Local Electron Affinity Research Articles

A novel method to evaluate insulation performance of SF6 alternative gases based on modified GIPF-Local properties model

Abstract The development of structure-activity relationships (SAR) emerges as a crucial strategy to establish the correlations between molecular micro-parameters and the macroscopic dielectric strength, thus markedly improving the gases screening efficiency. However, conventional prediction approaches have failed to characterize the molecule electronegativity accurately, resulting in inadequate prediction precision within extensive gas databases. To address this limitation, we have devised an insulation prediction method that incorporates corrections based on molecular local properties, including local ionization energy and local electron affinity. They enable us to determine both the electron acceptance and donation capacity of the whole molecular orbital electron cloud: a more accuracy assessment of electronegativity. Compared with previous method, our enhanced approach combined local properties with structural and GIPF parameters. It has yielded substantial improvements based on an extensive database comprising 91 gases, as evidenced by an increase in the correlation coefficient from 0.848 to 0.969. Utilizing the modified GIPF-Local properties model, over ten potential gases were found and categorized according to their application. This innovative screening strategy provides crucial insights for exploration and design of new eco-friendly gases as substitute for SF6 used in high-voltage power equipment.

Connection between nuclear and electronic Fukui functions beyond frontier molecular orbitals.

Based on the relationship between average local ionization energy Ī(r) and average local electron affinity Ā(r) with the electronic Fukui functions, i.e., f-(r) and f+(r), respectively, in this paper, we establish a connection between nuclear and electronic Fukui functions beyond frontier molecular orbitals. As a consequence of this connection, we obtain expressions of average nuclear Fukui functions interpreted as a variation of average nucleophilicity or electrophilicity (weighted by the electronic orbital Fukui functions) with respect to nuclear displacements, which goes beyond the highest occupied molecular orbital/or lowest unoccupied molecular orbital consideration. Furthermore, from this connection and considering the frontier molecular orbital approximation, we derive expressions of nuclear Fukui functions in terms of the atom-condensed electronic Fukui functions, which imply a locality in the chemical reactivity and could be used to study the variation of local nucleophilicity or electrophilicity with respect to nuclear displacements. Finally, this new way to interpret the nuclear Fukui function could be useful in the future to study the chemical reactivity related to molecular vibrations, internal rotations, bond dissociation, chemical reaction along the model of reaction coordinate, and so on.

A new series of asymmetric bis-isatin derivatives containing urea/thiourea moiety: Preparation, spectroscopic elucidation, antioxidant properties and theoretical calculations

In the present study, design, synthesis, characterization, and investigation of antioxidant properties of novel asymmetric bis-isatin derivatives (1-8) containing urea/thiourea moiety are reported for the first time. FT-IR, 1H-NMR, and 13C-NMR spectroscopic methods and elemental analysis were used to elucidate the structures of the synthesized compounds. Their CUPRAC and ABTS cation radical scavenging abilities were investigated for antioxidant activity. The bis-isatins containing urea moiety (compounds 1-4) did not show ABTS activity, while those containing the thiourea moiety (compounds 5-8) showed moderate ABTS activity. Besides, all bis-isatins were observed to exhibit CUPRAC activity at a low micromolar level The 1-(5-chloro-2-oxoindolin-3-ylidene)-5-(2-oxoindolin-3-ylidene)thiocarbonohydrazide (compound 5) showed the highest ABTS activity with IC50 value of 18.44 µM; on the other hand, the 1-(5-chloro-2-oxoindolin-3-ylidene)-5-(5-methoxy-2-oxoindolin-3-ylidene)carbonohydrazide (compound 2) had the strongest CUPRAC activity with A0.50 value of 0.600 µM. Both spectroscopic and antioxidant properties of the compounds were examined computationally, and the structure-activity relationship was investigated theoretically by comparing with experimental data. The ground state geometries and chemical reactivity parameters of the compounds were calculated using the B3LYP hybrid functional with 6-311++g(2d,2p) and 6-31g(d) basis sets. After determining the local electron affinity of the compounds, the Laplacian bond order and intrinsic bond strength indexes (independent gradient model-δgIGMH and IBSIIGMH descriptors based on Hirshfeld approach) of hydrogen bonds at possible reactive sites were calculated and associated with the antioxidant properties of the compounds.

A multi-agent quantum Monte Carlo model for charge transport: Application to organic field-effect transistors

We have developed a multi-agent quantum Monte Carlo model to describe the spatial dynamics of multiple majority charge carriers during conduction of electric current in the channel of organic field-effect transistors. The charge carriers are treated by a neglect of diatomic differential overlap Hamiltonian using a lattice of hydrogen-like basis functions. The local ionization energy and local electron affinity defined previously map the bulk structure of the transistor channel to external potentials for the simulations of electron- and hole-conduction, respectively. The model is designed without a specific charge-transport mechanism like hopping- or band-transport in mind and does not arbitrarily localize charge. An electrode model allows dynamic injection and depletion of charge carriers according to source-drain voltage. The field-effect is modeled by using the source-gate voltage in a Metropolis-like acceptance criterion. Although the current cannot be calculated because the simulations have no time axis, using the number of Monte Carlo moves as pseudo-time gives results that resemble experimental I/V curves.

The unrestricted local properties: application in nanoelectronics and for predicting radicals reactivity

The local electron affinity (EA(L)) and the local ionization energy (IE(L)) are successfully used for predicting properties of closed-shell species for drug design and for nanoelectronics. Here the respective unrestricted Hartree-Fock variants of EA(L) and IE(L), i.e., the unrestricted local electron affinity (UHF-EA(L)) and ionization energy (UHF-IE(L)), have been shown to be useful for predicting properties of open-shell species. UHF-EA(L) and UHF-IE(L) have been applied for explaining unique electronic properties of an exemplary nanomaterial carbon peapod. It is also demonstrated that UHF-EA(L) is useful for predicting and better understanding reactivity of radicals related to alkanes activation.

3D-QSAR Based on Quantum-Chemical Molecular Fields: Toward an Improved Description of Halogen Interactions

Current 3D-QSAR methods such as CoMFA or CoMSIA make use of classical force-field approaches for calculating molecular fields. Thus, they can not adequately account for noncovalent interactions involving halogen atoms like halogen bonds or halogen-π interactions. These deficiencies in the underlying force fields result from the lack of treatment of the anisotropy of the electron density distribution of those atoms, known as the "σ-hole", although recent developments have begun to take specific interactions such as halogen bonding into account. We have now replaced classical force field derived molecular fields by local properties such as the local ionization energy, local electron affinity, or local polarizability, calculated using quantum-mechanical (QM) techniques that do not suffer from the above limitation for 3D-QSAR. We first investigate the characteristics of QM-based local property fields to show that they are suitable for statistical analyses after suitable pretreatment. We then analyze these property fields with partial least-squares (PLS) regression to predict biological affinities of two data sets comprising factor Xa and GABA-A/benzodiazepine receptor ligands. While the resulting models perform equally well or even slightly better in terms of consistency and predictivity than the classical CoMFA fields, the most important aspect of these augmented field-types is that the chemical interpretation of resulting QM-based property field models reveals unique SAR trends driven by electrostatic and polarizability effects, which cannot be extracted directly from CoMFA electrostatic maps. Within the factor Xa set, the interaction of chlorine and bromine atoms with a tyrosine side chain in the protease S1 pocket are correctly predicted. Within the GABA-A/benzodiazepine ligand data set, PLS models of high predictivity resulted for our QM-based property fields, providing novel insights into key features of the SAR for two receptor subtypes and cross-receptor selectivity of the ligands. The detailed interpretation of regression models derived using improved QM-derived property fields thus provides a significant advantage by revealing chemically meaningful correlations with biological activity and helps in understanding novel structure-activity relationship features. This will allow such knowledge to be used to design novel molecules on the basis of interactions additional to steric and hydrogen-bonding features.

A Surface-Integral Model for Log POW

Log P(OW), the negative logarithm of the octanol-water partition coefficient, is omnipresent in computational drug design. Here, we present a surface-integral model for calculating log P(OW). The model is based on local properties calculated using AM1 semiempirical molecular orbital theory. These are the molecular electrostatic potential (MEP), local ionization energy (IE(L)), local electron affinity (EA(L)), local hardness (HARD), local polarizability (POL), and the local field normal to the surface (FN). We have developed a new scheme to calculate a local hydrophobicity based on binning the range of local surface properties instead of using polynomial expansions of the base terms. The model has been trained using approximately 9500 compounds available from the literature. It was validated on approximately 1350 compounds from the literature and an in-house validation set of 768 compounds from Boehringer-Ingelheim. The model performs similarly to or slightly better than the best commercially available models. We also introduce a model based purely on conformationally rigid compounds that performs well for flexible compounds if the Boltzmann weighted predictions for the different conformers are used. This is the first 3D QSPR model based on such a large databasis that is able to benefit from using conformational ensembles.

The local electron affinity for non-minimal basis sets

A technique known as intensity filtering is introduced to select valence-like virtual orbitals for calculating the local electron affinity, EA(L). Intensity filtering allows EA(L) to be calculated using semiempirical molecular orbital techniques that include polarisation functions. Without intensity filtering, such techniques yield spurious EA(L) values that are dominated by the polarisation functions. As intensity filtering should also be applicable for ab initio or density functional theory calculations with large basis sets, it also makes EA(L) available for these techniques.

ParaFrag—an approach for surface-based similarity comparison of molecular fragments

A frequent task in computer-aided drug design is to identify novel chemotypes similar in activity but structurally different to a given reference structure. Here we report the development of a novel method for atom-independent similarity comparison of molecular fragments (substructures of drug-like molecules). The fragments are characterized by their local surface properties coded in the form of 3D pharmacophores. As surface properties, we used the electrostatic potential (MEP), the local ionization energy (IE(L)), local electron affinity (EA(L)) and local polarizability (POL) calculated on isodensity surfaces. A molecular fragment can then be represented by a minimal set of extremes for each surface property. We defined a tolerance sphere for each of these extremes, thus allowing us to assess the similarity of fragments in an analogous manner to classical pharmacophore comparison. As a first application of this method we focused on comparing rigid fragments suitable for scaffold hopping. A retrospective analysis of successful scaffold hopping reported for Factor Xa inhibitors [Wood MR et al (2006) J Med Chem 49:1231] showed that our method performs well where atom-based similarity metrics fail.

New molecular descriptors based on local properties at the molecular surface and a boiling-point model derived from them.

New molecular descriptors based on statistical descriptions of the local ionization potential, local electron affinity, and the local polarizability at the surface of the molecule are proposed. The significance of these descriptors has been tested by calculating them for the Maybridge database in addition to our set of 26 descriptors reported previously. The new descriptors show little correlation with those already in use. Furthermore, the principal components of the extended set of descriptors for the Maybridge data show that especially the descriptors based on the local electron affinity extend the variance in our set of descriptors, which we have previously shown to be relevant to physical properties. The first nine principal components are shown to be most significant. As an example of the usefulness of the new descriptors, we have set up a QSPR model for boiling points using both the old and new descriptors.

Local molecular properties and their use in predicting reactivity.

Expressions for the local electron affinity, electronegativity and hardness are derived in analogy to the local ionization energy introduced by Sjoberg, Murray and Politzer. The local polarizability is also defined based on an additive atomic orbital polarizability model that uses Rivail's variational technique. The characteristics of these local properties at molecular surfaces and their relevance to electrophilic aromatic substitution, to S(N)2 reactivity and to the nucleophilicity of enolate ions are discussed.

A fresh look at aromatic dianions.

Isolated dianions of cyclooctatetraene and cyclobutadiene are, despite their aromatic character, unstable with respect to electron loss. Here we present the first ab initio study of the autodetachment lifetimes of several "aromatic" dianions. In particular, we investigate whether aromatic pi systems are associated with an enhanced stability with respect to electron loss and whether cross-conjugated (Y-aromatic) hydrocarbon dianions possess longer lifetimes than their linear-conjugated isomers. Moreover, our results are compared with bound state calculations for these metastable dianions, allowing us to analyze the validity of the latter approach. All considered, hydrocarbon dianions are found to possess surprisingly short autodetachment lifetimes on the order of a few femtoseconds, and therefore only electron scattering techniques seem to be capable of observing these species in the gas phase. The trends of the computed energies and lifetimes are best understood in terms of the mutual Coulomb repulsion of the extra charges. Small gas phase dianions can only be stable if the excess electrons can localize at least at two subgroups of high local electron affinity. Adding both extra electrons into one delocalized pi system leads to a very unstable species, and thus especially those structures associated with pronounced aromaticity are found to show high energies and short lifetimes.

On the band gap in peptide ?-helices

The variation in the band gap of a (Gly)30 α-helix with position in the peptide sequence has been investigated using AM1 semiempirical molecular orbital (MO) theory within an extensive singles configuration interaction (CI) treatment. The dipole-charge interaction along the length of the helix results in the highest local electron affinity near the N-terminus and the lowest local ionization potential near the C-terminus. The calculations suggest that the band gap, measured as the difference between the local ionization potential and electron affinity, decreases from the C-terminus to the N-terminus for the model (Gly)30 α-helix in vacuo. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 120–124, 2000

Mechanisms of electron field emission from diamond, diamond-like carbon, and nanostructured carbon

It is argued that the facile electron field emission from carbon systems occurs primarily because surface groups such as C–H can produce large changes in local electron affinity, so that electric fields from the anode can be focused toward unhydrogenated surface areas of high affinity, the fields ending on negative charges in an underlying depletion layer. The resulting downward band bending creates large surface fields which allow Fowler–Nordheim emission, while not exceeding the material’s breakdown field.

Estimation of the electron affinities of C 60, corannulene, and coronene by using the kinetic method

Novel anions that contain one molecule each of C60 and the polycyclic aromatic hydrocarbon coronene are generated in the gas phase by electron attachment desorption chemical ionization. Collision-induced dissociation reveals that these cluster ions are loosely bonded. Fragmentation of the mass-selected cluster anion yields, as the only products, the intact radical anions of the constituent molecules, namely, the C60 radical anion and the coronene radical anion, in almost identical relative abundances. This result is interpreted as evidence that the cluster ion can be considered as the anion radical of one molecule solvated by the other molecule. The known very high electron affinity of C60 (2.66 eV) and the comparable degree to which C60 and the PAH compete for the electron suggests that dissociation may be controlled by the electron affinity of a portion of the C60 surface, that is, in this case the kinetic method yields information on the local electron affinity of C60. The electron affinity of the bowl-shaped compound corannulene is estimated for the first time to be 0.50 ± 0.10 eV by the kinetic method by using a variety of reference compounds. Unlike coronene, corannulene reacts with C -• (60) in the gas phase to form a covalently bonded, denydrogenated cluster ion. Support for the concept of "local" electron affinity of C60 comes from a theoretical calculation on the electronic structure of C60 anions, which shows evidence for localization of the charge in the C60 molecule. The possibility of electron tunneling in the C60-coronene system is discussed as an alternative explanation for the unusual observation of equal abundances of C60 anions and coronene anions upon dissociation of the corresponding cluster ion.