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[en] Highlights: • Both the global and local aromaticity of 173 kinds of C24 fullerene isomers are analyzed. • General rules for predicting the local aromaticity of fullerenes are proposed. • In the hexaanionic state, pentagon pair sites exhibit larger local aromaticity. • The square–heptagon bond has a large destabilizing effect on fullerenes. The aromaticity of classical and nonclassical C24 fullerene isomers and their anions were studied using the topological resonance energy (TRE) method. Local aromaticity was studied using the bond resonance energy (BRE) method. On the basis of BRE values, the contributions of different types of chemical bonds to the molecular global aromaticity were analyzed and general rules for predicting the local aromaticity of fullerenes are proposed. It was found that pentagons, heptagons, and squares preferred hexagons as neighbors rather than squares as neighbors. In the hexaanionic state, pentagon pair sites exhibit larger local aromaticity than other places in the same molecule.
[en] The substituent effects on the pyridinolysis (XC5H4N) of Y-aryl ethyl chlorophosphates are investigated in acetonitrile at 35.0 .deg. C. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridines, exhibit large positive deviations from the Hammett plots but little positive deviations from the Bronsted plots. The substituent Y effects on the rates are really significant and the Hammett plots for substituent Y variations in the substrates invariably change from biphasic concave downwards via isokinetic at X = H to biphasic concave upwards with a break point at Y = 3-Me as the pyridine becomes less basic. These are interpreted to indicate a mechanistic change at the break point from a stepwise mechanism with a rate-limiting bond formation (ρXY = -6.26) for Y = (4-MeO, 4-Me, 3-Me) to with a rate-limiting leaving group expulsion from the intermediate (ρXY = +5.47) for Y = (4-Me, H, 3-MeO). The exceptionally large magnitudes of ρXY values imply frontside nucleophilic attack transition state
[en] 81Rb is produced in high specific activity and yield by the reaction 79Br(α,2n)81Rb. The 81Rb is purified and absorbed on an ion-exchange column in a minigenerator, which allows the elution, at a rapid rate, of the /SUP 81m/Kr daughter in a biologically compatible, sterile solution. Applications of /sup 81m/Kr are described. Chemical binding in tumor cells is being studied and the use of /sup 134m/Cs for myocardial scanning is being evaluated. (U.S.)
[en] Highlights: • A transformation from Si52− to Al5H52− by isoelectronic substitution of atoms is proposed. • The potential energy surfaces for the series Si5−n(AlH)n2− (n = 0–5) systems were explored. • A preservation of the chemical bonding pattern from Si52− to Al5H52 is proposed. • A theoretical VDEs for the series LiSi5−n(AlH)n− (n = 1–4) were calculated. We established that the transformation from Si52− to Al5H52− could be possible by the successive isoelectronic substitution of silicon atoms by AlH units. The potential energy surfaces for the series Si5−n(AlH)n2− (n = 0–5) systems were explored in detail, and the global minima maintained the same overall deltahedral structure as the one of the Si52− cluster. The conservation of the overall structure upon isoelectronic substitution was proven to happen due to the preservation of the chemical bonding pattern. Theoretical VDEs were calculated for the series LiSi5−n(AlH)n− (n = 1–4) systems to facilitate their experimental detection.
[en] A new universal method is developed for determination of nanostructure kinetic stability (KS) at high temperatures, when nanostructures can be destroyed by chemical bonds breaking due to atom thermal vibrations. The method is based on calculation of probability for any bond in the structure to stretch more than a limit value Lmax, when the bond breaks. Assuming the number of vibrations is very large and all of them are independent, using the central limit theorem, an expression for the probability of a given bond elongation up to Lmax is derived in order to determine the KS. It is shown that this expression leads to the effective Arrhenius formula, but unlike the standard transition state theory it allows one to find the contributions of different vibrations to a chemical bond cleavage. To determine the KS, only calculation of frequencies and eigenvectors of vibrational modes in the groundstate of the nanostructure is needed, while the transition states need not be found. The suggested method was tested on calculating KS of bonds in some alkanes, octene isomers and narrow graphene nanoribbons of different types and widths at the temperature T=1200 K. The probability of breaking of the C–C bond in the center of these hydrocarbons is found to be significantly higher than at the ends of the molecules. It is also shown that the KS of the octene isomers decreases when the double C=C bond is moved to the end of the molecule, which agrees well with the experimental data. The KS of the narrowest graphene nanoribbons of different types varies by 1–2 orders of magnitude depending on the width and structure, while all of them are by several orders of magnitude less stable at high temperature than the hydrocarbons and benzene.
[en] The quantum dimer model, relevant for short-range resonant valence bond physics, is rigorously shown to have long range order in a crystalline phase in the attractive case at low temperature and not too large flipping term. This term flips horizontal dimer pairs to vertical pairs (and vice versa) and is responsible for the word ‘quantum’ in the title. In addition to the dimers, monomers are also allowed. The mathematical method used is ‘reflection positivity’. The model and proof can easily be generalized to dimers or plaquettes in three-dimensions. (paper)