Electronic Isomers Research Articles

Valence Tautomerism: New Challenges for Electroactive Ligands

AbstractFor Abstract see ChemInform Abstract in Full Text.

Valence Tautomerism: New Challenges for Electroactive Ligands

AbstractValence tautomeric complexes combine redox‐active ligands and transition metal ions with two or more accessible oxidation states, exhibiting two nearly degenerated electronic states with localized electronic structures. Charge distribution in such electronic isomers has an appreciable sensitivity to the environment so an external perturbation, like photons, temperature and/or pressure, may lead to an intramolecular electron transfer between both redox active units and therefore to a reversible interconversion between the two degenerated electronic states. Moreover, since each electronic isomer exhibits different optical, electronic and/or magnetic properties, these complexes are being proposed as candidates for future use in molecular electronic devices and switches. Most of the valence tautomeric complexes reported thus far are based on quinone or quinone‐type ligands with a series of transition metal ions such as Co, Cu, Ni and Mn. Nevertheless, in the last few years, the number of electroactive ligands identified to be active in valence tautomeric complexes is being expanded by including new radical ligands such as polychlorotriphenylmethyl, phenoxyl or tetraphenylporphyrin radicals. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Structure, energies, vibrational spectra and reactions of the boron–silicon cluster molecules B2Si, BSi2 and B2Si2

Boron–silicon clusters are produced in various semiconductor-related processes. The exact structure and properties of these clusters are of great practical interest. No previous theoretical studies of these neutral clusters were found, and few experimental studies. The molecules B2Si, BSi2 and B2Si2 tend to form rings as the lowest energy isomers. These rings all have strong π bonding interactions. For each of the formulas there are also energetically low-lying excited electronic states and structural isomers. Standard heats of formation are obtained theoretically. These can be compared to experiment for BSi and BSi2. The B3LYP method gives results in good agreement with experiment for these molecules. From theoretical heats of formation it is predicted that formation of B2Si2 from the smaller clusters will be thermodynamically favoured.

Variable-temperature ion mobility time-of-flight mass spectrometry studies of electronic isomers of Kr2+ and CH3OH*+ radical cations.

Preliminary results from a liquid nitrogen-cooled ion mobility (IM) orthogonal-time-of-flight (o-ToF) mass spectrometer applied to the separation of electronic isomers of Kr2+ and methanol radical cations (conventional and distonic) are presented. Ab initio calculations were used to estimate the energies and energy barriers to interconversion between conventional (CH3OH*+) and distonic (CH2*OH2+) radical cations. In addition, computations and experiments are used to compare ion-neutral collision cross-sections for CH3OH*+ and CH2*OH2+ radical cations and suggest that the mobility separation is achieved by ion-neutral interactions between ions and neutral buffer gas.

Electronic structure and isomer shifts of neptunium compounds

The electronic structures of $\ensuremath{\alpha}\mathrm{Np}$ metal and 28 Np compounds are calculated with the generalized gradient approximation to density-functional theory, implemented with the full-potential linear-muffin-tin-orbital method. The calculations are compared to experimental isomer shifts providing a calibration of the ${}^{237}\mathrm{Np}$ isomeric transition with a value of $\ensuremath{\Delta}〈{r}^{2}〉=(\ensuremath{-}40.1\ifmmode\pm\else\textpm\fi{}1.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} {\mathrm{fm}}^{2}$ for the difference in nuclear radius between the excited isomeric level and the ground state. The isomer shift is primarily determined by the chemical environment. Decreasing the volume, either by external or chemical pressure, causes an $\stackrel{\ensuremath{\rightarrow}}{f}s+d$ charge transfer on Np, which leads to a higher electron contact density. The possible f-electron localization in Np compounds is discussed using self-interaction corrections, and it is concluded that f-electron localization has only a minor influence on the isomer shift.

Dynamics of wild-type HiPIPs: a Cys77Ser mutant and a partially unfolded HiPIP.

The temperature dependence of the mean square displacement of the (57)Fe nuclei due to motion faster than 100 ns are measured by temperature-dependent Mössbauer spectroscopy for oxidized and reduced HiPIPs from Ectothiorhodospira halophila, Chromatium vinosum WT and a Cys77Ser mutant. The behaviour is interpretable in the frame of the general model of protein dynamics distinguishing two temperature intervals. The character of harmonic and quasi-diffusional modes in HiPIPs is discussed. Dynamic information obtained from Mössbauer spectroscopy and Fe K-edge EXAFS are compared. Structure dynamics of the iron-sulfur cluster in the partially unfolded reduced HiPIP from C. vinosum was investigated by Mössbauer spectroscopy and EXAFS, indicating an intact metal centre and a protein backbone with a largely collapsed secondary structure. The role of the cofactor during protein folding is discussed. Differences in the dynamics between the native protein and the molten globule are found at physiological temperatures only. The structure and dynamic behaviour of the [Fe(4)S(4)]Cys(3)Ser cluster in the Cys77Ser mutant of the HiPIP from C. vinosum are analysed. The temperature dependence of electron relaxation in oxidized HiPIPs is investigated by Mössbauer spectroscopy and analysed theoretically, considering spin-spin and spin-lattice relaxation. The latter consists of contributions from direct phonon bottleneck and Orbach mechanisms. The data agree with former pulsed EPR results. Orbach relaxation is interpreted as due to transitions between electronic isomers of oxidized HiPIPs. With this interpretation, the energetic difference between both isomers equals the energy gap estimated from the temperature dependence of the Orbach relaxation.

Quantum Dots as Chemical Building Blocks: Elementary Theoretical Considerations

Quantum dots are clusters of atoms (or molecules) that are small enough that their electronic states are discrete. They can be prepared with a variety of compositions and covering ligands but are not quite identical. In particular, the dots will have a variable size. The study of the properties of individual dots is an active subject in its own right. Here we examine the electronic structure of assemblies of dots, where the dots are near enough that they interact. For the purpose of an elementary discussion, metallic dots are regarded as "atoms" with one valence orbital. The key point is that they are "designer" atoms because their electronic properties can be controlled through the synthetic method that is used to prepare the dots. Of direct concerns to us are the size of the dot and the nature of the ligands used to passivate the dots so that they do not coalesce. An important parameter is the energy cost, I, of adding an electron to a dot. The large size of the dots means that, unlike ordinary atoms, the Coulomb repulsion of the added electron is low. Other experimental control parameters are externally applied and include the ability to compress an assembly of dots, and thereby change the distance between them, or to subject them to static or alternating electromagnetic fields. The response to spectral probes for the electronic structure is discussed with special emphasis on new features, such as the onset of conjugation or the insulator-to-metallic transition, made accessible by the low charging energy of the dots. We propose a phase diagram of electronic isomers that can be accessed under realistic conditions.

Polylysine–porphycene conjugates as efficient photosensitizers for the inactivation of microbial pathogens

Porphycenes are electronic isomers of porphyrins which, when neutral, display no appreciable photosensitizing action towards Gram-negative bacteria. The covalent binding of oligomeric polylysine moieties, which are cationic at physiological pH values, endows porphycenes with a significant phototoxic activity against Gram-negative bacteria while retaining their photoefficiency against a variety of microbial pathogens, including Gram-positive bacteria, fungi and mycoplasmas. The effect of the polylysine moiety is dependent on both the polylysine concentration and the degree of oligomerization. A suitable interplay among the various parameters opens the possibility to obtain either a broad spectrum of antimicrobial activity or a selective action toward a specific pathogen while minimizing the damage to human fibroblasts.

Theoretical studies of azulene and its derivatives

The results of ab initio calculations with the 6-31G basis sets on azulene and its derivatives (including azulenequinones and diazoazulenequinones) are presented in accordance with considerations of their structures and bonding. Azulene is a non-alternant compound with ten π electrons and has either a Cs or C2v symmetry depending on the different carbon bonding. The semiempirical and HF ab initio calculations converge to a Cs symmetry and the DFT and MP2 calculations converge to a C2v symmetry as a ground state structure of azulene. The CIS calculations describe the excited state of azulene and the first excitation energy (S0 - S1) is 533 nm (CIS/6-31+G*), which could illustrate the azure color of azulene. According to the geometry analysis, there are 16 geometrical isomers in azulenequinone conjugated diketones of azulene. Ab initio calculation with the 6-31G basis set generates 1,5- and 1,7-azulenequinone being the most stable isomers of azulenequinone. Theoretically, the relative stability of the bromination product of azulenequinones indicates that 7-bromo-1,5-azulenequinone and 3-bromo-1,7- azulenequinone (for monobromoazulenequinones) and 3,7-dibromo-1,5- azulenequinone and 3,5-dibromo-1,7-azulenequinone (for dibromoazulenequinones) are more stable isomers. The product of diazotization of amino- bromoazulenes is diazoazulenequinone in which a diazo group replaces a ketone group. Isomeric 1,8- and 1,2-diazoazulenequinones are the most stable isomers of diazoazulenequinone according to the theoretical consideration. Due to the resonance and relative stability, diazoazulenequinone may easily extrude nitrogen and form the corresponding triplet ketocarbene intermediate and electronic isomers that undergo photoreaction with THF leading to a polyether bridged azulene (crown type ether). The cyclic reactions in diazoazulenequinone are also studied.Key words: azulene, azulenequinone, diazoazulenequinone, ab initio.

Ternary adduct {[W(CN)8]3−,[Pt(en)2]2+,[W(CN)8]3−} in aqueous solution and crystal structure of [Pt(en)2]3[W(CN)8]2·4H2O with infinite linear W · · · Pt chains

A new donor–acceptor system based on [W(CN)8]3− and [Pt(en)2]2+ has been characterised in aqueous solution and in the solid state. Spectroscopic studies indicate formation of the ternary adduct {[W(CN)8]3−,[Pt(en)2]2+,[W(CN)8]3−} in aqueous solution. The aggregate has been described as a mixed-valence species with weak-to-moderate coupling between metal centres. The solid [Pt(en)2]3[W(CN)8]2·4H2O exhibits the simultaneous presence of {WV, PtII, WV} ←→ {WIV, PtIV, WIV} electronic isomers and can be interpreted in terms of ground state borderline electronic localised-to-delocalised species. The structure of [Pt(en)2]3[W(CN)8]2·4H2O consists of one-dimensional infinite chains formed by tungsten and platinum moieties linked through hydrogen bonding from ethane-1,2-diamine nitrogens and water molecules to nitrogens of all cyano ligands.

Electronic isomers in [(CO2)nROH]− cluster anions. II. Ab initio calculations

Ab initio MO calculations have been performed for the [(CO2)nROH]− (R=H and CH3) anions with n=1 and 2. Three stable structures are found for [(CO2)H2O]−, and two structures for [(CO2)CH3OH]−. All the [(CO2)ROH]− structures are characterized by the charge localization on the CO2 moiety, which interacts with ROH through an O–H⋯O linkage. It is also revealed that the addition of ROH to CO2− leads to the formation of a potential barrier against autodetachment higher than that of a bare CO2−, which results in the increasing stability of [(CO2)ROH]− species. For n=2 the calculations predict the existence of two types of isomers having different degrees of the excess electron localization: CO2−⋅ROH(CO2) and C2O4−⋅ROH isomers. These “electronic isomers” are calculated to be close in energy, while their calculated vertical detachment energies (VDEs) differ by more than 1 eV. The ab initio results are discussed in comparison with recent experimental ones derived from photoelectron spectra of [(CO2)nROH]−.

Electronic isomers in [(CO2)nROH]− cluster anions. I. Photoelectron spectroscopy

Photoelectron spectra of [(CO2)n−1ROH]− (R=H and CH3) with 2⩽n⩽7 have been measured at a photon energy of 4.66 eV. Analysis of the photoelectron band envelopes has revealed that the spectra of [(CO2)n−1H2O]− with 3⩽n⩽5 consist of two band components. The maximum of each component corresponds to the vertical detachment energy (VDE) of the relevant anionic species. In each spectrum the VDE values for the two components differ by ≈1 eV. For example, the [(CO2)4H2O]− spectrum is characterized by two VDE values of 2.63±0.04 and 3.71±0.06 eV. From the VDE difference, we conclude that the observed two components arise from isomers having different electronic structures, and that these “electronic isomers” can be designated as C2O4−⋅H2O(CO2)n−3 and CO2−⋅H2O(CO2)n−2. Coexistence of electronic isomers occurs also in [(CO2)n−1CH3OH]−, but only at n=3. The [(CO2)n−1CH3OH]− anions with n≠3 display photoelectron spectra composed of a single broad band, which corresponds to photodetachment from CO2−⋅CH3OH(CO2)n−2 structure. The CO2−⋅ROH(CO2) isomers (n=3) are found to be distinguishable from C2O4−⋅ROH by selective photodepletion at 532 nm during the time of flight in the mass spectrometer, implying that the two electronic isomers of [(CO2)2ROH]− coexist without interconversion. In contrast, it is revealed that [(CO2)4H2O]− (n=5) are fluctuating between C2O4−⋅H2O(CO2)2 and CO2−⋅H2O(CO2)3 structures. Based on these experimental findings, combined with results obtained by recent ab initio calculations, a possible isomerization mechanism operative in [(CO2)n−1ROH]− is proposed.

4,4‘-Dipyridylamine-Bridged Binuclear Complexes of Pentacyanoferrate and Pentaammineruthenium

The oxidation of the mixed valence compound (CN)5FeIIpyNHpyRuIII(NH3)5 (M) proceeds first by the rapid equilibrium of M with its electronic isomer (CN)5FeIIIpyNHpyRuII(NH3)5 (M‘), followed by the o...

IR Sensing of the Electronic Structure in the Mixed-Valence States of Iron Carbonyl-Attached Biferrocene and Terferrocene

Abstract Ferrocene, 1,1,″-biferrocene and 1,1″ : 1′″,1″″-terferrocene derivatives involving a (cycopentadiene)Fe(CO)3 moiety were synthesized; the ν(CO) peak frequency shifts during their redox processes were monitored by the IRAS technique. All the complexes give two ν(CO) peaks, which shift to higher wavenumbers (12—18 cm−1) by the oxidation from the most reduced (all Fe’s are Fe(II)) to the most oxidized (all Fe’s are Fe(III)) forms. The biferrocene derivative in the monocationic form gives peaks at wavenumbers between those for neutral and dicationic forms, probably because the rate of electron-transfer affording two electronic isomers, Red–Ox–[FeCO] and Ox–Red–[FeCO], where Red, Ox, and [FeCO] refer to reduced and oxidized forms of a ferrocene unit, and a [(η4-cyclopentadiene)Fe(CO)3] moiety, respectively, is slower than or comparable to the time scale of IR (1011—1012 s−1). The terferrocene derivative shows ν(CO) frequency changes corresponding to the three-step 1e− oxidation processes. The shift is most significant in the second oxidation step, implying that the largest change in charge is that of the ferrocene unit nearest to Fe(CO)3. This is in principle consistent with the elucidation of the oxidation process via thermodynamically favorable electronic isomers based on the interaction energy between neighboring oxidized and reduced sites: Red–Red–Red–[FeCO] → Red–Ox–Red–[FeCO] → Ox–Red–Ox–[FeCO] → Ox–Ox–Ox–[FeCO].

Negative-ion photoelectron spectroscopy of (CS 2) n−: coexistence of electronic isomers

Photoelectron spectra of (CS 2) n − (1 ≤ n≤6) are measured at the photon energy of 4.66 eV. From the photoelectron spectra, the vertical detachment energy of (CS 2) n − is determined as a function of cluster size. The spectra for n ≥ 2 are found to consist of two different types of photoelectron bands which arise from isomers having different electronic structures. These ‘electronic’ isomers possess structures represented as CS 2 − ⋯ (CS 2) n−1 and C 2S 4 − ⋯ (CS 2) n−2 .

Manganese Phthalocyanine Coordination Chemistry: Recent Results and Present Status

A number of substituted manganese phthalocyanines ( PcMn ), which are easily soluble in organic solvents, have been synthesized with good yields (up to 76%) and high purity. PcMn with manganese +2 oxidation state without axial ligands was obtained for the first time and thoroughly characterized. It was shown that PcMn forms, PcMn III X , PcMn II or [ LPcMn III]2 O , depending on substituents nature, can be stable in usual conditions. It was found that some PcMn II derivatives can be oxidized by water to PcMn III OH . The stages of substituted PcMn electrochemical reduction and oxidation have been studied and the correlation of redox potentials with electronic properties of substituents has been established. For the first time, the idea has been proposed and confirmed that PcMn II, when treated by organic bases in solution forms an equilibrium mixture of three electronic isomers — Pc +· Mn I, PcMn II and Pc −· Mn III. A new scheme of PcMn photoreactions has been proposed and the possibility of using PcMn as a model of a photosystem component has been discussed. All the data are summarized in the complete scheme of photo and dark transformations between valence and coordination PcMn derivatives in solution.

Atomic design of π-conjugated metal complex oligomers and polymers

As a model to understand electronic communication between transition metal centres in π-conjugated metal complex polymers, redox properties of soluble oligo(1,1’-ferrocenylene)s were investigated. The dependence of redox potentials on the number of redox nuclei can be explained using not only the neighboring site interaction energies,U OR,U OO andU RR, but also the second neighbouring site interaction energy,U OXR. IR-spectroelectrochemistry of the (cyclopentadiene) Fe(CO)3-attached ferrocene, biferrocene and terferrocene has proved the theoretical prediction on the thermodynamically favorable forms of electronic isomers in the mixed valence states based on the neighboring site interaction energy.

Electronic Structure and Substitution and Redox Reactivity of Imidazolate-Bridged Complexes of Pentacyanoferrate and Pentaammineruthenium.

The mixed-valence compound [(NC)(5)Fe(II)-Im-Ru(III)(NH(3))(5)](-),M(i), was prepared in solution and as a solid sodium salt from [Fe(CN)(5)H(2)O](3)(-) and [Ru(NH(3))(5)Im](2+). The binuclear complex shows two bands at 366 nm (epsilon = 3350 M(-)(1) cm(-)(1)) and 576 nm (epsilon = 1025 M(-)(1) cm(-)(1)), assigned as LMCT transitions, as well as a near-IR band at 979 nm (epsilon = 962 M(-)(1) cm(-)(1)) associated with an intervalence transition. By calculation of the Hush model parameters alpha(2) and H(ab) (delocalization and electronic coupling factors, respectively), the complex is defined as a valence-trapped Fe(II)-Ru(III) system; this is confirmed by the measured redox potentials at -0.20 V and 0.30 V, associated with redox processes at the ruthenium and iron center, respectively. The formation stability constant of the mixed-valence ion was obtained through independent measurements of k(f) and k(d), the formation and dissociation specific rate constants, respectively. The stabilization of M(i) with respect to disproportionation into the isovalent states, as well as toward the formation of the electronic isomer, Fe(III)-Im-Ru(II), was also estimated. The fully reduced (R(i)) and fully oxidized (O(i)) binuclear complexes were prepared in solution and characterized by UV-vis spectroscopy. The kinetics of the reactions of R(i) and M(i) with peroxydisulfate were measured and a mechanistic analysis was performed, showing the relevance of electronic isomerization in completing the full conversion to O(i), through the assistance of the Ru(II)(NH(3))(5)(2+) center in the oxidation of the neighboring Fe(II)(CN)(5)(3)(-) moiety. The latter results are compared with those obtained with related complexes comprising different X(5)M-L moieties bound to Ru(II)(NH(3))(5)(2+). A linear correlation is displayed by plotting ln k(et) against E degrees (Ru), associated with the intramolecular oxidation rate constant of Ru(II) in the ion pair (binuclear species + peroxydisulfate) and the reduction potential of the corresponding Ru(III,II) couple in the ion pair.

Mixed-Valence Cyanopyridine-Bridged Complexes of Pentacyanoferrate and Pentaammineruthenium: Electronic Structure, Stability, and Redox Reactivity

The mixed-valence binuclear complexes, [(NC)5Fe(pyCN)Ru(NH3)5]n (4- and 3-cyanopyridine isomers, with nitrile-N and pyridine-N binding to Ru and Fe, respectively) were prepared as solid compounds through stoichiometric oxidation of the fully reduced (II,II) binuclear complexes, R, with peroxydisulfate. By analysis of IR spectra, the solids were observed to be a mixture of the predominant electronic isomers with a FeII, RuIII distribution, with minor amounts of the FeIII, RuII isomers. In aqueous solution, R was oxidized with peroxydisulfate to M, the mixed-valence complex, and to Ox, the fully oxidized complex. The M complex shows an intervalence band at 938 nm; by application of the Hush model, it is described as a valence-trapped RuII, FeIII complex; the latter electronic distribution is supported by UV−visible, electrochemical, and kinetic data, but a minor amount of the isomer with a FeII, RuIII distribution is also present in the equilibrium. The M complex is unstable toward dissociation and further ...

Observation and calculation of internal structure in scanning tunneling microscopy images of related molecules.

The scanning tunneling microscope has been used to distinguish among a series of related molecules on Pt(111) by details of their observed structures. The electronic isomers naphthalene and azulene can be recognized, as can three structural isomers of monomethylazulene, as well as dimethyl- and trimethylazulene. The number and position of substituents affect the diffusion rates, sticking coefficients, and orientations. A simple methodology based on extended H\uckel theory, including the substrate, generates electron and hole density plots which show very good agreement with the data.