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排序方式: 共有143条查询结果,搜索用时 171 毫秒
101.
Solans-Monfort X Sodupe M Mó O Yañez M Elguero J 《The journal of physical chemistry. B》2005,109(41):19301-19308
The interaction of a large set of bases covering a wide range of the basicity scale with HZSM5 medium-size zeolites has been investigated through the use of two model clusters, namely 5T and 7T:63T. The 5T cluster has been treated fully ab initio at the B3LYP level, whereas the 63T cluster has been treated with the ONIOM2 scheme using the B3LYP:MNDO combination for geometry optimizations and B3LYP:HF/3-21G for adsorption energies. The optimized geometries of the different hydrogen bond (HB) and ion pair (IP) complexes obtained with both models are rather similar. However, there are significant dissimilarities as far as the adsorption energies are concerned, in particular when dealing with IP clusters whose intrinsic stability is largely underestimated when the simpler 5T model is used. 5T clusters could be used to obtain reasonable estimates of adsorption energies provided these are scaled by a factor of 1.1 for HB complexes and 1.4 for IP complexes. The zeolite cavity favors the proton transfer process, similarly to that found by third polar partners in gas-phase HB trimers. The intrinsic basicity of the base and its adsorption energy within the zeolite are correlated. From this correlation, is possible to conclude that, in general, bases with proton affinities (PA) larger than 200 kcal mol(-1) should lead to the formation of IPs, whereas bases with PA smaller than this value should form HB complexes. 相似文献
102.
Mó O Yáñez M Eckert-Maksić M Maksić ZB Alkorta I Elguero J 《The journal of physical chemistry. A》2005,109(19):4359-4365
Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X). For A-X bonds involving electronegative atoms (A = C, N, O, F, P, S, and Cl) the bond energies achieve a maximum around Z(X) = 5. The same behavior is observed when BDEs are plotted against the electronegativity chi(X) of the atom X. Thus, for A-X bonds (A = Li, Be, B, Na, Mg, Al, Si), the BDEs for a fixed A increases, grosso modo, as the electronegativity differences between X and A increase, with some exceptions, which reflect the differences in the relaxation energies of the radicals produced upon the bond cleavage. A similar trend, albeit less pronounced, is found for single A-X bonds, where A = C, N, O, F, P, S, and Cl. However, there is an additional feature embodied in the enhancement of the strength of the A-boron bonds due to the ability of boron to act as a strong electron acceptor. The trends in bond lengths and charge densities at the bond critical points are in line with the aforementioned behavior. 相似文献
103.
Guillemin JC Riague el H Gal JF Maria PC Mó O Yáñez M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(7):2145-2153
The gas-phase acidity of CH3-CH2XH (X=S, Se, Te), CH2=CHXH (X=S, Se, Te) and PhXH (X=S, Se) compounds was measured by means of Fourier transform ion cyclotron resonance mass spectrometry. To analyze the role that unsaturation plays on the intrinsic acidity of these systems, a parallel theoretical study, in the framework of the G2 and the G2(MP2) theories, was carried out for all ethyl, ethenyl (vinyl), ethynyl, and phenyl O-, S-, Se-, and Te-containing derivatives. Unsaturated compounds are stronger acids than their saturated analogues, because of the strong pi-electron donor ability of the heteroatoms that contributes to a large stabilization of the unsaturated anions. Ethynyl derivatives are stronger acids than vinyl compounds, while phenyl derivatives have an intrinsic acidity intermediate between that of the corresponding vinyl and ethynyl analogues. The CH2=CHXH vinyl compounds (enol-like) behave systematically as slightly stronger acids than their CH3-C(H)X (keto-like) tautomers. Vinyl derivatives are stronger acids than ethyl compounds, because the anion stabilization attributable to unsaturation is greater than that undergone in the neutral compounds. Conversely, the enhanced acidity of the ethynyl derivatives with respect to the vinyl compounds is due to two concomitant effects, the stabilization of the anion and the destabilization of the neutral compound. The acidities of ethyl, vinyl, and ethynyl derivatives containing heteroatoms of Groups 14, 15, and 16 of the periodic table are closely related, and reflect the differences in electronegativity of the CH3CH2-, CH2=CH-, and CH[triple chemical bond]C- groups. 相似文献
104.
Abboud JL Herreros M Notario R Mó O Yáñez M Regitz M Elguero J 《The Journal of organic chemistry》1996,61(22):7813-7818
The experimental determination of physical properties of tetra-tert-butylphosphocubane (TtBuPC) is of paramount importance for understanding the reactivity of this fascinating molecule.The gas-phase basicity of TtBuPC measured by FT-ICR spectroscopy (GB = 221.8 kcal mol(-)(1), PA = 230.5 kcal mol(-)(1)) is surprisingly high although protonation in solution is only achieved under drastic conditions. A molecular orbital treatment, including electron correlation effects, predicts the unsubstituted parent compound phosphacubane (PC) to be a carbon base. Carbon protonation implies a P-C bond fission which alleviates the strain of the system. A similar behavior is also predicted for the tetramethyl derivative. However, TtBuPC is found to be a phosphorus base, because the strong repulsive interactions which appear between the tert-butyl substituents destabilize significantly the C-protonated form. These effects decrease dramatically when just one tert-butyl group is removed and both P- and C-protonated species become almost degenerate. As for other strained systems, the PAs of PC and TtBuPC are only adequately reproduced when G2-type [6-311+G(3df,2p)] basis sets are used. 相似文献
105.
Timea Claudia Ghitea Amina El-Kharoubi Mariana Ganea Erika Bimbo-Szuhai Tiberiu Sebastian Nemeth Gabriela Ciavoi Monica Foghis Luciana Dobjanschi Annamaria Pallag Otilia Micle 《Molecules (Basel, Switzerland)》2021,26(2)
(1) The metabolic syndrome (MS) promotes acute and chronic infections, due to the pro-inflammatory condition given by TNFα and IL6 or by affecting the microbiota. MS is also correlated with insulin resistance, causing inflammation and infections throughout the organism. (2) The purpose of this study was to track the effect of using the essential oil of Origanum vulgare L. (EOO) as an antibacterial treatment, compared to allopathic treatment with antibiotics in MS patients. A group of 106 people with MS was divided into four subgroups: L1—staphylococcal infection group, L2—Escherichia coli infection group, L3—streptococcal infection group with EOO treatment, and CG—control group without infection or treatment. (3) EOO is responsible for the antibacterial effect, and reduced minor uncomplicated infections. After a 10-day treatment, intestinal side effects were absent, improving the phase angle. (4) The results suggest that EOO may exhibit an antibacterial effect, similar to the antibiotic treatment, without promoting MS-specific dysbiosis, and it also improves the phase angle in patients, which is used as an index of health and cellular function. 相似文献
106.
Trujillo C Mó O Yañez M Tortajada J Salpin JY 《The journal of physical chemistry. B》2008,112(17):5479-5486
The gas-phase reactions between Ca(2+) and selenourea were investigated by means of electrospray/tandem mass spectrometry techniques. The MS/MS spectra of [Ca(selenourea)](2+) complexes show intense peaks at m/z 43, 121, 124, and 146 and assigned to monocations produced in different coulomb explosion processes. The structures and bonding characteristics of the stationary points of the [Ca(selenourea)](2+) potential energy surface (PES) were theoretically studied by DFT calculations carried out at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level. The analysis of the topology of this PES allows identification of H(2)NCNH(+), CaSeH(+), selenourea(+). and CaNCSe(+) ion peaks at m/z 43, 121, 124, and 146, respectively. The reactivity of selenourea and the topology of the corresponding potential energy surface mimic that of thiourea. However, significant dissimilarities are found with respect to urea. The dissociative electron-transfer processes, not observed for urea, is one of the dominant fragmentations for selenourea, reflecting its much lower ionization energy. Similarly, the coulomb explosions yielding CaXH(+) + H(2)NCNH(+) (X = O or Se), which for urea are not observed, are very favorable for selenourea. Finally, while in urea the loss of NH(3) competes with the formation of NH(4+), for selenourea the latter process is clearly dominant. 相似文献
107.
Viorel Sasca Elena Maria Mosoarca Livia Avram Ramona Tudose Otilia Costisor 《Journal of Thermal Analysis and Calorimetry》2008,94(2):355-360
The thermal decomposition of Mannich base N,N′-tetra(4-antipyrylmethyl)-1,2-diaminoethane (TAMEN), and its Ni(II), binuclear
complex, Ni2(TAMEN)Cl4, in air and in nitrogen atmosphere, were investigated. X-ray powder diffractometry, infrared spectroscopy and simultaneous
thermogravimetry-differential thermal analysis (TG-DTA), have been used to characterize and to study the thermal behavior
of these compounds. The results provided information concerning the stoichiometry, crystallinity, thermal stability and decomposition
mechanism of the compound. 相似文献
108.
Sanz P Mó O Yáñez M Elguero J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(14):4225-4232
The properties of the intramolecular hydrogen bond (IMHB) in tropolone, aminotropone, and aminotroponimine have been compared with those in the corresponding saturated analogues at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In general, all those compounds in which the seven-membered ring is unsaturated exhibit a stronger IMHB than their saturated counterparts. Nevertheless, this enhanced strength is not primarily due to resonance-assisted hydrogen-bond effects, but to the much higher intrinsic basicity and acidity of the hydrogen-bond acceptor and donor groups, respectively, in the unsaturated compounds. These acidity and basicity enhancements have a double origin: 1) the unsaturated nature of the moiety to which the hydrogen-bond donor and acceptor are attached and 2) the cyclic nature of the compounds under scrutiny. As has been found for hydroxymethylene and aminomethylene cyclobutanones, and cyclobutenones and their nitrogen-containing analogues, the IMHB strength follows the [donor, acceptor] trend: [OH, C=NH]>[OH, C=O]>[NH(2), C=NH]>[NH(2), C=O] and fulfills a Steiner-Limbach correlation similar to that followed by intermolecular hydrogen bonds. 相似文献
109.
Liposomes-entrapped chondroitin sulphate: ultrastructural characterization and in vitro biocompatibility 总被引:1,自引:0,他引:1
Trif M Moldovan L Moisei M Craciunescu O Zarnescu O 《Micron (Oxford, England : 1993)》2008,39(7):1042-1045
The purpose of this study was ultrastructural characterization of liposomes-entrapped chondroitin sulphate and to prove their in vitro biocompatibility in a human dermal fibroblast culture system, in order to use liposome-entrapped chondroitin sulphate in the treatment of inflammatory disorders. Chondroitin sulphate entrapped in liposomes appears as electron-dense particles in ultra-thin section. Comparative studies using chondroitin sulphate, empty liposomes and liposome-chondroitin sulphate systems were performed in order to evaluate their effect on growth and morphology of fibroblasts after 48 h of culture. Light microscopy indicated that chondroitin sulphate, empty liposomes and liposome-chondroitin sulphate systems do not induce appreciable cytotoxic effects, and cells maintain normal morphology when compared to control fibroblasts. 相似文献
110.
Rafal Zuzak Manish Kumar Otilia Stoica Diego Soler-Polo Jiri Brabec Katarzyna Pernal Libor Veis Remi Blieck Antonio M. Echavarren Pavel Jelinek Szymon Godlewski 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2024,136(9):e202317091
The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open-shell character has so far been missing. Here, we present the on-surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many-body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet-triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene. 相似文献