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Showing papers in "Journal of Molecular Structure-theochem in 2005"


Journal ArticleDOI
TL;DR: The state of the art for the quantum chemistry of the excited state is reviewed in this paper, focusing in the advantages and disadvantages of the most commonly employed computational methods, from the single configurational procedures like CI-Singles (CIS), propagator approaches, and Coupled-Cluster (CC) techniques, to the more sophisticated multiconfigurational treatments, with particular emphasis on perturbation theory, the CASPT2 approach.
Abstract: The present contribution contains an overview of quantum-chemical methods and strategies to compute and interpret spectroscopic and photochemical phenomena in molecular systems. The state of the art for the quantum chemistry of the excited state is reviewed, focusing in the advantages and disadvantages of the most commonly employed computational methods, from the single configurational procedures like CI-Singles (CIS), propagator approaches, and Coupled-Cluster (CC) techniques, to the more sophisticated multiconfigurational treatments, with particular emphasis on perturbation theory, the CASPT2 approach. Also, a short summary on the performance, lights, and shadows of the popular TDDFT methods is included. The role of the differential correlation effects on quantum-chemical calculations is analyzed, especially for the location of potential energy surface crossings. The contribution finally addresses the importance that theoretical constructs as conical and non-conical intersections play in non-adiabatic photochemistry. The nice photochemistry of cytosine is used as an illustrative example of theoretical photochemistry, a continuously expanding field of research.

198 citations


Journal ArticleDOI
TL;DR: In this paper, a Gaussian polarization function optimized at the Mϕller-Plesset second-order (MP2) level was added to the double zeta (DZ) set.
Abstract: Contracted basis sets of double zeta (DZ) valence quality for H, He, first- and second-row atoms are constructed from fully-optimized Gaussian basis sets generated in this work. A single set of Gaussian polarization functions (d symmetry) optimized at the Mϕller-Plesset second-order (MP2) level was added to the DZ set. Then, this set is augmented with diffuse (s and p symmetries) and polarization (d symmetry) functions that were optimized for the anion at the HF and MP2 levels, respectively. The accuracy of the sets generated in this work is compared with the widely used correlation consistent basis sets of Dunning et al.

153 citations


Journal ArticleDOI
TL;DR: In this article, the electronic and molecular structure of 1,3,4-thiadiazole and its derivatives, with different nucleophilic and electrophilic alkyl substituents (R1 and R2), obtained through quantum-chemistry calculations conducted by means of the gaussian 03W set of programs.
Abstract: The damages by corrosion generate not only high costs for inspection, repairing and replacement, but in addition these constitute a public risk. Thus the necessity of developing novel substances that behave like corrosion inhibitors. In general, the organic compounds have demonstrated a great effectiveness in inhibiting the watery corrosion of many metals and alloys. It has been demonstrated that the compounds that have nitrogen and sulfur in their structure provide a greater inhibition than those than only contain one of the atoms at a time, being this property attributable to its molecular structure. The objective of this work is to provide information about the electronic and molecular structure of several heterocyclic organic compounds: 1,3,4-thiadiazole and its derivatives, with different nucleophilic and electrophilic alkyl substituents (R1 and R2), obtained through quantum-chemistry calculations conducted by means of the gaussian 03W set of programs. It is an attempt to find the correlation between the molecular structure of these compounds and their possible behavior like corrosion inhibitors. It is important to take into account the effect of films formed by products of corrosion. Thus, the incorporation of metallic atoms in the calculations provides greater information about the relation between the reactivity of compounds and the metallic surface, in addition to the formation of stable complexes. The behavior of these organic substances in the presence of several solvents, among them water, facilitates the understanding of the processes of inhibition of the corrosion.

141 citations


Journal ArticleDOI
TL;DR: In this article, the intrinsic limits of ELF have been discussed, and it has been used with considerable success for analyzing the chemical bond in molecules and crystals, which seems to have some intrinsic limits, which are discussed.
Abstract: ELF is a function of the 3D-coordinates which is large in the regions where orbitals localize. It is closely related to LOL and the Fermi hole mobility function. While it has been used with considerable success for analyzing the chemical bond in molecules and crystals, it seems to have some intrinsic limits, which are discussed.

136 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated correlations between experimentally obtained inhibition efficiencies for pitting corrosion of Al 7075 alloy in NaCl solution and parameters such as electronic charges on active centres (qO, qN), dipole moment (μ), highest occupied (EH) and lowest unoccupied (EL) molecular orbital energies and the differences between them (EL−EH), HOMO-LUMO gap were investigated.
Abstract: Quantum chemical calculations were performed on six amino acids and six hydroxy carboxylic acids. Possible correlations between experimentally obtained inhibition efficiencies (ΔE) for pitting corrosion of Al 7075 alloy in NaCl solution and parameters such as electronic charges on active centres (qO, qN), dipole moment (μ), highest occupied (EH) and lowest unoccupied (EL) molecular orbital energies and the differences between them (EL−EH), HOMO–LUMO gap were investigated. Possible inhibition mechanism for pitting corrosion of Al 7075 in NaCl solution was proposed from the obtained relations between quantum chemical parameters computed using AM1 and PM3 methods and experimentally obtained inhibition efficiencies.

93 citations


Journal ArticleDOI
Huai Cao1, Weixian Cheng1, Cong Li1, Xulin Pan1, Xiaoguang Xie1, Taohong Li1 
TL;DR: A theoretical calculation based on density functional theory has been performed to understand the antioxidant activity of rosmarinic acid in view of a molecular structure according to the geometry of the ground state molecule and its free radical, the HOMO and LUMO, the O-H bond dissociation energy (BDE), and the single electron density distribution of the radicals as mentioned in this paper.
Abstract: A theoretical calculation based on the density functional theory (DFT) has been performed to understand the antioxidant activity of rosmarinic acid in view of a molecular structure According to the geometry of the ground state molecule and its free radical, the HOMO and LUMO, the O–H bond dissociation energy (BDE), and the single electron density distribution of the radicals, we interpreted the capacity of scavenging the radicals for rosmarinic acid by its structure Compared with flavonoids, the higher antioxidant activity of rosmarinic acid may be from the abstraction of hydrogen atoms of the ortho -position hydroxyls on the rings A and B This abstraction can occur continuously to form a semiquinone structure, or even to form a quinone structure The activity of the ring A is similar with the ring B However, the ring B is a stronger electron donor than the ring A, and, the radical formed from the H-abstraction of the ring A is more stable than that of the ring B We also found that there is a good relationship between the BDE, which is used to show the stability of the parent molecule and the unpaired electron delocalization correlated to the stability of the free radical These theoretical researches will be helpful to the development for the antioxidant compounds

91 citations


Journal ArticleDOI
TL;DR: In this paper, a QSAR study on N-containing corrosion inhibitors has been made using quantum chemical approach assisted by a novel topological index (Xv#2) and two models were obtained and their efficiency was examined using statistics.
Abstract: In this paper, QSAR study on N-containing corrosion inhibitors has been made using quantum chemical approach assisted by a novel topological index (Xv#2). Two models were obtained and their efficiency was examined using statistics. The results indicate that inhibition efficiencies relate to some parameters of corrosion inhibitors, such as Ehomo, polarizability, Dipole, frontier orbital charge density, the interaction mode between inhibitors and metal surface (feedback donor–acceptor coordination bonds), Xv#2 (steric hindrance of molecules). Predicted values are consistent with the experimental ones on the whole. The residuals range within the experimental error. So the models can be used to predict inhibition efficiencies of the same type molecules.

91 citations


Journal ArticleDOI
TL;DR: An extension of the Miertus-Scrocco-Tomasi (MST) continuum method to the Integral Equation Formalism (IEF) is presented in this article.
Abstract: An extension of the Miertus–Scrocco–Tomasi (MST) continuum method to the Integral Equation Formalism (IEF) is presented. In particular, we report here the parametrization of the MST(IEF) model to the solvation in water, octanol, chloroform and carbon tetrachloride. A detailed comparison is made between the results obtained from the MST(PCM) formalism corrected by using diverse charge normalization schemes and those obtained from the IEF formalism. The IEF method is found to provide results in close agreement with those obtained within the PCM framework by taking into account the anisotropy of the solute's charge distribution in the charge normalization of the apparent surface charge that simulates the solvent reaction field. Besides the standard HF/6-31G(d) level of theory considered in previous parametrizations of the MST(PCM) model, we also report here the results obtained for the B3LYP hybrid density functional method. These results indicate that there are small differences in the electrostatic component of the solvation free energy determined from HF and B3LYP levels of theory, which might be captured by suitable adjustment of the atomic surface tensions in the van der Waals component. Overall, the results support the strategy used in the development of the MST model and its suitability as a robust approach to the study of molecules in solution.

90 citations


Journal ArticleDOI
TL;DR: In this paper, the DFT-based analysis of the Si n / Si n − (n=2−10) species has been examined using seven hybrid and pure density functional theory (DFT) methods.
Abstract: The silicon clusters structures, electron affinities, and dissociation energies of the Si n / Si n − (n=2–10) species have been examined using seven hybrid and pure density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Four different types of energy separations presented in this work are the adiabatic electron affinity (EAad), zero-point vibrational energies (ZPVE) corrected EAad (EAzero), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Si–Si dissociation energies De (Sin→Sin−1+Si) for Sin, and both De ( Si n − → Si n − 1 + Si − ) and ( Si n − → Si n − 1 − + Si ) for Si n − species have also been reported. The most reliable adiabatic electron affinities, obtained at the DZP++ BPW91 level of theory, are 2.16 (2.15) eV for Si2, 2.32 (2.32) eV for Si3, 2.24 (2.25) eV for Si4, 2.51 (2.51) eV for Si5, 2.11 (2.12) eV for Si6, 2.06 (2.07) eV for Si7, 2.86 (2.85) eV for Si8, 2.28 (2.28) eV for Si9, 2.45 (2.46) eV for Si10. (EAzero values are in parentheses). While BP86, B3P86 and BPW91 predict to the most reliable dissociation energies. The dissociation energies for Sin→Sin−1+Si are predicted to be 3.26 (3.23) eV for Si2, 3.96 (3.92) eV for Si3, 4.39 (4.33) eV for Si4, 3.68 (3.62) eV for Si5, 4.12 (4.08) eV for Si6, 4.07 (4.01) eV for Si7, 2.76 (2.73) eV for Si8, 4.28 (4.22) eV for Si9, 4.33 (4.28) eV for Si10 with error of 0.13 (0.16) eV (corrected with ZPVE in parentheses). And the dissociation energies of Si n − → Si n − 1 − + Si are predicted to be 3.95 (3.92) eV for Si 2 − , 4.14 (4.11) eV for Si 3 − , 4.29 (4.24) eV for Si 4 − , 3.98 (3.92) eV for Si 5 − , 3.72 (3.68) eV for Si 6 − , 4.01 (3.96) eV for Si 7 − , 3.59 (3.54) eV for Si 8 − , 3.69 (3.63) eV for Si 9 − , and 4.51 (4.46) eV for Si 10 − .

82 citations


Journal ArticleDOI
TL;DR: In this article, a new quantitative structure-property relationship (QSPR) model was obtained using Radial Basis Function (RBF) neural networks and a set of four-parameter descriptors, P MVðterÞðRter Þ, LF, DXSB and P PEI.
Abstract: A novel approach to the prediction of the glass transition temperature (Tg) for high molecular polymers is presented. A new quantitative structure–property relationship (QSPR) model is obtained using Radial Basis Function (RBF) neural networks and a set of four-parameter descriptors, P MVðterÞðRterÞ, LF, DXSB and P PEI. The produced QSPR model (R 2 Z0.9269) proved to be considerably more accurate compared to a multiple linear regression model (R 2 Z0.8227). q 2004 Elsevier B.V. All rights reserved.

82 citations


Journal ArticleDOI
TL;DR: In this article, all the plausible conformations of β-aminoacrolein (AMAC) have been investigated by the Bekes-Lee-Yang-Parr (B3LYP) nonlocal density functional with extended 6-311++G** basis set for studying the stability order of conformers and the various possibilities of intramolecular hydrogen bonding formation.
Abstract: All the plausible conformations of β-aminoacrolein (AMAC) have been investigated by the Bekes-Lee-Yang-Parr (B3LYP) nonlocal density functional with extended 6-311++G** basis set for studying the stability order of conformers and the various possibilities of intramolecular hydrogen bonding formation. In general the ketoamine (KA) conformers of AMAC, by mean average, are more stable than the corresponding enolimine (EI) and ketoimine (KI) analogues and this stability is mainly due to the π-electron resonance in these conformers that established by NH2 functional group. The contribution of resonance to the stability of chelated KA conformers is about 75.6 kJ/mol, which is greater than that of the hydrogen bond energy ( E HB =35.0 kJ/mol). The relative decreasing order of the various hydrogen bond energies was found to be: O–H⋯N imine (strong)>N amine -H⋯O keto (normal)>N imine -H⋯O hydroxyl (weak) > N imine -H⋯O keto (weak). Hydrogen bond energies for all systems were obtained from the method that we called related rotamers method (RRM). The topological properties of the electron density contributions for various type of intramolecular hydrogen bond have been analyzed in term of the Bader theory of atoms in molecules (AIM). The results of these calculations support the previous calculations, which obtained by the related rotamer methods.

Journal ArticleDOI
TL;DR: Aromatic character of nucleic acid bases was characterized by means of magnetic and geometric properties in this article, where optimized geometries at DFT/6-311+G** level were used for calculation of nucleus independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA).
Abstract: Aromatic character of nucleic acid bases was characterized by means of magnetic and geometric properties. Optimized geometries at DFT/6-311+G** level were used for calculation of nucleus independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) values. Aromaticities of both pyrimidine and imidazole rings were analyzed. Solvent impact on HOMA values was estimated. Predictions of both methods are in qualitative agreement and suggest more aromatic character of purines rather than pyrimidines. Aromaticity decreases in the following sequence: adenine>guanine>cytosine>thymine≈uracil. Significant impact of protonation on the aromatic character was noticed. Protonation of adenine reduced its aromatic character of both pyrimidine and imidazole rings. However, protonation of guanine and all pyrimidines leads to increase of aromaticity compared to neutral species.

Journal ArticleDOI
TL;DR: In this article, the FTIR spectra of 5-hydroxytryptamine (serotonin, 5-HT) in solution and as a solid form were obtained in the range of 4000-400 cm−1.
Abstract: 5-Hydroxytryptamine (serotonin, 5-HT) is a hormone and a neurotransmitter found in the central nervous system. The conformational flexibility of the alkyl side chains in 5-HT plays an important role in its binding to receptor sites. The FTIR spectra of 5-hydroxytryptamine in solution and as a solid form were obtained in the range of 4000–400 cm−1. Geometry optimization based on molecular mechanics (using MM+ force field), semi-empirical quantum mechanical calculations (using AM1, PM3 and MINDO) and HF and B3LYP levels using the 3-21G and 6-31(d) basic sets were performed. The calculated geometric parameters were compared to the corresponding X-ray structure of 5-hydroxytryptamine. For the optimized structures by semi-empirical, ab initio and DFT calculations, vibrational spectra were also generated. The assignments of the observed bands corresponding to 5-HT were made on the basis of such calculation and the comparison with related molecules.

Journal ArticleDOI
TL;DR: In this article, the authors used the CHIH-DFT model chemistry to calculate the electronic properties and reactivity of quercetin and found that it exhibits diamagnetic properties in its basal state, in addition to an aromatic character.
Abstract: Quercetin is one of the most abundant flavonoids in vegetal tissues and presents high antioxidant activity. It is known that it exerts its antioxidant activity through the capacity to donate protons and electrons to inhibit reactive oxygen species (ROS) that act like free radicals. As the biological activity of the flavonoids is governed mainly by electronic interactions with biomolecules, the study of the electronic and reactivity properties in the molecule of quercetin is considered to be very important. For the calculation of the electronic properties and reactivity, we used the CHIH-DFT model chemistry. The results show that quercetin is a relatively polar molecule, soluble in water, and which presents diamagnetic properties in its basal state, in addition to an aromatic character. It presents a greater tendency to give electrons than to attract them, which confirms its antioxidant capacity. The more important reactive sites, that can be related to the antioxidant properties, are located mainly in rings B and C.

Journal ArticleDOI
TL;DR: Based on the theoretical linear solvation energy relationship (TLSER) model, the obtained structural parameters were taken as theoretical descriptors to establish the novel QSPR model for predicting n-octanol/water partition coefficients (lg-Kow) of PCBs as discussed by the authors.
Abstract: Optimized calculation of 209 PCBs was carried out at B3LYP/6-31G* level in gaussian 98 program. Based on the theoretical linear solvation energy relationship (TLSER) model, the obtained structural parameters were taken as theoretical descriptors to establish the novel QSPR model for predicting n-octanol/water partition coefficients (lg Kow) of PCBs. The new model achieved in this work contains three variables, polarizability α, ELUMO and EHOMO, of which r2=0.9484, SD=0.18, with larger t values. In addition, the variation inflation factors (VIF) of variables in the present model are all less than 5.0, suggesting high accuracy of the lg Kow predicting model. And the results of cross-validation test (q2=0.9455) and method validation also showed the model of this study exhibited optimum stability and better predictive power than semiempirical method.

Journal ArticleDOI
TL;DR: In this article, a theoretical study of oligothiophenes (from the dimer to the octamer) in their neutral and oxidized states based on DFT (B3LYP/6-31G*) calculations and focusing the discussion on the influence of chain length on conformational and optoelectronic properties.
Abstract: In this paper, we report a theoretical study of oligothiophenes (from the dimer to the octamer) in their neutral and oxidized states based on DFT (B3LYP/6-31G*) calculations and focusing the discussion on the influence of chain length on conformational and optoelectronic properties. The bridging effect by CH 2 , SiH 2 , C O and C C(CN) 2 on the optoelectronic properties of the octamer will then be investigated. The obtained results are close to available experimental ones. While structural parameters of neutral oligomers do not change significantly, the oligomers become planar and show a quinoidal character after oxidation. The bridging by C C(CN) 2 significantly decreases the octamer's band gap.

Journal ArticleDOI
TL;DR: In this paper, the structure of alanine, serine and cysteine radical cations, as well as their Ca-R fragmentations at the B3LYP/6-31CCG(d,p) level of theory were studied.
Abstract: The structure of glycine, alanine, serine and cysteine radical cations, as well as their Ca‐R fragmentations have been studied at the B3LYP/6-31CCG(d,p) level of theory. For all systems the loss of COOH % is found to be the most favourable process. However, when R1 size increases, fragmentations that lead to R1 or R C 1 start become competitive. The energy of a Ca‐R fragmentation can be related to the ionization potential of the two generated fragments, in such a way that the preferred process is the one that leaves the positive charge in the fragment with a lower ionization potential. q 2005 Elsevier B.V. All rights reserved.

Journal ArticleDOI
TL;DR: A theoretical study of two isomeric molecules based on the thiatriazolic ring has been performed by resorting to a new model chemistry known as CHIH-DFT as mentioned in this paper, where the properties most relevant to their potential action as corrosion inhibitors have been calculated: dipole moment (m), EHOMO, ELUMO, gap energy (DE), and several parameters that provide information about the chemical reactivity: electronegativity (c), global hardness (h) and the fraction of electrons transferred from the inhibitor molecule to the metallic atom (DN).
Abstract: A theoretical study of two isomeric molecules based on the thiatriazolic ring, namely 4-amino-1,2,3,5-thiatriazole (4AT) and 5-amino1,2,3,4-thiatriazole (5AT), has been performed by resorting to a new model chemistry known as CHIH-DFT. The properties most relevant to their potential action as corrosion inhibitors has been calculated: dipole moment (m), EHOMO, ELUMO, gap energy (DE) and several parameters that provide information about the chemical reactivity: electronegativity (c), global hardness (h) and the fraction of electrons transferred from the inhibitor molecule to the metallic atom (DN). Besides, the local reactivity was analyzed through the Fukui indices. All calculation have been performed by considering Density Functional Theory (DFT) using the GAUSSIAN03W suite of programs. q 2004 Elsevier B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, three hybrid density functional theory (B3PW91, B3P86) methods with 6-311G** and 6-31+G** basis sets were compared.
Abstract: Bond dissociation energies for removal of the nitrogen dioxide moiety in nitrobenzene; 3-amino-nitrobenze; 4-amino-nitrobenze; 1,3-dinitrobenzene; 1,4-dinitrobenzene; 2-methyl-nitrobenzene; 4-methyl-nitrobenzene; and 1,3,5-trinitrobenzene nitroaromatic molecules, are calculated using the three hybrid density functional theory (B3LYP, B3PW91, B3P86) methods with 6-311G** and 6-31+G** basis sets. By comparing the computed energies and experimental results, it is find that the B3LYP and B3PW91 methods are unable to predict satisfactory results of bond dissociation energy (BDE), however, the B3P86 method is able to give the best agreement with experimental BDE data for these nitroaromatic molecules, especially with 6-311G** basis set.

Journal ArticleDOI
TL;DR: In this article, the effects of hydrogen bonds in an ammonium dihydrogenphosphate (ADP) crystallographic frame on its optical properties such as static polarizabilities and first hyperpolarizabilities, are quantitatively studied by quantum chemical calculations.
Abstract: Effects of hydrogen bonds in an ammonium dihydrogenphosphate (ADP) crystallographic frame on its optical properties such as static polarizabilities and first hyperpolarizabilities, are quantitatively studied by quantum chemical calculations. Chemical bonding characteristics of the ADP crystal are analyzed by the bond valence model. The present results show that hydrogen bonds play an important role in crystal optical properties.

Journal ArticleDOI
TL;DR: The examination of similarities/dissimilarities among the coding sequences of the first exon of β-globin gene of different species illustrates the utility of the 3D graphical representation of DNA sequences.
Abstract: Recently we proposed a 3D graphical representation of DNA sequences [B. Liao, T. Wang, 3-D graphical representation of DNA sequences and their numerical characterization, Journal of Molecular Structure: THEOCHEM 681 (2004) 209–212]. Based on this representation, we outline one such approach by constructing a 3-component vector whose components are the leading eigenvalues of the L / L matrices associated with DNA sequences. The examination of similarities/dissimilarities among the coding sequences of the first exon of β-globin gene of different species illustrates the utility of the approach.

Journal ArticleDOI
TL;DR: In this article, a Quantitative Structure-Activity Relationship (QSAR) study of 67 polycyclic aromatic hydrocarbons (PAHs) is performed and a prediction rule for the phototoxicity of these compounds is proposed.
Abstract: A Quantitative Structure–Activity Relationship (QSAR) study of 67 polycyclic aromatic hydrocarbons (PAHs) is performed and a prediction rule for the phototoxicity of these compounds is proposed. The geometry of all PAHs was optimized by semi-empirical method and used to calculate the following electronic descriptors: the highest occupied molecular orbital energy ( E HOMO ), the lowest unoccupied molecular orbital energy ( E LUMO ) and the GAP ( E HOMO − E LUMO ) between them. The relationships between these molecular descriptors and the photo-induced toxicity are non-linear, and Gaussian type functions were used to linearize them. The data set was autoscaled and partial least squares (PLS) with leave-one-out cross-validation was used for building the regression model. After the model was validated, the phototoxicity was predicted for a set of molecules in which experimental measurements are unknown. Pentaphene, benzo[b]chrysene and dibenz[a.j]anthracene are among the compounds potentially phototoxic as predicted by the model. A new GAP range (7.2±0.7 eV) is proposed for the classification of phototoxic compounds, and a larger cutoff suggested for the normalized lethal time as log(1/ALT)≤−2.95.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the changes in aromaticity and planarity along the reaction path of the Diels-Alder reaction between ethene and 1,3-butadiene.
Abstract: In this work, we analyze the changes in aromaticity and planarity along the reaction path of the Diels–Alder reaction between ethene and 1,3-butadiene. To this end, a new index that quantifies the planarity of a given ring is defined. As expected, the planarity of the ring being formed in the Diels–Alder cycloaddition increases along the reaction path from reactants to product. On the other hand, the aromaticity of the ring formed is measured using several well-established indices of aromaticity such as the nucleus independent chemical shift (NICS), the harmonic oscillator model of aromaticity (HOMA), and the para-delocalization index (PDI), as well as a recently defined descriptor of aromaticity: the aromatic fluctuation index (FLU). The results given by the NICS and PDI indices, at variance with those obtained by means of the HOMA and FLU indicators of aromaticity, confirm the existence of an aromatic transition state for this reaction. The reasons for the failure of some of the descriptors of aromaticity employed are discussed. The results support the multidimensional character of aromaticity. q 2005 Elsevier B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the hydration of magnesium dication is studied using the density functional theory BPW91/6-311++G(3d,3p) and ab initio RHF/6 -31G(d,p) methods.
Abstract: The hydration of magnesium dication is studied using the density functional theory BPW91/6-311++G(3d,3p) and ab initio RHF/6-31G(d,p) methods. The polarizabilities, enthalpies and Gibbs free energies of Mg++ hydration are computed, up to nine added molecules of water. The entropic contribution to the hydration phenomenon is discussed using frequency analyses. The mechanism of hydration is also analyzed using the electronic chemical potential, hardness and electrophilicity concepts.

Journal ArticleDOI
TL;DR: In this paper, a new OPLSS-AA-SEI force field was obtained by optimizing the Lennard-Jones parameters and charges taking part in 1-4 interactions.
Abstract: Molecular dynamics simulations were used to study pure liquid ethylene glycol (EG) and EG-water binary mixtures as a function of the molar fraction. Calculations were performed in the isothermal and isobaric (NpT) ensemble. New improvement in the OPLSS-AA-SEI force field was obtained optimizing the Lennard-Jones parameters and charges taking part in 1–4 interactions. With the new force field density and heat of vaporization of pure liquid ethylene glycol were obtained in good agreement with experimental data. The value of the O-C-C-O dihedral was also monitored in the course of force field optimization process. The new optimized force field was named OPLSA-AA-SE-M. Concerning the pure liquid simulation, features observed on the radial distribution functions (RDF) show characteristics of intermolecular and intramolecular hydrogen bonding. The RDF integrations show that each EG molecule in the pure liquid has an average coordination number of five molecules in its first coordination shell. Therefore, the average oxygen-hydrogen contact number due to hydrogen bonding is seven: two of them arising from intramolecular and five from interactions with neighboring molecules. Dihedral angle distributions were calculated, showing the predominance of EG molecules with the O-C-C-O dihedral corresponding to gauche conformations. In the study of water-ethylene glycol binary mixtures, the average structural parameters observed for EG molecules are very similar to the ones found in the pure liquid simulation. RDF analyses also present characteristic features of hydrogen bonding. Particularly, no significant changes were observed on the dihedral angle distributions. The structures of EG-EG and EG-water dimers calculated with OPLS-AA-SEI-M force field and DFT at B3LYP 6-311 g (3df,3pd) level are in very good agreement.

Journal ArticleDOI
TL;DR: It is demonstrated that domain-average Fermi hole (DAFH) analysis, which has previously been used at the Hartree-Fock level, remains useful after the proper introduction of electron correlation, and provides further insight into the geometry dependence of the extent of electron sharing in polar and non-polar systems.
Abstract: Summary This paper reports the application of an index, which we propose to call the shared-electron distribution index, for quantifying the changes to the extent of electron sharing during the making or breaking of chemical bonds. For this purpose we use this index, which is based on the correlated pair density, to monitor the effects of increasing the bond length in four representative diatomic molecules. We do of course employ wave functions which describe correctly the dissociation process. It is found for the nonpolar bonds in H 2 and N 2 that the degree of electron sharing increases monotonically with shortening of the bond length, but that there appears to be no molecule-specific significance to the location of the point of inflexion. The variation of the extent of electron sharing with internuclear distance is somewhat more complex for the polar bonds in LiH and BeH, but it reflects directly the known variation of the nature of the wave functions.

Journal ArticleDOI
TL;DR: In this paper, the vibrational frequencies and potential energy distribution (PED) of melaminium cations were calculated by B3LYP method and the assignment of the bands has been made on the basis of the calculated PED.
Abstract: The melaminium cation can occur in three form: singly, doubly or triply protonated. The geometry and nonlinear optical properties (NLO) for all forms were computed by ab initio method. The restricted open Hartree–Fock (ROHF) computational method with 6-311+G basis set was used. Comprehensive studies of electronic and molecular structures have been performed. The theoretical vibrational frequencies and potential energy distribution (PED) of melaminium cations were calculated by B3LYP method. The calculated frequencies were compared with measured one for newly obtained compounds where melaminium cations are common component. The assignment of the bands has been made on the basis of the calculated PED. The restricted Hartree–Fock (RHF) methods were used for calculation of the hyperpolarizability for each ion. For comparison, analogous calculations were carried out for the closed-shell neutral melamine. The theoretical results can be used in design of new molecular compounds with nonlinear optical properties.

Journal ArticleDOI
TL;DR: In this article, a range of novel bimetallic molecular systems, coupled with group eight metal carbonyls, were designed and the full geometry optimisations were performed using DFT method at B3LYP/LanL2DZ level of theory using the gaussian 98W.
Abstract: Organometallic and coordination complexes are interesting candidates for NLO purposes. Among these materials bimetallic complexes belong to a new class of second order NLO materials. The objective of this investigation was to design a range of novel bimetallic molecular systems, which show high nonlinear optical activity. In this investigation, main attention was drawn to the ferrocenyl systems coupled with group eight metal carbonyls, where charge transfer occurs from ferrocene to the metal carbonyl moiety. These novel ferrocene bimetallic systems show high first static hyperpolarizability values. We report novel organometallic systems with high β values. The high accuracy density functional theory (DFT) was used for the investigation of NLO properties of these complexes. The full geometry optimisations of designed bimetallic systems were performed using DFT method at B3LYP/LanL2DZ level of theory using the gaussian 98W. The calculations of the first static hyperpolarizabilities of these systems were performed at the same level of theory.

Journal ArticleDOI
TL;DR: It was revealed that curcumin could efficiently sequester Cu(II) and the Cu( II)-curcumin complexes were more active than the parentCurcumin in scavenging radicals by donating proton or electron, therefore, cur cumin is a multipotent agent to combat AD.
Abstract: Curcumin has been recognized as an efficient natural product to treat Alzheimer's disease (AD) by scavenging radicals and blocking Aβ aggregation. As metal ions, e.g. Cu(II), are also implicated in AD and curcumin is a metal chelator, it is interesting to investigate curcumin's metal-chelating property, which stimulates our interest to evaluate curcumin's Cu(II)-chelating ability and the effect of Cu(II) on its radical-scavenging activity by theoretical calculation. It was revealed that curcumin could efficiently sequester Cu(II) and the Cu(II)-curcumin complexes were more active than the parent curcumin in scavenging radicals by donating proton or electron. Therefore, curcumin is a multipotent agent to combat AD, with the activities of scavenging ROS, blocking Aβ aggregation and chelating metal ions.

Journal ArticleDOI
TL;DR: In this paper, the structures of a series of metallatranes, N(CH2CH2O)3M-X (I-IV), where M=Si, Ge, Sn and Pb, and X=H, F, Cl, Br, HO, CH3, (CH3)2N, ( CH3)3N+ were studied using DFT calculations with full geometry optimization.
Abstract: The structures of a series of metallatranes, N(CH2CH2O)3M–X (I–IV), where M=Si, Ge, Sn and Pb, and X=H, F, Cl, Br, HO, CH3, (CH3)2N, (CH3)3N+ were studied using DFT calculations with full geometry optimization. The obtained results are in good agreement with gas electron diffraction data. The transannular bond M←N length depends on the nature of M and axial X substituent and the strength of M←N bond increases in Si