Electron Interaction Energy Research Articles

Modeling intercalation through the sandwich-type interactions between benzene and 14 polyaromatic molecules: DFT andab initio results

We use second order Moller Plesset perturbation theory and several density functional theory methods to calculate the counterpoise corrected electronic interaction energies between benzene and a series of polyaromatic molecules. These systems serve as a simple model for DNA intercalation. We show that addition of nitrogen atoms to the polyaromatic molecules always increases sandwich-type interactions, and that, of the density functional theory methods studied, only SVWN can mimic the interaction energies and optimal separations obtained with perturbation theory. SVWN reproduces the optimal molecular distances obtained with perturbation theory very well, and often comes within less than 10% of the interaction energy. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Short-range interaction energy for ground state [formula omitted

Explicit analytical formulae are presented for the exact R n -expansion of the short-range electronic interaction energy for ground state H 2 + up to n = 10. The corresponding calculated values for small values of the internuclear distance are in excellent agreement with Peek’s accurate results up to R = 0.30 a 0.

Positive and negative ion formation via slow electron collisions with 5-bromouridine

Cation and anion formation is studied experimentally upon inelastic low energy electron interactions with 5-bromouridine (BrUrd) using a crossed electron/neutral beams set-up combined with a quadrupole mass spectrometer. The BrUrd molecule belongs to the class of radiosensitizers that increase the sensitivity of DNA (or RNA) to ionizing radiation. In the case of positive ion formation the ionization efficiency curves are measured near the threshold and the corresponding appearance energies are determined using a non-linear least square fitting procedure. The anion yields are investigated in the electron energy range from about 0 to 14 eV. From the comparison of the present results for negative and positive ions information concerning the underlying mechanism of radiosensitizers is deduced.

Electron attachment to monomeric and dimeric forms of glycolaldehyde

Low energy electron interactions with gas phase monomeric and dimeric forms of glycolaldehyde are reported in the energy range from about 0 to 14 eV, with special emphasis on the formation and decay of transient negative parent ions. Differences between anion fragmentation obtained from monomeric and dimeric forms are observed and discussed. Dissociative electron attachment to the dimeric configuration is shown to have many energetically accessible reaction channels.

Low Energy Electron Interactions with Uracil: The Energetics Predicted by Theory

Low energy electrons (LEEs) induce strand breaks and base damage in DNA and RNA via fragmentation of molecular bonding. This includes the formation of hydrogen atoms from N−H and C−H bond dissociations in the bases thymine, cytosine and uracil, respectively. To better understand the dissociation of uracil induced by LEEs, we theoretically characterized the potential energy surfaces (PESs) along the N−H and C−H bonds of the uracil anion, as well as the energetics involved. The PESs show that an activation barrier of less than 1 eV exists for the N1−H dissociation with rather flat PES beyond N−H = ∼1.5 Å. The PESs for C5−H and C6−H show larger barriers, which increase monotonically with bond stretching. All the N−H and C−H bond dissociations are endothermic; the adiabatic PESs suggest the energy threshold for formation of hydrogen from N−H and C−H bonds are in the order: 0.78 (N1−H) < 1.3 (N3−H) < 2.2 (C6−H) < 2.7 eV (C5−H). The H-deleted uracil radicals (U-yl radical family) are found to have exceptionally high adiabatic electron affinities, namely, 3.46 (N1), 3.8 (N3), 2.35 (C5), and 2.67 eV (C6). During the H bond breaking process of an uracil transient anion, these electron affinities compensate the extra energy needed to break the N−H or C−H bonds. This process may therefore explain the large hydrogen yield found experimentally from uracil upon attachment of LEEs. Potential applications of this process for the synthesis of uracil analogues using LEE irradiation are suggested.

On the Interaction of Dihydrogen with Aromatic Systems

Second-order Moller−Plesset (MP2) calculations (using the approximate resolution of the identity, RI-MP2) in the TZVPP basis are performed to study the interaction of molecular hydrogen with the aromatic systems C6H5X (X = H, F, OH, NH2, CH3, and CN), C10H8 (naphthalene and azulene), C14H10 (anthracene), C24H12 (coronene), p-C6H4(COOH)2 (terephthalic acid), and p-C6H4(COOLi)2 (dilithium terephthalate). Various adsorption positions are studied for C6H5F. The most favorable configuration places H2 above the aromatic plane with its axis pointing toward the middle of the ring. The electronic (van der Waals) interaction energy for the differently substituted benzenes correlates with the ability of the substituents to enrich the aromatic system electronically. The largest interaction energy (among the singly substituted benzenes) is found for aniline (4.5 kJ mol-1). Enlarging the aromatic system increases the interaction energy; the value for coronene amounts to 5.4 kJ mol-1. Extending the basis set and includi...

A Coupled Cluster Study of van der Waals Interactions of the He Atom with CN, NO and O2 Radicals

The partially spin-adapted coupled cluster method with the restricted open-shell Hartree- Fock reference was applied to calculations of interaction energies between the helium atom and the three radicals, CN (2Σ), NO (2Π), and O2 (3Sg-). Basis set dependences with medium-augmented correlation consistent basis sets were alleviated by using extrapolations to the basis set limit which were based on aug-cc-pVTZ and aug-cc-pVQZ results. The two-dimensional potential energy surfaces were fitted by exponential and polynomial functions. Minima and transition states were located. Potential energy surfaces are very floppy, especially for HeCN. This complex exhibits the weakest van der Waals interaction, the electronic interaction energy being 92 μEh. Interaction energy in HeNO is 122 μEh, almost the same as was found for HeO2 (124 μEh). Considering zero-point-vibrational corrections, the dissociation energy of HeCN, HeNO, and HeO2 is 4.6, 6.6, and 7.3 cm-1, respectively. This sequence of the magnitude of interaction energies and the structural data for global and local minima and transition states were compared with available literature data. No simple link between the magnitude of intermolecular forces and dipole moments and dipole polarizabilities of CN, NO, and O2 was found. The low-order long-range model based on the induction and dispersion forces is completely useless in the assessment of the sequence of the size of intermolecular interactions of the HeCN, HeNO, and HeO2 complexes.

Ground state features of the Fr�hlich model

Following the ideas behind the Feynman approach, a variational wave function is proposed for the Fröhlich model. It is shown that it provides, for any value of the electron-phonon coupling constant, an estimate of the polaron ground state energy better than the Feynman method based on path integrals. The mean number of phonons, the average electronic kinetic and interaction energies, the ground state spectral weight and the electron-lattice correlation function are calculated and successfully compared with the best available results.

Low energy electron interaction with free and bound SF5CF3: Negative ion formation from single molecules, clusters and nanofilms

The interaction of free electrons with the potent greenhouse molecule SF5CF3 is studied under different degrees of aggregation: single molecules at collision free conditions, clusters within a supersonic molecular beam and condensed molecules. Electron collisions with single molecules are dominated by SF5− formation produced via dissociative electron attachment (DEA) within a resonance located below 2 eV. In clusters, undissociated parent anions SF5CF3− (and larger complexes containing undissociated anions) are observed in addition to the fragment ions. This indicates that (i) SF5CF3 possesses a positive adiabatic electron affinity and (b) low energy attachment is partly channeled into nondissociative processes when the molecule is coupled to an environment. Electron impact to condensed phase SF5CF3 exhibits a remarkably strong F− desorption signal appearing from a pronounced resonance located at 11 eV while in the gas phase at 11 eV only a weak DEA signal is observed. Electron induced desorption from sub-monolayers of SF5CF3 on an amorphous H2O ice surface is found to be more efficient compared to desorption of SF5CF3 from a Xe surface. The implications of these results for the heterogeneous photochemistry of SF5CF3 adsorbed on ice or dust particles in the Earth’s atmosphere are discussed.

Derivation of the density functional theory from the cluster expansion.

The density functional theory is derived from a cluster expansion by truncating the higher-order correlations in one and only one term in the kinetic energy. The formulation allows self-consistent calculation of the exchange correlation effect without imposing additional assumptions to generalize the local density approximation. The pair correlation is described as a two-body collision of bound-state electrons, and modifies the electron- electron interaction energy as well as the kinetic energy. The theory admits excited states, and has no self-interaction energy.

Two electrons in one-dimensional nanorings: Exact solutions and interaction energies

Exact series solutions for two electrons in one-dimensional nanorings have been obtained by expanding the Coulomb potential into power series and solving the corresponding equations in different regions. Electronic structures and interaction energies of two electrons in nanorings with different sizes have been calculated. The quantum-size effect of interaction energies and energy levels and the size-dependent magnetic oscillations of two-electron spectra have been revealed.

Magnetic-field-induced phase transitions in Wigner molecules

A theoretical analysis of formation and symmetry transformationsis presented for Wigner molecules with N = 2,..., 20 electronsconfined in quantum dots at high magnetic fields. Using the unrestricted Hartree–Fockmethod with the multicentre Gaussian basis, we have found that Wigner molecules withN ≥ 6abruptly change their shape and symmetry with an associated jump in the firstderivative of the ground-state energy, i.e. they undergo phase transitions. Inparticular, the phases of the Wigner molecules obtained just after emerging fromthe maximum-density droplet (MDD) phase possess a different symmetry fromthat formed at a high magnetic field. We show that the properties of theelectron–electron interaction energy demonstrate very well both the breakdown ofthe MDD and the quasi-classical character of the Wigner molecule in the highmagnetic field. Possible mechanisms of the MDD decay are discussed.

Three interacting electrons in vertically coupled quantum dots

We study a three-electron system in a double-layer quantum dot under a magneticfield by means of the exact diagonalization of the Hamiltonian matrix.Discontinuous ground-state energy transitions induced by an external magneticfield are reported. Series of magic numbers of the angular momentum whichminimize the ground-state electron–electron interaction energy have beenobtained.

Spatial generalization of the H atom nuclear cusp condition for kinetic energy density for closed shells in a bare Coulomb field

In a recent paper (N.H. March et al., Phys. Rev. A 63 (2000) 012520), it has been shown that there are nuclear cusp conditions for various energy densities of density functional theory, embracing exchange and correlation contributions as well as the full electron–electron interaction energy density. Here, for the single-particle kinetic energy density, an exact nuclear cusp condition for the H atom is shown to have a spatial generalization for an arbitrary number of closed shells in a bare Coulomb field.

Cyclotron spin-flip excitations in the extreme quantum limit

The cyclotron spin-flip excitations of a two-dimensional electron system in the extreme quantum limit have been investigated by inelastic light scattering. The electron interaction energy was directly determined from the gap between the charge- and spin-density modes in a broad region of electron filling factors. A comparison of the experimental results with the Hartree-Fock and generalized single mode approximations indicates a strong correlation between electrons at filling factors corresponding to both compressible and incompressible states.

Self-assembled complexes of oligopeptides and metalloporphyrins: measurements of the reorganization and electronic interaction energies for photoinduced electron-transfer reactions

Cationic porphyrins form ground state electrostatically associated complexes with anionic oligo-electrolytes such as those formed by a series of glutamic acid (E) residues. Temperature dependencies were measured of the rate constants for intra-complex electron transfer to the triplet state of Pd(II)TMPyP 4+ from a tyrosine (tyr, Y) or tryptophan (trp, W) moiety connected to a glutamic acid tetramer. In complexes such as YE 4, E 2YE 2, YE 4G 10E (G, glycine), and WE 4 these data were used to estimate the reorganization energy (λ) and electronic interaction energy ( H DA) relevant to the process. For all tyr–peptide complexes, λ values were found to be large (λ∼1.60±0.06 eV), reflecting a relatively high medium polarity in the vicinity of tyr residues. It further indicates that the tyr residues in all oligo-peptides are exposed to the aqueous medium in a similar way irrespective of the position of the aromatic moiety in the peptide chain. A significantly lower λ value (λ=1.08 eV) was derived for the tryptophan-containing peptide complex, indicating a relatively higher hydrophobic character of trp compared to tyr. The electronic coupling matrix elements ( H DA) derived for tyr–peptide complexes (5.1 meV for YE 4, 5.4 meV for YE 4G 10E and 7.5 meV for E 2YE 2) were larger than that found for WE 4 (1.1 meV). Molecular dynamics calculations were employed to obtain structural features of the porphyrin–peptide complexes. These showed average distances between the center of mass (COM) of the porphyrin ring and the center of mass of the amino acid aromatic ring of 816±140 pm (YE 4), 800±80 pm (E 2YE 2), 900±130 pm (YE 4G 10E) and 970±160 pm (WE 4). The molecular dynamics calculations were shown to be in good agreement with the experimentally determined electronic interaction energies, strongly suggesting that H DA is primarily responsible for the dependence of the electron-transfer rate constant ( k ET) on the donor–acceptor separation distance and relative orientation. The higher H DA (7.55 meV) derived for tyr incorporated into the middle of the peptide backbone (E 2YE 2) was presumed to be associated with a higher degree of orbital overlap due to a more favorable ring–ring orientation. Overlap parameters (β derived for all peptide–porphyrin complexes were similar (∼0.95±0.06 Å −1), being in good agreement with most literature values for similar systems. Finally, the intra-complex electron-transfer ratio ( k trp/ k tyr) derived from flash photolysis experiments and the corresponding ratio derived from Marcus' theory combined with experimental data from the temperature-dependence investigations and electrochemical measurements were found to be in excellent agreement. This same consistency was found for the couple E 4Y and E 2YE 2. The empirical expression (Moser and Dutton) governing the intraprotein electron-transfer rate constant in native systems combined with our experimental data ( k ET, λ, Δ G 0) yielded tunneling pathway distances in excellent agreement with those arising from the molecular modeling studies. The exception was for the long peptide YE 4G 10E, for which the Quenched Molecular Dynamic (QMD) sampling technique was complicated and is probably inadequate.

A theoretical study on the Coulomb oscillation in a two-dimensional quantum dot

The multiple level Anderson model and the exact many-body Hamiltionian are employed to describe the interacting electrons in the two-dimensional quantum dots, respectively. By diagonalizing the Hamiltonians of the two models, the numerical results of the conductance through a quantum dot with relatively large size in the weak coupling limit are obtained. The remarkable difference of the calculated conductances between the two models shows that the multiple level Anderson model is not applicable to such a quantum dot. By comparing the two-electron ground state energies calculated via the two models, the validity of the Anderson model to the quantum dots with different sizes is examined. It is found that only for very small quantum dot in which the confinement energy is far larger than electron interaction energy the Anderson model can give the accurate results.

A Computer Simulation Study on Acylation Reaction of Aromatic Hydrocarbons over Acidic Zeolites

In the case of acylation reaction of aromatic hydrocarbons it is observed that acylation of toluene and naphthalene in the presence of acidic zeolites such as H–ZSM-5, H–ZSM-12, H–Beta, H–Mordenite, and H–Y results in different products with variable selectivities. It is observed experimentally that benzoylation of toluene and naphthalene over zeolite H–Beta follows the selectivity order: 4-methylbenzophenone>2-methylbenzophenone> 3-methylbenzophenone and 2-benzoylnaphthalene>1-benzoylnaphthalene, respectively. Zeolite H–Beta shows the best selectivity among all other acidic zeolites. To explain this selectivity order a computer simulation study has been performed. Molecular mechanics were used to calculate the individual strain, dimensions of reactant, and product molecules; the dimensions of zeolite cages were compared. The results show that zeolite Beta is the best zeolite in terms of fitting of the reactant and product molecules. DFT was applied to study the electronic property and interaction energy of reactant and product molecules with acidic zeolite framework to rationalize the mechanism of acylation reaction over acidic zeolites. Molecular electrostatic potential maps were also plotted from DFT to rationalize the polarization of the reactants in the reaction process. The results were compared with experimental observation which justifies the role of acidic zeolites, in this particular reaction.

Physics of transformation from Schrödinger theory to Kohn-Sham density-functional theory: Application to an exactly solvable model

According to Hohenberg-Kohn-Sham density-functional theory (DFT), and its constrained search formulation, the Schr\odinger ground-state wave function \ensuremath{\Psi} is a functional of the ground-state electronic density \ensuremath{\rho}(r). But the explicit functional dependence of \ensuremath{\Psi} on \ensuremath{\rho} is unknown. It is, however, possible to describe Kohn-Sham (KS) DFT and its electron-interaction energy functional and functional derivative rigorously in terms of the wave function \ensuremath{\Psi}. This description involves a conservative field which is a sum of two fields, the first representative of electron correlations due to the Pauli exclusion principle and Coulomb repulsion, and the second of correlation-kinetic effects. The sources of these fields are expectations of Hermitian operators with respect to \ensuremath{\Psi}. The energy functional is expressed in integral virial form in terms of these fields, whereas the functional derivative is the work done to move an electron in the conservative field of their sum. In this paper we illustrate the physics of transformation from Schr\odinger to KS theory by application of this description to a ground state of the exactly solvable Hooke's atom. As such we determine properties such as the pair-correlation density, the Fermi and Coulomb holes, the Schr\odinger and KS kinetic-energy-density tensors and kinetic fields, and the electron-interaction and correlation-kinetic fields, potentials, and energies, the majority of these constituent properties of the transformation being obtained analytically. In this manner we demonstrate the separate contributions and significance of each type of electron correlation to the KS electron-interaction energy and its functional derivative. Based on this study and previous work, it is proposed that in the construction of approximate energy functionals and their derivatives for application to more complex systems, it is the fields that be directly approximated.

Highly Accurate Estimates of Hydrogen-Bond Energies Relying on Basis Set Convergence Patterns

We introduce a method for the determination of highly accurate estimates of the electronic interaction energies in uncharged hydrogen-bonded clusters, using basis set convergence patterns (=BSCP) of the aug-cc-pVxZ series of basis sets at MP2(FC) level of theory. The method is characterized by its robustness toward systematic deficiencies of MP2(FC)/aug-cc-pVxZ description of hydrogen bonds. The achieved reliability is due to a one parameter scaling ansatz that transforms the disadvantages of the basis set superposition error into precious information. The method is applied to the trimer of hydrogen fluoride and the trimer of water which were both fully geometry optimized up to MP2(FC)/aug-cc-pVQZ level of theory. The electronic interaction energy is predicted to be −15.05 kcal/mol for the trimer of hydrogen fluoride and −15.91 kcal/mol for the trimer of water.