Local Ionization Energy Research Articles

Computational prediction of relative group polarizabilities

AbstractEarlier work has shown that molecular polarizabilities correlate well with the quantity V/IS,ave, where V is the volume of the molecule and IS,ave is the average value of the local ionization energy on its surface. We now extend this to group polarizabilities; we computed V/IS,ave for four common chemical groups (NO2, CH3, NH2, and OH) in a variety of molecules. The transferabilities of IS,ave, V, and V/IS,ave are examined, as well as the correlation between V/IS,ave and literature polarizabilities of CH3, NH2, and OH. This permits the value for NO2 to be predicted (α = 2.77 Å3). The dependence of IS,ave, and therefore V/IS,ave, upon the remainder of the molecule is discussed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

The complementary roles of molecular surface electrostatic potentials and average local ionization energies with respect to electrophilic processes

AbstractWe focus upon two properties, the electrostatic potential V(r) and the average local ionization energy \documentclass{article}\pagestyle{empty}\begin{document}$\bar{I}(\mathbf{r})$\end{document}. When evaluated on molecular surfaces, VS(r) and \documentclass{article}\pagestyle{empty}\begin{document}$\bar{I}_{S}(\mathbf{r})$\end{document} can be useful tools for analyzing and predicting reactive behavior. VS(r) is most reliable with respect to noncovalent interactions; when electrophilic attack and some degree of charge transfer are involved, then the combination of VS(r) and \documentclass{article}\pagestyle{empty}\begin{document}$\bar{I}_{S}(\mathbf{r})$\end{document} can be quite effective. They play complementary roles: VS(r) reveals the regions of the molecule to which an electrophile would initially be attracted, and \documentclass{article}\pagestyle{empty}\begin{document}$\bar{I}_{S}(\mathbf{r})$\end{document} indicates the ease of charge transfer at these and other sites. Four examples of such complementarity are discussed, involving benzene derivatives, guanine and cytosine, furan and pyrrole, and binary hydrides. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002

Comparison of quantum chemical parameters and Hammett constants in correlating pK(a) values of substituted anilines.

Historically, Hammett constants have been extremely effective in describing the influence of substituents on chemical reactivity and other physical and chemical properties, whereas variables derived from quantum chemical calculations have generally been less effective. Taking the experimental pK(a)s of substituted anilines as a representative physicochemical property, five ab initio quantum chemical indices are compared for effectiveness as one-parameter regression descriptors for pK(a). All of the tested descriptors performed well for a set of 19 mono-, 13 di-, and 4 trisubstituted anilines, and two performed somewhat better than the traditional Hammett sigma constants. Among the calculated quantities, the best representation of the aniline pK(a)s is produced by the minimum average local ionization energy on the molecular surface.

Computational characterization of surfaces of model graphene systems

A series of two-dimensional sheets of hexagonal carbon rings, with hydrogens around the edges, has been investigated computationally as models for graphene. Bond distances were optimized at the density functional SVWN/6-31G ∗ level, and then HF/STO-5G electrostatic potentials and local ionization energies were calculated on the systems' surfaces. The potentials above the carbon rings are uniformly weakly negative, especially in the central portions of the systems. Overall, the analysis suggests low reactivity for these regions. On the other hand, certain of the outermost C–C bonds are expected to have considerable double bond character.

Asymptomatic carotid atherosclerosis in postmenopausal women users and non users of hormonal replacement therapy (HRT)

In the present study, the title compound named as (E)-1-(4-bromophenyl)-3-(4-(trifluoromethyl)phenyl)prop-2-en-1-one was synthesized and structurally characterized by single-crystal X-ray diffraction. The compound crystallizes in monoclinic crystal system in P21/c space group, unit cell parameters a = 16.7629 (12) Å, b = 13.9681 (10) Å, c = 5.8740 (4) Å, β = 96.3860 (12)° and Z = 4. Hirshfeld surface analysis revealed that the molecular structure is dominated by H⋯H, C⋯H/H⋯C, Br⋯F/F⋯Br and F⋯F contacts. The FT-IR spectrum was recorded and interpreted in details with the aid of Density Functional Theory (DFT) calculations and Potential Energy Distribution (PED) analysis. Average local ionization energies (ALIE) and Fukui functions have been used as quantum-molecular descriptors to locate the molecule sites that could be of importance from the aspect of reactivity. Degradation properties have been assessed by calculations of bond dissociation energies (BDE) for hydrogen abstraction and the rest of the single acyclic bonds, while molecular dynamics (MD) simulations were used in order to calculate radial distribution functions and determine the atoms with significant interactions with water. In order to understand how the title molecule inhibits and hence increases the catalytic efficiency of MOA-B enzyme, molecular docking study was performed.

Computational characterization of nucleotide bases: Molecular surface electrostatic potentials and local ionization energies, and local polarization energies

AbstractElectrostatic potentials and local ionization energies have been computed at the HF/6‐31G* level on the molecular surfaces of the five nucleotide bases. The potentials are analyzed in terms of their most positive and negative values as well as several statistically defined quantities that reflect their patterns over the entire surface. Considerable charge separation and variability are found for all five molecules. The results are consistent with the base pairing that is known to occur. The observed reactive behavior toward electrophiles can be interpreted in terms of the complementary roles of the surface potential and the local electron lability. Local polarization energies, corresponding to a test charge being placed at specific points above the molecules, are also calculated (HF/6‐31+G*), and their relationship to the local ionization energies is examined. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 245–254, 2001

Computed electrostatic potentials and average local ionization energies on the molecular surfaces of some tetracyclines

AbstractThe electrostatic potentials and average local ionization energies computed on the molecular surfaces of four tetracyclines have been investigated with the objective of identifying common features as well as subtle differences that may be related to their biological activities. The four are the parent molecule tetracycline, chlortetracycline, oxytetracycline, and doxycyline. The calculations were carried out at the HF/STO‐3G*//STO‐3G* level. Our electrostatic potential results show that each molecule has a large negative region that extends along its lower portion, consistent with its ability to complex Ca2+ and Mg2+ ions. Although the surface electrostatic potentials of the four tetracyclines show many similarities, our statistical measure of local polarity allows us to label doxycycline as the one with the lowest degree of local polarity, consistent with its longer half‐life in vivo. The regions in the tetracyclines with the most reactive electrons are the amide nitrogen lone pairs and certain carbons of the outermost rings. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 160–169, 2001

Characterization of aromatic hydrocarbons by means of average local ionization energies on their molecular surfaces

The average local ionization energies ı̄S(r) and the electrostatic potentials VS(r) have been computed on the molecular surfaces of nine aromatic hydrocarbons, at the Hartree–Fock HF/6-31G∗∗//HF/STO-3G and Kohn–Sham B3P86/6-31G∗∗//HF/STO-3G levels. The surface areas, the fractions covered by positive and negative potentials, and their average values are quite similar by both computational procedures. The average values of ı̄S(r) and its minima, ı̄S,min (which indicate the most reactive electronic sites), are smaller in magnitude at the Kohn–Sham level, but the differences from the Hartree–Fock are fairly uniform and the minima occur at essentially the same positions. In nearly all instances, therefore, the same qualitative predictions result from either approach. Most of the ı̄S,min are found in bond regions and indicate varying degrees of olefin-like behavior. Some ı̄S,min are located near carbons, and it is proposed that these reflect some radical character. Supporting evidence is cited.

Dissociative Intramolecular Charge Transfer after Local Two-Photon Ionization of Ferrocene Derivatives with Aromatic Chromophores

Experimental evidence for dissociative intramolecular charge transfer processes is found for gas-phase ferrocene-aniline derivatives after local ionization at the aniline chromophore site via intermediate states with predominant aniline character. The direct ionization at the ferrocene site can be excluded since there is a rapid dissociation of ferrocene in the intermediate state prior to ionization so that no ferrocene ions would be produced in this case. Because of the higher local ionization energy of the chromophore, charge transfer takes place after local ionization. The released energy and additional photon absorption leads to breaking of bonds and results in a dissociation into intact ferrocene or ferrocene-containing cations and neutral fragments.

ChemInform Abstract: Average Local Ionization Energies: Significance and Applications

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Comparison of density functional and Hartree-Fock average local ionization energies on molecular surfaces

We investigated whether an average local ionization energy defined within the framework of Hartree–Fock theory is physically meaningful in Kohn–Sham density functional theory, in view of the uncertainty in relating the latter orbital energies to electronic ionization energies. In a preliminary study of N2O using several different exchange/correlation functional combinations, it was found that the orbital energies in each case correlate roughly with experimental binding energies. Average local ionization energies are computed on the molecular surfaces of 11 monosubstituted benzene derivatives using both Hartree–Fock and density functional procedures. Both sets of results correctly predict the ortho/para- or meta-directing tendencies of the substituents, as well as their ring-activating or -deactivating effects. Excellent correlations were obtained with the Hammett substituent constants. This initial study suggests that Kohn–Sham average local ionization energies should be physically meaningful, on a relative basis. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 607–613, 1998

Modified Interaction Properties Function for the Analysis and Prediction of Lewis Basicities

Ab initio HF/6-31G* electrostatic potentials and average local ionization energies have been computed for 42 nitrogen, oxygen, and sulfur bases. The molecules are characterized by four computed quantities, Vmin, the spatial minimum in the electrostatic potential, PVmin, a polarization correction to Vmin, ĪS,min, the surface minimum in the average local ionization energy, and Π, a global index derived from the calculated surface electrostatic potential. It is shown that shifts in the O−H stretching frequencies for methanol−base complexes, complexation enthalpies for phenol−base and iodine−base interactions, and gas phase protonation enthalpies can be well correlated to the computed quantities of the bases by a single type of relationship. An analysis of the importance of the different quantities for correlating a specific interaction provides information about the nature of the interaction.

Local temperature in an electronic system.

It is argued that the most appropriate definition of the local temperature T(r) for the ground state of an electronic system is provided by the formula 3/2\ensuremath{\rho}(r)kT(r)=1/8${\ensuremath{\sum}}_{\mathit{i}}$[(\ensuremath{\nabla}${\mathrm{\ensuremath{\rho}}}_{\mathit{i}}$\ensuremath{\cdot}\ensuremath{\nabla}${\mathrm{\ensuremath{\rho}}}_{\mathit{i}}$)/${\mathrm{\ensuremath{\rho}}}_{\mathit{i}}$], where \ensuremath{\rho}(r) is the total electron density and the ${\mathrm{\ensuremath{\rho}}}_{\mathit{i}}$ are Kohn-Sham orbital densities. T(r) is everywhere non-negative. For atoms, T(r) is nearly stepwise constant. T(r) behaves very much like the Politzer average local ionization energy index. Accordingly, T(r) measures reactivity toward attack by an electron-attracting reagent. Exchange energies and Compton profiles are calculated for several atoms using this definition of the local temperature. \textcopyright{} 1996 The American Physical Society.

Computational Analysis of Substituent Effects inPara-Substituted Phenoxide Ions

Para-substituted phenoxide ions are characterized by a strong resonance interaction between the oxygen and the substituent when the substituent is electron-accepting. We have modeled these interactions by computing the minima of the electrostatic potential, Vmin, and the average local ionization energy, ĪS,min, near the unprotonated oxygens of para-substituted phenoxide ions. The calculations of Vmin and ĪS,min were carried out at different levels of theory using both ab initio and density functional methods. Vmin and ĪS,min have been found to correlate well with the empirical substituent constant, σp-. The calculated energy differences for the acidic dissociation of phenols and the corresponding experimental gas-phase acidities were found to agree very well. A close linear relationship was found between Vmin and the gas-phase acidities. The findings suggest that through-resonance is accounted for in our calculations and that Vmin and ĪS,min can be used as tools for predicting σp-.

Family-independent relationships between computed molecular surface quantities and solute hydrogen bond acidity/basicity and solute-induced methanol O–H infrared frequency shifts

Family-independent relationships between computed molecular surface properties and solute hydrogen bond acidity/basicity and solute-induced methanol O–H infrared frequency shifts are presented. The molecular surface quantities, computed at the ab initio HF/6-31G* level, are primarily related to the electrostatic potential of the molecule, but also include the average local ionization energy on its surface. Keywords: hydrogen bond acidity/basicity, O–H frequency shifts, molecular electrostatic potential.

Molecular surface electrostatic potentials and local ionization energies of Group V–VII hydrides and their anions: Relationships for aqueous and gas‐phase acidities

AbstractWe have computed ab initio HF/6‐31+G* electrostatic potentials and average local ionization energies on the molecular surfaces of the Group V–VII hydrides and corresponding anions of the first three rows of the periodic table. The surfaces were defined to be specified contours (0.002 or 0.001 au) of the molecular electronic density. The most negative potentials, VS,min, and lowest ionization energies ĪS,min, were located and determined. Their magnitudes separately satisfy limited correlations with gas‐phase protonation enthalpies and aqueous pKa values. Our results indicate that VS,min, and ĪS,min are complementary, the former reflecting electrostatic factors and the latter being related to charge transfer/polarization. More general relationships for protonation enthalpies are obtained when both VS,min and ĪS,min are explicitly included. Solution‐phase and gas‐phase acidities are shown to correlate very well if electrostatic effects are explicitly taken into account. © 1993 John Wiley & Sons, Inc.

Computational analysis of the dinitramine and chlorine derivatives of benzene and s-tetrazine

Optimized HF/3-21G ∗ geometries were computed for s-tetrazine (I) and for the dinitramine and chlorine derivatives of I and of benzene. These structures were used to calculate the electrostatic potentials and average local ionization energies of these molecules. HF/3-21G ∗ and MP2/6-31G ∗ geometries were also computed for pentazine ( VI), which is as yet unknown, and shown to correspond to energy minima. The electrostatic potential on the surface of I is strongly negative above the electron-attracting ring nitrogen atoms and positive above the ring and the hydrogen atoms, fully consistent with the structures of complexes that s-tetrazine is known to form with other molecules. Previously developed relationships were used to estimate the Hammett constants σ m and σ p of the dinitramine group, N(NO 2) 2, and the p K a values of s-tetrazine, its dinitramine and chlorine derivatives, and pentazine. N(NO 2) 2 is found to be strongly electron withdrawing through induction and more weakly donating through resonance. In the s-tetrazine derivatives, the electron attracting power of the ring nitrogen atoms significantly increases the extents of conjugation of the N(NO 2) 2 and Cl, both resonance donors. However, the dominant effect of these substituents upon the ring is inductive deactivation toward electrophiles.

Calculated electrostatic potentials and local surface ionization energies of para-substituted anilines as measures of substituent effects

Using an abinitio self-consistent-field molecular orbital approach, we computed 3-21G//STO-3G* and STO-5G//STO-3G* electrostatic potentials and average local ionization energies for 17 para-substituted anilines. Our results demonstrate that the most negative potentials (Vmin) and the local surface ionization energy minima (ĪS,min) associated with the amine nitrogen lone pairs are highly sensitive indicators of the electron-donating and electron-attracting tendencies of the para substituents. We find excellent linear relationships between the 3-21G//STO-3G* amine nitrogen Vmin and ĪS,min and the σp0 Hammett constants of the substituent X; the correlation coefficients are 0.99. Correlations of slightly lesser quality are shown to exist between Vmin, ĪS,min, and σp−, σp, and pKa. Estimates of previously unknown σp0 and pKa values are given. The presence of ring carbon ĪS,minmeta to the substituent X also provides a predictive capability for determining σm values.

Correlations between molecular electrostatic potentials and some experimentally-based indices of reactivity

The key role that electrostatics plays in molecular reactive behavior is demonstrated in this work, which surveys and further explores correlations that we found between the molecular electrostatic potential V(r) calculated by an ab initio SCF-MO approach (a gas-phase property) and experimentally-based indices of reactivity (derived from solution studies). In our relationships involving negative V(r), we find in all cases that spatial minima ( V min) provide correlations of higher quality than surface minima ( V S, min ). Relationships between V min and the hydrogenbond-acceptor parameter β, and V S,max and the hydrogen-bond-donor parameter α, confirm the physical validity of the empirically-derived solvatochromic parameters α and β. Correlations between the V min of NH 2-X molecules and the substituent constants σ I, and σ I + σ R (when σ R>0) for the substituents X show how electrostatic properties reflect the electron-attracting tendencies of substituent groups. Whereas good relationships exist between the p K a, values of some limited groups of molecules and their conjugate base V min, we find that the average local ionization energy Ī(r) is better suited as a general measure of aqueous acidity.

Applications of calculated local surface ionization energies to chemical reactivity

A recently introduced property, the average local ionization energy I ( r), is discussed in relation to chemical reactivity. I ( r), is defined within the framework of self-consistent-field molecular-orbital (SCF-MO) theory and is interpreted as the average energy required to remove an electron from any point r in the space of a molecule. A survey of applications of ab initio SCF I ( r), computed on molecular surfaces defined by the 0.002 e bohr −3 contour of the electronic density is presented. The positions at which the surface I ( r), has its lowest values ( I S,min) are indicative of sites most reactive toward electrophiles. An excellent correlation was found between the ring carbon ( I S,min) of monosubstituted benzenes and the Hammett constants. For a large variety of carbon, oxygen and nitrogen acids, we have shown that good correlations exist between p K a and the conjugate base ( I S, min) Finally, a good relationship is demonstrated to exist between the conjugate base ( I S,min) of substituted acetic acids and the inductive substituent constants σ i (which were derived from the p K a values). ( I S, min) found near the C-C bond midpoints of the three-membered ring portions of saturated strained hydrocarbons are interpreted as reflecting the σ aromaticity of these rings.