443 research outputs found

    Ordering graphs with small index and its application

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    AbstractWe consider the problem of ordering connected graphs by index (the largest eigenvalue). The asymptotic ordering for the connected graphs with index less than 2+5 is determined. Its application to the study of acyclic Kekulean molecules with big HOMO–LUMO separation is also given

    Why are the Nitro and Sulfone Groups Poor Hydrogen Bonders?

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    The interactions of water or methanol with nitromethane and dimethyl sulfone vs. comparison molecules, e.g. dimethylsulfoxide (DMSO), are reported. For nitromethane with water, the classical (edgewise) hydrogen bonded configuration is modestly stabilizing. However, the approach of water over the face of the nitro group is preferred in AM1 calculations. Generally, molecules such as sulfones and nitro compounds have lower energy bonding orbitals than sulfoxides. The energy of the n δ σ* interaction (e.g. nitro lone pair to O-H of water) thus is larger for the nitro and sulfone cases, and this interaction is less prevalent than other cases. Since the interaction of the nitro group with water is somewhat favorable, the reason for the insolubility of nitromethane is water was investigated. It was found that the energy of segregated sets of nitromethane and of water molecules was lower than the mixed nitromethane - water state. In contrast, the energy of the mixed DMSO-water hydrogen bonded mixed state is lower than segregated molecules

    Quantum chemical investigations on acetylenic carbon rich compounds as molecular construction kit

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    Thesis (Master)--İzmir Institute of Technology, Chemistry, İzmir, 2004Includes bibliographical references (leaves: 76-86)Text in English; Abstract: Turkish and Englishxi, 90 leavesGround and excited state behaviors of Radialenes, Expanded Radialenes and TEE monomer and dimer derivatives, which are carbon rich compounds were investigated by using quantum chemical calculations. Most of these advanced materials have non-linear optical properties and they can be used as molecular electronics. AM1 and DFT/B3LYP with 3-21G and 6-31G* basis sets methods were used for the ground state calculations of radialenes, expanded radialenes and TEE monomers and dimers. TDDFT/3-21G, TDDFT/6-31G* level of calculations were carried out for the excited state behaviors on AM1, DFT/3-21G* and DFT/6-31G* ground state structures. All the methods that we have used gave similar results with a very small discrepansies. Radialenes and expanded radialenes have planar ground state structures except the one with size 6; the three dimensional chair like geometry is slightly stable than the planar one. There is no effect of the size of radialenes on the the geometrical parameters. The introduction of ethynyls instead of hydrogens causes a red-shift about 100-150 nm. The maximum absorption wavelength usually increases with the size of radialenes with some exceptions for the planar structures. The effect of various acceptors such that p-NO2-benzene- , p-CH3-benzene-, p-CHO-benzene- and their locations which are mainly CIS, TRANS and GEMINAL with respect to donor positions to the TICT state on push-pull TEE derivatives were investigated by using excited state calculations. The probable donor units on the TEE derivates were considered as dimethyl amine and dimethyl aniline units. TICT property for the rotation of dimethylaniline group is observed for many of isomers. TICT state appeared both for cis and trans conformer of the donor substituted TEE dimer

    Theoretical Studies of Nucleophilic Additions to Alkynes and Stabilities of Anions.

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    Nucleophilic additions to alkynes were investigated using ab initio molecular orbital theory. Analyses of the energetic consequences of certain molecule distortions showed that the anti selectivity of nucleophilic additions to simple alkyl, aryl, and halogenated alkynes was consistent with trans bending of acetylene. This premise was verified by considering the addition of the hydride ion to acetylene which gave a single transition state resulting from anti addition. Comparing the activation energies for hydride addition to ethylene and acetylene showed that addition to acetylene improves relative to addition to ethylene with increasing basis set sophistication. Nucleophilic addition of ammonia to cyanoacetylene was also investigated to determine the variable stereochemical results experimentally found for these reactions. This study showed that syn and anti transition states and zwitterions existed for the addition of ammonia to cyanoacetylene. Anti addition has a smaller activation energies than syn addition. However, the syn zwitterion was more stable than the anti zwitterion. Thus, a kinetic product and a thermodynamic product are produced from this reaction. A series of (beta)-substituted vinyl anions were also considered theoretically for structural details and qualitative understanding of the types of interactions which control isomer stability. This study showed that (beta)-substituents stabilize the vinyl anion more than ethylene and lower the isomerization barriers for cis-trans isomerizations. Heteroallyl anions were also studied computationally. This studies showed that heteroallyl anions possess an intrinsic preference for the syn configuration. This preference was consistent with electrostatic interactions between the nitrogen lone-pair and the (pi)-charge on the carbon terminus. Lithiation and subsequent solvation using water as the solvent were also studied. These latter investigation showed that (pi) lithiation favored the anti configuration. While (sigma) lithiation and solvation reinstated the syn preference

    Low oxidation state Group 15 cations: Synthesis, reactivity and computational studies

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    Magnetic and Photoluminescent Sensors Based on Metal-Organic Frameworks Built up from 2-aminoisonicotinate

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    Red Guipuzcoana de Ciencia, Tecnologia e Innovacion OF218/2018 University of Basque Country GIU 17/13 Basque Government IT1005-16 IT1291-19 IT1310-19 Junta de Andalucia FQM-394 Spanish Ministry of Science, Innovation and Universities (MCIU/AEI/FEDER, UE) PGC2018-102052-A-C22 PGC2018-102052-B-C21 MAT2016-75883-C2-1-P European Union (EU) ESFIn this work, three isostructural metal-organic frameworks based on frst row transition metal ions and 2-aminoisonicotinate (2ain) ligands, namely, {[M(μ-2ain)2]·DMF}n [MII=Co (1), Ni (2), Zn (3)], are evaluated for their sensing capacity of various solvents and metal ions by monitoring the modulation of their magnetic and photoluminescence properties. The crystal structure consists of an open diamond-like topological 3D framework that leaves huge voids, which allows crystallizing two-fold interpenetrated architecture that still retains large porosity. Magnetic measurements performed on 1 reveal the occurrence of feld-induced spin-glass behaviour characterized by a frequency-independent relaxation. Solvent-exchange experiments lead successfully to the replacement of lattice molecules by DMSO and MeOH, which, on its part, show dominating SIM behaviour with low blocking temperatures but substantially high energy barriers for the reversal of the magnetization. Photoluminescence studied at variable temperature on compound 3 show its capacity to provide bright blue emission under UV excitation, which proceeds through a ligand-centred charge transfer mechanism as confrmed by timedependent DFT calculations. Turn-of and/or shift of the emission is observed for suspensions of 3 in diferent solvents and aqueous solutions containing metal ions

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