187,366 research outputs found
Role of face-to-face and edge-to-face aromatic interactions in the inclusion complexation of cyclobis(paraquat-p-phenylene): A theoretical study
No full textA B3LYP/6-31G(d,p) and MP2/6-31G(d,p)//B3LYP/6-31G(d,p) computational study of the gas-phase complexation of cyclobis(paraquat-p-phenylene) (14+) with four typical aromatic guests, namely, 1,4-dimethoxybenzene (2), 1,5-dimethoxynaphthalene (3), benzidine (4), and tetrathiafulvalene (5), has been carried out. The structure of the host has been successively split into two responsible substructures, respectively, for the face-to-face and edge-to-face interactions with the guests. The sum of the two interactions calculated at the B3LYP/6-31G(d,p) and MP2/6-31G(d,p)//B3LYP/6-31G(d,p) levels for each guest proved to be in good agreement with the overall binding energy of the host calculated at the corresponding level of theory. The results show that the binding of the complexes is primarily due to London dispersion interactions which require wave function-based correlation methods for an adequate description. Face-to-face interactions are about I order of magnitude more important than edge-to-face interactions in determining the overall binding energy. While edge-to-face interactions essentially depend on London dispersion forces, face-to-face interactions depend about one-half on electrostatic and frontier orbital contributions (the latter being more important) and the other half on London dispersion forces
SOME REMARKS ON THE ERCOLANI-SINHA CONSTRUCTION OF MONOPOLES
No full textWe develop the Ercolani-Sinha construction of SU(2) monopoles, which provides a gauge transform of the Nahm data.</p
Distributions of cyclic oligomers formed by irreversible step-growth polymerisation. Results from kinetic modeling
No full textThe distribution of cyclic oligomer concentrations (examined up to the cyclic pentamer) in the process of step-growth polymerisation decreases, in concentrated solution, with i(-2.5) if the chains follow Gaussian statistics (i: degree of polymerization). This picture is analogous to that shown by cyclic oligomer distributions obtained under thermodynamic control and is in contrast with that shown by cyclic oligomer distributions obtained by irreversible chain-growth polymerisation which is characterized by a gradient of -1.5. This result has been initially obtained by numerical integration of the differential rate equations pertinent to a kinetic model relative to the reaction of a bifunctional reactant A-B under batchwise conditions, and then confirmed by analytical integration of the differential rate equations under the condition that propagation is much faster than cyclisation. The case in which the monomeric ring is strained has been also investigated. Also in this case the obtained distribution is analogous to that shown under equilibrium conditions. From an examination of the distributions of the ring yields, the best conditions for the preparation of either the cyclic monomer or the cyclic dimer are suggested
Hückel Molecular Orbital Calculations on a Microcomputer
No full textCheap microcornputers can be profitably used by students to get acquainted with a number of chemical topics. Continuing our interest in educational software devoted to physical organic chemistry,we report a program performing HMO calculations on a Sinclair ZX Spectrum microcomputer
Simulation of irreversible cyclization of bifunctional chains. a computer-aided approach to the synthesis of many-membered rings and to the evaluation of effective molarities by preparative experiments
No full textTwo complementary kinetic models for the reaction of a bifunctional reactant A-B to give macrocycles under either batch-wise or influxion (Ziegler high dilution) conditions, are described. The degree of accuracy of both models is variable at will, depending on the value of the maximum polymerization degree accounted for by the models themselves. On the basis of these models, a computer program (CYCLES) has been developed, serving both as an useful tool for the optimization of reaction conditions in the synthesis of many-membered rings, and as the essential means of obtaining effective molarities by preparative experiments. The program CYCLES performs fourth-order Runge-Kutta numerical integration of the set of differential rate equations pertinent to each kinetic model for an arbitrary value of the degree of polymerization
Numerical integration of rate equations on a microcomputer
No full textMicrocomputer can offer a new dimension in chemical education. This article describes the application of a simple numerical integration method on a microcomputer, to help students understand chemical kinetic
La dialettica della violenza. Il caso Guenther Anders
No full textIl testo analizza la questione della violenza negli scritti ultimi di Guenther Anders, in un istante confrtìotno con le riflessioni di Arendt, Benjamin e soprattutto Jean Amery
Quantitative evaluation of template effect in the formation of cyclobis(paraquat-p-phenylene)
No full textTemplate effect of tetrathiafulvalene in the formation of cyclobis(paraquat-p-phenylene)
No full textThe template effect exerted by tetrathiafulvalene (TTF) in the ring-closure reaction of the trication 5(3+) yielding cyclobis(paraquat-p-phenylene) has been quantitatively evaluated in acetonitrile at 62 degrees C with UV-vis spectrophotometry. The rate of ring closure of the trication 5(3+) largely increases in the presence of the template (a maximum increase of ca. 80 times at [TTF] approximate to 0.14 M). The results are compared with those of other aromatic templates, 2 and 3, that were provided with polyethereal sidearms and indicate that the template ability of tetrathiafulvalene is comparable or better than that of the others
Convenient Alkylation of Pyrylium Salts by Nucleophilic Alkyl Radicals
No full text2,6-disubstituted pyrylium salts easily undergo homolytic substitution in y position by
reaction with alkyl radicals generated by oxidative cleavage of carboxylic acids with peroxydisulphate
ion. Yields are generally satisfactory with secondary and tertiary alkyl radicals; as far as primary alkyl
radicals are concerned, the reaction proved to be successful only with Me and Et
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