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PubMed2004

Symmetry and electronic structure of noble-metal nanoparticles and the role of relativity.

We present high resolution UV-photoelectron spectra of cold mass selected Cun-, Agn-, and Aun- with n=53-58. The observed electron density of states is not the expected simple electron shell structure, but is strongly influenced by electron-lattice interactions. Only Cu55- and Ag55- exhibit highly degenerate states. This is a direct consequence of their icosahedral symmetry, as is confirmed by density functional theory calculations. Neighboring sizes exhibit perturbed electronic structures, as they are formed by removal or addition of atoms to the icosahedron and therefore have lower symmetries. Gold clusters in the same size range show completely different spectra with almost no degeneracy, which indicates that they have structures of much lower symmetry. This behavior is related to strong relativistic bonding effects in gold, as demonstrated by ab initio calculations for Au55-.

Journal Article↗
PubMed2002

Electronic structure of the lowest excited triplet state of 5,12-naphthacenequinone.

Continuous-wave time-resolved EPR (cw-TREPR) and pulsed electron nuclear double resonance (ENDOR) studies have been carried out to clarify the electronic structure of the lowest excited triplet (Tl) state of 5,12-naphthacenequinone (5,12-NpQ) as well as 1,4-anthraquinone (1,4-AQ) and 6,13-pentacenequinone (6,13-PeQ). The Tl energy level and the D value of the zero-field splitting (ZFS) parameters only slightly decreased with the increasing pi-conjugated system. The Tl states of these linear para-acenequinones were assigned to the pi pi* character. In triplet 5,12-naphthacenequinone, more than 80% of the unpaired electron spins are localized on the naphthalene aromatic sub-system.

Anthraquinones↗
PubMed2005

Elucidating excited state electronic structure and intercomponent interactions in multicomponent and supramolecular systems.

Rational design of supramolecular systems for application in photonic devices requires a clear understanding of both the mechanism of energy and electron transfer processes and how these processes can be manipulated. Central to achieving these goals is a detailed picture of their electronic structure and of the interaction between the constituent components. We review several approaches that have been taken towards gaining such understanding, with particular focus on the physical techniques employed. In the discussion, case studies are introduced to illustrate the key issues under consideration.

Chemical Phenomena↗
PubMed2004

An exact reformulation of the diagonalization step in electronic structure calculations as a set of second order nonlinear equations.

A new formulation of the diagonalization step in self-consistent-field (SCF) electronic structure calculations is presented. It exactly replaces the diagonalization of the effective Hamiltonian with the solution of a set of second order nonlinear equations. The density matrix and/or the new set of occupied orbitals can be directly obtained from the resulting solution. This formulation may offer interesting possibilities for new approaches to efficient SCF calculations. The working equations can be derived either from energy minimization with respect to a Cayley-type parametrization of a unitary matrix, or from a similarity transformation approach.

Algorithms↗
PubMed2006

Electronic structure and stability of pentaorganosilicates.

The exceptional stability of recently reported pentaorganosilicates is investigated by bond energy analyses. Experimental coupling constants are used to probe their electronic structure, entailing bonds with mixed ionic-covalent character. Our analyses reconfirm that the axial bonds are more prone to heterolytic cleavage than are the equatorial bonds. Aryl substituents provide substantial electronic stabilization by charge delocalization, but cause steric crowding due to ortho-hydrogen repulsion. In contrast, silicates with two ax,eq biaryl groups are not congested. The remaining substituent is confined to an equatorial site, where it is insensitive to elimination. These concepts adequately explain the experimentally observed stability trends and are valuable for designing other stable pentaorganosilicates.

Journal Article↗
PubMed2005

The electronic structure of free water clusters probed by Auger electron spectroscopy.

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.

Journal Article↗
PubMed2005

Electronic structure and ionization energies of palladium and platinum N-heterocyclic carbene complexes.

Density functional methods have been used to calculate the geometries, electronic structure and ionization energies (IE) of N-heterocyclic carbene complexes of palladium and platinum, [M(CN2R2C2H2)2](M = Pd, Pt; R = H, Me, Bu t). Agreement with X-ray structures (R = Bu t) was good. Calculated IE agreed well with the photoelectron (PE) spectra (R = Bu t); metal bands were calculated to be within 0.25 eV of the experimental values, whereas the higher lying ligand bands deviated by up to 0.9 eV. Spin-orbit methods were needed to achieve this level of agreement for the Pt complex, but the calculations were found to underestimate the spin-orbit splitting somewhat. The principal metal-ligand bonding is between the carbene lone pair HOMO and a (d(z2)+ s) hybrid on the metal. The metal p(z) orbital contributes very little to the bonding. The metal d(xz,yz) orbitals mix primarily with the filled pi3 orbitals on the ligands and secondarily with the empty pi5 orbitals. Consequently they are little stabilized in comparison to the metal d(xy,x2- y2) orbitals, which are non-bonding in the complex. The first PE band for both the Pd and Pt complexes is from ionization of a (s - d(z2)) hybrid orbital. The IE is greater for Pt than for Pd on account of the post-lanthanide relativistic stabilization of the Pt 6s orbital.

Journal Article↗
PubMed2002

Electronic structure of reduced symmetry peripheral fused-ring-substituted phthalocyanines.

Reduced symmetry phthalocyanines are finding use in an increasing number of industrial applications. A detailed understanding of the electronic structure of the pi-system will greatly facilitate the design of new complexes, which fit the specifications required in many of these emerging high technology fields. NMR, electronic absorption, magnetic circular dichroism (MCD), and fluorescence emission and excitation spectra have been recorded for five generic metal phthalocyanine (MPc) derivatives in which additional benzene rings are fused either radially or obliquely onto at least one of the four peripheral benzo groups. The spectroscopy of four radially substituted compounds, zinc mononaphthotribenzotetraazaporphyrine (Zn3B1N), zinc monobenzotrinaphthotetraazaporphyrine (Zn1B3N), and two cis and trans zinc dibenzodinaphthotetraazaporphyrine (Zn2B2N) isomers, is compared to that of the obliquely fused structural isomer of Zn3B1N (Zn3BoN) and the D(4)(h)() symmetry parent compounds, ZnPc and zinc naphthalocyanine (ZnNc). The selection of Zn(II) as the central metal eliminates the possibility of charge transfer between the metal and ring. None of the complexes studied contain any sigma-bonded peripheral substituents. (1)H NMR signals of the seven compounds are assigned on the basis of the coupling patterns, integrated proton numbers, and decoupling experiments. The SIMPFIT program was used to perform spectral band deconvolution analyses of absorption and MCD spectra. ZINDO molecular orbital calculations are described, and the optical spectra are assigned on the basis of the MO models that have been developed previously to account for the spectral properties of metal porphyrin (MP(-2)) and metal phthalocyanine (MPc(-2)) complexes.

Algorithms↗
PubMed2005

O2-binding to heme: electronic structure and spectrum of oxyheme, studied by multiconfigurational methods.

We have studied the ground state of a realistic model of oxyheme with multiconfigurational second-order perturbation theory (CASPT2). Our results show that the ground-state electronic structure is strongly multiconfigurational in character. Thus, the wavefunction is a mixture of many different configurations, of which the three most important ones are approximately 1FeII-1O2 (70%), FeIV-2O2(2-) (12%) and 3FeII-3O2 (3%). Thus, the wavefunction is dominated by closed-shell configurations, as suggested by Pauling, whereas the Weiss 2FeIII-2O2- configuration is not encountered among the 10 most important configurations. However, many other states are also important for this multiconfigurational wavefunction. Moreover, the traditional view is based on an oversimplified picture of the atomic-orbital contributions to the molecular orbitals. Thus, the population analysis indicates that all five iron orbitals are significantly occupied (by 0.5-2.0 electrons) and that the total occupation is most similar to the 3FeII-3O2 picture. The net charge on O2 is small, -0.20 e. Thus, it is quite meaningless to discuss which is the best valence-bond description of this inherently multiconfigurational system. Finally, we have calculated the eleven lowest ligand-field excited states of oxyheme and assigned the experimental spectrum of oxyhemoglobin with an average error of 0.24 eV.

Electrons↗
PubMed1996

Electronic structure of bridging halogen atoms in metal halide dimers studied by the AM1 method and NQR spectroscopy.

A nuclear quadrupole resonance (NQR) study of the electronic structure of bridging halogen atoms in dimers of transition and non-transition metal halides has been performed. An explanation of the differences in the NQR frequencies for transition and non-transition metal halides is proposed. The population of the bridging halogen atom orbitals and the metal-halogen bond lengths have been estimated by the AM1 method. The aim of this paper was to obtain a relationship describing the lengths of the metal-halogen bonds.

Chemical Phenomena↗
PubMed2003

Electronic structure of carbon nanostripes.

Carbon nanostripes of graphene structure prepared on the stepped Ni(771) surface have been studied by angle-resolved photoemission. The electronic structure is anisotropic: parallel to the stripe direction, a graphite-type dispersion is measured, whereas the perpendicular direction displays two entangled band structures shifted in energy with respect to each other. These are experimentally identified as the microsurface-centered band structure and its umklapp scattered image caused by the superlattice.

Journal Article↗
PubMed2006

Potential energy landscape of the photoinduced multiple proton-transfer process in the green fluorescent protein: classical molecular dynamics and multiconfigurational electronic structure calculations.

The green fluorescent protein proton wire operating upon photoexcitation of the internally caged chromophore is investigated by means of classical molecular dynamics and multiconfigurational electronic structure calculations. The structure of the proton wire is studied for the solvated protein, showing that the wire is likely to be found in a configuration ready to operate as soon as the chromophore is photoexcited, and leading to a total of three proton translocations in the vicinity of the chromophore. Multiconfigurational CASSCF and CASPT2 calculations provide a detailed overview of the energy landscape of the proton wire for the ground electronic state S0, the photoactive 1pi pi* state, and the charge-transfer 1pi sigma* state. The results allow discussion of the operation of the wire in terms of the sequence of proton-transfer events and the participation of each electronic state.

Green Fluorescent Proteins↗
PubMed2005

The electronic structure of inorganic benzenes: valence bond and ring-current descriptions.

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X(6)H(6) (X = C (1), Si (2)), X(6) (X = N (3), P (4)), X(3)Y(3)H(6) (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B(3)Y(3)H(3) (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekulé-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekulé-like structures do not contribute. Of the heteronuclear compounds, only B(3)P(3)H(6) (6) has some benzene-like features with a significant contribution of two Kekulé-like structures to its VB wave function, an appreciable resonance energy, and a discernible diatropic ring current in planar geometry. However, relaxation of 6 to the optimal nonplanar chair conformation is accompanied by the onset of localization of the ring current.

Computer Simulation↗
PubMed2005

Electronic structures of trans-dioxometal complexes.

We have employed computational methods based on density functional theory to elucidate the effects of equatorial ligands on the electronic structures of trans-dioxometal complexes. In complexes with amine (sigma-only) equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) excitation energy increases with metal oxidation state: Mo(IV) < Tc(V) < Ru(vi) and W(IV) < Re(V) < Os(VI). Increasing transition energies are attributed to enhanced oxometal pi-donor interactions in the higher valent central metals. But in complexes with cyanide equatorial donors, the (1)A(1 g)(b(2 g))(2)-->(1)E(g)(b(2 g))(1)(e(g))(1) energy remains roughly independent of metal oxidation state, likely owing to the compensating increased pi-donation from the pi(CN) orbitals to the metal d(xy) orbitals as the oxidation state of the metal increases.

Journal Article↗