Core hole induced charge-transfer reactions in disubstituted benzenes

The application of the charge-transfer concept on core electron ionization in donor–acceptor molecules is analyzed. A single excitation picture, involving the highest occupied and lowest unoccupied molecular orbitals is compared with a full π-valence orbital active space model. The connections between the notion of charge transfer in donor–acceptor species with the shake-up picture used for core photoelectron spectra of small molecules and with the dynamical screening concept applied on surface–adsorbate spectra are discussed.     

Metal-porphyrin orbital interactions in highly saddled low-spin iron(III) porphyrin complexes

Substituent effects of the meso-aryl (Ar) groups on the 1H and 13C NMR chemical shifts in a series of low-spin highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArP)L2]+, where axial ligands (L) are imidazole (HIm) and tert-butylisocyanide ((t)BuNC), have been examined to reveal the nature of the interactions between metal and porphyrin orbitals. As for the bis(HIm) complexes, the crystal and molecular structures have been determined by X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH2 proton as well as meso and CH2 carbon shifts are fairly small in the bis(HIm) complexes. Since these complexes adopt the (d(xy))2(d(xz), d(yz))3 ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal dpi orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3e(g)-like orbitals. A fairly small substituent effect is understandable because the 3e(g)-like orbitals have zero coefficients at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial HIm is replaced by (t)BuNC. The Hammett plots exhibit a large negative slope, -220 ppm, for the meso-carbon signals as compared with the corresponding value, +5.4 ppm, in the bis(HIm) complexes. Since the bis((t)BuNC) complexes adopt the (d(xz), d(yz))4(d(xy))1 ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled dxy orbital interacts with the specific porphyrin orbitals that have large coefficients on the meso-carbon atoms. Thus, we have concluded that the major metal-porphyrin orbital interaction in low-spin saddle-shaped complexes with the (d(xz), d(yz))4(d(xy))1 ground state should take place between the d(xy) and a(2u)-like orbital rather than between the dxy and a(1u)-like orbital, though the latter interaction is symmetry-allowed in saddled D(2d) complexes. Fairly weak spin delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the decrease in spin population in the d(xy) orbital due to a smaller energy gap between the d(xy) and dpi orbitals. In fact, the energy levels of the d(xy) and dpi orbitals are completely reversed in the complex carrying a strongly electron-withdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin complex with an unprecedented (d(xy))2(d(xz), d(yz))3 ground state despite the coordination of (t)BuNC.     

X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character.     

Orbital signatures of methyl in L-alanine

Molecular orbital signatures of the methyl substituent in L-alanine have been identified with respect to those of glycine from information obtained in coordinate and momentum space, using dual space analysis. Electronic structural information in coordinate space is obtained using ab initio (MP2/TZVP) and density functional theory (B3LYP/TZVP) methods, from which the Dyson orbitals are simulated based on the plane wave impulse approximation into momentum space. In comparison to glycine, relaxation in geometry and valence orbitals in L-alanine is found as a result of the attachment of the methyl group. Five orbitals rather than four orbitals are identified as methyl signatures. That is, orbital 6a in the core shell, orbitals 11a and 12a in the inner valence shell, and orbitals 19a and 20a in the outer valence shell. In the inner valence shell, the attachment of methyl to glycine causes a splitting of its orbital 10a' into orbitals 11a and 12a of L-alanine, whereas in the outer valence shell the methyl group results in an insertion of an additional orbital pair of 19a and 20a. The frontier molecular orbitals, 24a and 23a, are found without any significant role in the methylation of glycine.     

Possible spin-Peierls transition in La2RuO5

The semiconductor–semiconductor transition of La₂RuO₅ is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. This transition has lately been reported to lead to orbital ordering and a quenching of the local spin magnetic moment. Our results give strong hints for a different orbital ordering scenario than the one previously proposed. In our type of ordering the local S = 1 moment at the Ru sites is preserved in the low-temperature phase. The unusual magnetic behaviour is interpreted by the formation of spin ladders resulting from the structural transformations occurring at the transition. The spin ladders are characterized by antiferromagnetic coupling along the rungs. The loss of the total spin moment is attributed to a spin-Peierls transition.     

Substituent Effect on the π Linkers in Triphenylamine Dyes for Sensitized Solar Cells: A DFT/TDDFT Study

A series of metal‐free organic donor–π bridge–acceptor dyes are studied computationally using density functional theory (DFT) and time‐dependent DFT (TDDFT) approaches to explore their potential performances in dye‐sensitized solar cells (DSSCs). Taking triphenylamine (TPA) and cyanoacrylic acid moieties as donor and acceptor units, respectively, the effects of different substituents of the π linkers in the TPA‐based dyes on the energy conversion efficiency of the DSSCs are theoretically evaluated through optimized geometries, charge distributions, electronic structures, simulated absorption spectra, and free energies of injection. The results show that the molecular orbital energy levels and electron‐injection driving forces of the TPA dyes can be tuned by the introduction of substituents with different electron‐withdrawing or ‐donating abilities. The electron‐withdrawing substituent always lowers the energies of both frontier orbitals, while the electron‐donating one heightens them simultaneously. The efficiency trend of these TPA derivatives as sensitizers in DSSCs is also predicted by analyzing the light‐harvesting efficiencies and the free energies of injection. The following substituents are shown to increase the efficiency of the dye: OMe, OEt, OHe, and OH.     

Spectroscopic and computational study of β-ethynylphenylene substituted zinc and free-base porphyrins

A series of tetraphenylporphyrins appended at the β-pyrrolic position with an ethynylphenylene- or ethynylpyridine-substituent have been subjected to spectroscopic and density functional theory (DFT) analyses. The mean absolute deviation between corresponding experimental and DFT-derived vibrational spectra is up to 10.2 cm(-1), suggesting that the DFT B3LYP/6-31G(d) method provides an accurate model of the β-substituted porphyrin systems. The configuration interactions that give rise to prominent electronic absorptions have been calculated using time-dependant DFT (TD-DFT) and have been rationalized with reference to the energy and topology of DFT calculated molecular orbitals. As the electron withdrawing capacity of the β-substituent increases the LUMO orbital gains appreciable amplitude over the substituent moiety and is stabilised. This represents a departure from the assumptions underpinning the Gouterman four-orbital model, resulting in atypical electronic absorption spectra. This phenomenon is also manifested in the enhancement patterns of the resonance Raman spectra insofar as B-band excitation engenders an enhancement of substituent based modes. These observations demonstrate that the β-substituent exerts an appreciable electronic influence on the porphyrin π-electron system and provides a means of introducing charge-transfer character to prominent electronic transitions.     

Lead-poisoned zinc fingers: quantum mechanical exploration of structure, coordination, and electronic excitations

Density functional theory (DFT) structure calculations and time-dependent DFT electronic excitation calculations on simple mononuclear lead structures confirm recent reports on the stabilization of tricoordinated structural domains in poisoned proteins. However, the possibility of the formation of tetracoordinated lead complexes should not be disregarded in studies on mechanisms of lead toxicity because structures with both coordination modes are plausible and might contribute to observed UV spectra. Reported calculations along with detailed molecular orbital analysis confirm that the intense UV signal at around 260 nm is an indicator of the ligand-to-metal charge transfer (LMCT) band where the electrons are transferred from the sulfur 3p orbital to the lead 6p orbital. The composition of the LMCT band reveals significant excitations not only from the Pb-S bonding orbitals but also from sulfur lone-pair orbitals to the Pb-S antibonding orbitals for which the electron density is largely localized on the Pb "6p-like" molecular orbitals. There is a solid indication that the stereochemically active pair orbital of lead is not strongly hybridized and remains largely of the 6s character in tricoordinated lead structures and is minimally hybridized in tetracoordinated lead structures. Computed UV spectra of lead model complexes are compared to experimental UV spectra of model lead peptides. The comparison shows a good agreement with the major spectral trends and changes observed in these experiments.     

Photoinduced charge transfer in short-distance ferrocenylsubphthalocyanine dyads

Two new ferrocenylsubphthalocyanine dyads with ferrocenylmethoxide (2) and ferrocenecarboxylate (3) substituents directly attached to the subphthalocyanine ligand via the axial position have been prepared and characterized using NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopies as well as X-ray crystallography. The redox properties of the ferrocenyl-containing dyads 2 and 3 were investigated using the cyclic voltammetry (CV) approach and compared to those of the parent subphthalocyanine 1. CV data reveal that the first reversible oxidation is ferrocene-centered, while the second oxidation and the first reduction are localized on the subphthalocyanine ligand. The electronic structures and nature of the optical bands observed in the UV-vis and MCD spectra of all target compounds were investigated by a density functional theory polarized continuum model (DFT-PCM) and time-dependent (TD)DFT-PCM approaches. It has been found that in both dyads the highest occupied molecular orbital (HOMO) to HOMO-2 are ferrocene-centered molecular orbitals, while HOMO-3 as well as lowest unoccupied molecular orbital (LUMO) and LUMO+1 are localized on the subphthalocyanine ligand. TDDFT-PCM data on complexes 1-3 are consistent with the experimental observations, which indicate the dominance of π-π* transitions in the UV-vis spectra of 1-3. The excited-state dynamics of the dyads 2 and 3 were investigated using time-correlated single photon counting, which indicates that fluorescence quenching is more efficient in dyad 3 compared to dyad 2. These fluorescence lifetime measurements were interpreted on the basis of DFT-PCM calculations.     

Orbital-resolved partial charge transfer from the methoxy groups of substituted pyrenes in complexes with tetracyanoquinodimethane--a NEXAFS study

It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a(1) of the isolated methoxy group) and acceptor (orbitals b(3g), a(u), b(1g), and b(2u), all located at the cyano group) with π*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.     

The sign and magnitude of 2hJ(F,F) and 1hJ(F,H) in FH...FH. A CLOPPA analysis of their distance dependence

The sign change of the intermolecular (2h)J(F,F) coupling in the (HF)2 dimer as a function of the F-F distance is discussed by means of the CLOPPA method. It is found that it is due to the competition of positive and negative contributions involving the interaction of the sigma lone pair of the acceptor nucleus with vacant molecular orbitals localized in the F-H...F moiety and with other molecular orbitals localized in the donor molecule. The origin of the sign of each contribution is fully determined by analyzing the response of the electronic system to the magnetic perturbation at the acceptor F nucleus. (2h)J(F,F) coupling in the FH...F-, which is positive for all F-F distances, is also analyzed in order to look for the differences with the former case.     

带有噻吩侧基的有机硼小分子电子受体光伏材料

有机小分子电子受体材料的侧基能够影响异质结有机太阳能电池的给体/受体匹配和器件性能。我们设计并合成了一个硼原子带有噻吩侧基的有机硼小分子(MBN-Th)。该分子的LUMO离域在整个骨架上,HOMO定域在中心核上,其独特的电子结构使该分子具有两个强的吸收峰(波长分别为490和726nm),因此分子具有宽的吸收光谱和强的太阳光吸收能力。与苯基侧基相比,噻吩侧基使分子的HOMO能级下移0.1 eV,LUMO能级保持不变,进而引起分子带隙减小和吸收光谱蓝移20nm。基于该有机硼小分子受体材料的异质结有机太阳能电池,实现了4.21%的能量转化效率和300–850nm的宽响应光谱。实验结果表明,硼原子上的噻吩侧基是调控有机硼小分子光电性质的有效方法,可以用于有机硼小分子受体材料的设计。     

Valence orbital response to pseudorotation of tetrahydrofuran: a snapshot using dual space analysis

The pseudorotation of tetrahydrofuran (THF) (C(4)H(8)O) has been studied using density functional theory, with respect to the valence orbital responses to the ionization potentials and to orbital electron and momentum distributions. Three conformations of THF, the global minimum structure C(s), local minimum structure C(2), and a transition state structure C(1), which are characteristic configurations on the potential energy surface, are examined using the SAOP/et-pVQZ//B3LYP/6-311++G** models with the aforementioned dual space analysis. It is noted in the ionization energy spectra that the minimum structures C(s) and C(2) are not directly connected by pseudorotation, but through the transition state structure C(1). As a result, some orbitals of the C(s) conformer are able to "correlate" to orbitals of the C(2) conformer without a strict symmetry constraint, i.e., orbital 7a' of the C(s) conformer is correlated to orbital 5b of the C(2) conformer. It is also noted that although the valence orbital ionization potentials are not significantly altered by the pseudorotation of THF, their spectra (mainly due to excitation) are quite different indeed. Detailed orbital analysis based on dual space analysis is given. The valence orbital behavior of the conformations is orbital dependent. It can be approximately divided into three groups: the "signature group" is associated with orbitals experiencing significant changes. The frontier orbitals are in this group. The "nearly identical group" includes orbitals without apparent changes across the conformations. Most of the orbitals showing a certain degree of distortion during the pseudorotation process belong to the third group. The present study demonstrates that a comprehensive understanding of the pseudorotation of THF and its dynamics requires multidimensional information and that the information gained from momentum space is complementary to that from the more familiar coordinate space.     

Density functional perturbational orbital theory of spin polarization in electronic systems. II. Transition metal dimer complexes

We present a theoretical scheme for a semiquantitative analysis of electronic structures of magnetic transition metal dimer complexes within spin density functional theory (DFT). Based on the spin polarization perturbational orbital theory [D.-K. Seo, J. Chem. Phys. 125, 154105 (2006)], explicit spin-dependent expressions of the spin orbital energies and coefficients are derived, which allows to understand how spin orbitals form and change their energies and shapes when two magnetic sites are coupled either ferromagnetically or antiferromagnetically. Upon employment of the concept of magnetic orbitals in the active-electron approximation, a general mathematical formula is obtained for the magnetic coupling constant J from the analytical expression for the electronic energy difference between low-spin broken-symmetry and high-spin states. The origin of the potential exchange and kinetic exchange terms based on the one-electron picture is also elucidated. In addition, we provide a general account of the DFT analysis of the magnetic exchange interactions in compounds for which the active-electron approximation is not appropriate.     

Theoretical investigation of the magnetic interactions of Ni9 complexes

On the basis of density-functional theory (DFT) calculations, a theoretical analysis of the exchange interactions in Ni9L2(O2CMe)8{(2-py)2CO2}4, was performed, where L is a bridging ligand, OH- (1) or N3- (2). Each magnetic interaction between the Ni spin centers is analyzed for 1 and 2 in terms of exchange integrals (J values), orbital overlap integrals (T values) and natural orbitals. It was found that a J3 interaction, which is a magnetic interaction via the bridging ligand orbitals, mainly controls the whole magnetic properties, and the dominant interaction is a sigma-type orbital interaction between Ni dz2 orbitals. Further investigations on the magnetostructural correlations are performed on the J3 interactions using simplest Ni-L-Ni models. These models reproduced the magnetic interactions qualitatively well not only for the Ni9 complexes but also for other inorganic complexes. Strong correlations have been found between the magnetic orbital overlaps (T values) and the Ni-L-Ni angle. These results revealed that the difference of the magnetic properties between OH- and N3- is caused by the orbital overlap integral (T values) of the sigma-type J3 interaction pathway. The magnetic interactions are also discussed from a Hubbard model by evaluating the transfer integral (t) and on-site Coulomb integrals (U), in relation to the Heisenberg picture.     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of the phenol molecule in the gas phase was studied by X-ray emission spectroscopy (using the O-Kα and C-Kα spectra). MNDO calculations were performed, which made it possible to construct theoretical spectra and interpret experimental spectra. The structure of the molecular orbitals of phenol was compared with those of benzene and water. The π-interaction of the phenyl fragment with the oxygen-containing substituent was investigated. The contribution of the 2p atomic orbital of the oxygen atom to the π-HOMO of phenol is considerably less than that to lower-lying orbitals. For Part 3, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2187–2193, December, 1997.     

Proton Transfer Influence on Geometry and Electron Density in Benzoic Acid–Pyridine Complexes

The influence of the proton transfer on the geometry of donor and acceptor molecule in benzoic acid–pyridine complexes is investigated by theoretical calculations at the B3LYP/6‐311++G** level of theory. Systematic shifts of the H‐atom in the H‐bond are reflected in the geometry of the COOH group and the lengths of aromatic ring bond lengths of the proton acceptor. Changes in electron densities have been studied by atoms in molecules analysis. A systematic natural bond orbital analysis has been performed to study the proton transfer mechanism. Two donor orbitals are engaged in the proton transfer process which is accompanied by a change in orbital delocalization of H‐atom that can switch between two donor orbitals so the path of proton transfer in intermolecular H‐bond is not determined by the orbital shape. Theoretical results have been confirmed by experimental results published previously.     

Experimental and theoretical study of substituent effects of iodonitrobenzenes

The electronic structures and substituent effects of o-, m-, and p-iodonitrobenzene have been studied by ultraviolet photoelectron spectroscopy (UPS). The observed bands were interpreted on the basis of empirical arguments and theoretical calculations. The analysis of electronic effects of the donor/acceptor substituent groups is essential for the reliable assignment of the observed photoelectron spectra. The investigation of pi- and n-orbital ionization potentials enabled us to describe the correlation between substituent effects and the relative reactivities of the iodonitrobenzenes. It was found that the energy order of the pi(2) and n(II) parallel orbitals is reversed as a result of the combined influence of the electron-withdrawing nitro group and the electron-donating iodine atom. Distinct changes of the pi and n bands occur in o-iodonitrobenzene. This characteristic depends on the conjugation between the pi orbitals of the benzene ring and the nitro group and the interaction of in-plane lone pairs of iodine and one of the oxygen atoms of the nitro group in the adjacent position. This might contribute to the high reactivity of o-iodonitrobenzene in a number of reactions.     

Gauge invariance of the spin-other-orbit contribution to the g-tensors of electron paramagnetic resonance

The spin-other-orbit (SOO) contribution to the g-tensor (DeltagSOO) of electron paramagnetic resonance arises due to the interaction of electron-spin magnetic moment with the magnetic field produced by the orbital motion of other electrons. A similar mechanism is responsible for the leading term in nuclear magnetic-shielding tensors sigma. We demonstrate that analogous to sigma, paramagnetic DeltagSOO contribution exhibits a pronounced dependence on the choice of the magnetic-field gauge. The gauge corrections to DeltagSOO are similar in magnitude, and opposite in sign, to the paramagnetic SOO term. We calculate gauge-invariant DeltagSOO values using gauge-including atomic orbitals and density-functional theory. For organic radicals, complete gauge-invariant DeltagSOO values typically amount to less than 500 parts per million (ppm), and are small compared to other g-tensor contributions. For the first-row transition-metal compounds, DeltagSOO may contribute several thousand ppm to the g-tensor, but are negligible compared to the remaining deviations from experiment. With popular choices for the magnetic-field gauge, the individual gauge-variant contributions may be an order of magnitude higher, and do not provide a reliable estimation of DeltagSOO.