A novel density functional study of the ground state properties of a localized trinuclear copper(II,II,III) mixed-valence system

Herein we report the analysis of a mixed-valence localized trinuclear copper(II,II,III) cluster by density functional theory. We focused on two peculiar aspects of this system. First we investigated the triplet ground state potential energy surface on a model system. To this end we computed, on the [E multiply sign in circle e] adiabatic potential energy surface, the potential energy profile along the e mode and constructed ab initio the full potential energy surface (the so called Mexican hat), by a fitting procedure. Next, we analyzed the magnetooptical properties of the minimum energy structures. In particular, we applied the single determinant method to compute the full manifold of states arising from the highest occupied molecular orbitals (magnetic orbitals). This procedure yielded results in agreement with previous calculations and with the available experimental data when using a model closer to the X-ray structure or when directly dealing with the complete structure of the system.     

Protein-ligand structure and electronic coupling of photoinduced charge-separated state: 9,10-anthraquinone-1-sulfonate bound to human serum albumin

To elucidate how the protein-ligand docking structure affects electronic interactions in the electron-transfer process, we have analyzed time-resolved electron paramagnetic resonance spectra of photoinduced charge-separated (CS) states generated by light excitation of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin at a hydrophobic drug-binding region. The spectra have been explained in terms of the triplet-triplet electron spin polarization transfer model to determine both the geometries and the exchange couplings of the CS states of AQ1S(2-?)-histidine-242 radical cation (H242(+?)) and AQ1S(2-?)-tryptophan-214 radical cation (W214(+?)). For the CS state of the former, it has been revealed that, due to the orthogonal relationship between the singly occupied molecular orbitals of AQ1S(2-?) and H242(+?), the electronic coupling (5.4 cm(-1)) is very weak, contributing to the prevention of energy-wasting charge recombination, even at a contact edge-to-edge separation.     

New class of Non‐Kekulé radical polymethines: Theoretical study

A new class of open‐shell polymethine monoradicals, possessing nonbonding molecular orbitals (NBMO) and very low excitation energies, was investigated theoretically. The radicals are derivatives of the stable 2‐azaphenalenyl radical. Since in the ground state the NBMO is strictly localized within a molecular fragment, the electron transitions are connected with a substantial charge‐transfer. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Does charge carrier dimensionality increase in mixed-valence salts of tetrathiafulvalene-terminated dendrimers?

[structure: see text] In four new dendrimers terminated by 12 electroactive tetrathiafulvalenyl substituents, the tridimensional character of the inter- and intradendrimeric charge and electron transfer, and hence of the electroconductivity, is evidenced by examination of the electronic spectra of their corresponding neutral state and cation radical, dication, and mixed-valence salts, including a closed-shell anion.     

细菌光合反应中心突变体原初电子转移机理的理 论研究

利用量子化学DFT从头计算方法,计算经过突变的细菌光合反应中心HM202L原始电子给体和其他色素分子的电子结构,然后对其原初电子转移机理进行探讨。结果表明：1)超分子D-2A的HOMO主要是由定域在其组成单元BChl~L分子上的原子轨道组成,而它的LUMO主要是由定域在其组成单元MBPheo~M分子上的原子轨道组成。这表明它在基态的激发态时分别存在超分子内的电荷分离态[BChl~L^--MBPheo~M^+]和[BChl~L^+-MBPheo~M^-]。同时也说明了D-2A阳离子态的正电荷完全分布在组成单元细菌叶绿素分子BChl~L上,与实验事实相符。2)HM202L细菌光合反应中心原初电子转移反应存在由ABCha~L^h^*驱动的电子转移反应。     

Electronic reorganization triggered by electron transfer: The intervalence charge transfer of a Fe3+/Fe2+ bimetallic complex

The key role of the molecular orbitals in describing electron transfer processes is put in evidence for the intervalence charge transfer (IVCT) of a synthetic nonheme binuclear mixed‐valence Fe³⁺/Fe²⁺ compound. The electronic reorganization induced by the IVCT can be quantified by controlling the adaptation of the molecular orbitals to the charge transfer process. We evaluate the transition energy and its polarization effects on the molecular orbitals by means of ab initio calculations. The resulting energetic profile of the IVCT shows strong similarities to the Marcus' model, suggesting a response behaviour of the ensemble of electrons analogue to that of the solvent. We quantify the extent of the electronic reorganization induced by the IVCT process to be 11.74 eV, a very large effect that induces the crossing of states reducing the total energy of the transfer to 0.89 eV. © 2015 Wiley Periodicals, Inc.     

Intramolecular electronic communication in a dimethylaminoazobenzene–fullerene C60 dyad: An experimental and TD‐DFT study

An electronically push–pull type dimethylaminoazobenzene–fullerene C₆₀ hybrid was designed and synthesized by tailoring N,N‐dimethylaniline as an electron donating auxochrome that intensified charge density on the β‐azonitrogen, and on N‐methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4′ positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor‐bridge‐acceptor dyad were studied experimentally and by performing TD‐DFT calculations. The TD‐DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV‐vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the N?N group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n_(N?N) (nonbonding) and π _(N?N) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C₆₀ were analyzed by comparing the TD‐DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)‐N,N‐dimethyl‐4‐(p‐tolyldiazenyl)aniline (AZNME). The n_(N?N) → π*_(N?N) and π_(N?N) → π*_(N?N) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of π_(N?N) → π*_(N?N) excitation energy closer to the n → π*_(N?N) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid‐like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the π_(N?N) → π*_(N?N) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD‐DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Structure and properties of non-classical polymers. IV. Magnetic properties of polymers with superexchange interaction

It is shown that, for a new class of polymers, ferromagnetic superexchange may arise. The model polymers of the new class have specific electronic structure. In addition to the delocalized system of coupled π electrons, these polymers have singly occupied molecular orbitals localized within each monomer unit. The localized electrons are indirectly exchange coupled, mediated via delocalized orbitals. The resultant exchange interaction is ferromagnetically signed. It depends strongly on the energy gap of the delocalized π-electron system. The suggested model is close to the superexchange of some rare earth magnetics where magnetic f electrons interact indirectly due to delocalized s electron system. The theory of Ruderman, Kittel, Kasuya and Yosida is used in the quantitative treatment of the exchange interaction.     

Natural bond orbital analysis in the ONETEP code: Applications to large protein systems

First principles electronic structure calculations are typically performed in terms of molecular orbitals (or bands), providing a straightforward theoretical avenue for approximations of increasing sophistication, but do not usually provide any qualitative chemical information about the system. We can derive such information via post‐processing using natural bond orbital (NBO) analysis, which produces a chemical picture of bonding in terms of localized Lewis‐type bond and lone pair orbitals that we can use to understand molecular structure and interactions. We present NBO analysis of large‐scale calculations with the ONETEP linear‐scaling density functional theory package, which we have interfaced with the NBO 5 analysis program. In ONETEP calculations involving thousands of atoms, one is typically interested in particular regions of a nanosystem whilst accounting for long‐range electronic effects from the entire system. We show that by transforming the Non‐orthogonal Generalized Wannier Functions of ONETEP to natural atomic orbitals, NBO analysis can be performed within a localized region in such a way that ensures the results are identical to an analysis on the full system. We demonstrate the capabilities of this approach by performing illustrative studies of large proteins—namely, investigating changes in charge transfer between the heme group of myoglobin and its ligands with increasing system size and between a protein and its explicit solvent, estimating the contribution of electronic delocalization to the stabilization of hydrogen bonds in the binding pocket of a drug‐receptor complex, and observing, in situ, the n → π* hyperconjugative interactions between carbonyl groups that stabilize protein backbones. © 2012 Wiley Periodicals, Inc.     

The role of excited Rydberg States in electron transfer dissociation

Ab initio electronic structure methods are used to estimate the cross sections for electron transfer from donor anions having electron binding energies ranging from 0.001 to 0.6 eV to each of three sites in a model disulfide-linked molecular cation. The three sites are (1) the S-S sigma(*) orbital to which electron attachment is rendered exothermic by Coulomb stabilization from the nearby positive site, (2) the ground Rydberg orbital of the -NH(3)(+) site, and (3) excited Rydberg orbitals of the same -NH(3)(+) site. It is found that attachment to the ground Rydberg orbital has a somewhat higher cross section than attachment to either the sigma orbital or the excited Rydberg orbital. However, it is through attachment either to the sigma(*) orbital or to certain excited Rydberg orbitals that cleavage of the S-S bond is most likely to occur. Attachment to the sigma(*) orbital causes prompt cleavage because the sigma energy surface is repulsive (except at very long range). Attachment to the ground or excited Rydberg state causes the S-S bond to rupture only once a through-bond electron transfer from the Rydberg orbital to the S-S sigma(*) orbital takes place. For the ground Rydberg state, this transfer requires surmounting an approximately 0.4 eV barrier that renders the S-S bond cleavage rate slow. However, for the excited Rydberg state, the intramolecular electron transfer has a much smaller barrier and is prompt.     

Interpreting ultrafast molecular fragmentation dynamics with ab initio electronic structure calculations

High-level ab initio electronic structure calculations are used to interpret the fragmentation dynamics of CHBr(2)COCF(3), following excitation with an intense ultrafast laser pulse. The potential energy surfaces of the ground and excited cationic states along the dissociative C-CF(3) bond have been calculated using multireference second order perturbation theory methods. The calculations confirm the existence of a charge transfer resonance during the evolution of a dissociative wave packet on the ground state potential energy surface of the molecular cation and yield a detailed picture of the dissociation dynamics observed in earlier work. Comparisons of the ionic spectrum for two similar molecules support a general picture in which molecules are influenced by dynamic resonances in the cation during dissociation.     

Optimal virtual orbitals to relax wave functions built up with transferred extremely localized molecular orbitals

Extremely localized molecular orbitals (ELMOs), namely orbitals strictly localized on molecular fragments, are easily transferable from one molecule to another one. Hence, they provide a natural way to set up the electronic structure of large molecules using a data base of orbitals obtained from model molecules. However, this procedure obviously increases the energy with respect to a traditional MO calculation. To gain accuracy, it is important to introduce a partial electron delocalization. This can be carried out by defining proper optimal virtual orbitals that supply an efficient set for nonorthogonal configurations to be employed in VB-like expansions.     

双核簇合物M~2S~2(μ-S)~2(dtp)~2(M=Mo,W)的电子结构及元件组装活性的abinitio研究

本文对双核簇合物M~2S~2(μ-S)~2(dtp)~2(M=Mo, W)及其簇芯碎片M~2S~4^2^+进行了相对论赝势从头算和Boys定域化分子轨道计算, 根据所计算的正则分子轨道(CMO)和定域分子轨道(LMO)以及Mulliken布居分析, 探讨了作为簇元元件的M~2S~4(dtp)~2的成键性质和电子结构, 利用广义微扰理论, 定性分析了M~2S~4^2^+型簇合物的元件组装活性位和活性区, 研究了它们发生[2+1]=3和[2+2]=4型元件组装的成簇机理。     

Intervalence (charge-resonance) transitions in organic mixed-valence systems. Through-space versus through-bond electron transfer between bridged aromatic (redox) centers

Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar(*)(+)/Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3(*)(+) with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T(*)(+)/T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar(*)(+)/Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T(*)(+)/T centers by X-ray crystallography, (c) "doubling" of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a(*)(+) by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar(*)(+)/Ar redox centers. The direct comparison of intramolecular rates of electron transfer (k(ET)) between identical T(*)(+)/T centers in 3(*)(+) and 2a(*)(+)( )()indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3(*)(+) and 2a(*)(+)( )()from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus-Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7(*)(+), in which the aromatic redox S center is sterically encumbered by annulation.     

QUANTUM MECHANICAL ANALYSIS OF ELECTRONIC TRANSITIONS IN SOME CYANINE DYE PHOTOSENSITIZERS

Abstract— INDO and CNDO/2 calculations of some cyanine dye photosensitizers are reported. The redistribution of electron charge density following various electronic transitions is analyzed and found to be localized mainly in the conjugated chains. The charge redistribution is shown to be strongly influenced by presence of the counter ion, which also proves to be a main factor in determination of the ionization energy values and symmetry of the molecular orbitals. The results suggest an ionization process different from the one considered in previous approaches, because the electron is shown here to leave the negative ion, with the conjugated cation acting as a "photon trap". This situation is shown to provide a link between different models of optical sensitization. Results and assumptions from other methods, including the 'electron gas' models for the conjugated chain, are discussed in view of the counter-ion effect and the chain–localized transitions found here.