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251.
Weakly bound linear and bent dimers, FH—X (where X = CO, OC, CNH, NCH, N2O and ON2), are investigated using the DFT B3LYP and ab initio MP2 methods with the same basis sets (6–311++G(3df,2pd)). The strengths of the H—C or H—N H‐bonds in dimers FH—CO, FH—CNH, and FH—N2O are compared with those of the H—O or H—N H‐bonds in dimers FH—OC, FH—NCH, and FH—ON2. The results obtained for the H‐bond distances, the elongation effect of the HF bond, the red shift of the HF stretching frequency, and the energy difference between the dimer and the charge transfer reveal that the H‐bonds of the first group of dimers are stronger than those of the second. The Gibbs energies calculated for the six dimer formations indicate that the weakly bound dimers are unstable at room temperature (T = 298 K) (FH—X's → FH + X's, ΔG < 0). 相似文献
252.
In April 1998 Raimond Castaing left the world of electrons, of ions and others particles, his wife and his family, his numerous
students, for the world of stars.
Raimond Castaing (Fig. 1) had a very strong personality. No one will forget their first meeting with him and all his students
remember how brilliant he was as a teacher. A lot of anecdotes about his famous hot temper are still circulating among his
friends and his former students. But in this paper, we would like to evoke Castaing’s memory through his achievements in Instrumental
Physics, from the time of his doctoral thesis to later developments with his students, which were all centred on the imaging
of the microstructures of materials and their quantitative chemical analysis. 相似文献
253.
Christian Bellec Daniel Bertin Roger Colau StPhane Deswarte Pierre Maitte Claude Viel 《Journal of heterocyclic chemistry》1979,16(8):1657-1659
Nitration of α-acylstilbenes with dinitrogen tetroxide leads to corresponding Z-α-acyl-β-nitrostilbenes. By chemical or electochemical reduction of these compounds, trisubstituted isoxazoles were prepared in good yields. 相似文献
254.
Jean Ravez Annie Perron Jean Pierre Chaminade Paul Hagenmuller Loïc Rivoallan 《Journal of solid state chemistry》1974,10(3):274-281
Two series of phases with tetragonal bronze-like structure and composition BaxLi5?2xT5O15 (T = Nb, Ta) have been isolated in the systems BaNb2O6LiNbO3 and BaTa2O6LiTaO3. All these phases show ferroelectric-paraelectric transitions. The Curie temperature increases with the lithium content. The value of TC for Ba2.03Li0.94Nb5O15 is the highest ever observed for this type of structure: the obtained phases are potentially good materials for the harmonic generation of the 0.53-μm radiation. The optical yield of the niobate Ba2.14Li0.71Nb5O15 is about 2.5 times that of Ba2NaNb5O15 and 250 times that of the K.D.P. The crystallographic and dielectric data of the system Ba2.14Li0.71Nb5O15Ba2.14Li0.71Ta5O15 characterize three domains, which are respectively antiferroelectric, ferroelectric, and paraelectric. The Curie temperature and the optical yield decrease with increasing tantalum content. 相似文献
255.
De‐Chun Zhang 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(6):o351-o352
In the title compound, N‐(2‐methoxyphenyl)‐4‐nitrobenzylideneamine, C14H12N2O3, the two phenyl rings make a dihedral angle of 48.0 (2)° and the nitro group is at an angle of 6.5 (1)° with respect to its attached phenyl ring. In the crystal structure, molecules are related as centrosymmetric pairs through π–π interactions and are further connected through strong C—H?O hydrogen bonds [C?O 3.4259 (17) Å and C—H?O 167°], forming molecular stacks along [100]. These stacks associate further through longer C—H?O interactions, forming two‐dimensional networks. In the c direction, there are only weak van der Waals interactions. The relationship between the molecular planarity and its centrosymmetry is also briefly described. 相似文献
256.
Julio Belmar Claudio Jimnez C. Ruiz‐Prez F. S. Delgado Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(10):o599-o601
In the title compound, C15H20N2O, the bond distances and angles are consistent with the presence of the hydroxy tautomer. This tautomer was unambiguously determined by the clear presence of a H atom bonded to oxygen, as well as the total absence of any residual electron density around the N atom in the heterocycle, thus precluding any possibility of desmotropism. 相似文献
257.
Zun‐Ting Zhang Yan‐Chang Wang 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(1):o51-o53
In the lattice of the title compound (systematic name: 5,6,7‐trihydroxy‐4′‐methoxyisoflavone monohydrate), C16H12O6·H2O, the isoflavone molecules are linked into chains through R43(17) motifs composed via O—H⋯O and C—H⋯O hydrogen bonds. Centrosymmetric R42(14) motifs assemble the chains into sheets. Hydrogen‐bonding and aromatic π–π stacking interactions lead to the formation of a three‐dimensional network structure. 相似文献
258.
Résumé Les auteurs montrent que le moment dipolaire d'une molécule peut, avec une bonne approximation, se réduire à la somme vectorielle des moments atomiques et d'un moment de charges ponctuelles centrées sur les atomes. Les charges ainsi définies sont indépendantes du système d'axes de coordonnées.
Equipe de Recherche Associée au C.N.R.S. n 22.
Remerciements. Ce travail a été en partie présenté au 2ème Colloque International des Chimistes Théoriciens d'expression latine (Paris, septembre 1970).Les auteurs remercient les participants à ce Colloque et en particulier Madame A. Pullman et Monsieur P. Claverie pour leurs remarques et leurs suggestions. Leur gratitude va également au Centre National de la Recherche Scientifique pour l'aide matérielle qu'il apporte à ces recherches. 相似文献
Population analysis by LCAO calculations: Charges and atomic moments
The authors show that the dipole moment of a molecule can be reduced to the vector sum of the atomic moments and of a moment due to point charges centred on atoms with a good accuracy. The so defined charges are independent of the coordinate system.
Zusammenfassung Die Autoren zeigen, daß das Dipolmoment von Molekülen als Vektorsumme der Atommomente und eines Moments, das von Punktladungen an den Atomen herrührt, dargestellt werden kann. Die dabei definierten Ladungen sind unabhängig vom Koordinatensystem.
Equipe de Recherche Associée au C.N.R.S. n 22.
Remerciements. Ce travail a été en partie présenté au 2ème Colloque International des Chimistes Théoriciens d'expression latine (Paris, septembre 1970).Les auteurs remercient les participants à ce Colloque et en particulier Madame A. Pullman et Monsieur P. Claverie pour leurs remarques et leurs suggestions. Leur gratitude va également au Centre National de la Recherche Scientifique pour l'aide matérielle qu'il apporte à ces recherches. 相似文献
259.
Klaus Meyersen Jean Y. C. Wang 《Journal of polymer science. Part A, Polymer chemistry》1967,5(8):1845-1853
The cyclocopolymerization of maleic anhydride and four 1,5- and 1,6-dienes (bicyclopentene, bicyclohexene, dicyclopentenyl ether, and dicyclohexenyl ether) and one tetraene (quartercyclopentene) is described. Soluble, low molecular weight copolymers were obtained from all five compounds. Their compositions approach 2:1 copolymer ratios. Fused ring structures are proposed as the main repeating units. Among the compounds listed, bicyclopentene copolymerized most easily and gave good conversions for monomer ratios of 2:1. Quartercyclopentene and dicyclopentyl ether, the other five-membered ring compounds, also polymerized to good-to-fair yields. However, a monomer ratio of about 4:1 was required to obtain conversions comparable to a 2:1 maleic anhydride—bicyclopentene polymerization. The six-membered systems, bicyclopentene and dicyclopentenyl ether, gave consistently low conversions, even with a 4:1 monomer ratio. The influence of the initiator system, initiator concentration, and reaction medium was studied on copolymerizations of bicyclopentene. Best results were obtained in acetic anhydride with azobisisobutyronitrile as the initiator. 相似文献
260.
Jean Pascal Eloundou 《European Polymer Journal》2002,38(3):431-438
Dipolar relaxations in a reactive epoxy-amine system based on diglycidyl ether of bisphenol A with 4,4′-diaminodiphenylsulfone were studied with the Havriliak-Negami function. The system was cured isothermally at 140 °C using simultaneous kinetic and microdielectric studies.The relaxation time was calculated from the frequency fmax of the peak of the loss factor ε′′ versus frequency f. A linear relation exists between the logarithm of the relaxation time τ and the glass transition temperature Tg. Then, logτ follows the Di Benedetto equation revisited by Pascault and Williams allowing the prediction of the relaxation time τ during cure.The unrelaxed permittivity at high frequency εu, the relaxed or static permittivity εs and the skewness parameter β were found independent on the conversion or the curing time. The distribution parameter α decreases as curing time t increases. The difficulty of orientation of dipoles in the electric field due to vitrification is responsible of these behaviours. 相似文献