Electronic and vibrational spectra of mixed-valence complexes with a metal--halide chain

The electronic and vibrational spectra of compounds with a M(II)---X—M(IV) (where M = Pt, Pd, Ni and X = Cl,Br,I) chain are briefly reviewed and some new results reported. Emphasis is given on the size effects. The interpretation of the optical absorption spectra is based on the assumption that the gap energy increases as the size of the particles decreases. A red shift predicted for the resonance Raman excitation profiles is in agreement with the experiments.     

Quantification of the mixed-valence and intervalence charge transfer properties of a cofacial metal–organic framework via single crystal electronic absorption spectroscopy

Gaining a fundamental understanding of charge transfer mechanisms in three-dimensional Metal–Organic Frameworks (MOFs) is crucial to the development of electroactive and conductive porous materials. These materials have potential in applications in porous conductors, electrocatalysts and energy storage devices; however the structure–property relationships pertaining to charge transfer and its quantification are relatively poorly understood. Here, the cofacial Cd(ii)-based MOF [Cd(BPPTzTz)(tdc)]·2DMF (where BPPTzTz = 2,5-bis(4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole, tdc²⁻ = 2,5-thiophene dicarboxylate) exhibits Intervalence Charge Transfer (IVCT) within its three-dimensional structure by virtue of the close, cofacial stacking of its redox-active BPPTzTz ligands. The mixed-valence and IVCT properties are characterised using a combined electrochemical, spectroelectrochemical and computational approach. Single crystal electronic absorption spectroscopy was employed to obtain the solid-state extinction coefficient, enabling the application of Marcus–Hush theory. The electronic coupling constant, H_ab, of 145 cm⁻¹ was consistent with the localised mixed-valence properties of both this framework and analogous systems that use alternative methods to obtain the H_ab parameter. This work demonstrates the first report of the successful characterisation of IVCT in a MOF material using single crystal electronic absorption spectroscopy and serves as an attractive alternative to more complex methods due to its simplicity and applicability.

Gaining a fundamental understanding of charge transfer mechanisms in three-dimensional Metal–Organic Frameworks (MOFs) is crucial to the development of electroactive and conductive porous materials.     

Delocalization in platinum-alkynyl systems: a metal-bridged organic mixed-valence compound

A complex featuring two triarylamine redox centers bridged by Pt, trans-bis(triethylphosphine)-bis{4-[bis(4-methoxyphenyl)amino]phenylethynyl} platinum(II), has been synthesized as a model system for pi-conjugated Pt-containing polymers. Analysis of the intervalence charge-transfer band displayed by its mixed-valence monocation affords a quantitative assessment of electronic delocalization through the Pt bridge; this is found to be only slightly smaller than that determined for a benzene-bridged analogue. These results are supported by density functional theory calculations, which show that the active orbitals involved in the electron-transfer process in both cases have similar delocalization through the bridging unit.     

X-ray structure analysis and the intervalent electron transfer in organic mixed-valence crystals with bridged aromatic cation radicals

X-ray crystallography identifies the aromatic donor group D = 2,5-dimethoxy-4-methylphenyl to be a suitable redox center for the construction of organic mixed-valence crystals owing to its large structural change attendant upon 1e oxidation to the cation-radical (D*(+)). The combination of cyclic voltammetry, dynamic ESR line broadening, and electronic (NIR) spectroscopy allows the intervalence electron transfer between the redox centers in the mixed-valence system D-br-D*(+) [where br can be an aliphatic trimethylene or an aromatic (poly)phenylene bridge] to be probed quantitatively. Independent measures of the electronic coupling matrix element (H) for D/D*(+) electron exchange via Mulliken-Hush theory accord with the X-ray crystallographic data-both sufficient to consistently identify the various D-br-D*(+) according to the Robin-Day classification. Thus, the directly coupled biaryl D-D*(+) is a completely delocalized cation in class III with the charge distributed equally over both redox centers. The trimethylene- and biphenylene-bridged cations D(CH(2))(3)D*(+) and D(ph)(2)D*(+) with highly localized charge distributions are prototypical class II systems involving moderately coupled redox centers with H approximately equal to 400 cm(-1). The borderline region between class II/III is occupied by the phenylene-bridged cation D(ph)D*(+); and the X-ray, CV, and NIR analyses yield ambivalent H values (which we believe to be) largely a result of an unusually asymmetric (20/80) charge distribution that is polarized between the D/D*(+) redox centers.     

Femtosecond dynamics of electron transfer in a neutral organic mixed-valence compound

In this article we report a femtosecond time-resolved transient absorption study of a neutral organic mixed-valence (MV) compound with the aim to gain insight into its charge-transfer dynamics upon optical excitation. The back-electron transfer was investigated in five different solvents, toluene, dibutyl ether, methyl-tert-butyl ether (MTBE), benzonitrile and n-hexane. In the pump step, the molecule was excited at 760 nm and 850 nm into the intervalence charge-transfer band. The resulting transients can be described by two time constant. We assign one time constant to the rearrangement of solvent molecules in the charge-transfer state and the second time constant to back-electron transfer to the electronic ground state. Back-electron transfer rates range from 1.5 × 10¹² s⁻¹ in benzonitrile through 8.3 × 10¹¹ s⁻¹ in MTBE, around 1.6 × 10¹¹ s⁻¹ in dibutylether and toluene and to 3.8 × 10⁹ s⁻¹ in n-hexane.     

Crystal and electronic band structure of IrTe2: Evidence of anionic bonds in a CdI2-like arrangement

The Rietveld structure determination of IrTe₂ showed that this phase differs from the CdI₂?like arrangement, with Te–Te multiple bonds (mean d_Te-Te = 3.528 Å) corresponding to Te^?1.5 anions, in agreement with the charge balance Ir³⁺(Te^?1.5)₂, the Iridium ion being d⁶ low spin. This new polymeric-CdI₂ structural type is shared by several other tellurides, namely: RhTe₂, CoTe_1.7, NiTe_1-x, PdTe₂, and PtTe₂, and its occurrence explains the abnormally low c/a ratio (1.38) of the hexagonal cell parameters. IrTe₂ band structure shows a strong degree of hybridization and mixing between the d cationic and p anionic levels. From the negative total overlap population between neighbour Ir³⁺ cations at the Fermi level, it can be inferred that the metallic conductivity of the phase involves the Tellurium anions through their 3D network. The crystal orbital overlap population curves for the Te–Te bonds indicate that electrons donated to the compound should fill anti-bonding levels, allowing for a breaking of these links, and possibly restoring a classical anionic stacking.     

Molecular deformation and vibrational structure of the first U.V. band in linear polyenes

We try to reproduce the vibrational structure of the first U.V. band of linear polyenes, using the simple model proposed by McCoy and Ross. The Hückel orbitals give satisfactory results for the first terms of the series but a bad asymptotic behaviour when the dimension of the system increases. Bond alternation improves slightly the agreement with experimental spectra, but a discrepancy remains: we discuss its possible origin.

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Zusammenfassung Es wird versucht, die Schwingungsstruktur der ersten UV-Bande der linearen Polyene mit Hilfe des Modells von McCoy und Ross wiederzugeben. Die Hückel-Orbitale geben zufriedenstellende Resultate nur für die ersten Glieder der Serie, aber schlechtes asymptotisches Verhalten für große n. Bindungsalternierung verbessert die Übereinstimmung geringfügig. Der Ursprung der Diskrepanz wird zu erklären versucht.

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Résumé Nous essayons de rendre compte de l'intensité des bandes vibrationnelles de la première transition U.V. des polyenes linéaires, à l'aide du modèle simple de McCoy et Ross. La méthode de Hückel donne des résultats satisfaisants pour les premiers composés de la série, mais un mauvais comportement asymptotique quand la dimension du systeme augmente. L'alternance des liaisons améliore légèrement les résultats mais laisse subsister une divergence pour les chaînes longues: nous discutons l'origine possible de cette divergence.

     

Extraction-spectrophotometric determination of anionic surfactants with a flow-injection system

Anionic surfactants are preconcentrated from 5-ml samples by extraction as the ion-pair with ethyl violet into toluene. The absorbance of aliquots of the toluene phase is measured at 610 nm in a flow-injection system. A phase converter is located prior to the injection valve to convert a water stream to, pumped with ordinary pump tubing, to a toluene stream. The working range was 0.01–1.0 mg l^?1 and the reproductibility (r.s.d, n = 10) was 2% for 0.4 mg l^?1 sodium dodecyl sulphate. The non-aqueous flow-injection system serves to miniaturize the extraction from separatory funnel (200 ml) to test tube (10 ml) scale without loss of precision or validity.     

The vibrational spectra of complex oxides with a perovskite-type structure

Some aspects of the crystal chemistry and vibrational spectroscopy of complex oxides with a perovskite-type structure have been considered. The results of investigations of rhenium, tungsten, niobium, and tantalum complex oxides with a perovskite-type structure by vibrational spectroscopy methods (IR absorption, spontaneous Raman scattering) have been surveyed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2062–2070, December, 1994.     

An energy-decomposition technique for the analysis of band structure and its application to polyphosphazene and its halogenated derivatives

An energy-decomposition technique is presented for the analysis of band structures of polymers and is applied to the π orbitals of polyphosphazene and its halogenated derivatives. In this treatment the orbital energy is decomposed into the contributions from individual atoms and bonds in the structure. Thereby, the shape of the energy band is elucidated as a function of atomic orbital interactions as well as structures of polymers. The trends of the k dependences for the individual contributions thus analyzed are consistent with those to be expected from the characters of the crystal orbitais. This method seems to be suitable for determining the effects of various substituents on the shape of the energy bands of polymers.     

A theoretical study of the second band of the photoelectron spectrum of benzene with an analysis of the vibrational structure

Ab initio calculations have been performed to study the second band of the photoelectron spectrum of benzene with analysis of the vibrational structure. The ²E_{2 g} and ²A_{2 u} states contribute to this band. In this study each contribution to the band is discussed. We propose that the onset of the band should be assigned to the 0-0 transition of the ²A_{2 u} state. Received: 20 August 1998 / Accepted: 15 January 1999 / Published online: 7 June 1999     

A new organically-templated cobalt borophosphate with a novel borophosphatic anionic partial structure

A novel cobalt borophosphate, (NH₄)₂(C₄H₁₂N₂)[Co₂B₄P₆O₂₄(OH)₂]·H₂O with the mixed cations has been synthesized under mild hydrothermal conditions. Its crystal structure was determined by the single-crystal X-ray diffraction (tetragonal, I4₁/a (no. 88), a=14.207(3) Å, c=24.956(6) Å, V=5037.09(2) Å³, Z=8). The crystal structure consists of a new type of three-dimensional borophosphatic anionic partial framework, which is built from the condensation of the fundamental building unit (FBU) [B₂P₃O₁₄(OH)]. The CoO₆ octahedra are enchased in such borophosphate network to form a complex open framework with a three-dimensional intersecting channel system, the voids of which are occupied by ammonium, dipronated piperazine ions and water molecules, respectively. The magnetic measurement of the title compound has also been investigated.     

Anharmonic vibrational spectroscopy calculations with electronic structure potentials: comparison of MP2 and DFT for organic molecules

Density functional theory (DFT) technique is the most commonly used approach when it comes to computation of vibrational spectra of molecular species. In this study, we compare anharmonic spectra of several organic molecules such as allene, propyne, glycine, and imidazole, computed from ab initio MP2 potentials and DFT potentials based on commonly used BLYP and B3LYP functionals. Anharmonic spectra are obtained using the direct vibrational self-consistent field (VSCF) method and its correlation-corrected extension (CC-VSCF). The results of computations are compared with available experimental data. It is shown that the most accurate vibrational frequencies are obtained with the MP2 method, followed by the DFT/B3LYP method, while DFT/BLYP results are often unsatisfactory. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Abnormal inversion splitting in NH2D: rotational analysis of the nu5 bending vibrational band system

A high-resolution Fourier transform infrared spectrum of the nu(5) bending vibrational band system region of the partially deuterated ammonia molecule NH(2)D has been measured and rotationally analyzed. The spectrum consists of strong a-type transitions between the states of same vibrational symmetry and weaker c-type transitions between the states of different vibrational symmetry. The Hamiltonian model used includes interaction terms between the rotational states of both upper and lower inversion doublets. The vibrational term values for the symmetric and the antisymmetric component of the upper-inversion doublet are 1,605.637 965(620) cm(-1) and 1,590.993 82(100) cm(-1), respectively, where the numbers in parentheses are one-standard deviations in the least significant digit. These figures are close to the corresponding values 1,605.62 cm(-1) and 1,590.72 cm(-1) obtained recently from results based on high-level ab initio calculations. The order of the vibrational term values is abnormal in the ammonia family, as typically the symmetric state is lower in wavenumber than the antisymmetric one.     

Fast and highly efficient removal of anionic organic dyes with a new Cu modified nanoclinoptilolite

Adsorption of anionic dyes onto most of zeolites with net negative charge may be restricted. In this article, a natural nanoclinoptilolite was modified with Cu and the obtained nanomaterial was used as an effective adsorbent for removal of methyl red as an anionic model azo dye up to 90% in 20 min.This new adsorbent was characterized utilizing X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. Also, effects of methyl red concentration, mass of sorbent and pH on the removal percent were examined. Moreover, the adsorption mechanism was investigated by plotting the Langmuir and Freundlich adsorption isotherms. The results showed that the data can be fitted with both models. The most adsorption capacity obtained from Langmuir isotherm was about 200 mg/g. Moreover, the Cu modified nanoclinoptilolite was successfully employed for adsorption of another anionic dye, bromothymol blue. The results confirmed that this new adsorbent can be effectively applied for removing of anionic dyes from waste waters.     

Gegenbauer polynomials in a theoretical study of the vibrational structure of the Ca+–H2 system

This work presents an application of Gegenbauer polynomials in vibrational calculations. We illustrated that by example calculations of vibrational structure of the Ca⁺–H₂ exciplex, in the state correlated with 3D calcium ion state. For this case Gegenbauer polynomials are used for formation of a basis set for a bending mode. We showed that this basis set leads to a faster convergence of results than a basis set formed from Legendre polynomials. Additionally we compared vibrational structure obtained in this manner with results of discrete variable representation-distributed Gaussian basis (DVR–DGB) method.     

Hydrosilylation of alkynes with a cationic rhodium species formed in an anionic micellar system

[reaction: see text] Rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system has been developed. A combination of [RhCl(nbd)](2) and bis(diphenylphosphino)propane (dppp) effects (E)-selective hydrosilylation in the presence of sodium dodecyl sulfate (SDS) in water. The (E)-selectivity strongly indicates the formation of a cationic rhodium species via dissociation of the Rh-Cl bond by the action of anionic micelles. The addition of sodium iodide provided (Z)-alkenylsilanes predominantly.     

Probing the transition between the localised (class II) and localised-to-delocalised (class II-III) regimes by using intervalence charge-transfer solvatochromism in a series of mixed-valence dinuclear ruthenium complexes

Intervalence charge-transfer (IVCT) solvatochromism studies on the diastereoisomeric forms of [{Ru(bpy)(2)}(2)(mu-BL)](5+) (bpy=2,2'-bipyridine; BL=a series of di-bidentate polypyridyl bridging ligands) reveal that the solvent dependencies of the IVCT transitions decrease as the "tail" of the bridging ligand is extended, and the extent of delocalisation increases. Utilising a classical theoretical approach for the analysis of the intervalence charge-transfer (IVCT) solvatochromism data, the subtle and systematic variation in the electronic properties of the bridging ligands can be correlated with the shift between the localised (class II) and localised-to-delocalised (class II-III) regimes. The investigation of the diastereoisomeric forms of two series of complexes incorporating analogous structurally rigid (fused) and nonrigid (unfused) bridging ligands demonstrates that the differences in the IVCT characteristics of the diastereoisomers of a given complex are accentuated in the latter case, due to a stereochemically induced redox asymmetry contribution. The marked dependence of the IVCT transitions on the stereochemical identity of the complexes provides a quantitative measure of the fundamental contributions of the reorganisational energy and redox asymmetry to the intramolecular electron-transfer barrier at the molecular level.