Relative Orbital Energies Research Articles

Theoretical Investigations of Dye-Sensitized Solar Cells

This presentation considers theoretical investigations of dye-sensitized solar cells (DSSC). Theoretical methods were applied to investigate the interactions between titanium dioxide nanoparticles and sensitizers. The ONIOM model was used to obtain the geometries of different conformers of dye molecules with TiO2 and their binding energies. TD-DFT calculations were carried out to obtain the absorption spectra and the relative orbital energy levels of sensitizers and TiO2. The electronic couplings between different sensitizers and TiO2 were calculated using the fragment charge difference method. The redox potentials of the sensitizers are calculated to complete the full working cycle of a DSSC. We observed that the -COOH group is not the only possible binding site, and the sensitizers are more likely to be adsorbed horizontally on the TiO2 surface instead of being perpendicular to the surface having the -COOH group as a linker. The TiO2 nanoparticle was found to have minor influence on the absorptions of the sensitizers with the spectra shift smaller than 0.2 eV. TiO2 has more influence on the absorptions of softer and larger molecules because the interactions between sensitizers and TiO2 twist the conjugated chromophore structures. Compared to the neutral form, the deprotonated anion conformers of the sensitizers have larger binding energy and lower LUMO level against conduction band of TiO2. The gap between the LUMO of sensitizers and conduction band edge of TiO2 might indicate the coupling strength between the sensitizers and TiO2. Several binding groups have shown promising properties for interacting with the TiO2 nanoparticle and generally deprotonated anion forms of the dyes were strongly bonded to the TiO2 nanoparticle. The model and associated calculated results provide close agreement with experimental data and give crucial atomistic information of the relevant processes in dye-sensitized solar cells.

Fluorine Effects for Tunable C–C and C–S Bond Cleavage in Fluoro-Julia–Kocienski Intermediates

Classical Julia–Kocienski fluoroolefination represents an indispensable platform for the construction of monofluoroalkenes. Nevertheless, its complex multistep mechanistic manifold along with the u...

The Effect of Substitution of Gallium on the Transport Properties of Polycrystalline GeTe

We have investigated the electron transport properties of polycrystalline Ge1−xGaxTe alloys. Ga‐incorporation in GeTe has a nonmonotonic effect on its electrical conductivity which decreases slightly at low doping levels, but significantly at high doping levels (x 0.06) due to a concomitant reduction of hole‐concentration and mobility. Plausible Ga‐doping mechanisms determining these trends have been discussed. The Seebeck coefficient of GeTe does not enhance with Ga content as much as with Sb, Bi or In content at similar hole‐concentrations. Based on the theoretical Pisarenko plots and estimated effective carrier masses in Ge1−xGaxTe alloys, the observed behavior of Seebeck coefficient is rationalized on the basis of relative orbital energies and energy difference between valence band valleys in GeTe.

Luminescence Comparison of Homoleptic and Heteroleptic 6-membered Iridium(III) Complexes

The new iridium complexes, heteroleptic Ir(4-Me-2,3-dpq)2(LX) (4-Me-2,3-dpq = 4-methyl-2,3-diphenylquinolinato ligand; LX = 2-(2-hydroxyphenyl)benzoxazolate (BOX) and 2-(2-hydroxyphenyl)benzothiazolate (BTZ)) and homoleptic Ir(LX)3, were synthesized and their luminescence properties were investigated. The lithium salts of the ligands, BOX and BTZ, were previously applied to OLED as the blue fluoresecent dyes. These ligands can form 6-membered metallacycles with the metal center. Herein, BOX and BTZ were introduced as ligands to the iridium complexes for possible application to phosphorescent OLEDs. According to the previously reported literature, the ancillary ligand could have a profound effect on the emitting color of its complexes depending on the relative orbital energy levels of the main ligands (CˆN) and the ancillary ligand(LX). For the heteroleptic iridium complexes, 4-Me-2,3-dpq which could form a 5-membered metallacycle with the iridium center was introduced as a main ligand. We compared the luminescence and stability of the homoleptic and heteroleptic iridium complexes with respect to their structures. We investigated photoabsorption and photoluminescence properties of the iridium complexes and studied their band gaps with cyclic voltammetry. The emitting colors and the luminous efficiencies of the complexes were compared to find the involvement effect of the five- and six-membered rings in the complexes.

Electron delocalization in the S 1 and T 1 metal-to-ligand charge transfer states of trans -substituted metal quadruply bonded complexes

The singlet S(1) and triplet T(1) photoexcited states of the compounds containing MM quadruple bonds trans-M(2)(T(i)PB)(2)(O(2)CC(6)H(4)-4-CN)(2), where T(i)PB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I(')), and trans-M(2)(O(2)CMe)(2)((N[(i) Pr ])(2)CC ≡ CC(6)H(5))(2), where M = Mo (II) and M = W (II(')), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I(') but localized for II and II(') involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized (3)MLCT for I(') and localized (3)MLCT for II('). These differences arise from consideration of the relative orbital energies of the M(2)δ or M(2)δ* and the ligand π(∗) as well as the magnitudes of orbital overlap.

Tuning Aryl, Hydrazine Radical Cation Electronic Interactions Using Substitutent Effects

Absorption spectra for 2,3-diaryl-2,3-diazabicyclo[2.2.2]octane radical cations (2(X)(*+)) and for their monoaryl analogues 2-tert-butyl-3-aryl-2,3-diazabicyclo[2.2.2]octane radical cations (1(X)(*+)) having para chloro, bromo, iodo, cyano, phenyl, and nitro substituents are reported and compared with those for the previously reported 1- and 2(H)(*+) and 1- and 2(OMe)(*+). The calculated geometries and optical absorption spectra for 2(Cl)(*+) demonstrate that p-C6H4Cl lies between p-C6H4OMe and C6H5 in its ability to stabilize the lowest energy optical transition of the radical cation, which involves electron donation from the aryl groups toward the pi*(NN)(+)-centered singly occupied molecular orbital of 2(X)(*+). Resonance Raman spectral determination of the reorganization energy for their lowest energy transitions (lambda(v)(sym)) increase in the same order, having values of 1420, 5300, and 6000 cm(-1) for X = H, Cl, and OMe, respectively. A neighboring orbital analysis using Koopmans-based calculations of relative orbital energies indicates that the diabatic aryl pi-centered molecular orbital that interacts with the dinitrogen pi system lies closest in energy to the bonding pi(NN)-centered orbital and has an electronic coupling with it of about 9200 +/- 600 cm(-1), which does not vary regularly with electron donating power of the X substituent.

POTENTIAL UNUSUAL CHEMICAL BONDING IN MIXED METAL STRUCTURES OF THE 3d TRANSITION SERIES CONTAINING THE ELEMENTNi

A heuristic proposal is made in this communication that involves potentially unusual chemical bonding between the 3d transition series element Ni and the 3d transition series metals preceding it in the Periodic Table. The bonding in such mixed metal dimers, and their potential realizations as some simple three-dimensional (3D) extended structures in a one-to-one stoichiometry, as sphaelerite, rocksalt or CsCl structure-types, is proposed to include two principal components that involve either covalent or ionic contributions. Ordinary covalent contributions from a σ bond (or band) formed from overlap of singly occupied 4s atomic orbitals on each metal center are proposed to be operative in these molecules (or their extended realizations). However, in addition to this 4s–4s σ bond (band), an unprecedented transfer of a d electron from the incomplete, open 3d9subshell of the transition metal involved in the bonding that precedes Ni in the Periodic Table, into the 3d9subshell on the Ni center, which results in completion of this 3d subshell in Ni , is here implicated as a further driving force for the formation of these species. The resulting molecular and extended structures MNi , where M is a 3d transition metal preceding Ni in the Periodic Table (i.e. M = Sc , Ti , V , Cr , Mn , Fe and Co ), are therefore proposed to be bonded together by a combination of covalent and ionic forces, such that the Ni center acts as an anion of charge 1-, a so-called nickelide anion Ni-, while its counterpart is a univalent cation M+. This proposal is consistent with commonly employed electronegativity scales of chemical bonding, and with considerations of the relative orbital energies of the 3d subshells of the elements across the 3d transition series period, as well as with known trends in covalent and ionic bonding in the Periodic Table of the elements. It is also found to be in agreement with the spin multiplicities calculated in the nickel dimers MNi ( M = Sc , Ti , V , Cr , Mn , Fe and Co ) if Hund's rule is assumed to be operative.

Electronic Absorption and Magnetic Circular Dichroism Spectra for the Linear Two-Coordinate Bis(tri-tert-butylphosphine)platinum(0) Complex in 2-Methylpentane at 295 and 80 K

Electronic absorption and 8.0 T magnetic circular dichroism (MCD) spectra are reported for Pt(P(t-Bu)(3))(2) in 2-methylpentane solutions at 295 K and in glasses at 80 K. The intense bands observed in the 2.4-5.0 microm(-1) (1 microm(-1) = 10(4) cm(-1)) region are assigned as 5d --> 6p/metal-to-ligand charge-transfer transitions to dipole-allowed Sigma(u)(+) and Pi(u) spin-orbit states of 5d(10) --> 1pi(u) excited configurations. The enhanced resolution of the spectra at low temperatures reveals several previously unresolved spectral features in both absorption and MCD spectra, which strengthen the assignments and relative orbital energy ordering concluded from them.

Orbital control of π-facial selectivity in Diels–Alder reactions of cyclopentadienes having C(O)YR substituents at the 5-positions

π-Facial selectivity in Diels–Alder reactions of cyclopentadienes having C(O)YR substituents at the 5-positions was predicted on the basis of the orbital mixing rule and was substantiated experimentally. The selectivity was disclosed to be highly dependent on the relative orbital energy between π HOMO of the diene and n Y of the substituent.

Carbon–carbon π antibonding effects on the thermochemistry of alkanes, elucidated by angular overlap and MO calculations

It is postulated that the enthalpies of formation of alkanes can be rationalised quantitatively in terms of an additive bond energy scheme with a constant C–H term (taking the methane value of 415.8 kJ mol−1) while the C–C bond energy is dependent upon the π-antibonding effects of the highest occupied molecular orbitals (HOMOs). This antibonding effect can be treated by angular overlap methods with two adjustable parameters, eπ′(HH) (which represents the magnitude of the effect where the 2pπ orbitals concerned are engaged in σ-overlap only with hydrogen 1s orbitals) and eπ′(HC) (where one 2pπ orbital is σ-bonded to hydrogen atoms and the other to carbon atoms); eπ′(CC) is deemed to be zero, so that the C–C bond energy in the absence of any π antibonding effect takes the diamond value (357.4 kJ mol−1). The values for eπ′(CH) and eπ′(HH) are obtained from the experimental enthalpies of formation of C2–C6 alkanes having no 1,4 steric interactions. It is then possible to calculate bond energy terms for each of the types of bonds CX –CY [X, Y = P(primary), S(secondary), T(tertiary), Q(quaternary)]. These yield calculated enthalpies of formation for alkanes CnH2n+2 (n = 2–8) in excellent agreement with experiment in the absence of significant steric interactions. Corrections for 1,4 steric repulsion are needed for CS–CQ , CT–CT and CT–CQ bonds but not for CS–CT . The HOMOs in CH4−nMen (n = 2, 3) obtained from semi-empirical MO calculations (PM3) have the same relative orbital energies (b2>a1>b1 for n = 2, a1>e for n = 3) as found in the angular overlap calculations, and there is a clear correlation between the antibonding effects as measured by MO eigenvalues and the empirical thermochemical parameters. There is also an unexpectedly good agreement between the atomic fractional charges in alkanes calculated by Mulliken population analysis of the PM3 MOs and those obtained by electronegativity equilibration.

Perfluorocarbenes Produced by Thermal Cracking. Barriers to Generation and Rearrangement

Thermal crackings of substituted oxiranes to generate various perfluoroalkylcarbenes are examined using ab initio density functional theory. Such reactions are generalizations of a current technology for the preparation of difluorocarbene. Barriers for the generation of fluoro(perfluoroalkyl)carbenes by this approach are computed to be higher than those for generation of difluorocarbene; the difference is attributed primarily to the lower stability of the respective singlet carbenes. Once generated, however, the carbenes are reasonably stable with respect to unimolecular rearrangement, so that high selectivity for reaction with olefins, such as might be present in an unsaturated polymer, may be expected under typical experimental conditions. With respect to rearrangements of perfluorocarbenes, 1,2-alkyl shifts are lowest in energy, 1,2-fluorine atom shifts are higher, and 1,3-fluorine atoms shifts are highest of all; this ordering reflects the relative orbital energies of the sigma bonds broken in the respective migrations. In instances where thermal conditions required for oxirane cracking may be incompatible with other functionalities in the reaction medium, theory predicts that extrusion of SiF(4) from perfluorosilylalkanes may be an attractive alternative for perfluorocarbene generation.

Bond Characterization of Chromium−Fischer Carbene Complexes: A Combined Study of Experiment and Theory

Four pentacarbonyl chromium−carbene complexes, (CO)5CrC(XR‘)R, have been studied via X-ray diffraction and molecular orbital calculations. One of the carbene complexes (XR‘ = OCH3, R = −(C⋮CPh)) has been investigated extensively at 110 K by X-ray diffraction using Mo Kα radiation. The electron density distribution of this compound as well as the asphericity in electron density around the Cr atom is clearly demonstrated by deformation density and the Laplacian of electron density. The comparison between experiment and theory is made in terms of deformation density, topological properties, and d orbital populations of Cr. Further chemical bond characterization is based on quantum mechanical molecular orbital calculation and properties associated with bond critical points. The π bond character of a metal carbene can be best represented by a Cr−C−X three-centered four-electron bond with the π electron density mainly located at either the dyz orbital of Cr or the pz orbital of X in the carbene ligand. This makes the carbene carbon an electrophilic site in the pπ direction. The resemblances and differences between amino−(XN) and alkoxy−(XO) carbene complexes are of special interest. Because the energy of π*C-N orbital is fairly close to that of the π*C⋮O (carbonyl on Cr), the π bond character is delocalized toward the M−Ccarbonyl at the trans position i.e., OCM−CN in the amino carbene case; this is in accord with the shortening of bond lengths of the trans M−Ccarbonyl for many amino−carbene complexes. The π bond-delocalization is also illustrated by the Fermi hole distribution. The bond dissociation energies of these carbene complexes are calculated at the CASSCF level and with the density functional method (DFT). The relative orbital energies are also compared with photoelectron spectroscopy (PES) data. The values based on DFT using the transition-state approximation give the best agreement with the experimental results.

The calculation of the ligand field energies of cobalt porphyrins from E.S.R. data

Griffith's original equations were applied with modifications to the study of the E.S.R. of cobalt porphyrins. From the first-order equations for g ⊥, A ‖, and A ⊥, the relative orbital energies of d x 2 - y 2(b 1), d z 2(a 1), and dxz , dyz (e) could be calculated. The relative orbital energy of dxy (b 2) calculated from the second-order equation for g ‖ was always too high. This failure was attributed to the breakdown of the perturbation treatment due to the closeness of 2 E, 4 E and 4 A 2 excited states to the 2 A 1 ground state. The calculation of the g ‖ value from assumed value of the energy of b 2 led to the assignment of the signs of hyperfine coupling constants as A ‖ > 0, A ⊥ > 0 for uncoordinate cobalt porphyrins and A > 0, A < 0 for solvent coordinated complexes.

The electronic structures of the core and valence ion states of metal oxides

Abstract SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO 6 10− , AlO 4 5− and SiO 4 4− show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO 6 10− , CrO 6 9− and TiO 6 8− relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e g than for t 2g orbitals and increases as the separation of the e g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO 2 (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr 2 O 3 and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e g bonding → e g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure. A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO 2 . For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO 6 9− to FeO 6 10− to TiO 6 8− . as the O → M charge transfer energy declines. High energy XPS satellites in TiO 2 may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr 2 O 3 arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO 6 10− , CrO 6 9− and TiO 6 8− are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO 6 10− Fe 2p core hole state to Co 3 O 4 ) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e g → 3e g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.

Studies in eight-co-ordination. Part III. The single-crystal electronic spectrum, electron spin resonance, and magnetic anisotropy of, and crystal-field in, tetra-(6-aminohexanoic acid)copper(II) diperchlorate

The single-crystal polarised electronic spectra, e.s.r., and magnetic anisotropy of tetra-(6-aminohexanoic acid)copper(II) diperchlorate are reported. These properties are interpreted by use of a point-charge crystal-field model and are seen to be consistent with the relative orbital energies, dxy > dx2–y2 > dz2 > dxz yz. The crystal-field parameters obtained are shown to agree closely with those for CaCu(AcO)4,6H2O when allowance is made for the different Cu–O distances involved.