Mostar index

We propose and investigate a new bond-additive structural invariant as a measure of peripherality in graphs. We first determine its extremal values and characterize extremal trees and unicyclic graphs. Then we show how it can be efficiently computed for large classes of chemically interesting graphs using a variant of the cut method introduced by Klav?ar, Gutman and Mohar. Explicit formulas are presented for several classes of benzenoid graphs and Cartesian products. At the end we state several conjectures and list some open problems.     

Designed To React: Terminal Copper Nitrenes and Their Application in Catalytic C−H Aminations

Heteroscorpionate ligands of the bis(pyrazolyl)methane family have been applied in the stabilisation of terminal copper tosyl nitrenes. These species are highly active intermediates in the copper‐catalysed direct C?H amination and nitrene transfer. Novel perfluoroalkyl‐pyrazolyl‐ and pyridinyl‐containing ligands were synthesized to coordinate to a reactive copper nitrene centre. Four distinct copper tosyl nitrenes were prepared at low temperatures by the reaction with SO₂tBuPhINTs and copper(I) acetonitrile complexes. Their stoichiometric reactivity has been elucidated regarding the imination of phosphines and the aziridination of styrenes. The formation and thermal decay of the copper nitrenes were investigated by UV/Vis spectroscopy of the highly coloured species. Additionally, the compounds were studied by cryo‐UHR‐ESI mass spectrometry and DFT calculations. In addition, a mild catalytic procedure has been developed where the copper nitrene precursors enable the C?H amination of cyclohexane and toluene and the aziridination of styrenes.     

N,O-diacylhydroxylamines-structures in crystals and solutions

The structures of four N,O-diacylhydroxylamines (RCOHNOCOR', R, R'= Me, Ph) were determined in the solid state by X-ray diffraction and studied by NMR and IR spectroscopies in solution. The interpretation of the results was supported by ab-initio calculations of various tautomers and conformers, rotational barriers and chemical shifts. The results indicate the absence of OH tautomers (R-C(OH)=N-O-C(O)-R', N-acyloxyimidic acid); the NH tautomers (R-C(O)-NH-O-C(O)-R', O-acylhydroxamic acid) are present in DMSO solutions as equilibrium mixtures of a few conformers, their exchange being the likely source of 15N and 13C NMR line broadening.     

Synthesis of coumarin-nucleoside conjugates via Huisgen 1,3-dipolar cycloaddition

An efficient synthesis of fluorescent coumarin-nucleoside conjugates via Cu(I) catalysed Huisgen 1,3-dipolar cycloaddition is described. Starting from azidonucleosides and coumarin derivatives, products are obtained in good yields. The fluorescent properties of the newly prepared coumarin-nucleoside conjugates are determined.     

Reduction pathway of end-on terminally coordinated dinitrogen. V. N-N bond cleavage in Mo/W hydrazidium complexes with diphosphine coligands. Comparison with triamidoamine systems

N-N cleavage of the dialkylhydrazido complex [W(dppe)2(NNC5H10)] (B(W)) upon treatment with acid, leading to the nitrido/imido complex and piperidine, is investigated experimentally and theoretically. In acetonitrile and at room temperature, B(W) reacts orders of magnitude more rapidly with HNEt3BPh4 than its Mo analogue, [Mo(dppe)2(NNC5H10)] (B(Mo)). A stopped-flow experiment performed for the reaction of B(W) with HNEt3BPh4 in propionitrile at -70 degrees C indicates that protonation of B(W) is completed within the dead time of the stopped-flow apparatus, leading to the primary protonated intermediate B(W)H+. Propionitrile coordination to this species proceeds with a rate constant k(obs(1)) of 1.5 +/- 0.4 s(-1), generating intermediate RCN-B(W)H+ (R = Et) that rapidly adds a further proton at Nbeta and then mediates N-N bond splitting in a slower reaction (k(obs(2)) = 0.35 +/- 0.08 s(-1), 6 equiv of acid). k(obs(1)) and k(obs(2)) are found to be independent of the acid concentration. The experimentally observed reactivities of B(Mo) or B(W) with acids in nitrile solvents are reproduced by DFT calculations. In particular, geometry optimization of models of solvent-coordinated, Nbeta-protonated intermediates is found to lead spontaneously to separation into the nitrido/imido complexes and piperidine/piperidinium, corresponding to activationless heterolytic N-N bond cleavage processes. Moreover, DFT indicates a spontaneous cleavage of nonsolvated B(W) protonated at Nbeta. In the second part of this article, a theoretical analysis of the N-N cleavage reaction in the Mo(III) triamidoamine complex [HIPTN3N]Mo(N2) is presented (HIPTN3N = hexaisopropylterphenyltriamidoamine). To this end, DFT calculations of the Mo(III)N2)triamidoamine complex and its protonated and reduced derivatives are performed. Calculated structural and spectroscopic parameters are compared to available experimental data. N-N cleavage most likely proceeds by one-electron reduction of the Mo(V) hydrazidium intermediate [HIPTN3N]Mo(NNH3)+, which is predicted to have an extremely elongated N-N bond. From an electronic-structure point of view, this reaction is analogous to that of Mo/W hydrazidium complexes with diphos coligands. The general implications of these results with respect to synthetic N2 fixation are discussed.     

Phase transition, thermal expansion and electrical properties of BiCu2VO6

Phase transition in BiCu₂VO₆ has been studied by variable temperature powder and single crystal X-ray diffraction. A reversible single-crystal-to-single-crystal phase transition has been identified and the high-temperature β-BiCu₂VO₆ polymorph structurally characterized. β-BiCu₂VO₆ is monoclinic I-centered and related to the α-form by a subgroup-supergroup relationship. Bi atoms are coordinated to oxygen so as to give rise to (BiO₂)⁻ chains parallel to the c-axis. The magnetic Cu-O sublattice forms a complex system of quasi one-dimensional ladders, built up by five- and six-coordinate Cu atoms. Dynamic disorder in the high temperature structure can be described in terms of librational motion of VO₄ tetrahedral group. AC impedance measurements suggest predominantly electronic conduction in this material.     

Differential pulse voltammetric study of the complexation of Cd(II) by the phytochelatin (γ-Glu---Cys)2Gly assisted by multivariate curve resolution

A multivariate curve resolution method by alternating least-squares (MCR-ALS) is applied to differential pulse voltammograms measured on the Cd(II)+(γ-Glu---Cys)₂Gly system as a model of metal–phytochelatin interactions at concentrations of both components in the range 10⁻⁷–10⁻⁵ mol l⁻¹. The course of complexation is different when peptide is titrated with metal from that when metal is titrated with peptide. The combined analysis of both matrices from titrations of peptide with metal and of metal with peptide allowed the resolution of the system. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggested the presence of four different types of bound Cd(II) and made possible the formulation of a complexation model.     

Enantiopure hydroxylactones from d-xylose. A novel approach to the enantiodivergent synthesis of (+)- and (−)-muricatacin suitable for the preparation of 7-oxa analogues

A new route towards enantiopure hydroxylactones 3 and ent-3, the final chiral precursors in an enantiodivergent synthesis of (+)- and (−)-muricatacin, has been developed starting from d-xylose.     

Role of LHCII organization in the interaction of substituted 1,4-anthraquinones with thylakoid membranes

The chlorophyll fluorescence, photochemical activity and surface electric properties of thylakoid membranes with different stoichiometry of pigment-protein complexes and organization of the light-harvesting chlorophyll a/b protein complex of photosystem II (LHCII) were studied in the presence of substituted 1,4-anthraquinones. Data show strong dependence of the quenching of the chlorophyll fluorescence on the structural organization of LHCII. The increase of the LHCII oligomerization, which is associated with significant reduction of the transmembrane electric charge asymmetry and electric polarizability of the membrane, correlates with enhanced quenching effect of substituted 1,4-athraquinones. Crucial for the large quinone-induced changes in the membrane electric dipole moments is the structure of the quinone molecule. The strongest reduction in the values of the dipole moments is observed after interaction of thylakoids with 3-chloro-9-hydroxy-1,4-anthraquinone (TF33) which has the highest quenching efficiency. The quinone induced changes in the photochemical activity of photosystem II (PSII) correlate with the total amount of the supramolecular LHCII-PSII complex and depend on the number of substituents in the 1,4-anthraquinone molecule.