Electron Delocalization Effects Research Articles

Why is sulfuric acid a much stronger acid than ethanol? Determination of the contributions by inductive/field effects and electron-delocalization effects.

Two different and complementary computational methods were used to determine the contributions by inductive/field effects and by electron-delocalization effects toward the enhancement of the gas-phase deprotonation enthalpy of sulfuric acid over ethanol. Our alkylogue extrapolation method employed density functional theory calculations to determine the deprotonation enthalpy of the alkylogues of sulfuric acid, HOSO2-(CH2CH2)n-OH, and of ethanol, CH3CH2-(CH2CH2)n-OH. The inductive/field effect imparted by the HOSO2 group for a given alkylogue of sulfuric acid was taken to be the difference in deprotonation enthalpy between corresponding (i.e., same n) alkylogues of sulfuric acid and ethanol. Extrapolating the inductive/field effect values for the n = 1-6 alkylogues, we obtained a value of 51.0 ± 6.4 kcal mol(-1) for the inductive/field effect for n = 0, sulfuric acid, leaving 15.4 kcal mol(-1) as the contribution by electron-delocalization effects. Our block-localized wavefunction method was employed to calculate the deprotonation enthalpies of sulfuric acid and ethanol using the electron-localized acid and anion species, which were compared to the values calculated using the electron-delocalized species. The contribution from electron delocalization was thus determined to be 18.2 kcal mol(-1), which is similar to the value obtained from the alkylogue extrapolation method. The two methods, therefore, unambiguously agree that both inductive/field effects and electron-delocalization effects have significant contributions to the enhancement of the deprotonation enthalpy of sulfuric acid compared with ethanol, and that the inductive/field effects are the dominant contributor.

Impact of Electron Delocalization on the Nature of the Charge-Transfer States in Model Pentacene/C60 Interfaces: A Density Functional Theory Study

Electronic delocalization effects have been proposed to play a key role in photocurrent generation in organic photovoltaic devices. Here, we study the role of charge delocalization on the nature of the charge-transfer (CT) states in the case of model complexes consisting of several pentacene molecules and one fullerene (C60) molecule, which are representative of donor/acceptor heterojunctions. The energies of the CT states are examined by means of time-dependent density functional theory (TD-DFT) using the long-range-corrected functional, ωB97X, with an optimized range-separation parameter, ω. We provide a general description of how the nature of the CT states is impacted by molecular packing (i.e., interfacial donor/acceptor orientations), system size, and intermolecular interactions, features of importance in the understanding of the charge-separation mechanism.

Single N–C Bond Becomes Shorter than a Formally Double N═C Bond in a Thiazete-1,1-dioxide Crystal: An Experimental and Theoretical Study of Strong Crystal Field Effects

3-Diethylamino-4-(4-methoxyphenyl)-1,1-dioxo-4H-1λ6,2-thiazete-4-carbonitrile (DTC) is a synthetic compound that exhibits a significant similarity with β-sultamic drugs. Its core chemical moiety is an uncommon four-membered thiazete-1,1-dioxide heterocycle. Former crystallographic investigations carried out at room temperature on different DTC polymorphs and a chemically related compound showed a very unusual structural feature: within the conjugated −N–C═N–SO2– system, the formally single N–C bond is, on average, 0.018 Å shorter than the formally double N═C bond. In this work, the charge density distribution of DTC has been explored by both single-crystal X-ray diffraction at T = 100(2) K and quantum mechanical simulations, with the aim of gaining insights into the subtle interplay between structure, electron delocalization, and crystal field polarization effects. To this end, both local and nonlocal topological descriptors provided by the Quantum Theory of Atoms in Molecules have been employed. Topological and structural changes of crystalline and in vacuo DTC have been related to the smaller or larger importance of resonance forms in the −N–C═N–SO2– moiety. A rationale for the mentioned C–N/C═N bond length inversion is found in terms of the large DTC dipole moment enhancement occurring in the crystal, which stabilizes highly polar resonant forms that exploit more favorable electrostatic interactions with neighboring molecules. In turn, this causes a significant electronic rearrangement within the molecule that results in an unusual and counterintuitive bond length alternation pattern. Possible implications from the viewpoint of the accurate in silico modeling of crystal structures are discussed.

The stability of B15N15Hx nano-rings is affected by electron delocalization

Density functional theory (DFT) computations are employed to study the relation between the electron delocalization, defined within the context of quantum theory of atoms in molecules, and the stability of B15N15Hx (x=4, 8, 12, 16, 20) nano-rings at B3LYP/6-31G(d) level. It is shown that the increasing the electron delocalization correlates with the increasing the stability and the effect of electron delocalization on the total electronic energy of B15N15Hx nano-rings is completely linear with a squared regression coefficient equal 0.98. This is an interesting notion that the electron delocalization in boron-nitride nano-rings bonded to hydrogen atoms is the main factor in reducing the electronic energy.

Evidence of A–B site cooperation in the EuFeO3 perovskite from 151Eu and 57Fe Mössbauer spectroscopy, EXAFS, and toluene catalytic oxidation

EuFeO3 perovskite was considered as a case study of the cooperative effects of the rare earth and transition metal elements in the total catalytic oxidation of aromatic hydrocarbons. For this purpose, a EuFeO3 perovskite was prepared using the citrate route. Extensive characterization was performed via ex situ and in situ methods. EXAFS revealed that oxygen vacancies are formed in the nearest neighborhood of Fe and next-nearest neighborhood of Eu. Combining 151Eu and 57Fe Mössbauer experiments also suggested local oxygen defects in the proximity of the Eu cations during the reaction, as well as cooperative electron delocalization effects between Eu and Fe. XPS has shown the +3 oxidation state of both Eu and Fe cations and the relatively strong ionicity of the Fe–O chemical bonds, as suggested by the weakened 2p3/2 satellite in the Fe2p spectrum. A systematic change of this satellite with the A cation in AFeO3 perovskites was also discussed. The thermal treatment of the catalyst at 623K, in air or toluene, was accompanied by a partial reduction of Fe and a partial oxidation of Eu, respectively. The interaction between the two elements appeared to be influenced mainly by the temperature and only slightly by the sample environment during further treatments. Catalytic oxidation of toluene on this perovskite led only to the production of carbon dioxide and water with no side-product formation from partial oxidation reactions. All the characterization and catalytic results preferably agree with a Volkenstein mechanism.

Molecular structure, vibrational spectra (FTIR and FT Raman) and natural bond orbital analysis of 4-Aminomethylpiperidine: DFT study

The FT-IR and FT-Raman spectra of 4-Aminomethylpiperidine have been recorded using Perkin Elmer Spectrophotometer and Nexus 670 spectrophotometer. The equilibrium geometrical parameters, various bonding features, the vibrational wavenumbers, the infrared intensities and the Raman scattering activities were calculated using Hartree–Fock and density functional method (B3LYP) with 6-311+G(d,p) basis set. Detailed interpretations of the vibrational spectra have been carried out with the aid of the normal coordinate analysis. The spectroscopic and natural bonds orbital (NBO) analysis confirms the occurrence of intra molecular hydrogen bonds, electron delocalization and steric effects. The changes in electron density in the global minimum and in the energy of hyperconjugative interactions of 4-Aminomethylpiperidine (4AMP) were calculated. The theoretical UV–Visible spectrum of the compound was computed in the region 200–400nm by time-dependent TD-DFT approach. The calculated HOMO and LUMO energies show that charge transfer occur within the molecule. The dipole moment (μ) and polarizability (α), anisotropy polarizability (Δα) and hyperpolarizability (β) of the molecule have been reported.

Thermodynamic study of 2-aminothiazole and 2-aminobenzothiazole: Experimental and computational approaches

This work reports an experimental and computational thermochemical study of two aminothiazole derivatives, namely 2-aminothiazole and 2-aminobenzothiazole.The standard (p°=0.1MPa) molar energies of combustion of these compounds were measured by rotating bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15K, were derived from the temperature dependence of the vapor pressures of these compounds, measured by the Knudsen-effusion technique and from high temperature Calvet microcalorimetry. The conjugation of these experimental results enabled the calculation of the standard molar enthalpies of formation in the gaseous state, at T=298.15K, for the compounds studied. The corresponding standard Gibbs free energies of formation in crystalline and gaseous phases were also derived, allowing the analysis of their stability, in these phases.We have also estimated the gas-phase enthalpies of formation from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory, the estimates revealing very good agreement with the experimental ones.The importance of some stabilizing electronic interactions occurring in the title molecules has been studied and quantitatively evaluated through Natural Bonding Orbital (NBO) of the corresponding wavefunctions and their Nucleus Independent Chemical Shifts (NICS) parameters have been calculated in order to rationalize the effect of electronic delocalization upon stability.

Reprint of: Energetics of 2- and 3-coumaranone isomers: A combined calorimetric and computational study

Condensed phase standard (p°=0.1MPa) molar enthalpies of formation for 2-coumaranone and 3-coumaranone were derived from the standard molar enthalpies of combustion, in oxygen, at T=298.15K, measured by mini-bomb combustion calorimetry. Standard molar enthalpies of sublimation of both isomers were determined by Calvet microcalorimetry. These results were combined to derive the standard molar enthalpies of formation of the compounds, in gas phase, at T=298.15K. Additionally, accurate quantum chemical calculations have been performed using DFT methods and high level composite ab initio calculations. Theoretical estimates of the enthalpies of formation of the compounds are in good agreement with the experimental values thus supporting the predictions of the same parameters for isobenzofuranone, an isomer which has not been experimentally studied. The relative stability of these isomers has been evaluated by experimental and computational results. The importance of some stabilizing electronic intramolecular interactions has been studied and quantitatively evaluated through Natural Bonding Orbital (NBO) analysis of the wave functions and the nucleus independent chemical shift (NICS) of the studied systems have been calculated in order to study and establish the effect of electronic delocalization upon the relative stability of the isomers.

Structure and electronic properties of (+)-catechin: aqueous solvent effects

We report a study of the structure of (+)-catechin, which belongs to the family of the flavan-3-ols-one of the five most widely distributed phenolic groups. The biological activities and pharmaceutical utility of these compounds are related to antioxidant activity due to their ability to scavenge free radicals. A breakthrough in the study of the conformational space of this compound, so far absent in the literature, is presented herein. A detailed analysis of the electronic distribution, charge delocalization effects, and stereoelectronic effects is presented following application of the theory of atoms in molecules (AIM) and natural bond orbital analysis. The stability order, and the effects of electron delocalization in the structures were analyzed in depth. The molecular electrostatic potential (MEP) was also obtained, assessing changes in the electronic distribution in aqueous solution, the effects of the solvent on the intrinsic electronic properties, and molecular geometry. The effect of the aqueous solvent on MEP was also quantified, and rationalized by charge delocalization mechanisms, relating them to structural changes and topological properties of the electronic charge density. To further analyze the effects of the aqueous solvent, as well as investigating the molecular and structural properties of these compounds in a biological environment, the polarizabilities for all conformers characterized were also calculated. All results were interpreted on the basis of our accumulated knowledge on (4α→6", 2α→O→1")-phenylflavans in previous reports, thus enriching and deepening the analysis of both types of structure.

Direct Evaluation of the Hyperconjugative Interactions in 1,1,1-Trihaloethane (CH3CX3, X = F, Cl, and Br)

Following the computational strategy proposed by Mulliken in 1939 ( J. Chem. Phys. 1939, 7 (5), 339-352), when the concept of hyperconjugation was coined, we evaluated the hyperconjugative stabilization energy in 1,1,1-trihaloethane using the block-localized wave function (BLW) method. The BLW method is the simplest and most efficient variant of ab initio valence bond (VB) theory and can derive the strictly electron-localized state wave function self-consistently. The latter serves as a reference for the quantification of the electron delocalization effect in terms of the resonance theory. Computations show that the overall hyperconjugative interactions in 1,1,1-trihaloethane, dominated by σ(CH) → σ'(CX) with minor contribution from σ(CX) → σ'(CH), ranges from 9.59 to 7.25 kcal/mol in the staggered structures and decreases in the order Br > Cl > F. This is in accord with the (1)H NMR spectra of CH3CX3. Notably, the hyperconjugation effect accounts for 35-40% of the rotation barriers in these molecules, which are dominated by the conventional steric repulsion. This is consistent with the recent findings with 1,2-difluoroethane (Freitas, Bühl, and O'Hagan. Chem. Comm. 2012, 48, 2433-2435) that the variation of (1)J(CF) with the FCCF torsional angle cannot be well explained by the hyperconjugation model

Polygonal Current Model: An Effective Quantifier of Aromaticity on the Magnetic Criterion

To explain peculiar effects of electron delocalization on the magnetic response of planar cyclic molecules, a basic model that accounts for their actual geometrical structure has been developed by integrating the differential Biot-Savart law. Such a model, based on a single polygonal circuit with ideal features, is shown to be applicable to electrically neutral or charged monocyclic compounds, as well as linear polycyclic condensed hydrocarbons. Two theoretical quantities, easily computed via quantum chemistry codes (the out-of-plane components of the magnetizability, ξ∥, and the magnetic shielding σ∥(h) of points P on the symmetry axis orthogonal to the molecular plane, at distance h from the center of mass) are shown to be linearly connected, for example, for monocyclic structures, via the relationship σ∥(h) = ±(μ0/2π)ξ∥D(h), where D(h) is a simple function of geometrical parameters. Equations of this type are useful to rationalize scan profiles of magnetic shielding and nucleus-independent chemical shift along the highest symmetry axis. For a regular polygon, D(h) depends approximately on the third inverse power of the distance d of the vertices from the center, and ξ∥ is proportional to the area of the polygon, that is, ∼d(2); hence, the shielding σ∥(0) and the related nucleus-independent chemical shift NICS∥(0) are unsafe quantifiers of magnetotropicity; they are biased by a spurious geometrical dependence on d(-1), incorrectly exhalting them in cyclic systems with smaller size. A more reliable magnetotropicity measure for a cyclic compound, in the presence of a magnetic field Bext applied at right angles to the molecular plane, is defined within the polygonal current model by the current susceptibility or current strength, ∂I/∂Bext = -ξ∥/Aeff, expressed in nanoampère per tesla, where Aeff is a properly defined area enclosed with the polygonal circuit. An extended numerical test on a wide series of mono- and polycyclic compounds and a comparison with corresponding ab initio current susceptibilities prove the superior quality of this indicator over other commonly employed aromaticity/antiaromaticity benchmarks on the magnetic criterion.

Energetics of 2- and 3-coumaranone isomers: A combined calorimetric and computational study

Condensed phase standard (p°=0.1MPa) molar enthalpies of formation for 2-coumaranone and 3-coumaranone were derived from the standard molar enthalpies of combustion, in oxygen, at T=298.15K, measured by mini-bomb combustion calorimetry. Standard molar enthalpies of sublimation of both isomers were determined by Calvet microcalorimetry. These results were combined to derive the standard molar enthalpies of formation of the compounds, in gas phase, at T=298.15K. Additionally, accurate quantum chemical calculations have been performed using DFT methods and high level composite ab initio calculations. Theoretical estimates of the enthalpies of formation of the compounds are in good agreement with the experimental values thus supporting the predictions of the same parameters for isobenzofuranone, an isomer which has not been experimentally studied. The relative stability of these isomers has been evaluated by experimental and computational results. The importance of some stabilizing electronic intramolecular interactions has been studied and quantitatively evaluated through Natural Bonding Orbital (NBO) analysis of the wave functions and the nucleus independent chemical shift (NICS) of the studied systems have been calculated in order to study and establish the effect of electronic delocalization upon the relative stability of the isomers.

High-Resolution Infrared and Electron-Diffraction Studies of Trimethylenecyclopropane ([3]-Radialene)

Combined high-resolution spectroscopic, electron-diffraction, and quantum theoretical methods are particularly advantageous for small molecules of high symmetry and can yield accurate structures that reveal subtle effects of electron delocalization on molecular bonds. The smallest of the radialene compounds, trimethylenecyclopropane, [3]-radialene, has been synthesized and examined by these methods. The first high-resolution infrared spectra have been obtained for this molecule of D3h symmetry, leading to an accurate B0 rotational constant value of 0.1378629(8) cm(-1), within 0.5% of the value obtained from electronic structure calculations (density functional theory (DFT) B3LYP/cc-pVTZ). This result is employed in an analysis of electron-diffraction data to obtain the rz bond lengths (in Å): C-H = 1.072(17), C-C = 1.437(4), and C═C = 1.330(4). The results indicate that the effects of rehybridization and π-electron delocalization affects each result in a shortening of about 0.05 Å for the C-C bond in radialene compared to ethane. The analysis does not lead to an accurate value of the HCH angle; however, from comparisons of theoretical and experimental angles for similar compounds, the theoretical prediction of 117.5° is believed to be reliable to within 2°.

Plasmon resonances and plasmon‐induced charge transport in linear atomic chains

In linear hydrogen atomic chains, plasmon resonances and plasmon‐induced charge transport are studied by time‐dependent density functional theory. For the large linear chain, it is a general phenomenon that, in the longitudinal excitation, there are high‐energy resonances and a large low‐energy resonance. The energy of the large low‐energy resonance conforms to the results calculated by the classical Drude model. In order to explain the formation mechanism of the high‐energy resonances, we present a simple harmonic oscillator model. This model may reasonably account for the relationship between low‐energy and high‐energy resonances, and has a certain degree of universality. As the interatomic distance decreases, the current shows a gradual transition from insulator to metal. The current enhancement mainly depends on the local field enhancement associated with plasmon excitation, and the enhanced electron delocalization effect as a result of the decrease of the interatomic distance. © 2013 Wiley Periodicals, Inc.

Effect of methyl groups substitution on the strength of intramolecular hydrogen bonding of naphthazarin: DFT and NBO studies

A density functional theory (DFT) study-based method B3LYP/6-311++G** was carried out to investigate the methyl groups substitution effect on the structure and the strength of intramolecular hydrogen bonding in naphthazarin (NZ) (5,8-dihydroxy-1,4-naphthoquinone). The full geometry optimization of molecular structures, the difference between the energies of hydrogen-bonded and non-hydrogen-bonded rotamers, and the proton chemical shift of the hydroxyl groups in NZ and its methyl substituents obtained at the B3LYP/6-311++G** level. The vibrational frequencies of all samples and their deuterated analogues were calculated at the same theoretical level. The 1H chemical shifts for NZ and its methyl substituents were computed at the B3LYP/6-311++G** level using the gauge-including atomic orbital method. Furthermore, in order to investigate the changes in bond order, electron density, electron delocalization, and steric effects caused by methyl substituents, natural bond orbital analysis were carried out at the B3LYP/6-311++G** level. After comparing these effective parameters in methyl substituents with those of their parent, NZ, we concluded that, in general, intramolecular hydrogen bonding strength increases by substituting methyl groups in the different positions of NZ.

An experimental and theoretical approach of spectroscopic and structural properties of the bis(diethyldithiocarbamate)–cobalt(II)

Theoretical and experimental bands have been assigned for the Fourier Transform Infrared spectrum (FT-IR) and FT-Raman of the bis(diethydithiocarbamate)Co(II) complex, [Co(DDTC)2]. The calculations have been based on the DFT/B3LYP method, second derivative spectrum and band deconvolution analysis. The UV–Vis experimental spectra of [Co(DDTC)2] was measured in the solid state and in an acetonitrile solution. The calculated electronic spectrum was estimated using the TD/PBE1PBE and TD/B3LYP methods 6-311G (d,p) basis set for all atoms.The Bond Orbital Analysis was carried out with the DFT:B3LYP/PBE1PBE methods, revealing electronic delocalization effects involving CoS and CN bonds and their neighboring groups. The observed valence configurations for the alpha and beta electrons of the cobalt atom were (4s)0.46(3d)7.69 (B3LYP) and (4s)0.46(3d)7,68 (PBE1PBE), as expected for the planar structure around the Co(II) cation. The calculated infrared and UV–Vis spectra, based on the proposed geometrical structure of the bis(diethyldithiocarbamate)cobalt(II) complex, showed an excellent agreement with the experimental spectra.

Adsorption, Dissociation, and Hydrogenation of CO2 on WC(0001) and WC-Co Alloy Surfaces Investigated with Theoretical Calculations

We applied periodic density functional theory to investigate the adsorption of CO2 on WC(0001) and various WC-Co alloy surfaces and discuss the reaction trend of dissociation of CO2 and hydrogenation on these surfaces. We employed an electron localization function (ELF) to investigate the effect of electron localization or delocalization on various WC-Co alloy surfaces with a varied Co coverage; the partial-delocalization surfaces (WC-Co (0.25 ML) surface) exhibit the largest energy of adsorption of CO2 (−1.61 eV) on all calculated surfaces. When we increased the Co ratio to form a WC-2Co (0.50 ML) surface, the activation energy for the dissociation CO2→CO+O decreased to 0.57 eV. The energy of CO2 hydrogenation to form formate (CO2+H → HCOO) decreases with Co coverage due to increased electron delocalization.

The generalized block-localized wavefunction method: A case study on the conformational preference and C–O rotational barrier of formic acid

A Lewis structure corresponding to the most stable electron-localized state is often used as a reference for the measure of electron delocalization effect in the valence bond (VB) theory. As the simplest variant of ab initio VB theory, the generalized block-localized wavefunction (BLW) method defines the wavefunction for an electron-localized state with block-localized orbitals without the orthogonalization constraint on different blocks. The validity of the method can be critically examined with experimental evidences. Here the BLW method has been applied to the investigation of the roles of both the π conjugation and σ hyperconjugation effects in the conformational preference of formic acid for the trans (Z) conformer over the cis (E) conformer. On one hand, our computations showed that the deactivation of the π conjugation or σ hyperconjugation has little impact on the Z-E energy gap, thus neither is decisive and instead the local dipole-dipole electrostatic interaction between the carbonyl and hydroxyl groups is the key factor determining the Z-E energy gap. On the other hand, the present study supported the conventional view that π conjugation is largely responsible for the C-O rotation barrier in formic acid, though the existence of hyperconjugative interactions in the perpendicular structure lowers the barrier considerably.

From aromaticity to self-organized criticality in graphene

The unique properties of graphene are rooted in its peculiar electronic structure where effects of electron delocalization are pivotal. We show that the traditional view of delocalization as formation of a local or global aromatic bonding framework has to be expanded in this case. A modification of the π-electron system of a finite-size graphene substrate results in a scale-invariant response in the relaxation of interatomic distances and reveals self-organized criticality as a mode of delocalized bonding. Graphene is shown to belong to a diverse class of finite-size extended systems with simple local interactions where complexity emerges spontaneously under very general conditions that can be a critical factor controlling observable properties such as chemical activity, electron transport, and spin-polarization.

Revealing Electron Delocalization through the Source Function

The source function (SF) introduced in late 90s by Bader and Gatti quantifies the influence of each atom in a system in determining the amount of electron density at a given point, regardless of the atom's remote or close location with respect to the point. The SF may thus be attractive for studying directly in the real space somewhat elusive molecular properties, such as "electron conjugation" and "aromaticity", that lack rigorous definitions as they are not directly associated to quantum-mechanical observables. In this work, the results of a preliminary test aimed at understanding whether the SF descriptor is capable to reveal electron delocalization effects are corroborated by further examination of the previously investigated benzene, 1,3-cyclohexadiene, and cyclohexene series and by extending the analysis to some benchmark organic systems with different unsaturated bond patterns. The SF can actually reveal, order, and quantify π-electron delocalization effects for formal double, single conjugated, and allylic bonds, in terms of the influence of distant atoms on the electron density at given bond critical points. In polycyclic aromatic hydrocarbons, the SF neatly reveals the mutual influence of the benzenoid subunits. In naphthalene it provides a rationale for the changes observed in the local aromatic character of one ring when the other is partially hydrogenated. The SF analysis describes instead biphenyl as made up by two weakly interacting benzene rings, only slightly perturbed by the combination of mutual steric and electronic effects. Eventually, a new SF-based indicator of local aromaticity is introduced, which shows excellent correlation with the aromatic index developed by Matta and Hernández-Trujillo, based on the delocalization indices. At variance with this latter and other commonly employed quantum-mechanical (local) aromaticity descriptors, the SF-based indicator does not require the knowledge of the pair density, nor the system wave function, being therefore promising for applications to experimentally derived charge density distributions.