Showing papers on "HOMO/LUMO published in 1993"

‘‘Ab initio’’ liquid water

Abstract: An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules.

Electron excitations in solid C 60 : Energy gap, band dispersions, and effects of orientational disorder

Abstract: Electron excitation energies and photoemission spectra in undoped, solid ${\mathrm{C}}_{60}$ are calculated using a quasiparticle approach. The effects of orientational disorder and electron correlations are studied. We find a band gap of 2.15 eV, in good agreement with experiment, and \ensuremath{\sim}1 eV widths for the highest-occupied-molecular-orbital (HOMO) and lowest-unoccupied-molecular-orbital (LUMO) bands. Calculated angle-resolved, inverse photoemission spectra for the LUMO bands show very little angular dependence, explaining recent experimental work on epitaxial thin films. The present results suggest that undoped, solid ${\mathrm{C}}_{60}$ is a standard band insulator.

Pathways for photoinduced electron transfer within a mixed-metal bisporphyrin

Abstract: The photophysical properties of an oblique bisporphyrin, comprised of gold(III) and zinc(II) porphyrinic subunits separated by a 2,9-diphenyl-1, 10-phenanthroline spacer moiety, have been studied. Upon selective excitation of either porphyrin, rapid electron transfer occurs from the zinc porphyrin to the appended gold porphyrin and the ground-state system is restored by relatively slow reverse electron transfer. The rates of the various electron transfer steps correlate with both reaction exergonicity and temperature but only poorly with the solvent polarity because of limited applicability of the dielectric continuum model to this system and solvent-induced changes in redox potentials. The magnitude of electronic coupling between the reactants shows a marked dependence on the energy gap between relevant orbitals on the porphyrin and the spacer group, consistent with through-bond interaction. It is concluded that reaction via the zinc porphyrin excited singlet and triplet states proceeds via electron transfer through the LUMO of the spacer, while the gold porphrin triplet excited state reacts via hole transfer through the HOMO of the spacer moiety. Reverse electron transfer seems to show no preferred pathway and may involve through-space electron transfer. 43 refs., 6 figs., 5 tabs.

Photoelectron spectroscopy and electronic structure of Ca@C60

Abstract: The electronic structure of an isolated Ca@C60 molecule was probed by photoelectron spectroscopy (PES) of Ca@C 60 − , generated by laser desorption of a preformed Ca@C60/fullerene thin film. The PES spectrum of Ca@C60 was found to be similar to that of C 60 − , except that Ca@C60 has an electron affinity of 3.0 eV, about 0.3 eV higher than that of C60. The spectrum suggests that Ca atom donates its two 4s electrons to the C60 t lu lowest unoccupied molecular orbital. Thus, the interaction between the central Ca atom and C60 is quite ionic, and Ca@C60 can essentially be expressed as Ca2+@C 60 2− .

Infrared fingerprints of nine fullerene C82 isomers: a semiempirical prediction

Abstract: We use a combination of the quantum chemical force field for [pi] electrons (QCFF/PI) method and the modified neglect of differential overlap-parametrization method 3 (MNDO-PM3) to predict some properties of the nine isomers of C[sub 82] that satisfy the isolated pentagon rule. All the structural parameters are optimized with both methods. The two calculations furnish a coherent picture for the relative stabilities, energy ordering, and energy differences among the nine clusters. A C[sub 2] isomer is the most stable while the two C[sub 30] isomers are the least stable. The two methods also agree in the energy trends of the HOMOs and LUMOs. The nine C[sub 82] molecules have HOMOs at higher energies and LUMOs at lower energies than C[sub 60] and C[sub 70]. On a qualitative basis, they are therefore better electron donors and better electron acceptors. The HOMO-LUMO energy gap is smaller for the less stable species, with the energy of the LUMO more strongly affected upon changing the isomer. In an effort to contribute to the identification of the different isomers, we predict the infrared spectra of all nine molecules by a combination of the QCFF/PI force fields and MNDO-PM3 derivatives of the dipole moments. 27 refs.,more » 10 figs., 1 tab.« less

Theoretical analysis of the geometric and electronic structure of small-band-gap polythiophenes: poly(5,5'-bithiophene methine) and its derivatives

Abstract: Results of geometry optimizations and band-structure calculations are reported on poly(6,6'-bithiophene methine), a small-band-gap conjugated polymer made of alternating aromatic and quinoid thiophene rings. Derivatives are also investigated in which two methyl groups are added to, or a benzene ring fused on, positions 3 and 4 of either the aromatic or guinoid ring of the backbone. It is shown that the small band gap calculated for poly(5,5'-bithiophene methine), 1 eV, arises from a mixing of the highest occupied molecular orbital and lowest unoccupied molecular orbital wavefunctions of polythiophene chains possesing, on the one hand, totally aromatic and, on the other hand, totally quinoid geometric structures

Synthesis and crystal structure of [Co(CO)4(H7Si8O12)]. A new type of monosubstituted octanuclear silasesquioxane with a silicon–cobalt bond

Abstract: Reaction of octacarbonyldicobalt [Co2(CO)8] with octahydrosilasesquioxane H8Si8O12 in toluene leads to [Co(CO)4(H7Si8O12)], a new monosubstituted, octanuclear silasesquioxane with a silicon–cobalt bond. The product has been analysed by 1H, 13C, 29Si NMR and IR spectroscopies, mass spectrometry and microanalysis. The crystal structure of [Co(CO)4(H7Si8O12)] has been determined by single-crystal X-ray diffraction and is compared with the structures of H8Si8O12, [Co(CO)4(SiCl3)] and [Co(CO)4(SiF3)]. The H7Si8O12 fragment exhibits structural distortions consistently around, and originating from the Si atom to which the Co(CO)4 fragment binds. The remaining part of the cage essentially retains C3 symmetry, but is, nevertheless, in principle closer to the ideal Oh symmetry for this type of cage, than is crystalline H8Si8O12, for which a non-crystallographic molecular symmetry of Th is present. The comparison of the structures of the Co(CO)4 fragments shows that these are consistently distorted and deviate from ideal C3v symmetry. The distortions are essentially of the same kind in the three compounds and differ only in magnitude. The Si–Co distance in [Co(CO)4(H7Si8O12)] is 2.285 A. The experimental findings have been compared with extended-Huckel molecular orbital calculations. Interaction between the Co(dz2) and the Si(Pz) orbitals leads to the bond between the two fragments. Overlap population analysis indicates small but significant bonding interaction between Si and Ceq and antibonding interaction between the Co and the nearest O(Si). Fractional molecular orbital analysis indicates that the electronic structure of [Co(CO)4(H7Si8O12)] consists of three parts: orbitals belonging only to one of the two fragments and orbitals shared by both fragments. The highest-occupied molecular orbital (HOMO) of [Co(CO)4(H7Si8O12)] consists of oxygen lone pairs localised on H7Si8O12. The lowest-unoccupied molecular orbital (LUMO) is identical with the LUMO of Co(CO)4 and the first electronic transitions observed in the near UV are of H7Si8O12(oxygen lone pair) to Co(CO)4 fragment charge-transfer type.

Elastic and vibrationally inelastic cross sections and energy loss spectra for electron collisions with GeH4

Abstract: Absolute vibrationally elastic cross sections for e-GeH4 collisions have been determined for electrons of 1, 2, 2.5, 3, 5, 7.5, 10, 15, 20, 60 and 100 eV incidence energy over a scattering angular range of 10 degrees -130 degrees . The observed angular distributions correspond, at least qualitatively, to theoretical formulations using the continuum-multiple-scattering method, the parameter-free static-exchange-polarization approximation, and a multichannel Schwinger variational calculation. Vibrational excitation functions and the energy distribution of elastically scattered electrons reveal a shape resonance at about 2-2.5 eV. Calculations show that the resonance scattering state belongs to the t2 representation of the Td point group, which is also the representation of the lowest unoccupied molecular orbital (LUMO) of GeH4. Energy loss spectra recorded in the electronic threshold region of GeH4 reveal enhanced inelastic scattering at large angles in the loss range 7-9 eV characteristic of excitation to the lowest triplet state.

Electronic structure of Li(9‐crown‐3)2: A molecule with a Rydberg‐type ground state

Abstract: We report unrestricted Hartree–Fock (UHF) results for the molecule Li(9C3)2, obtained with several basis sets, including diffuse functions, up to 6‐31G++**. These results show that (1) the spatial distribution of the highest occupied molecular orbital (HOMO), or valence, electron is predominantly ‘‘outside’’ the Li(9C3)2 complex. That is, the complex is rather accurately described by the formula Li+(9C3)2e−; (2) the spin density at the Li nucleus drops by about three‐orders of magnitude, as compared to the free lithium atom; (3) the character of the ground state, and the first few excited states, is predominantly Rydberg‐type, centered on the lithium, with the valence electron ‘‘loosely’’ bound to the complex. We argue that this complex is thermodynamically stable, and show graphically what happens to the spin density as the 9‐crown‐3 (9C3) molecules complex the neutral lithium atom. As far as we are aware, this is the first example of a neutral molecule with a Rydberg‐type ground‐state. We also discuss the relation of our results to the properties of the crystalline electrides.

Estimation of electron affinity based on structure activity relationships

Abstract: Electron affinity for a wide range of organic molecules was calculated from molecular structure using the chemical reactivity models developed in SPARC. These models are based on fundamental chemical structure theory applied to the prediction of chemical reactivities for organic molecules strictly from molecular structure. The energy differences between the LUMO state and the HOMO state for a molecule of interest are factored into mechanistic components including the field, sigma induction and resonance contributions to these energy differences. The RMS deviation between observed and calculated electron affinities was found to be less than 0.14 e.v. for a large set of organic molecules.

Low-Energy Electronic States Related to Contact Electrification of Pendant-Group Polymers: Photoemission and Contact Potential Difference Measurement

Abstract: Low-energy electronic states, 3-6 eV below the vacuum level, relevant to contact electrification of pendant-group polymers were investigated by means of photoemission and contact potential difference (CPD) measurements under atmospheric conditions. As a result, it was shown that the thresholds of photoemission of the polymers are related to their highest occupied molecular orbital (HOMO) levels derived from modified neglect of diatomic overlap (MNDO) molecular orbital calculation, and that the thresholds exist near the energy corresponding to their work function inferred from CPD measurement. Furthermore, it was found that the contact electrification of the polymers is related to their threshold of photoemission. These results are interpreted in terms of a molecular ion model proposed by Duke et al. [Phys. Rev. B 18 (1978) 5717].

The Electronic Structure of One-Dimensional (1-D), 2-D, and 3-D Silicon Clusters

Abstract: The electronic structure of one-dimensional (1-D), 2-D, and 3-D silicon clusters have been calculated using the Discrete Variational (DV)- X α Molecular-Orbital method. The calculated results are discussed with respect to the effect of the size of the cluster on the energy level of molecular orbitals. The energy-gap ( E g ) between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) decreases with the increase of cluster size and the number of dimensions. It is found that including silicon 3 d orbitals as basis sets decreases the E g value. The results also show that the components of silicon 3 d orbitals in the unoccupied levels near LUMO are over 50 per cent. The calculated results predict that the E g value will be close to the band gap of crystalline silicon when a 3-D cluster contains more than 1000 silicon atoms.

A highly regioselective reaction of allylic acetates with silylated carbon nucleophiles directed by a sulfenyl group. Scope, limitation, and mechanistic aspects

Abstract: α-(Sulfenylmethyl)allyl acetates reacted with silylated carbon nucleophiles in the presence of a catalytic amount of TMSOTf to give products substituted at the α-position of the sulfenylmethyl group in moderate to good yields with high regioselectivity. The theoretical calculation on an intermediate cationic species indicated that an episulfonium ion was a stable form; the observed regioselectivity was rationalized qualitatively on the basis of the coefficients of LUMO of the cation. Some transformations of the products were also demonstrated.

Ring-bipolaron exciton in neutral fullerene C60

Abstract: Neutral fullerene C 60 is studied by the electron-lattice-coupling tight-binding model. When one or two electrons are excited from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of a C 60 molecule, the lattice relaxes and the lowest-energy state of C 60 is discovered to be a ringbipolaron exciton which is qualitatively consistent with the self-trapped polaron exciton observed in the luminescence experiment of neutral C 60 . The symmetry of optical excitation C 60 is reduced from I h to Dsa and it is proposed that four NMR lines would be observed in this material

Brominated short-chain basket-handle porphyrins and their copper(II) derivatives: spectral and electrochemical studies on the effect of β-substitution versus distortion

Abstract: Pyrrole-brominated short-chain basket-handle porphyrins and their copper(II) derivatives have been synthesized and characterized. These porphyrins are significantly distorted and the β-substitution by bromines further enhances the degree of distortion. Optical absorption and electrochemical sudies reveal considerable shifts in the energies of absorption maxima and electrode potentials. An analysis of these shifts indicates significant changes in the energies of the highest-occupied (HOMO)[a1u(π) and a2u(π)] and lowest-unoccupied molecular orbitals (LUMOs)[eg(π*)] of the porphyrin ring upon β-substitution and distortion. Specifically: (a) both distortion and β-substitution decreases the energy gap between the HOMO and LUMO due to different stabilization/destabilization mechanisms and (b) the magnitude of separation between a1u(π) and a2u(π) is large for β-substitution while it is small for distortion relative to the corresponding planar unsubstituted derivative. The absorption and redox potential shifts show a non-linear behaviour with the number of bromine substituents and this is ascribed to the combined effect of antagonistic inductive interactions and steric hindrance. ESR spectral studies indicate weakening of the Cu–N σ bond without much change in the electronic structure of Cu2+.

Use of a modified romberg formalism for the extrapolation of molecular properties from oligomers to polymers. Polyalanine diamide in its (γL)n or (C7)n conformation

Abstract: Computed torsional angles, HOMO/LUMO energies, total energy per unit, polymerization energy, dipole moment, and selected net charges are reported for the For-(Ala) 4 -NH 2 tetrapeptide in its (γ L ) 4 or (Cr 7 equ ) 4 conformation. These molecular properties are compared with those of shorter members of the oligomer series, and an extrapolation of the computed numerical values for infinite backbone length was attempted using a modified Romberg algorithm