Involving Charge Transfer Research Articles

Reactions of 7,7,8,8-Tetracyanoquinodimethane with Poly-ynyl Ruthenium and Iron Complexes

The reaction of tetracyanoquinodimethane (TCNQ) with Ru(C≡CC≡CH)(dppe)Cp* (5) at the outer (from Ru) C≡C triple bond gives η1-(butadienyl)ethynyl Ru{C≡CC[CH═C(CN)2]═C6H4═C(CN)2}(dppe)Cp (8), which reacts with a second equivalent of diynyl-Ru complex to give {Ru(dppe)Cp*}{C≡CC[═C6H4═C(CN)2]CH═CHC[═C(CN)2]C≡C}{Ru(dppe)Cp*} (9). The Ph-substituted complexes M{C≡CC≡CPh}(dppe)Cp* (M = Fe 6-Fe, Ru 6-Ru) and Ru{(C≡C)3Ph}(PPh3)2Cp (7) react with TCNQ to give the η1-(butadienyl)ethynyls M{C≡CC[CPh═C(CN)2]═C6H4═C(CN)2}(dppe)Cp (10-Fe, 10-Ru) and Ru{C≡CC[C(C≡CPh)═C(CN)2]═C6H4═C(CN)2}(PPh3)2Cp (11), respectively. Single-crystal X-ray diffraction molecular structure determinations for 8–11 have been carried out. In the Fe series, we suggest that the initial step of the mechanism involves electron transfer to form the [TCNQ]−• salt of the diynyl-iron cation, followed by C–C bond formation to give a zwitterionic intermediate. Isolated products can be rationalized by further reaction involving [2 + 2]-cycloaddition of one of the C═C(CN)2 groups of TCNQ to a C≡C triple bond of the metal poly-ynyl complex and a subsequent ring-opening reaction of the resulting (unobserved) cyclobutenyl intermediate. On the basis of X-ray diffraction data, redox potential determinations, and 57Fe Mössbauer and UV–vis spectroscopies, the electronic structures of the new compounds contain significant contributions from polarized mesomers involving charge transfer from the electron-rich metal–ligand fragment to the cyanocarbon ligand via the conjugated unsaturated carbon linker.

Observation of Extremely High Vibrational Excitation in O2 from Inelastic Scattering of Rydberg H Atom with O2.

The state-resolved differential cross sections for the Rydberg-atom (RA) inelastic scattering process H*(n = 46) + O2(v = 0, j = 1,3) → H*(n') + O2(v', j') have been measured by using the H-atom Rydberg tagging time-of-flight (HRTOF) technique. Extensive vibrational excitation of O2 products has been observed at the two collision energies of 0.64 and 1.55 eV. Experimental results show that the O2 products in the low vibrationally excited states are clearly forward-scattered, whereas those in the highly vibrationally excited states are mainly backward-scattered. Partially resolved rotational structures were also observed and assigned. The striking observation of extremely high energy transfer from translational to vibrational excitation at the backward direction could be explained involving charge transfer between proton and O2 molecule and possibly complex formation during the scattering process.

Contributions of Various Noncovalent Bonds to the Interaction between an Amide and S‐Containing Molecules

N-Methylacetamide, a model of the peptide unit in proteins, is allowed to interact with CH(3) SH, CH(3)SCH(3), and CH(3)SSCH(3) as models of S-containing amino acid residues. All of the minima are located on the ab initio potential energy surface of each heterodimer. Analysis of the forces holding each complex together identifies a variety of different attractive forces, including SH⋅⋅⋅O, NH⋅⋅⋅S, CH⋅⋅⋅O, CH⋅⋅⋅S, SH⋅⋅⋅π, and CH⋅⋅⋅π H-bonds. Other contributing noncovalent bonds involve charge transfer into σ* and π* antibonds. Whereas some of the H-bonds are strong enough that they represent the sole attractive force in several dimers, albeit not usually in the global minimum, charge-transfer-type noncovalent bonds play only a supporting role. The majority of dimers are bound by a collection of several of these attractive interactions. The SH⋅⋅⋅O and NH⋅⋅⋅S H-bonds are of comparable strength, followed by CH⋅⋅⋅O and CH⋅⋅⋅S.

Calculating Off-Site Excitations in Symmetric Donor-Acceptor Systems via Time-Dependent Density Functional Theory with Range-Separated Density Functionals.

Time-dependent density functional theory with range-separated hybrid functionals is used to calculate off-site excitations involving transitions between spatially separated orbitals in weakly coupled systems. Although such off-site excitations involve charge transfer, orbital degeneracy in symmetrical systems results in linear combinations of off-site excitations with equal weights and therefore zero net charge-transfer character. Like other types of off-site excitations, such "hidden" charge-transfer excitations are not accurately captured by conventional density functionals. We show that the recently introduced Baer-Neuhauser-Livshitz range-separated hybrid functional accurately characterizes such hidden off-site excitation energies via applications to the ethene dimer model system and to dye-functionalized silsesquioxanes.

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

AbstractTransition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge‐injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition‐metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor‐like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal‐oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Field-dependent exciton dissociation in organic heterojunction solar cells

In organic heterojunction solar cells, the generation of free charge carriers takes place in a multistep process which involves charge transfer (CT) states, that is, bound electron-hole pairs at the interface between donor and acceptor molecules. Past efforts to model the CT-state dissociation during solar cell operation were not able to consistently reproduce the experimentally observed field and temperature dependence. This discrepancy between model and experiment was partly due to the field-dependent free charge carrier collection process, which plays an important role in the widely used bulk heterojunction cell configuration and superimposes a possible field-dependent charge carrier generation process. In order to distinguish between generation and collection of free charge carriers, we propose the planar heterojunction cell configuration as a model system to study the field-dependent charge carrier generation process in organic heterojunction solar cells. We apply this model system to check current CT-state dissociation models against experimental data. Although the models can quantitatively account for the photocurrent's dependence on the applied voltage and the device thickness, they fail to account for the virtually negligible temperature dependence of the field-dependent charge-generation process. This discrepancy is traced back to a common feature of the models: an Arrhenius-like temperature dependence, distinctive of all processes involving a thermally activated jump over an energy barrier. As a solution to the problem, we introduce an exciton dissociation model based on a field-dependent tunnel process and demonstrate its consistency with the experimental observations. Our results indicate that the current microscopic picture of the charge-generation process in organic heterojunction solar cells being limited by the CT-state dissociation process needs to be reconsidered.

Reply to the ‘Comment on “Dynamic factors in the reactions between the magic cluster Al13− and HCl/HI”: A wavefunction instability problem.’ by Young-Kyu Han, Phys. Chem. Chem. Phys. 2012, DOI: 10.1039/C2CP23908F

The reaction between a metal cluster Al13− and an inorganic molecule HI involves charge transfer and density functional calculations has deficiency to estimate the activation barrier due to the self-interaction error. Coupled-cluster calculation with single and double excitation (CCSD) based on spin-unrestricted Hartree-Fock wavefunction can improve the calculated barrier and the choice of basis set is important for the calculation. Energy difference can be as large as 4.9 kcal mol−1 for the reaction by using aug-cc-pvdz and 6-31++G** basis sets respectively.

Benzoquinolateplatinum(ii) complexes as building blocks in the synthesis of Pt–Ag extended structures

The reaction between (NBu(4))[Pt(bzq)(C(6)F(5))(2)] (1, bzq = 7,8-benzoquinolate) and AgClO(4) in a 1 : 1 molar ratio, in acetone, gives the polymer [{Pt(bzq)(C(6)F(5))(2)}Ag](n) (2). The reaction of 2 with equimolecular amounts of PPh(3) and SC(4)H(8) (tht) produces the bimetallic complexes [{Pt(bzq)(C(6)F(5))(2)}AgL] (L = PPh(3) (3), tht (4)). For L = py, decomposition takes place and [Pt(bzq)(C(6)F(5))py] (5) is obtained. All these complexes have been characterized by X-ray diffraction. The most interesting features of complexes 2-4 is the presence of Pt-Ag bonds, with Pt-Ag distances of ca. 2.75 Å. Besides, the silver centres establish short η(1) bonding interactions with the C(ipso) of the bzq ligands, with distances Ag-C of ca. 2.45 Å. Complex 2 is a one-dimensional infinite chain in which the fragments "Pt(bzq)(C(6)F(5))(2)(-)" and Ag(+) alternate. On the other hand, complexes 1 and 3-5 show intermolecular pairing through π···π interactions between the aromatic rings of the bzq ligand, having interplanar separations of ca. 3.5 Å. Complex 2 dissolves in donor solvents (acetone, THF) as discrete bimetallic solvated fragments [{Pt(bzq)(C(6)F(5))(2)}AgS(n)] (S = solvent), similar to complexes 3 and 4. The persistence of the Pt-Ag bond in 2-4, supported by multinuclear NMR spectroscopy, causes a significant blue-shift in the lowest-lying absorption in relation to 1. This fact is attributed (TD-DFT) to a remarkable modification of the orbitals contributing to the HOMO, which changes the character of the transition from (1)LC/(1)MLCT in 1 to admixture (1)L'LCT/(1)MLCT in the bimetallic complexes. The low energy feature (490-530 nm) of 2 in solid state is attributed to CT from the Pt fragments to the Ag centers. Complexes 2-4 are only emissive in rigid media (solid and glasses). In the solid state, the metallic chain 2 exhibits a bright orange emission (560 nm, 298 K; 590 nm, 77 K), assigned to an excited state involving charge transfer from the platinum fragment with a remarkable contribution of C(6)F(5) (Ar(f)) rings to the Pt-Ag bond ((3)LMM'CT/(3)L'M'CT). However, 3 and 4 exhibit in solid state at 298 K a vibronic band, which is clearly resolved in two close non-equilibrated bands at 77 K in 3, tentatively ascribed to a mixture of (3)MLCT/(3)L'LCT transitions modified by the formation of the Pt-Ag bond. In glassy solution (77 K) 2-4 display a vibronic emission ascribed primarily to (3)LC character.

Significant enhancement in efficiency of NKX-2807 Coumarin dye by applying external electric field in dye sensitizer solar cell: Theoretical study

We present calculations of various properties of the ground and excited electronic states of Coumarin NKX-2807. An extensive Density Functional Theory (DFT) and Time Dependent DFT study of this sensitizer was performed, investigating on the electronic structure and optical properties of the dye both in the gas phase and in ethanol solution. The absorption bands are assigned to π → π ∗ transitions. We also examine the ground and excited-state dipole moments and polarizability of this Coumarin dye. Solvents (ethanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are studied in detail. Electric fields influence any process or transition that involves charge transfer, effect of a strong external electric field on the absorption spectrum is evaluated. The effect of the electric field is observed as a broadening of the absorption band if there is a change in the dipole moment and polarizability upon excitation. The results of this work show that field application, parallel to the dipole moment, can be very effective to increase DSSC efficiency. Our results open the possibility of computationally screening the various predictions on the electronic structure, optical response and in consequence their influences on the efficiency, thus paving the way to an effective molecular engineering of further enhanced sensitizers for solar cell applications.

A high-light-harvesting-efficiency of NKX-2593 and NKX-2883 Coumarin dyes in a local electric field: Can a local electric field enhance dye sensitizer solar cells efficiently?

The electronic structures and optical absorption spectra of organic dye sensitizers NKX-2593 and NKX-2883 were studied by using density functional theory (DFT), and the vertical excitation energies were calculated within the framework of the time-dependent DFT (TD-DFT) approach both in the gas phase and in ethanol solution. The solvent effects provide changes in both the geometries and the absorption spectra. Electric fields influence any process or transition that involves charge transfer, as a result of Stark effect. Stark spectroscopy is a general expression describing the study of spectral changes in electric fields, and it has been verified to be a broadly functional approach for characterizing the change in dipole moment and polarizability for electronic transitions. The results of this work show that field application, parallel to the dipole moment, can be very effective to increase DSSC efficiency. Our results open the possibility of computationally screening the various predictions on the electronic structure, optical response and in consequence their influences on the efficiency, thus paving the way to an effective molecular engineering of further enhanced sensitizers for solar cell applications.

Photoprotection in Plants Involves a Change in Lutein 1 Binding Domain in the Major Light-harvesting Complex of Photosystem II

Nonphotochemical quenching (NPQ) is the fundamental process by which plants exposed to high light intensities dissipate the potentially harmful excess energy as heat. Recently, it has been shown that efficient energy dissipation can be induced in the major light-harvesting complexes of photosystem II (LHCII) in the absence of protein-protein interactions. Spectroscopic measurements on these samples (LHCII gels) in the quenched state revealed specific alterations in the absorption and circular dichroism bands assigned to neoxanthin and lutein 1 molecules. In this work, we investigate the changes in conformation of the pigments involved in NPQ using resonance Raman spectroscopy. By selective excitation we show that, as well as the twisting of neoxanthin that has been reported previously, the lutein 1 pigment also undergoes a significant change in conformation when LHCII switches to the energy dissipative state. Selective two-photon excitation of carotenoid (Car) dark states (Car S(1)) performed on LHCII gels shows that the extent of electronic interactions between Car S(1) and chlorophyll states correlates linearly with chlorophyll fluorescence quenching, as observed previously for isolated LHCII (aggregated versus trimeric) and whole plants (with versus without NPQ).

Interaction between Encapsulated Excited Organic Molecules and Free Nitroxides: Communication Across a Molecular Wall

Communication between two molecules, one confined and excited (triplet or singlet) and one free and paramagnetic, has been explored through quenching of fluorescence and/or phosphorescence by nitroxides as paramagnetic radical species. Quenching of excited states by nitroxides has been investigated in solution, and the mechanism is speculated to involve charge transfer and/or exchange processes, both of which require close orbital interaction between excited molecule and quencher. We show in this report that such a quenching, which involves electron-electron spin communication, can occur even when there is a molecular wall between the two. The excited state molecule is confined within an organic capsule made up of two molecules of a deep cavity cavitand, octa acid, that exists in the anionic form in basic aqueous solution. The nitroxide is kept free in aqueous solution. (1)H NMR and EPR experiments were carried out to ascertain the location of the two molecules. The distance between the excited molecule and the paramagnetic quencher was manipulated by the use of cationic, anionic, and neutral nitroxide and also by selectively including the cationic nitroxide within the cavity of cucurbituril. Results presented here highlight the role of the lifetime of the encounter complex in electron-electron spin communication when the direct orbital overlap between the two molecules is prevented by the intermediary wall.

Pathways of diffusion mixed with subsequent reactions with examples of hydrogen extraction from hydride-forming electrode and oxygen reduction at gas diffusion electrode

This paper covers the role of the rate-determining step (RDS) in anodic hydrogen extraction from hydride-forming electrode. In general, hydrogen extraction from the electrode proceeds through the following steps: (1) hydrogen diffusion within the electrode, (2) hydrogen transfer from absorbed state to adsorbed state, (3) electrochemical oxidation of hydrogen to hydrogen ion involving charge transfer, and (4) hydrogen ion conduction through the electrolyte. In most theoretical and experimental investigations, it has been assumed that the RDS of anodic hydrogen extraction is hydrogen diffusion through the electrode. In real situation, however, the overall rate of hydrogen extraction is simultaneously determined by the rates of two or more reaction steps including hydrogen diffusion. The present work provides the overview of anodic hydrogen extraction in case that diffusion is coupled with interfacial charge transfer, interfacial hydrogen transfer, and hydrogen ion conduction through the electrolyte as well as the purely diffusion-controlled hydrogen extraction. In addition, the mixed controlled diffusion model was also exemplified with oxygen reduction at gas diffusion electrode of fuel cell system.

Role of nitrogen substitution in phenyl ring on excited state intramolecular proton transfer and rotamerism of 2-(2′-hydroxyphenyl)benzimidazole: A theoretical study

A comparative study of 2-(2'-hydroxy-3'-pyridyl)benzimidazole (2',3'-HPyBI), 2-(3'-hydroxy-4'-pyridyl)benzimidazole (3',4'-HPyBI), 2-(4'-hydroxy-3'-pyridyl)benzimidazole (4',3'-HPyBI), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (3',2'-HPyBI), and 2-(5'-hydroxy-4'-pyrimidinyl)benzimidazole (5',4'-HPymBI) with 2-(2'-hydroxyphenyl)benzimidazole (HPBI) was performed theoretically to evaluate the effect of nitrogen substitution in the phenolic ring on the photophysics and rotamerism of HPBI. Density functional theory (DFT) and configuration interaction singles (CIS) combined with time-dependent DFT were employed for ground and excited state studies, respectively. Different possible molecular forms were considered for each molecule viz., cis-enol, trans-enol, open-enol, and keto forms. The computational results revealed that cis-enol is the most stable form in the ground state for all the molecules except in 2',3'-HPyBI. In 2',3'-HPyBI, K-2 keto is the most stable form. Water molecule assisted interconversions between different forms of 2',3'-HPyBI were examined theoretically. Excitation and emission energies for all the forms have been calculated theoretically and the values are in good agreement with the available experimental data. The calculations show that intramolecular proton transfer (ESIPT) is endothermic in the ground state while it is exothermic in the first excited singlet state (except 5',4'-HPymBI). The barrier for the excited state ESIPT reaction increases with nitrogen substitution. Torsional rotation between the benzimidazole and the pyridinyl∕pyrimidinyl rings in the S(1) state depicts that twisted-keto structures involve charge transfer from the hydroxypyridinyl∕hydoxypyrimidinyl to the benzimidazole ring. However, the formation of twisted-keto is not energetically favored in these systems.

Role of Conformational Isomerism in Solvent-Mediated Charge Transfer in Chiral (S) 1,2,3,4-Tetrahydro-3-isoquinoline Methanol (THIQM): Condensed-Phase to Jet-Cooled Spectroscopic Studies

Intramolecular charge-transfer reaction in chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM) has been investigated in the condensed phase and in jet-cooled conditions by means of laser-induced fluorescence, dispersed emission, resonance-enhanced two-photon ionization, and IR-UV double resonance experiments, as well as quantum chemical calculations. In the condensed phase, THIQM only shows local emission in nonpolar and protic solvents and dual emission in aprotic polar solvents, where the solvent-polarity dependent Stokes shifted emission is ascribed to a state involving charge transfer from the nitrogen lone pair to the benzene π-cloud. Ab initio calculations reveal two low-energy conformers, which are observed in jet-cooled conditions. In the most stable conformer, THIQM(I), the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position, while the second most stable conformer, THIQM(II), corresponds to the opposite NH···O hydrogen bond, with the nitrogen lone pair in the axial position. The two low-energy jet-cooled conformers of THIQM evidenced from the laser-induced fluorescence and dispersed emission spectra only show structured local emission. Complexes with usual solvents reproduce the condensed phase properties. The jet-cooled complex with aprotic polar solvent acetonitrile shows both local emission and charge transfer emission as observed in solution. The jet-cooled hydrate mainly shows local emission due to the unavailability of the nitrogen lone pair through intermolecular hydrogen bonding.

Dynamic factors in the reactions between the magic cluster Al−13 and HCl/HI

The icosahedral Al is a "magic" cluster with remarkable stability due to its high symmetry and closed valence shells. Its reactivity has provided a molecular model for understanding oxidation and dissolution processes in bulk metals. By first principles calculations, we demonstrated the importance of dynamic factors in the Al + HX reactions, with HX being either HCl or HI. There was a barrier to the dissociative adsorption of HX on the surface of an Al cluster, which involved charge transfer from Al. Furthermore, the H atom could be bonded to the cluster in multiple ways, similar to the top, bridge and hollow adsorption sites on Al(111) surface. With a large amount of energy (∼40 kcal mol(-1)) deposited during the formation of Al(13)HX(-), the H atom could easily migrate among these sites, similar to the diffusion of hydrogen on metal surfaces. These factors were therefore important considerations in the formation and dissociation of Al(13)HX(-), and more generally in reactions involving other metal clusters.

Visible and ultraviolet spectroscopy of gas phase protein ions

Optical spectroscopy has contributed enormously to our knowledge of the structure and dynamics of atoms and molecules and is now emerging as a cornerstone of the gas phase methods available for investigating biomolecular ions. This article focuses on the UV and visible spectroscopy of peptide and protein ions stored in ion traps, with emphasis placed on recent results obtained on protein polyanions, by electron photodetachment experiments. We show that among a large number of possible de-excitation pathways, the relaxation of biomolecular polyanions is mainly achieved by electron emission following photo-excitation in electronically excited states. Electron photodetachment is a fast process that occurs prior to relaxation on vibrational degrees of freedom. Electron photodetachment yield can then be used to record gas phase action spectra for systems as large as entire proteins, without the limitation of system size that would arise from energy redistribution on numerous modes and prevent fragmentation after the absorption of a photon. The optical activity of proteins in the near UV is directly related to the electronic structure and optical absorption of aromatic amino acids (Trp, Phe and Tyr). UV spectra for peptides and proteins containing neutral, deprotonated and radical aromatic amino acids were recorded. They displayed strong bathochromic shifts. In particular, the results outline the privileged role played by open shell ions in molecular spectroscopy which, in the case of biomolecules, is directly related to their reactivity and biological functions. The optical shifts observed are sufficient to provide unambiguous fingerprints of the electronic structure of chromophores without the requirement of theoretical calculations. They constitute benchmarks for calculating the absorption spectra of chromophores embedded in entire proteins and could be used in the future to study biochemical processes in the gas phase involving charge transfer in aromatic amino acids, such as in the mediation of electron transfer or redox reactions. We then addressed the important question of the sensitivity of protein optical spectra to the intrinsic properties of protein ions, including conformation, charge state, etc., and to environmental factors. We report optical spectra for different charge states of insulin, for ubiquitin starting from native and denaturated solutions, and for apo-myoglobin protein. All these spectra are compared critically to spectra recorded in solution, in order to assess solvent effects. We also report the spectra of peptides complexed with metal cations and show that complexation gives rise to new optical transitions related to charge transfer types of excitation. The perspectives of this work include integrative approaches where UV-Vis spectroscopy could, for example, be combined with ion mobility spectrometry and high level calculations for protein structural characterization. It could also be used in spectroscopy to probe biological processes in the gas phase, with different light sources including VUV radiation (to probe different types of excitations) and ultra short pulses with time and phase modulation (to probe and control the dynamics of de-excitation or charge transfer events), and with the derivatization of proteins with chromophores to modulate their optical properties. We also envision that photo-excitation will play an important role in the future to produce intermediates with new chemical and reactive properties. Another promising route is to conduct activated electron photodetachment dissociation experiments.

Protein Free Energy Corrections in ONIOM QM:MM Modeling: A Case Study for Isopenicillin N Synthase (IPNS).

The protein environment can have significant effects on the enzyme catalysis even though the reaction occurs locally at the reaction center. In this paper, we describe an efficient scheme that includes a classical molecular dynamics (MD) free-energy perturbation (FEP) correction to the reaction energy diagram, as a complement to the protein effect obtained from static ONIOM (QM:MM) calculations. The method is applied to eight different reaction steps, from the O2-bound reactant to formation of a high-valent ferryl-oxo intermediate, in the nonheme iron enzyme isopenicillin N synthase (IPNS), for which the QM:MM energy diagram has previously been computed [ Lundberg , M. et al. J. Chem. Theory Comput. 2009 , 5 , 220 - 234 ]. This large span of the reaction coordinate is covered by dividing each reaction step into microsteps using a virtual reaction coordinate, thus only requiring ONIOM information about the stationary points themselves. Protein effects are important for C-H bond activation and heterolytic O-O bond cleavage because both these two steps involve charge transfer, and compared to a static QM:MM energies, the dynamics of the protein environment changes the barrier for O-O bond cleavage by several kcal/mol. The origin of the dynamical contribution is analyzed in two terms, the geometrical effect caused by the change in average protein geometry (compared to the optimized geometry) in the room temperature MD simulation with the solvent, and the statistical (entropic) effect resulting from fluctuations in the interactions between the active site and the protein environment. These two effects give significant contributions in different steps of the reaction.

Spatial distribution of electron densities during optical excitation ofC60

We calculate the spatial distribution of the oscillating electron density for the low energy optical excitationsin C60, using ab initio time-dependent density functional theory. We contrastthis with the unscreened free response of the Kohn–Sham eigenfunctions,in which the charge transfer is between hemispheres and the simple densityoscillation patterns are very similar for electrons outside and inside theC60 radius. In the screened case, the oscillation patterns involve charge transfer on the scale ofinteratomic distances. The exterior and interior densities play different roles in theoscillator strength and screening. Almost all of the dipole oscillation for opticalabsorption strength is from exterior electrons, a consequence of the superposition ofπ andσ electron oscillations. The lowest transition with appreciable strength is dominated by screening fromπ electrons.

Design of the lattice parameter of embedded nanoparticles

It is found that the bonding of nanoparticles in nancomposites can be influenced by interactions with a surrounding matrix phase. A model involving charge transfer between phases is presented, and supported by DFT-simulations. The model explains observations in nanocomposite nc-TiC x /a-C of additional interface states and lattice expansion of TiC x . It is suggested that this approach can be extended to other types of nanocomposites, and that it opens for new possibilities in materials design.