydrolysis in aqueous solutions. This problem was solved partially by the addition of calcium ions (0.04 M), increasing the shelf-life to at least 6 h. In this paper the defluorination reaction was studied in detail to elucidate its mechanism. Therefore, L and D enantiomers of 2-[18F]FMP and 4-[18F]FMP were synthesized, as well as 2-[18F]fluoromethyl-phenethylamine and 4-[18F]fluoromethyl-phenethylamine, both decarboxylated ‘mimetic’ molecules of the amino acid analogues. Radiosynthesis, using a customized Scintomics automatic synthesis hotboxthree module, resulted in a high overall yield and a radiochemical purity of >99%. The defluorination rates of all compounds were studied by HPLC. The L enantiomer of n.c.a 2-[18F]FMP defluorinated seven times faster than the D enantiomer and 2-[18F]fluoromethyl-phenethylamine. Both enantiomers of 4-[18F]FMP and 4-[18F]fluoromethyl-phenethylamine were stable. From these data, the reaction mechanism, involving two distinct intramolecular interactions, was derived. First, the interaction between the amine and the benzylic fluorine weakens the carbon–fluorine bond. Secondly, the formation of a second hydrogen bridge between the carboxyl group and one of the benzylic hydrogen atoms renders the fluorine atom even more susceptible to hydrolysis. The latter interaction induces an additional chiral center. The probability of its formation differs considerably between L and D enantiomers of n.c.a. 2-[18F]FMP, which explains the difference in hydrolysis rate. Copyright © 2010 John Wiley & Sons, Ltd.

Energy Levels and Redox Properties of Aqueous Mn2+/3+ from Photoemission Spectroscopy and Density Functional Molecular Dynamics Simulation

Polymer nanocomposites containing carbon nanotubes (NTs) can find applications in a wide variety of fields, owing to the many beneficial properties of carbon NTs such as their very good mechanical characteristics and a high intrinsic conductivity. The latter of these two properties justifies the use of carbon NTs for the preparation of conducting nanocomposites. To achieve good conductive qualities, NTs must form a percolating network, which requires a good dispersion of the NTs. Therefore, specialised techniques are often required, such as the recently developed latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-moulded. Recently, polydimethylsiloxane (PDMS) was also shown to readily disperse carbon nanotubes after mere high shear mixing. Using the double percolation concept, where the nanotubes are restricted to only one phase of a phase separated co-continuous polymer blend, can further aid to lower the percolation threshold and enable the development of novel (semi-)conducting nanocomposites. In this work, first the interaction is modelled between carbon NTs and sodium dodecyl sulphate (SDS) as surfactant and polystyrene (PS) as polymer matrix, i.e. the system obtained when using the abovementioned latex technology. This analysis shows that addition of an extra component, such as low molar mass PS can increase polymer wetting of the NTs within the matrix, improving the electronic properties of the obtained nanocomposite. In a second part, the potential use of PDMS and polymethylphenylsiloxane (PMPS) in blends with PS as a matrix for novel conducting carbon NT nanocomposites was studied. Due to the large size of systems containing NTs and polymer chains, the semi-empirical AM1 method was chosen for modelling purposes. Theoretical results were compared to experimental analysis with modulated temperature differential scanning calorimetry (MTDSC), rheometry, conductivity measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM). With regard to pure PDMS and PMPS, the experimentally obtained results indicated good NT dispersion, behaviour that was not seen before for PMPS. Rheology and conductivity measurements indicate the formation of a percolating network. AM1 calculations indicate that while the PS/PDMS system is immiscible, which is well-known, the PS/PMPS shows interactions that may lead to miscibility. The partially miscible nature of this system was confirmed with MTDSC, SEM and AFM. The partial miscibility of PS and PMPS was not studied before, and together with the ability of PMPS to disperse NTs easily, may lead to new (semi-)conducting siloxane-based carbon NT nanocomposites.

The linear response kernel: inductive and resonance effects quantified.

To achieve good (conductive) qualities in nanocomposites, carbon nanotubes (NTs) must form a percolating network in the polymer matrix, which requires they are dispersed well. In order to achieve this, specialised and complex techniques are often required, such as latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-molded. Recently however, polydimethylsiloxane (PDMS) was shown to readily disperse carbon nanotubes after mere mechanical mixing. In this work, the properties of PDMS and especially polymethylphenylsiloxane (PMPS) nanocomposites are examined. As lowering the amount of filler that results in percolation is an important goal, the application of the double percolation concept on blends of PMPS with polystyrene (PS) was also studied. For this research, experimental techniques such as modulated temperature differential scanning calorimetry (MTDSC), rheometry, conductivity measurements and scanning electron microscopy (SEM) were supplemented with theoretical semi-empirical AM1 calculations.

Polymer nanocomposites containing carbon nanotubes (NTs) can find applications in a wide variety of fields, owing to the many beneficial properties of carbon NTs such as their very good mechanical characteristics and a high intrinsic conductivity. The latter of these two properties justifies the use of carbon NTs for the preparation of conducting nanocomposites. To achieve good conductive qualities, NTs must form a percolating network, which requires a good dispersion of the NTs. Therefore, specialised techniques are often required, such as the recently developed latex technology, where surfactants are used to form aqueous polymer and nanotube emulsions, which are subsequently mixed, freeze-dried and compression-moulded 1,2. Recently, polydimethylsiloxane (PDMS) was also shown to readily disperse carbon nanotubes after mere high shear mixing 3. Using the double percolation concept, where the nanotubes are restricted to only one phase of a phase separated co-continuous polymer blend, can further aid to lower the percolation threshold and enable the development of novel (semi-)conducting nanocomposites 4. In this work, first the interaction is modelled between carbon NTs and sodium dodecyl sulphate (SDS) as surfactant and polystyrene (PS) as polymer matrix, i.e. the system obtained when using the abovementioned latex technology. This analysis shows that addition of an extra component, such as low molar mass PS can increase polymer wetting of the NTs within the matrix, improving the electronic properties of the obtained nanocomposite 5. In a second part, the potential use of PDMS and polymethylphenylsiloxane (PMPS) in blends with PS as a matrix for novel conducting carbon NT nanocomposites was studied. Due to the large size of systems containing NTs and polymer chains, the semi-empirical AM1 method was chosen for modelling purposes. Theoretical results were compared to experimental analysis with modulated temperature differential scanning calorimetry (MTDSC), rheometry, conductivity measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM). With regard to pure PDMS and PMPS, the experimentally obtained results indicated good NT dispersion, behaviour that was not seen before for PMPS. Rheology and conductivity measurements indicate the formation of a percolating network. AM1 calculations indicate that while the PS/PDMS system is immiscible, which is well-known, the PS/PMPS shows interactions that may lead to miscibility. The partially miscible nature of this system was confirmed with MTDSC, SEM and AFM. The partial miscibility of PS and PMPS was not studied before, and together with the ability of PMPS to disperse NTs easily, may lead to new (semi-)conducting siloxane-based carbon NT nanocomposites. References [1] Grossiord, J. Loos, O. Regev, and C.E. Koning, Chem. Mater., 18, 1089 (2006); [2] O. Regev, P. ElKati, J. Loos, and C.E. Koning. Adv. Mater., 16, 248 (2004); [3] A. Beigbeder, M. Linares, M. Devalckenaere, P. Degee, M. Claes, D. Beljonne, R. Lazzaroni, and P. Dubois, Adv. Mater., 5, 1003 (2008); [4] A. Göldel, G. Kasaliwal, and P. Pötschke. Macromol. Rapid Commun. 30, 423 (2009); [5] G. Van Lier, G. Van Assche, H.E. Miltner, N. Grossiord, C.E. Koning, P. Geerlings, B. Van Mele, Phys. Chem. Chem. Phys., 11, 11121 (2009)

A metal has a typical grey "metallic" look. Different colours can appear when the metallic surface is covered with a thin transparent layer. This is of course the result of interference, and consequently the colour depends on the optical thickness of the layer. Experimental observations can be completely predicted by theoretical modelling. Using the Fresnel equations the colour can be calculated within excellent agreement of the experimental observations. Fresnel, of course, assumes perfectly flat surfaces. Roughness complicates matters: the optical path within the coating no longer depends on the local thickness of the coating only, but also on the angle of scattering at the underlying metal, both varying from point to point. In this presentation we describe how the roughness can be taken into account in predicting the colour. The scattered light was calculated using the "Modeled Integrated Scattering Tool", a computer program developed at the "National Institute of Standards and Technology" (USA). The non-uniformity of the coatings was taken into account by considering different coatings thicknesses. The resulting colour is calculated by taking the average of the obtained reflections. Finally the colours were measured with a spectrophotometer. It turned out that the modelled and the measured colours agree very well, confirming the validity of the used model.

Theoretical Chemistry Accounts, 127 (3), ISSN:1432-881X, ISSN:1432-2234

Equilibrium properties of aqueous metal aqua-/hydroxo species."

In Staphylococcus aureus thioredoxin (Trx) it has been shown that mutation of the conserved active site tryptophan residue (Trp28) has a large effect on the protein stability, on the pKa of the nucleophilic cysteine and on the redox potential. Since these effects can either be due to the partially unfolding of the Trp28Ala mutant or to the absence of the indole side chain of Trp28 as possible interaction partner for the active site cysteines, the origin of the experimentally observed effects is not known and is beyond experimental approach. With theoretical pKa and density functional theory reactivity analysis on model systems where Trp28 has been replaced by an alanine within the structural environment of Trx it is shown that Trp28 does not affect the redox parameters of Trx. As such, the experimentally observed redox effects of the Trx W28A mutant might be due to structural changes induced by partial unfolding.

Branched alkane hydrocarbons are thermodynamically more stable than straight-chain linear alkanes. This thermodynamic stability is also manifest in alkane bond separation energies. To understand the physical differences between branched and linear alkanes, we have utilized a novel density functional theory (DFT) definition of steric energy based on the Weizäcker kinetic energy. Using the M06-2X functional, the total DFT energy was partitioned into a steric energy term (Es[ρ]), an electrostatic energy term (Ee[ρ]), and a fermionic quantum energy term (Eq[ρ]). This analysis revealed that branched alkanes have less (destabilizing) DFT steric energy than linear alkanes. The lower steric energy of branched alkanes is mitigated by an equal and opposite quantum energy term that contains the Pauli component of the kinetic energy and exchange-correlation energy. Because the steric and quantum energy terms cancel, this leaves the electrostatic energy term that favors alkane branching. Electrostatic effects, combined with correlation energy, explains why branched alkanes are more stable than linear alkanes.

"[2+2]Cycloaddition of Carbon Disulfide to NCN-Chelated Organoantimony (III) and Organobismuth(III) Sulfides: Evidence for Terminal Sb-S and Bi-S Bonds in Solution.

In our series of studies on quantifying chirality, a new chirality measure is proposed in this work based on the Kullback−Leibler information entropy. The index computes the extra information that the shape function of one enantiomer carries over a normalized shape function of the racemate, while in our previous studies the shape functions of the R and S enantiomers were used considering one as reference for the other. Besides being mathematically more elegant (symmetric, positive definite, zero in the case of a nonchiral system), this new index bears a more direct relation with chirality oriented experimental measurements such as circular dichroism (CD) and optical rotation measurements, where the racemate is frequently used as a reference. The five chiral halomethanes holding one asymmetric carbon atom and H, F, Cl, Br, and I as substituents have been analyzed. A comparison with our calculated optical rotation and with Avnirs Continuous Chirality Measure (CCM) is computed. The results show that with this index the emphasis lies on the differences between the noncoinciding substituents.

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"On the Derivative of the Lieb Definition for the Energy Functional in Spin-Polarized Density Functional Theory with Respect to the Spin Number".

2-t-Butyl-4,5-didehydropyrimidine, generated by oxidation of 3-amino-5-t-butyl-3H-v-triazolo[4,5-d] pyrimidine, was allowed to react with a variety of reagents. Trapping experiments with furan and two tetracyclones gave the expected adducts in low to moderate yields. On treatment with anthracene and 1,3-cyclohexadiene, complex mixtures were obtained from which the adducts could not be isolated. Cycloaddition of phenyl azide to the intermediate yielded 3-phenyl-5-t-butyl-3H-v-triazolo[4,5-d]pyrimidine as the major product together with the unexpected 2-t-butyl-9H-pyrimido[4,5-b]indole in lesser amount. The structure of these two compounds was established by comparison with authentic specimens whose synthesis is described. Cycloaddition also occurred with 2,3,5-tri-O-benzoyl-β-D-ribofuranosyl azide to give an 8-azanucleoside in low yield. Oxidation of the precursor in ethanol gave solely 4-ethoxy-2-t-butylpyrimidine. Oxidation in the presence of iodine, in dichloromethane or benzene, afforded products arising from attack on the solvent, i.e. 4-chloro-5-iodo-2-t-butylpyrimidine and 5-iodo-4-phenyl-2-t-butylpyrimidine respectively. In addition, 5-iodo-2-t-butyl-4(3H)-pyrimidinone was obtained in both cases. Mechanisms for these reactions are proposed. The electronic structure of 4,5-didehydropyrimidine has been calculated by an ab initio 3-21G quantum chemical method. Both the Molecular Electrostatic Potential and the Fukui function give a very reasonable account of the strong orientation effects observed in the additions to 2-t-butyl-4,5-didehydropyrimidine.

Organoantimony(III) sulfide (1) containing the NCN chelating ligand L [L = 2,6-(CH(2)NMe(2))(2)C(6)H(3)] displays unusual dual behavior, being dimeric (LSbS)(2), with the central Sb(2)S(2) core, in the solid state but monomeric in solution. Sulfide 1 reacts with elemental S to give the unprecedented cyclic bis(pentasulfide) LSb(mu-S(5))(2)SbL (2) with the central 12-membered ring Sb(2)S(10).

Experimentalists and quantum chemists are living in a different world. A wealth of theoretical enzymology-related publications is hardly known by experimentalists, and vice versa. Our aim is to bring both worlds together and to show the powerful possibilities of a multidisciplinary approach to study Subtle details of complicated enzymatic processes to a broad readership. MD simulations and QM/MM approaches often focus on the calculation of reaction paths based on activation energies, which is a time-consuming task. A valuable alternative is the reactivity descriptors founded in conceptual DFT like softness, electrophilicity, and the Fukui function, which describe the kinetic aspects of a reaction in terms of the response to perturbations in N and/or v(r), typical for a chemical reaction, of the reagents in the ground state. As such, the relative energies at the beginning of the reaction predict a sequence of activation energies only based on the properties of the reactants (Figure 5). In 2003, Geerlings et al. published a key review giving a detailed description of the principles and concepts of conceptual DFT and highlighting its success to study generalized acid/base reactions including addition, substitution, and elimination reactions. Since the time that this review appeared, conceptual DFT has proven its strength in literally hundreds of papers with application to organic and inorganic reactions. Its role in unravelling enzymatic reaction mechanisms, in handling experimentally difficult accessible biochemical problems, and in the interpretation of biochemical experimental observations is emerging and very promising

Experimentalists and quantum chemists are living in a different world. A wealth of theoretical enzymology-related publications is hardly known by experimentalists, and vice versa. Our aim is to bring both worlds together and to show the powerful possibilities of a multidisciplinary approach to study subtle details of complicated enzymatic processes to a broad readership. MD simulations and QM/MM approaches often focus on the calculation of reaction paths based on activation energies, which is a time-consuming task. A valuable alternative is the reactivity descriptors founded in conceptual DFT like softness, electrophilicity, and the Fukui function, which describe the kinetic aspects of a reaction in terms of the response to perturbations in N and/or upsilon(r), typical for a chemical reaction, of the reagents in the ground state. As such, the relative energies at the beginning of the reaction predict a sequence of activation energies only based on the properties of the reactants (Figure 5 ). In 2003, Geerlings et al. published a key review giving a detailed description of the principles and concepts of conceptual DFT and highlighting its success to study generalized acid/base reactions including addition, substitution, and elimination reactions. Since the time that this review appeared, conceptual DFT has proven its strength in literally hundreds of papers with application to organic and inorganic reactions. Its role in unravelling enzymatic reaction mechanisms, in handling experimentally difficult accessible biochemical problems, and in the interpretation of biochemical experimental observations is emerging and very promising.

Although extensive theoretical and experimental research has been conducted on fluorinated fullerenes, little detailed information exists on their solubility in different solvents. However, this solubility is crucial for their processability and possible application. In this work, we predict the solubility of fluorinated C(60) in various polar and non-polar solvents, based on a correlation between experimentally measured solubilities for C(60) from the literature and theoretically predicted solubilisation energies. These solubilisation energies are predicted using the polarizable continuum model (PCM) at the ab initio Hartree-Fock 6-31+G* level of theory. In particular, the solubilities are predicted for C(60)F(2)(n) (n = 1-10) isomers, part of the addition route to saturnene C(60)F(20). An increasing solubility is found for more polar solvents with higher degree of fluorination. With these results, we can determine the minimal fluorination degree necessary for possible solubilisation in a given solvent, and offer new perspectives to separate and purify species with different degrees of fluorination.

A detailed investigation of the kinetic irreversibility-reversibility concept is presented on the basis of the analysis of four side reactions occurring in the polymerization of poly(vinyl chloride), the intramolecular 1,5- and 1,6-backbiting and 1,2- and 2,3-Cl shift side reactions. Density functional theory-based reactivity indices combined with an analysis of the reaction force are invoked to probe this concept. The reaction force analysis is used to partition the activation and reaction energy and characterize the behavior of reactivity indices along the three reaction regions that are defined within this approach. It has been observed that in the reactant and product regions mainly geometric rearrangements take place, whereas in the transition state region changes in the electronic bonding pattern occur; here most changes of the electronic properties are observed. The kinetic irreversibility-reversibility of the reactions is confirmed and linked to the differences in the Fukui function and dual descriptor of the radical centers associated with the initial and final species.

When a molecule is placed as a guest inside a zeolite pore, its electronic structure will be altered, among others by the effect of the so-called “confinement”. It has been established that the compression of the molecular orbitals influences a system’s reactivity. In this work we use a simple potential barrier method to quantify the importance of confinement effects on chemical reactivity. In the first part, excitation energies and molecular orbital energy gaps are evaluated for molecules placed in cavities of different sizes, resembling a zeolite pore. Our results for ethylene and formaldehyde reveal an increase in excitation energy and the gap between the occupied and the unoccupied levels. In the case of the larger molecules naphthalene and anthracene, the HOMO–LUMO gap shows very little sensitivity to the confinement. To investigate the role of confinement effects on local aspects of chemical reactivity and on regioselectivity, we evaluated its effect on the Fukui function and the molecular electrostatic potential, reactivity indices that are central in the description of orbital and charge controlled reactions. The results indicate that confinement can influence the regioselectivity and that the reactivity of anions is expected to change, due to the artificial binding of the excess electron.

This paper presents an approach for obtaining accurate interaction energies at the density functional theory level for systems where dispersion interactions are important. This approach combines Becke and Johnson's [J. Chem. Phys. 127, 154108 (2007)] method for the evaluation of dispersion energy corrections and a Hirshfeld method for partitioning of molecular polarizability tensors into atomic contributions. Due to the availability of atomic polarizability tensors, the method is extended to incorporate anisotropic contributions, which prove to be important for complexes of lower symmetry. The method is validated for a set of 18 complexes, for which interaction energies were obtained with the B3LYP, PBE, and TPSS functionals combined with the aug-cc-pVTZ basis set and compared with the values obtained at the CCSD(T) level extrapolated to a complete basis set limit. It is shown that very good quality interaction energies can be obtained by the proposed method for each of the examined functionals, the overall performance of the TPSS functional being the best, which with a slope of 1.00 in the linear regression equation and a constant term of only 0.1 kcal/mol allows to obtain accurate interaction energies without any need of a damping function for complexes close to their exact equilibrium geometry.

The recently proposed density-based quantification of the steric effect, based on an alternative energy partition scheme where the total electronic energy is decomposed into contributions from three independent effects, steric, electrostatic, and fermionic quantum, is investigated at the atomic and functional group levels in this work. Reasonable trends and linear relationships between theoretical and experimental scales of the steric effect at both group and entire molecular levels have been observed, providing further evidence that the newly defined quantity can serve as an intrinsic measurement of the steric effect for molecular systems.

The dissociation mechanism of the thioredoxin (Trx) mixed disulfide complexes is unknown and has been debated for more than twenty years. Specifically, opposing arguments for the activation of the nucleophilic cysteine as a thiolate during the dissociation of the complex have been put forward. As a key model, the complex between Trx and its endogenous substrate, arsenate reductase (ArsC), was used. In this structure, a Cys29Trx-Cys89ArsC intermediate disulfide is formed by the nucleophilic attack of Cys29Trx on the exposed Cys82ArsC-Cys89ArsC in oxidized ArsC. With theoretical reactivity analysis, molecular dynamics simulations, and biochemical complex formation experiments with Cys-mutants, Trx mixed disulfide dissociation was studied. We observed that the conformational changes around the intermediate disulfide bring Cys32Trx in contact with Cys29Trx. Cys32Trx is activated for its nucleophilic attack by hydrogen bonds, and Cys32Trx is found to be more reactive than Cys82ArsC. Additionally, Cys32Trx directs its nucleophilic attack on the more susceptible Cys29Trx and not on Cys89ArsC. This multidisciplinary approach provides fresh insights into a universal thiol/disulfide exchange reaction mechanism that results in reduced substrate and oxidized Trx., Author Summary Thioredoxins are found in all types of cells and control several essential functions of life, including promotion of cell growth, inhibition of apoptosis, and modulation of inflammation. Thioredoxin has two ‘free’ cysteines in its active site, which are used to break disulfide bonds in oxidized substrate proteins. In the first step, an intermediate thioredoxin-protein complex is formed, which is broken in the second step, releasing the substrate protein in its reduced state. In other words, the disulfide is being transferred from the substrate protein to thioredoxin, or the electrons coming from thioredoxin are shuttled to the protein substrate. The exact reaction mechanism, i.e., the detailed succession of steps in which the reaction takes place, of how this mixed disulfide is broken is not known and has been debated over the last twenty years. With a multidisciplinary approach, combining computational and experimental work, we provide fresh insights into how conformational changes activate the catalytic cysteines with which this universal reduction mechanism is completed with the correct regioselectivity. This work illustrates the strengths of computational approaches in probing phenomena which are otherwise very difficult to investigate experimentally.

Density functional theory (DFT) is a very popular and widely used methodology to investigate the electronic structure of many chemical systems having a variety of complexities.1–7 This theory uses the electron density function ρ(r) as the basic quantity from which all atomic and molecular properties...

Comparative Characterization of two-electron wave functions using Information Theory Measures

On the basis of the zero-temperature grand canonical ensemble generalization of the energy E[N,N_s,v,B] for fractional particle N and spin N_s numbers, the energy surface over the (N,N_s) plane is displayed and analyzed in the case of homogeneous external magnetic fields B(r). The (negative of the) left/right-side derivatives of the energy with respect to N, N_up, and N_down give the fixed-N_s, spin-up, and spin-down ionization potentials/electron affinities, respectively, while the derivative of E[N,N_s,v,B] with respect to N_s gives the (signed) half excitation energy to a state with N_s increased (or decreased) by 2. The highest occupied and lowest unoccupied Kohn-Sham spin-orbital energies are identified as the corresponding spin-up and spin-down ionization potentials and electron affinities. The excitation energies to the states with N_s+2, N_s-2, can be obtained as the differences between the lowest unoccupied and the opposite-spin highest occupied spin-orbital energies, if the (N,N_s) representation of the Kohn-Sham spin-potentials is used. The cases where the convexity condition on the energy does not hold are also discussed. Finally, the discontinuities of the energy derivatives and the Kohn-Sham potential are analyzed and related., Comment: 35 pages, to appear in JCP; text made more precise, Aufbau discussed, T_s derivative discontinuities given too, two Appendices added