1,720,994 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
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
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
Quantitative evaluation of template effect in the formation of cyclobis(paraquat-p-phenylene)
No full textHückel 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
Effect of 1-methylcyclopropene on kiwifruit softening
No full textPremature softening induced by ethylene is a serious commercial problem limiting the storage life of kiwifruit. The objective of this study was to evaluate the effectiveness of 1-MCP in extending storage and shelf life and preventing postharvest softening of kiwifruit. Exposure to 100 nl l-1 1-MCP at room temperature for 12 h or at low temperature ( 5°C) for 24 h was applied with a single or double treatment before or/and after storage. The fruit were also treated with a single application at 250 nl l-1 1-MCP. Following storage at 0°C, the fruit were kept at 20°C. The results show that exposure to 100 and 250 nl l-1 1-MCP delayed softening in kiwifruit after shelf life. An application before storage was more effective than treatment after storage. Double treatment was slightly more effective compared to a single application. Furthermore, a single application of 1-MCP at room and low temperature was equally effective in delaying the loss of firmness. In a separate experiment, kiwifruit were treated three days after harvest, after curing, with 100 nl l-1 1-MCP at room temperature for 12 h and stored in air at 0°C with or without ethylene absorber. Following storage, the fruit were kept at 20°C. 1-MCP treated fruit were significantly firmer than untreated fruit after storage and shelf life. Kiwifruit treated and stored without ethylene absorber had comparable or superior values of firmness to untreated fruit stored with ethylene absorption. In both trials, 1-MCP had no effects on soluble solids content and acidity. No significant differences were observed in fruit rot, caused by Botrytis cinerea
Kinetic treatment of irreversible cyclooligomerization of bifunctional chains and its relevance to the synthesis of many-membered rings
No full textThe reaction of a bifunctional reactant A-B under batchwise conditions to give macrocycles has been simulated by numerical integration of the proper system of differential rate equations. This was set up considering all the possible processes of concurrent cyclization and polymerization up to a polymerization degree of 12. In order to obtain meaningful yield data for the cyclooligomers, realistic sets of effective molarities (EM) have been considered. The results show how the yields of the various cyclooligomers vary as a function of the initial monomer concentration and provide useful guidelines to achieve maximum selectivity in the synthesis of a given ring. It is clearly shown that the yield of any cyclooligomer is a function of the whole set of EM values. This observation is the basis of a procedure here developed to evaluate EM data from experimental yields of cyclooligomers. Merits and limitations of more simplified kinetic schemes, which had been previously suggested, are discussed in light of the present results. © 1988 American Chemical Society
The problem of regioselectivity in nucleophilic additions to pyridinium and related cations - role of generalized anomeric effect
No full textA MNDO and AM1 study of the isomerization of pyrans, thiopyrans, dihydropyridines, and their methoxy derivatives was carried out so as to clarify the nature of the effects causing the observed thermodynamic regioselectivity in nucleophilic addition to pyridinium, pyrylium and thiopyrylium cations. The results show that, in contrast to 2H-thiopyrans, 2H-pyrans and 1,2-dihydropyridines are significantly stabilized by a generalized anomeric effect when the group bonded to the 2 position is OMe. It is pointed out that in the case of reversible additions, a clear distinction should be made between the kinetic and the thermodynamic regioselectivity because, in contrast with current usage, the latter cannot be interpreted in terms of frontier orbital theory but by the presence or absence of a generalized anomeric effect. It is also pointed out that predictions of kinetic regioselectivity based on frontier orbital theory are not supported by literature data. The kinetic regioselectivity seems to be governed by the relative electron density at the carbon under attack, independent of the hard or soft character of the nucleophile
Syntheses of Many-Membered Rings. Computer Simulated Kinetics of Irreversible Cyclization of Bifunctional Chains under Ziegler High Dilution Conditions
No full textThe reaction of a bifunctional reactant A--B under Ziegler high-dilution (influxion)conditions to give families of oligomeric macrocycles has been simulated by numerical integration of
the proper system of differential rate equations. This was set up considering all the possible processes of concurrent cyclisation and polymerisation up to a polymerisation degree of 12. In order to obtain meaningful data for the yields of the cyclooligomers, realistic sets of effective molarities (EM) have been
considered. The results show how the yields of the various cyclooligomers vary as a function of the monomer feed rate, and provide useful guidelines to achieve maximum selectivity in the synthesis of a given ring. Merits and limitations of more simplified kinetic schemes dealing with the monomeric ring
only are outlined by the present calculations. A comparison with the batch-wise procedure shows that the two techniques give somewhat analogous results in terms of yields and selectivities. The criteria of choice between the two procedures, which are established on the basis of practical convenience, are indicated
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