Molecular Orbital Self-consistent Field Research Articles

Electron impact elastic and excitation cross-sections of the isomers of C4F6 molecule for plasma modeling

We report the calculations of elastic (along with its symmetry components) and electronic excitation cross sections by electron impact of the three isomers of C4F6, namely, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluoro-2-butyne (2-C4F6), and hexafluorocyclobutene (c-C4F6) belonging to the point groups C2, D3d, and C2v, respectively, using the R-matrix approach. The electron energy range is from 0.01 eV to 12 eV. We have employed the cc-pVTZ basis set for C and F atoms to generate self-consistent field molecular orbitals to construct the target states for all the isomers included in our calculations. All the target states are constructed by including correlation effects in a configuration interaction (CI) approach. The target properties such as vertical excitation energies and dipole moment of all the isomers are in reasonable agreement with the literature values. Differences in the cross sections of these isomers are strongly influenced by the effect of correlation and polarization effects and their geometrical extent. We have included the ground state and many excited states of each isomer in the trial wave function of the entire scattering system. The resulting elastic cross sections are compared with the available experimental results. The agreement is reasonably good for energies above 5 eV. The shape resonances detected at 2.57, 2.95, and 3.20 eV for c-C4F6, 1,3-C4F6, and 2-C4F6 isomers are associated with the negative anion formation of C3F3- as observed in the mass spectrometry experiments. We have also performed 1-state CI calculation for all the isomers that include only the correlated ground state. The position of resonances shifts to lower energies as the number of target states is increased compared to 1-state calculation for all the isomers. The elastic cross section for 2-C4F6 isomer is larger than the other isomers because of its larger spatial extent. The present cross section data are important for plasma simulation and modeling, especially related to fluorocarbon plasma.

Ergodicity and rapid electron delocalization – The dynamical mechanism of atomic reactivity and covalent bonding

AbstractSurprisingly, the historical development of the understanding of the concept of covalent bonding is incomplete and the physical mechanism responsible for bonding is still the subject of debate among chemists. We argue that this is primarily due to the key role played by quantum mechanics and the peculiarity of the Coulomb interaction operating between the electrons and nuclei in molecules. A study of the simplest molecules Hand H2as well as the π‐electron structure of planar conjugated hydrocarbon molecules leads to the conclusion that delocalization of electron dynamics is the key mechanism of covalent bonding. We find that the new concept of quantum ergodicity defined in terms of the globality of the energy eigenfunctions relates directly to covalent bonding. This is illustrated in the Höckel model of the π‐electrons in polyene molecules. An understanding results associating covalent bonding most fundamentally with the relaxation of nonergodic dynamical constraints upon the electron dynamics in atoms and molecules. The strain energy present due to these constraints can be crudely estimated by the comparison of the results of Thomas‐Fermi (TF) density functional calculations, which can be carried out both with and without these constraints. We present results for the light atoms H through Ar indicating that there is a very considerable strain in most atoms with the exception of the inert gas atoms. For the atoms H through Ne, we can verify this picture by direct comparison of ergodic TF and self‐consistent field‐molecular orbital (SCF‐MO) results. Comparison with atomization energies for some small molecules shows that due to repulsive mechanisms only a small fraction of the covalent strain energy is actually realized as binding energy in most molecules. The hydride molecules appear to be most efficient in utilizing the strain energy. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Computation of structural and electronic properties of single-wall II–VI compound nanotubes

A theoretical investigation on the structural and electronic properties of single-wall II–VI compound CdS and ZnO nanotubes (NTs) of different length (2–6 ring long) and chirality has been carried out by performing semiempirical molecular orbital self-consistent field calculations at the level of parametric, PM3 (modified neglect of diatomic overlap, parametric method number 3) method. The calculation and results are presented for the NTs diameter, length, bond length, bond angle, surface area, volume, heat of formation, interfrontier molecular energy gap, dipole moment, electric charge distribution and electrostatic potential surface. Both CdS and ZnO make nanotubes of uniform diameter across the length in armchair model only. The interfrontier energy gap in case of CdS is found to be almost identical in both the configurations (zigzag and armchair), whereas in ZnO, it is more in the armchair model as compared to the zigzag model. We have also established some linear equations that can predict binding energy and heat of formation for longer nanotubes for a given number of rings. According to the results obtained, we conclude that all the II–VI compound NTs studied in present work show insulating properties.

Electronic structure of organic diradicals: Evaluation of the performance of coupled-cluster methods

The performance of (i) the reduced multireference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that employs a modest-size MR CISD wave function as an external source for the most important (primary) triples and quadruples in order to account for the nondynamic correlation effects in the presence of quasidegeneracy, (ii) the RMR CCSD(T) method that adds a perturbative correction for the remaining (secondary) triples to the RMR CCSD energy, and (iii) the recently developed partially linearized MR CCSD method, which determines primary triples and quadruples using a subset of linear CC equations projected onto the corresponding higher-than-doubly excited configurations, are tested by considering the singlet-triplet splitting for several diradicals, ranging from a prototypical methylene radical to trimethylenemethane, and benzyne and pyridynium cation isomers. Both RHF and multiconfigurational self-consistent field molecular orbitals are employed. The equilibrium geometries for the lowest-lying singlet and triplet states are determined using both the density functional theory (DFT) and various CC approaches, and a comparison with both the experiment and other theoretical results, wherever available, is made. The RMR CCSD(T) results provide the most satisfactory description in all cases. The dependence of the MR diradical character on a spatial separation of radical centers, as well as the artifactual DFT geometry in the case of benzyne and pyridynium meta-isomers, is also pointed out.

The single excitation perturbation expansion theory based on the locally projected molecular orbitals for molecular interaction: Comparison with the counterpoise corrected energy

The single excitation second order perturbation expansion (SPT) based on the locally projected self-consistent field molecular orbitals for molecular interaction is numerically tested for isomers of uracil–water and for 24 water clusters. For the extensive basis sets, the calculated binding energy BE SPT becomes close to the counterpoise corrected binding energy BE SCF CP . The required cpu time is less than that for a single supermolecule calculation. For smaller basis sets, by restricting the excited orbitals to the strictly monomer basis set (SMBS), BE SPT becomes approximate to BE SCF CP . The use of the SMBS for the excited orbitals has a clear advantage over the orthogonal but delocalized excited orbitals.

Structural and electronic properties of single-wall ZnO nanotubes

The structural and electronic properties of armchair and zigzag models of single-wall ZnO nanotubes have been investigated by performing semiempirical molecular orbital self-consistent field calculations at the level of AM1 method within the RHF formulation. It has been found that these structures are stable and endothermic. The armchair model has zero net dipole moment, whereas the zigzag model has nonzero net dipole moment. The interfrontier molecular energy gap of these systems are different; the gap of armchair model is about 0.2 eV, whereas the gap of zigzag model is about 4.4 eV. Armchair ZnO nanotubes seem to be conductors, however zigzag ZnO nanotubes seem to be semiconductors.

Molecular orbital studies on brominated diphenyl ethers. Part II—reactivity and quantitative structure–activity (property) relationships

Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants and are increasingly turning up in the environment. Their structural similarities to polychlorinated biphenyls and thyroid hormones suggest they may be a risk to human health. The present study examines the reactivity of brominated diphenyl ethers (BDEs) on the basis of the electronic structures as calculated by semiempirical AM1 self-consistent field molecular orbital (SCF-MO) method. Frontier orbital energies were used to elucidate the reactivity of BDEs in electrophilic, nucleophilic and photolytic reactions. From an examination of the frontier electron densities, the regioselectivity, or orientation, of metabolic reactions of BDEs was predicted. Furthermore, satisfactory quantitative structure–activity (property) relationship (QSAR and QSPR) models were derived to calculate gas chromatographic and ultraviolet spectral properties and luciferase induction activities from the AM1-computed electronic parameters.

Structural and electronic properties of single-wall GaN nanotubes: semi-empirical SCF-MO calculations

The structural and electronic properties of armchair and zigzag models of single-wall GaN nanotubes have been investigated by performing semi-empirical molecular orbital self-consistent field calculations at the level of PM3 method within the RHF formulation. It has been found that these structures are stable and endothermic. The armchair model has zero net dipole moment, whereas the zigzag model has nonzero net dipole moment. It has been found that GaN armchair tube with even number of hexagonal rings on the circumference has more excess charge on it and thus this tube may have better conducting properties, they may be candidates to be used as good conducting nanowires.

Structural and electronic properties of rubreneperoxides

The structural and electronic properties of rubrene and three rubreneperoxide isomers have been investigated by performing molecular mechanics and semi-empirical AM1 molecular-orbital self-consistent field calculations. It has been found that isolated rubrene and rubreneperoxide isomers have non-planar structure, and are stable but endothermic.

Structural and electronic properties of MTA

The structural and electronic properties of mellitic trianhydride (MTA) have been investigated by performing semiempirical (AM1, PM3, MNDO3) and ab initio molecular orbital self-consistent field calculations. The density of states and the IR spectrum of MTA have been calculated. It has been found that the isolated MTA molecule has a planar structure, and that it is exothermic.

Structural and electronic properties of single-wall BN nanotubes

The structural and electronic properties of armchair and zigzag models of single-wall BN nanotubes have been investigated by performing semi-empirical molecular orbital self-consistent field calculations at the level of the Austin Model 1 (AM1) method within the RHF formulation. It has been found that these structures are stable and exothermic. The armchair model has a zero net dipole moment, whereas the zigzag model has a nonzero net dipole moment. The interfrontier molecular energy gap of these systems are close to each other, thus the gap is independent of the chirality of the tube.

Structural and electronic properties of borazine cyclacenes

The structural and electronic properties of borazine cyclacenes (BNn) have been investigated by performing semiempirical molecular orbital self-consistent field calculations at the level of AM1 method within the RHF formulation. It has been found that as the number of borazine rings increases in the arenoid belt the structures become more exothermic.

Natural bond orbital analysis of steric interactions

We describe an ab initio procedure for extracting the Pauli exchange antisymmetry (“steric”) contributions to molecular potential energy in the framework of self-consistent-field molecular orbital (SCFMO) theory. This “natural steric analysis” method is based on natural bond orbital (NBO) representation of the SCFMO wave function, which allows the steric exchange energy to be approximated as an energy difference between “preorthogonal” and final NBOs, analogous to the procedure of Sovers et al. [J. Chem. Phys. 49, 2592 (1965)]. We show how the total NBO steric exchange energy can in turn be approximated in terms of pairwise-additive interactions between localized bonding units, comparable to the empirical steric potentials of molecular mechanics models. The accuracy of NBO steric analysis is tested in applications to various rare-gas interactions (expected to be of pure steric exchange type), and excellent agreement is found with the full ab initio potential curves over a wide range of separations. Limitations of the simple pairwise-additive model of steric exchange interactions are noted and characterized.

Multireference configuration interaction study of the mixed Valence–Rydberg character of the C2H4 (π,π*)1V state

The spatial extension of the C2H4 (π,π*)1V state is investigated by means of low selection threshold multireference configuration interaction (CI) calculations employing two atomic orbital (AO) basis sets with different numbers of polarization and Rydberg functions. The results are shown to be nearly independent of the choice of one-electron basis (ground N, triplet T, and singlet V self-consistent field molecular orbitals (SCF MOs)) in forming the many-electron basis for the configuration interaction indicating that the AO basis limit has been closely approached in each case. The calculations indicate that the value for the 〈ΨV|Σxi2|ΨV〉≡〈x2〉V matrix element falls in the 18±1 a02 range, 50% larger than the corresponding values computed for 〈x2〉N and 〈x2〉T, respectively, for the corresponding N and T states. This result is interpreted to be a consequence of the mixing of diabatic (π,π*)1 valence and (πd,π)1 Rydberg states in the Franck–Condon region of the V–N transition. The corresponding excitation energy is computed to lie in the 7.90–7.95 eV range, indicating that there is a distinct nonverticality in the measured absorption spectrum which is caused in part by nonadiabatic interactions between the V and (π,3py)1 Rydberg states as a result of torsional motion of the C2H4 molecule.

Extended SSH model approach to dopyballsC 58 B 2 andC 58 N 2

The Su-Schrieffer-Heeger (SSH) model has been extended to study electronic properties of DopyballsC 58 B 2 andC 58 N 2. It is proposed that self-consistent solutions for electronic wave functions should satisfy some symmetry conditions even though the icosahedral symmetryC 60 has lost by doping. The present numerical results are compared with those obtained from a selfconsistent-field molecular-orbital (SCF-MO) method, most of them being in good agreement with each other. However, for aC 58 B 2 isomer in which twoB atoms are on opposite sides of the cage, our result for electronic energy levels differs quite from the SCF-MO result. We discuss this difference from a viewpoint of the symmetry requirement.

Structural and electronic properties of C59X (X=B,N): The extended Su-Schrieffer-Heeger model.

The molecules C{sub 59}{ital X} ({ital X}=B,N) are investigated by the extended Su-Schrieffer-Heeger model. The obtained results for the energy levels and molecular structures are compared with those from a self-consistent-field molecular-orbital (SCF-MO) method. We have found that by choosing suitable empirical parameters, most of the features included in the results of the SCF-MO method can be well reproduced. Other structural and electronic properties such as the excess electron density and the amplitude of the midgap states have also been studied.

A generalized multistructural wave function

A generalized multistructural (GMS) wave function is presented which combines the advantages of the self-consistent-field molecular orbital (SCF-MO) and valence bond (VB) models, preserving the classical chemical structures but optimizing the orbitals in a self-consistent way. This wave function is particularly suitable to treat situations where the description of the molecular state requires localized wave functions. It also provides a very convenient way of treating the electron correlation problem, avoiding large configuration interaction (CI) expansions. The final wave functions are much more compact and easier to interpret than the ones obtained by the conventional methods, using orthogonal orbitals. Applications of the GMS wave functions to a variety of systems are presented.

Bisulfide complexes of zinc and cadmium in aqueous solution: Calculation of structure, stability, vibrational, and NMR spectra, and of speciation on sulfide mineral surfaces

Ab initio self-consistent-field molecular orbital (SCF-MO) calculations have been employed to determine the structures and properties of Zn and Cd bisulfides and their hydrates. Calculated M-S bond lengths, R(M-S), for M( SH) n ( OH 2) 2− n a species are consistently about 0.05 Å longer than the experimental ones but show the right trends with n and a. Calculated Zn-S symmetric and average stretching frequencies scale linearly with R(Zn-S) −1, so that Zn-S distances can be estimated from vibrational spectra and vice versa. The total NMR shieldings of Zn and Cd in M( SH) 2− n n species show a shallow minimum for n= 3, and the shift compared to free M 2+ can be well fitted to a function increasing linearly with n and decreasing exponentially with R(M-S). The calculated shielding anisotropy for Cd(SH) − 3 is in good agreement with observed values for the C 3 h symmetry species Cd(SR) − 3 in solids. Complexation of water to Zn(SH) 2 produces slightly larger Zn-S distances and smaller Zn shieldings due to the deshielding effect of the tightly bound water. For Zn(SH) − 3, on the other hand, hydration is less exothermic since it requires greater Zn-S bond elongation and results in a longer Zn-O bond. Reduction of the magnitude of deshielding from S, coupled with only a small deshielding from O, causes the Zn NMR shielding of Zn(SH) 3OH − 2 to be larger than that of Zn(SH) − 3. Such effects of hydration on structure and shielding are much like those previously seen in studies of Zn and Cd chlorides. Comparison of the isomeric species Zn(SH) 3OH −2 and ZnS(SH) 2(OH 2) −2 shows the former to be more stable by more than 50 kJ/mol. The two species also differ spectroscopically, with ZnS(SH) 2(OH 2) −2 having a higher stretching frequency and a lower Zn NMR shielding. Equivalently, for the species Zn(SH) 3(OH 2) −, the H 2O is calculated to be more acidic than the SH −. Both H 2O and SH − are more acidic in the complexes studied herein than when occurring as the free molecules. The quantitative change is, in fact, greater for SH −, but not enough to make it more acidic than H 2O. Comparisons of the energies of intermediate members in some relevant chemical series indicate that Zn(SH) − 3 is enhanced in stability compared to Zn(SH) 2 and Zn(SH) −2 4, and that Zn(SH) 3(OH) −2 is enhanced in stability compared to Zn(SH) −2 4 and Zn(OH) −2 4; whereas Zn(SH) 2Cl −2 2 is unstable energetically compared to Zn(SH) −2 4 and ZnCl −2 4. This supports the occurrence of Zn(SH) − 3 and Zn(SH) 3(OH) −2 species and argues against that of Zn(SH) 2Cl −2 2. Zn(SH) −3 also provides a model for Zn sites on the surface of solid ZnS, showing the geometric relaxation expected compared to the bulk. Hydration of the ZnS surface can be modeled by the species Zn(SH) 3(OH 2) −1. Such Zn and Cd species have large NMR shielding anisotropies, and the Cd species may be observable using modern NMR techniques.

Ab initio potential energy surfaces for unimolecular reactions during thermal conversion of acetylene

Unimolecular reactions involved in the pyrolysis of acetylene have been studied using ab initio molecular orbital-self-consistent field (MO-SCF) methods. A multiconfiguration (MC-SCF) method has proved adequate to describe the reaction studied. In the case of acetylene → vinylidene isomerization we also performed multireference double configuration interaction (MRDCI) calculations: we found that vinylidene is a real minimum on the surface and it could play an important role in other, more endothermic, reactions.

The bonding mechanism of NO to Cu(111) An ab initio molecular orbital cluster model study

Abstract Ab initio self consistent field molecular orbital (SCF-MO) wavefunctions have been obtained and analyzed for the interaction of NO above a Cu(111) surface, modelled by a five-atom cluster. The analysis shows that the bond is ionic, with charge transfer from the metal to NO which is chemisorbed as NO−, and that bent orientations are preferred. The N-O stretching frequencies for the atop, bridge and three-fold (fcc) adsorption positions of this cluster have been obtained and related to those found for the species experimentally observed by HREELS in the NO/Cu(111) system.