Investigation of the stereochemistry and electronic structure in a series of dipyridyls

Conclusions The barriers to internal rotation in dipyridyls were estimated from the parameters of the PMR spectra and by calculation, considering the electronic delocalization and the nonbonded interaction of the ortho-protons.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimi-cheskaya, No. 7, pp. 1510–1517, July, 1973.     

The stereochemistry of isoquinoline Reissert compounds: a unique platform for observation of steric and electronic interactions

Isoquinoline Reissert compounds (2-acyl-1,2-dihydroisoquinaldonitriles) with either 3-H (1) or 3-CH₃ (2) substituents and various N-acyl groups have been examined in detail by ¹H and ¹³C NMR spectroscopy and X-ray crystallography. In all cases the trans amide conformation, with reference to the carbonyl oxygen and the 3-position of the isoquinoline ring, predominates in solution. In the solid state the nitrile moieties are pseudo-axial and the amides exist almost exclusively in the trans form, except for the case of 2-isobutyryl-3-methyl-1,2-dihydroisoquinaldonitrile (2c), which exists exclusively as the cis amide form in the solid state. In N-aroyl 3-CH₃ compounds with two ortho-aroyl substituents both amide isomerism and hindered aryl/carbonyl rotation are observed by ¹H NMR spectroscopy. In other N-aroyl derivatives only hindered aryl/carbonyl rotations are observed by NMR and in N-alkanoyl compounds amide isomerism is observable only at very low temperatures. X-ray crystallography reveals the two rotamers in the solid state in four cases of ortho-substituted benzoyl compounds; with one exception, the rotamer with the larger ortho-aroyl substituent syn to the pseudo-axial cyano group is favored. Unusual solubility and reactivity patterns observed with these compounds are rationalized in terms of the interplay between steric and electronic factors.     

The electronic properties of conjugated ions. II

The calculations presented here demonstrate the relative importance of the excess charge effect for energy calculations, and its lack of importance for spin density calculations, in conjugated ions and radicals.Part 1 is considered to be Ref. [1].     

Nanostructured Ti-W mixed-metal oxides: structural and electronic properties

In this article, the structural and electronic properties of Ti-W binary mixed oxide nanoparticles are investigated by using X-ray diffraction, Raman, X-ray absorption spectroscopies (XAS; near edge XANES and extended EXAFS), UV-vis spectroscopy, and density functional calculations. A series of Ti-W binary oxide samples having W content below 20 atom % and with particle size between 8 and 13 nm were prepared by a microemulsion method. The atoms in these nanoparticles adopted the anatase-type structure with a/b lattice constants rather similar to those of the single TiO(2) reference and with a c cell parameter showing a noticeable expansion upon doping. Within the anatase structure, W occupies substitutional positions, while Ti atoms only suffer minor structural perturbations. A change in the W local order at first neighboring distance is observed when comparing samples having a W content below and above 15 atom %. Charge neutrality is mostly achieved by formation of cation vacancies located at the first cation distance of W centers. Upon addition of W to the TiO(2) structure, the Ti charge is not strongly modified, while changes in the W-O interaction appear to drive a modest modification of the W d-electron density throughout the Ti-W series. A combination of these geometrical and electronic effects produced Ti K- and W L(I)/L(III)-edge XANES/EXAFS spectra with distinctive features. UV-vis spectra show a nonlinear decrease of the band gap in the Ti-W solid solutions with a characteristic turning point at a W content of ca. 15 atom %. The relationship between local/long-range order and electronic parameters is discussed on the basis of these experimental results.     

1,1'-bipyrroles: synthesis and stereochemistry

1,1'-Bipyrrole is synthesized in four steps from hydrazine. A colorless solid, mp 52 degrees C, it sublimes readily at room temperature and forms X-ray quality crystals in which the rings are not coplanar but are nearly orthogonal.     

Synthesis,stereochemistry and fluorescence properties of polystyrenes having carbazole substituents

Poly(styrene)s bearing carbazole substituents at various positions were synthesised by homopolymerisation of 3-(9-carbazolylmethyl)styrene and 4-(9-carbazolylmethyl)styrene and by chemical modification of preformed poly(styrene)s prepared in the presence of different initiators. Main chain stereochemistry of the polymer samples was investigated by ¹³C-NMR and related to the initiation process. The fluorescence emission spectra were independent of the stereochemistry of polymer backbone and exhibited only monomer emission. In no case was there any evidence of excimer fluorescence. In contrast, energy migration appeared to be very efficient, as evaluated by measurements of the degree of fluorescence polarisation relative to that of a structurally related model compound     

Intramolecular n-alkenylnitrone-additions: Regio- and stereochemistry

Efficient intramolecular cycloadditions of N-3-alkenyl- and N-4-alkenylnitrones proceed with opposite regioselection which is modified by dipolarophile-substituent effects. Polycyclic isoxazolidines are obtained in a highly stereocontrolled fashion, consistent with an endo-addition of the Z-nitrones.     

Hydroxyl Ammonium and Diethyl Ammonium Glutaratouranylates: Synthesis and Structure

The structures of two new coordination polymers, namely, hydroxyl ammonium glutaratouranylate NH₃OH[UO₂(C₅H₆O₄)(C₅H₇O₄)] · H₂O (I) and diethyl ammonium glutaratouranylate NH₂(C₂H₅)₂[UO₂(C₅H₆O₄) (C₅H₇O₄)] · 2H₂O (II), have been characterized by single-crystal X-ray diffraction. It has been established that the structures of complexes I and II contain [UO₂(C₅H₆O₄)(C₅H₇O₄)]–infinite chains with the crystallochemical formula AQ⁰²B⁰¹ (\(\rm{A = UO_2^{2+}}, Q^{02} = C_5H_6O_4^{2-}, B^{01} = C_5H_7O_4^{-}\)). Despite the identical compositions, uranyl glutarate chains in the studied structures appreciably differ from each other by their geometry and linking. The specific features of nonbonded interactions in the structures of complexes I and II have been characterized by the Voronoi–Dirichlet method of molecular polyhedra.     

Structural and electronic properties of heptachlor

In this work, the molecular geometry of heptachlor is investigated using ab initio HF, DFT, LDA, and GGA methods. The natural bond orbital (NBO) analysis is performed at the B3LYP/6-311++G(d,p) level of theory. The first order hyperpolarizability β_total, the mean polarizability Δα, the anisotropy of the polarizability Δα, and the dipole moment μ, are calculated by B3LYP/6-311++G(d,p) and HF/6- 311++G(d,p) methods. The first order hyperpolarizability (β_total) is calculated based on the finite field approach. UV spectral parameters along with HOMO, LUMO energies for heptachlor are determined in vacuum and the solvent phase using HF, DFT, and TD-DFT/B3LYP methods implemented with the 6-311++G(d,p) basis set. Atomic charges and electron density of heptachlor in vacuum and ethanol are calculated using DFT/B3LYP and TD-DFT/B3LYP methods and the 6-311++G(d,p) basis set. In addition, after the frontier molecular orbitals (FMOs), the molecular electrostatic potential (MEP), the electrostatic potential (ESP), the electron density (ED), and the solvent accessible surface of heptachlor are visualized as a results of the B3LYP/6-311++G(d,p) calculation. Densities of states (DOS), the external electric field (EF) effect on the HOMO-LUMO gap, and the dipole moment are investigated by LDA and GGA methods.     

Conformational and electronic properties of a microperoxidase in aqueous solution: a computational study.

A theoretical study of the conformational properties of a small heme peptide in aqueous solution is carried out by classical, long-timescale molecular dynamics simulations. The electronic properties of this species, that is, the relative energies of its excited electronic states and the redox potential, are reproduced and related to the conformational behavior using the perturbed matrix method and basic statistical mechanics. Our results show an interesting coupling between the conformational transitions and the electronic properties. These investigations, beyond the biophysically relevant results addressing the long-standing question of the actual role of the enzyme structure on the enzyme activity, are also of some methodological interest since they offer a further computational perspective for including the electronic degrees of freedom into the modeling of rather complex molecular systems.     

Luminescent bichromophoric spiroindolones--synthesis and electronic properties

Novel bichromophoric spirocyclic indolones have been synthesized by an insertion-coupling-isomerization-Diels-Alder domino reaction. The emission colors are strongly affected by the substituents: N-Boc leads to intense blue fluorescence, N-dansyl causes turquoise emission, whereas a 1-anthryl substituent on the butadiene results in yellow luminescence. The latter behavior is rationalized by TDDFT computations as a result of significant geometrical changes.     

Configuration analysis and electronic states properties

The excited-states electronic properties of unsaturated small-ring hydrocarbons with four- and six--electrons have been studied by the Molecules in Molecules method and compared with experimental data and PPP results interpreted through a configuration analysis procedure. The results show that the MIM method is applicable with good reliability.

---

Zusammenfassung Die elektronischen Eigenschaften der angeregten Zustände von ungesättigten kleinen Kohlenwasserstoffringen mit vier und sechs -Elektronen wurden mit Hilfe der Molekül- im-Molekül-Methode untersucht und die Ergebnisse mit experimentellen Daten und PPP Resultaten, die durch eine Konfigurationsanalyse interpretiert wurden, verglichen. Die Ergebnisse zeigen, daß die MIM-Methode mit großer Zuverlässigkeit anwendbar ist.

---

---

Financial aid from the Italian C.N.R. is gratefully acknowledged.     

Phosphole modified pentathienoacene: synthesis, electronic properties and self-assembly

A series of new ladder π-conjugated materials, phosphole modified pentathienoacene (PO-PTA), are synthesized and characterized. Single-crystal X-ray results demonstrate that methyl-disubstituted PO-PTA forms a face-to-face dimer structure driven by π-π interactions. The investigations of optical properties showed that the oxidized phosphole moiety in this ladder system can effectively narrow the band gap. PO-PTA is a promising building block in π-conjugated polymers and oligomers for optoelectronic applications. The derivative of PO-PTA, obtained by introducing four long alkyl chains, can self-assemble into one-dimensional (1D) fibers based on intermolecular π-π interactions, dipole-dipole interactions and van der Waals interactions. Interestingly, the uniform and well-ordered monolayers were also obtained for PO-PTA derivative on a HOPG (highly oriented pyrolytic graphite) surface.     

O2 and vacancy diffusion on rutile(110): pathways and electronic properties.

The binding structures and diffusion pathways of molecular oxygen on a defective TiO2(110) surface are studied by means of a recently developed first-principles string molecular dynamics approach. A variety of molecular and dissociated O2 adsorption states are identified and the kinetics of their interconversion is analyzed. These results, as well as calculations of the electronic properties and of scanning tunneling microscopy (STM) images, are used to discuss recent experimental observations of the interactions between surface oxygen vacancies and the adsorbed oxygen molecule.