Change In Orbital Energy Research Articles

Stark shifts, band-edge transitions, and intrinsic optical dipoles in spherical InP quantum dots under electric fields

The band-edge electronic structure of spherical InP dots under electric fields is investigated using an atomistic pseudopotential approach. Field-induced changes in orbital energies, single-particle wave functions, transition intensities, and excitonic gaps are calculated. We find that interstate couplings profoundly affect the Stark shifts and optical transitions in InP quantum dots. As a result, band-edge transitions can be enhanced instead of being quenched by the field. We also demonstrate that both dipole and polarizability contributions should exist in the Stark shifts of zinc-blende dots.

Computational study of organized cycloaddition reactions with pyrrole moiety as a diene

Computational studies were performed to determine the reactivity of pyrrole as the diene in cycloaddition reactions, and to investigate the necessary chemical transformations that would make pyrrole an effective diene. Also, it was of interest to establish what chemical transformations would lead exclusively to an exo or to endo cycloadducts. The study was performed by using the pyrrole ring bond order uniformity, transition state six-membered ring bond order uniformity, frontier molecular orbital energy changes (inertia principle), and activation energies. Synthetic routes for the preparation of one isomer over another were proposed.

A Simple Molecular Orbital Treatment of the Barrier to Internal Rotation in the Ethane Molecule

The origin of the barrier to internal rotation in the ethane molecule is explored in terms of elementary molecular orbital (MO) considerations. Emphasis is placed on the antibonding effect, i.e. the result that an antibonding MO is more destabilized than its bonding counterpart is stabilized, relative to the parent atomic orbitals (AOs). It is shown that, in the case of two equivalent AOs, this effect is approximately proportional to the square of the overlap integral. By constructing the ethane Mos from those of two methyl fragments, it is shown that the most important orbital energy changes consequent upon rotation about the C-C bond can be expressed in terms of the antibonding effect arising from the filled twofold-degenerate p-bonding and -antibonding MOs. This can be reduced to the dependence on the rotation angle of the vicinal H-H overlap integrals, which are calculated explicitly, showing that the antibonding effect is minimised in the staggered conformation.

Electronic Structures of Metal Complexes Possessing Stacking Configurations of Conjugated Macrocycles.

Electronic structures of multilayered phthalocyanine (H2pc) complexes, including phthalocyaninato lanthanide double-decker complexes (dimers), triple-decker complexes (trimers), their radicals, and a tetramer biradical, were investigated through spectroscopic measurements, synthesis of new complexes and theoretical calculations. (1) Excited states of double-decker phthalocyaninato complexes were described using exciton-type and charge-resonance-type excitations. Assignments were given for split Q bands and B bands of lanthanide and tin phthalocyaninate dimers. (2) A new soluble crown-ether substituted trimer was synthesized to obtain detailed spectroscopic data. Previously unknown absorption bands were found in near IR region. The changes in the electronic spectra from a monomeric phthalocyaninato complex through the trimeric complex were accounted for. (3) In order to describe electronic structure of molecular assembly systems in a manner where roles of component units are explicitly represented, localized orbitals (LOs) defined to have a maximum population on a unit were used. The LOs were employed for CI calculations, thereby contributions of intra- and inter-ligand excitation s in excited states of the dimer and trimer were elucidated. Orbital energy changes on respective macrocycies of the dimer and trimer were evaluated using the LOs. (4) A hole in a radical species of the dimer was concluded to be delocalized over the macrocycles by a comparison of electronic spectra of a newly synthesized hetero-dimer, [Lu (nc) (pc) ] (H2nc: naphthalocyanine), and two homo-dimers. Based on the hole delocalization, a description of ground and excited states of the dimer radicals was given. (5) A trimer radical generated by chemical oxidation showed an intense band at 5000 cm-1. Energy ratio of the band to the counterpart of the dimer (ca.1/√2 ) as well as the difference of the behavior of the hole was explained. The model also correctly predicted the presence of a single intense band in Q-band region. (6) From a newly synthesized crown-ether substituted dimer radical, a supramolecular structure consisting of two dimer radicals was generated. It was shown from electronic and ESR spectra that the supramolecular structure was in biradical state where the two holes are localized on respective dimer sites.

Optimal control of a spacecraft with a solar sail

Analysis of the results indicates that: a) all the optimal trajectories may be divided into two sections: the phased section, corresponding to motion over a trajectory close to the initial one with a small change in initial energy (amounting to around two-thirds of the total journey time); and the orbital transfer, with considerable change in orbital energy, as a result of active control of the solar-sail orientation (around one-third of the total journey time). In the second section, the spacecraft first moves closer to the Sun and then makes the transfer to Mars orbit; b) the duration of orbital transfer is 581 dyas according to the optimal plan, which includes sections of deceleration and movement from the initial orbit over a distance of 0.2 dimensional length units toward the Sun; with the constraint ϑ<π/2, the deceleration is eliminated, and the distance toward the Sun is reduced (to 0.1 dimensionless units), with a corresponding increase in journey time; c) taking account of the nonideal reflecting surface (e=0.85 rather than e=1) increases the journey time to 615 days without change in the other characteristics of the orbital transfer.

Electron density distribution functions and the ASED–MO theory

AbstractA diatomic charge density distribution function may be partitioned into free atom and delocalization components. Integrating the electrostatic force on one of the nuclei as the atoms join to bond yields a repulsive atom superposition energy Er(R) and an attractive delocalization energy Ed(R) which, when added, equal the Born‐Oppenheimer potential energy, E(R). Bond stretch force constants may be calculated to quite good accuracy from the Poisson equation ▽E(R) = 4πZbpa(r = R), where Zb is the charge of nucleus b at a distance of R from nucleus a and pa is the charge‐density distribution function of isolated atom a. This equation follows if the delocalization density component is rigid during molecular vibrations and Ed = c/R, where c is a constant. Ed(R) is well approximated by the change in orbital energy, δEMO(R), for the bond‐forming reaction a + b → a ‐ b as obtained from a modified extended Hückel procedure. The resulting E() = Er(R) + δEMO(R) generalizes immediately to polyatomic molecules and solids and can be used for the calculation of structures and other properties and their molecular orbital interpretation. The current understanding of the atom superposition and electron delocalization molecular orbital (ASED‐MO) theory is presented in this article. It is pointed out that a parallel exists with the density functional theory, wherein the Born‐Oppenheimer potential function may be constructed from energy points E(R) that are functionals of the molecular charge density distribution function. Calculations of molecular structures with density functional theory has also become possible only through the introduction of electron orbitals. However, the approaches are otherwise different, for density functional theory works with the total energy of the molecule and the ASED approach works with the molecular binding energy obtained by integrating the electrostatic force on a nucleus, which is zero for the isolated atoms. © 1994 John Wiley &amp; Sons, Inc.

Theory of molecular orbital energy shifting induced by electrostatic external effects

The Self Consistent Reaction Field (SCRF) theory of electrostatic medium effects representation is used as the basis to derive the molecular orbital energy shifting induced by electrostatic external effects, coming from the interaction with a highly polarizable medium. A Perturbation–Variation methodology is used to perform the theoretical analysis. Under a first order perturbation approach, the shifting induced on the molecular electronic levels appears to be mainly dependent on the electronic polarization of the particular molecular orbital involved. As a result, it appears that the molecular orbital energy levels do not shift in a trivial constant amount by solvation, as it has been currently assumed. The result of our theoretical analysis shows that the solute electronic polarization term may be a relevant contribution to the molecular orbital energy changes induced by the environment. As an illustration of the usefulness of the present analysis, the alpha effect towards nucleophilic reactivity in solution of the OOH− and OH− ions is revisited.

Evolutionary scenarios for intermediate-mass stars in close binaries

view Abstract Citations (122) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolutionary Scenarios for Intermediate-Mass Stars in Close Binaries Iben, Icko, Jr. ; Tutukov, Alexander V. Abstract The consequences are explored of assuming that, in common envelope events involving mass transfer in close binaries, the energy required to eject the envelope from the system is comparable with the change in orbital energy of the system. Situations in which the primary fills its Roche lobe for the first time (1) after it has exhausted central hydrogen, but before it has ignited helium, and (2) after it has already developed an electron-degenerate carbon-oxygen core are studied in detail. The properties of a system consisting of a CO degenerate dwarf and a helium star, with angular momentum loss due to gravitational wave radiation maintaining the helium star in Roche lobe contact, are explored. Finally, the formation frequency of non-Roche lobe filling helium stars with companions of various types is examined to see whether they could account for some fraction of observed sdB and sdO stars. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/165011 Bibcode: 1987ApJ...313..727I Keywords: Astrophysics; Binary Stars; Mass Transfer; Stellar Evolution; Dwarf Stars; Helium; Roche Limit; Stellar Envelopes; Stellar Mass; Stellar Mass Accretion; Stellar Mass Ejection; Astrophysics; STARS: BINARIES; STARS: EVOLUTION full text sources ADS | data products SIMBAD (1)

Electronic structure studies on silylalkanes, alkylsilanes and silysilanes

The authors report ab initio calculations on the H, C, Si cluster molecules Si2H6, C2H6, SiH3CH3, Si5H12, (SiH3)4C, (CH3)4Si and C5H12. The method involved the use of pseudopotentials to replace the Si core. Total energy differences show that, at least at the SCF level of theory, Coulombic stabilisation largely compensates for loss of maximal overlap, thus rendering the C-Si bond very slightly stable against disproportionation. The orbital energy changes and charge populations imply that bonding differences can be understood very largely in terms of local interactions, so that the delocalisation properties occurring in silanes must arise from second-order interactions involving relatively low-lying excited states. The tetrasilyl methane molecule is predicted to be rather stable.

Photoelectron Spectra of Hydrogen-bonded Complexes

Abstract The photoelectron spectra of typical hydrogen-bonded complexes, CF3COOH–(C2H5)2NCH3, CF3COOH–(n-C3H7)3N, CF3CF2COOH–(C2H5)2NCH3, and CF3CF2COOH–(n-C3H7)3N were observed in the gas phase. It was found that the nonbonding orbital of the proton acceptor is stabilized significantly by hydrogen-bond formation while the three higher occupied orbitals of the proton donor are destabilized. The large orbital energy changes due to hydrogen-bond formation strongly support the charge-transfer model for the hydrogen bond.

An ab initio molecular orbital study of intramolecular H-bonding: 1,3 propanediol

An ab initio molecular orbital calculation has been carried out for three different conformations of 1,3 propanediol, one of which permits intramolecular H-bond studied by ab initio quantum mechanical methods. The ΔE for H-bonding formation is compated to be 0.9 kcal/mole and the charge redistributions and molecular orbital energy changes are compared to those found in intermolecular H-bonds.