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Javier de Abajo Jos G. de la Campa Angel Alegría Jos M. Echave 《Journal of Polymer Science.Polymer Physics》1997,35(2):203-212
Two different series of poly(ester imide)s, which are distinguished from each other in the orientation of the ester linkages and show well-differentiated thermotropic behavior, are investigated by means of model calculations and dielectric relaxation spectroscopy. Model calculations show that the orientation of the ester linkages has a strong influence on the rotational energy barriers. The dielectric relaxation spectra of both series shows three relaxation regions in the temperature range between 100 and 400 K that have been identified as the α-, β- and γ-relaxation processes. A difference of about two orders of magnitude between the characteristic rates of the γ-relaxation is the main feature observed in the dielectric response. However, the β-relaxation shows very similar behavior for both series. The differences in the relaxation behavior in the solid state are interpreted on the basis of the rotational barriers deduced from the model calculation results. © 1997 John Wiley & Sons, Inc. 相似文献
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Pierdominici-Sottile G Echave J Palma J 《The journal of physical chemistry. B》2006,110(23):11592-11599
We have obtained AMBER94 force-field parameters for the TTQ cofactor of the enzyme methylamine dehydrogenase (MADH). This enzyme catalyzes the oxidation of methylamine to produce formaldehyde and ammonia. In the rate-determining step of the catalyzed reaction, a proton is transferred from the methyl group of the substrate to residue Asp76. We used the new parameters to perform molecular dynamics simulations of MADH in order to characterize the dynamics of the active site prior to the proton-transfer step. We found that only one of the oxygen atoms of Asp76 can act as an acceptor of the proton. The other oxygen interacts with Thr122 via a strong hydrogen bond. In contrast, because of the rotation the methyl group of the substrate, the three methyl hydrogen atoms are alternately in position to be transferred. The distance that the proton has to travel presents a broad distribution with a peak between 1.0 and 1.1 A and reaches values as short as 0.8 A. The fluctuation of the distance between the donor and the acceptor has the largest frequency component at 50 cm(-1), but the spectrum presents a rich structure between 10 and 400 cm(-1). The more important peaks appear below 250 cm(-1). 相似文献
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A method is proposed for disentangling exponentials of operators that belong to finite-dimensional Lie algebras. A straightforward matrix representation is combined with a widely used parameter-differentiation method, giving rise to a simpler and more systematic procedure. The SU(1, 1), SU(2) and double photon algebras are explicitly considered as illustrative examples.Work partly supported by Fundación Antochas Project No. 1108911 and by the USA-Argentina Cooperative Research Project NSF INT-8802247 and CONICET No. 163/88. 相似文献
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José Campos-Martínez Estela Carmona-Novillo Julián Echave Marta I. Hernández Juliana Palma 《Molecular physics》2013,111(21):1729-1735
Rate constants have been calculated using quasi-classical trajectories (QCT) for the vibrational relaxation of highly excited O2:O2(X3Σ?,vi) + O2(X3Σ?,0) → O2(X3Σ?,vf) + O2(X3Σ?) with 22 ≤vi≤28. The present full-dimensional QCT results agree very well with recent quantum reduced dimensionality calculations, giving further support to the hypothesis that the observed experimental jump in depletion rates would be due partially to enhanced vibrational relaxation. 相似文献
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Haizea Echave Dr. Rosa López Prof. Dr. Claudio Palomo 《Angewandte Chemie (International ed. in English)》2016,55(10):3364-3368
The first enantioselective direct cross‐aldol reaction of α‐keto amides with aldehydes, mediated by a bifunctional ureidopeptide‐based Brønsted base catalyst, is described. The appropriate combination of a tertiary amine base and an aminal, and urea hydrogen‐bond donor groups in the catalyst structure promoted the exclusive generation of the α‐keto amide enolate which reacted with either non‐enolizable or enolizable aldehydes to produce highly enantioenriched polyoxygenated aldol adducts without side‐products resulting from dehydration, α‐keto amide self‐condensation, aldehyde enolization, and isotetronic acid formation. 相似文献
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Pérez de Alejo R Ruiz-Cabello J Cortijo M Rodriguez I Echave I Regadera J Arrazola J Avilés P Barreiro P Gargallo D Graña M 《Magnetic resonance imaging》2003,21(8):901-912
An accurate computer-assisted method able to perform regional segmentation on 3D single modality images and measure its volume is designed using a mixture of unsupervised and supervised artificial neural networks. Firstly, an unsupervised artificial neural network is used to estimate representative textures that appear in the images. The region of interest of the resultant images is selected by means of a multi-layer perceptron after a training using a single sample slice, which contains a central portion of the 3D region of interest.The method was applied to magnetic resonance imaging data collected from an experimental acute inflammatory model (T(2) weighted) and from a clinical study of human Alzheimer's disease (T(1) weighted) to evaluate the proposed method. In the first case, a high correlation and parallelism was registered between the volumetric measurements, of the injured and healthy tissue, by the proposed method with respect to the manual measurements (r = 0.82 and p < 0.05) and to the histopathological studies (r = 0.87 and p < 0.05). The method was also applied to the clinical studies, and similar results were derived of the manual and semi-automatic volumetric measurement of both hippocampus and the corpus callosum (0.95 and 0.88). 相似文献
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J. Campos-Martínez E. Carmona-Novillo J. Echave M.I. Hernández R. Hernández-Lamoneda J. Palma 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,4(2):159-168
A reduced dimensionality model is used to study the reaction OO
O
3
(
X1A1
) + O(
3
P
) by means of time-dependent and time-independent quantum-mechanical methods. State-selected probabilities and rate constants
are obtained for the reactive process as well as for the inelastic collision in which the vibrationally excited oxygen loses
one or more quanta. It is found that the experimentally observed jump in depletion rates above a critical value of v could be partially explained by the vibrational relaxation rather than reaction. Reaction only becomes important for relatively
high translational energies and therefore the calculated rates are too small at the temperatures of interest. It is concluded,
however, that the reaction saddle point region in the potential energy surface plays a crucial role in the enhancement of
vibrational relaxation.
Received: 3 February 1998 / Revised: 27 March 1998 / Accepted: 15 May
1998 相似文献
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