NMR detection of intermolecular NH.OP hydrogen bonds between guanidinium protons and bisposphonate moieties in an artificial arginine receptor

Hydrogen bonding plays a major role in the selective recognition of guanidinium groups by receptor molecules. The present NMR investigation provides direct experimental evidence of hydrogen bonds in an artificial arginine receptor complex consisting of alpha-N-benzoylarginine ethyl ester and a bisphosphonate tweezers molecule. trans-Hydrogen bond 2hJHP couplings between the phosphonate moieties and individual guanidinium protons as well as the amide proton have been detected by [1H,31P]-HMBC and [31P,1H]-INEPT experiments. The detected hydrogen bonding network in the investigated artificial arginine receptor shows a symmetrical end-on interaction of the guanidinium moiety, which enables concerted rotations and deviates from the structure proposed for the biological arginine fork.     

The molecular geometries of some cyclic nitramines in the gas phase

The structural parameters of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), (CH₂NNO₂)₃, 1,3-dinitro-1,3-diazacyclopentane (DDCP), CH₂(CH₂NNO₂)₂, andN-nitropyrrolidine (NP), (CH₂)₄NNO₂, have been determined by electron diffraction.The six-membered ring of RDX has a chair form with axial positions of the nitro groups and close to planar bond geometry of the amine nitrogen atoms. The overallC ₃ symmetry of the molecule is in agreement with the experimental data.The conformation of the five-membered ring in DDCP is a half-chair ofC ₂ symmetry, while that in NP is an envelope ofC _S symmetry. The nitro groups are in equatorial positions in both molecules. The conformations of pyrrolidine and imidazolidine cycles show interesting features.The pyramidal geometry of the amine nitrogen atom bonds flattens in going from pyrrolidine andN-chloropyrrolidine to NP and DDCP and then to RDX and to dimethylnitramine (DMNA), (CH₃)₂NNO₂.     

Rational Krylov methods for fractional diffusion problems on graphs

BIT Numerical Mathematics - In this paper we propose a method to compute the solution to the fractional diffusion equation on directed networks, which can be expressed in terms of the graph...     

Triazene proton affinities: A comparison between density functional,Hartree–Fock,and post-Hartree–Fock methods

The consistency of three density functional computational implementations (DMol, DGauss, and deMon) are compared with high-level Hartree–Fock and Møller–Plesset (MP) calculations for triazene (HN?NNH₂) and formyl triazene (HN?NNHCOH). Proton affinities on all electronegative sites are investigated as well as the geometries of the neutral and protonated species. Density functional calculations employing the nonlocal gradient corrections show agreement with MP calculations for both proton affinities and geometries of neutral and protonated triazenes. Local spin density approximation DMol calculations using numerical basis sets must employ an extended basis to agree with other density functional codes using analytic Gaussian basis sets. The lowest energy conformation of triazene was found to be nonplanar; however, the degree of nonplanarity, as well as some bond lengths, is dependent on the basis set, electron correlation treatment, and methods used for the calculation. © 1994 by John Wiley & Sons, Inc.

1 This article is a U.S. Government work and, as such, is in the public domain in the United States of America.

     

Two-point Padé approximants for formal Stieltjes series

We prove that certain two-point Padé approximants occupying the diagonal of the Padé table form monotone sequences of lower and upper bounds uniformly converging to a Stieltjes function. The results can be applied to the theory of inhomogeneous media for the calculation of the bounds on the effective transport coefficients of heterogeneous materials.     

Synthesis of unsaturated adenine derivatives,potential inhibitors of S-adenosyl-L-homocysteine hydrolase

The synthesis of 9-substituted adenine derivatives, containing an unsaturated fragment at the end of the acyclic chain, is described; it consists of the alkylation of adenine salts with unsaturated -haloethers, tosylates, and epoxides. It was shown that compounds of this series possess moderate antiviral activity.Volgograd Medical Academy, Scientific Research Institute of Pharmacology, Volgograd 400066. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 826–830, June, 1996. Original article submitted February 14, 1995.     

Isomorphism of two realizations of quantum affine algebraU_q (\widehat{\mathfrak{g}\mathfrak{l}{\text{(}}n{\text{)}}})

We establish an explicit isomorphism between two realizations of the quantum affine algebra given previously by Drinfeld and Reshetikhin-Semenov-Tian-Shansky. Our result can be considered as an affine version of the isomorphism between the Drinfield/Jimbo and the Faddeev-Reshetikhin-Takhtajan constructions of the quantum algebra .     

LiNa2AlF6: a powder structure solution

Lithium sodium aluminium fluoride was obtained as a white powder by melting a stoichiometric mixture of AlF₃, NaF and LiF at 1223 K, and then cooling to 923 K and sintering at this temperature for 4 h. The ab initio crystal structure determination was carried out using X‐ray powder diffraction techniques. The monoclinic structure of LiNa₂AlF₆ can be related to cubic elpasolite. The Li and Al atoms lie on inversion centres. The main octahedral AlF₆ structural elements are not deformed, but are rotated slightly with respect to the unit‐cell axes. The Li atoms have octahedral coordinations, whereas the Na atoms have cubo‐octahedral coordinations. The Na coordination polyhedron is distorted in comparison with that of elpasolite.     

Synthesis and properties of monodisperse oligofluorene-functionalized truxenes: highly fluorescent star-shaped architectures

This paper describes the strategy toward novel monodisperse, well-defined, star-shaped oligofluorenes with a central truxene core and from monofluorene to quaterfluorene arms. Introduction of solubilizing n-hexyl groups at both fluorene and truxene moieties results in highly soluble, intrinsically two-dimensional nanosized macromolecules T1-T4. The radius for the largest oligomer of ca. 3.9 nm represents one of the largest known star-shaped conjugated systems. Cyclic voltammetry experiments reveal reversible or quasi-reversible oxidation and reduction processes (Eox = +0.74 to 0.80 V, Ered = -2.66 to 2.80 eV vs Fc/Fc+), demonstrating excellent electrochemical stability toward both p- and n-doping, while the band gaps of the oligomers are quite high (EgCV = 3.20-3.40 eV). Close band gaps of 3.05-3.29 eV have been estimated from the electron absorption spectra. These star-shaped macromolecules demonstrate good thermal stability (up to 400-420 degrees C) and improved glass transition temperatures with an increase in length of the oligofluorene arms (from Tg = 63 degrees C for T1 to 116 degrees C for T4) and show very efficient blue photoluminescence (lambdaPL = 398-422 nm) in both solution (PhiPL = 70-86%) and solid state (PhiPL = 43-60%). Spectroelectrochemical experiments reveal that compounds T1-T4 are stable electrochromic systems which change their color reversibly from colorless in the neutral state (approximately 340-400 nm) to colored (from red to purple color; approximately 500-600 nm) in the oxidized state.     

Dissolution of gold in the DMSO—RX systems. The concept of a donor-acceptor electron transport system

Summary The liquid phase oxidation of gold in donor-acceptor organic and aqueous-organic media has been studied. The compounds [AuCl(Me₂S)], [AuBr(Me₂S)], [AuBr₃(Me₂S)], [Me₃S][AuBr₄], [Me₃S][AuBr₄(Me₂S)]·H₂O, [Me₃SO]-[AuBr₄]·H₂O, [Me₃S][Au₂Br₇(Me₂S)₂]·3H₂O, [Me₃S]₂-[Au₂Br₈]·2DMSO·H₂O, [Me₂(Bu)SO][AuBr₄]·H₂O and [Me₃S]Br were isolated by dissolution of Au⁰ in DMSO-RX mixtures (R = H or Bu; X = Cl or Br). The products were characterized by elemental analysis and i.r. spectroscopy. The nature of the Au⁰-DMSO-RX systems and the oxidant species are discussed in terms of a newly-developed concept of donor-acceptor electron transport (DAET) systems.