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991.
F. Pignatiello M. De Rosa P. Ferraro S. Grilli P. De Natale A. Arie 《Optics Communications》2007,277(1):14-18
A moiré interferometer is used to measure the thermal expansion of two ferroelectric crystals, LiNbO3 and KTiOPO4. The crystal samples are patterned with a chromium reflective grating and used as a diffractive component in a reflective grating interferometer. The thermal expansion of all the three axes of congruent LiNbO3 and of x and y axes of the flux-grown KTiOPO4 were measured from room temperature to 200 °C. For this temperature range the thermal expansion coefficient has been modeled by a second-order polynomial and its coefficients have been estimated by accurate analysis of the resulting moiré fringe pattern. 相似文献
992.
Rafael J. Rasse Rosa M. Domínguez Armando Herize Maria Tosta Doris Brusco Gabriel Chuchani 《Journal of Physical Organic Chemistry》2007,20(1):44-48
A homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k1 (s?1) = (11.01 ± 0.31) ? (109.5 ± 2.8) kJ mol?1 (2.303 RT)?1 and for 3‐methyl‐1‐buten‐3‐ol log k1 (s?1) = (11.50 ± 0.18) ? (116.5 ± 1.4) kJ mol?1 (2.303 RT)?1. Electron delocalization of the CH2?CH and C6H5 appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
993.
Santiago V. Luis M. Isabel Burguete Rosa V. Salvador 《Journal of inclusion phenomena and macrocyclic chemistry》1991,10(3):341-353
Complexes between crown ethers having ring sizes of 15–24 atoms and 5 to 8 oxygens with HgX2 species (X = Cl, I, CN, SCN) have been prepared and studied. Interactions of the substituents X bound to mercury with groups or fragments in the crown compound are essential in determining the characteristics and stability of the complexes formed. Complexes with crowns of 18C6 ring sizes are the most favourable except for HgX2 compounds for which the size of X is larger than the macrocycle ring, as is the case for X = CF3. 相似文献
994.
Diastereoselective synthesis of fulleropyrrolidines from suitably functionalized chiral cyclobutanes
Illescas BM Martín N Poater J Solà M Aguado GP Ortuño RM 《The Journal of organic chemistry》2005,70(17):6929-6932
The first diastereoselective synthesis of fulleropyrrolidines endowed with diastereomerically pure functionalized cyclobutanes is reported. The new C(60)-based cyclobutane derivatives 7a,b and 9 are suitably functionalized for further incorporation into peptide surrogates. 相似文献
995.
Martin D Illa O Baceiredo A Bertrand G Ortuño RM Branchadell V 《The Journal of organic chemistry》2005,70(14):5671-5677
[reaction: see text] The reactivity of phosphino(trimethylsilyl)carbenes 1 with several organic acids has been examined in order to evaluate the pKa values of the conjugate acids. Carbenes 1 react efficiently with C-organic acids such as 1,3-dimesitylimidazolium chloride, phenylacetylene, acetonitrile, and acetyltrimethylsilane, which have pKa's in DMSO in the range 18-31. However, the reaction of the conjugate acids 1H+ with the anion perturbs the determination of the genuine basicity of 1. Theoretical calculations have been performed in order to quantify the basicity of phosphino(trimethylsilyl)carbenes 1 and to compare them with that of N-heterocyclic carbenes 2. The pKa of 1H+ in DMSO has been computed to be in the 23.0-23.4 range, so that 1 is not strong enough as a base to spontaneously deprotonate organic acids such as phenylacetylene, acetonitrile, or acetyltrimethylsilane. However, its conjugate acid 1H+ is a strong electrophile and easily reacts with the nucleophilic conjugate bases of these acids leading to the formation of the corresponding phosphorus ylides. 相似文献
996.
Oró-Solé J Frontera C Martínez B Beltrán-Porter D Palacín MR Fuertes A 《Chemical communications (Cambridge, England)》2005,(26):3352-3354
A new phase has been observed during the sodium intercalation of hafnium nitride chloride as intermediate between the host beta-HfNCl and the already reported Na(0.29)HfNCl with Tc of 24 K; the new intermediate shows interlayer spacings ranging from 9.48 to 9.67 A, corresponds to a second stage intercalate of HfNCl and is superconducting with a critical temperature of 20 K. 相似文献
997.
In this study, we describe a technique which can be used to characterize some relevant properties of 26 cylindrical samples (15 x 30 cm2) of concrete. The characterization has been performed, according to Spanish regulations in force, by some destructive and ultrasound-based techniques using frequencies of 40 kHz. Samples were manufactured using different water/cement ratios (w/c), ranging from 0.48 to 0.80, in order to simulate different values of compressive strength at each sample. We have correlated the propagation velocity v of ultrasonic waves through the samples to compressive strength R values. As some other authors remark, there exists an exponential relationship between the two above parameters. We have found that a highly linear relationship is present between R and w/c concentration at the samples. Nevertheless, when the same linear model is adopted to describe the relationship between v and w/c, the value of r decreases significantly. Thus, we have performed a multiple regression analysis which takes into account the impact of different concrete constituents (water, cement, sand, etc.) on ultrasound propagation speed. One of the most relevant practical issues addressed in our study is the estimation of the hardening curve of concrete, which can be used to quantify the viability of applying the proposed method in a real scenario. Subsequently, we also show a detailed analysis of the temporal evolution of v and R through 61 days, beginning at the date where the samples were manufactured. After analyzing both parameters separately, a double reciprocal relationship is deduced. Using the above parameters, we develop an NDE-based model which can be used to estimate hardening time of concrete samples. 相似文献
998.
Fortes MA Rosa ME Vaz MF Teixeira PI 《The European physical journal. E, Soft matter》2004,15(4):395-406
We have carried out a systematic study of buckling-like mechanical instabilities in simple two- (2D) and three-dimensional (3D) symmetric foam clusters sandwiched between parallel planar walls. These instabilities occur when the wall separation w is reduced below a critical value, w*, for which the foam surface energy E reaches its minimum, E*. The clusters under investigation consist of either a single bubble, or of twin bubbles of fixed equal sizes (areas A in 2D or volumes V in 3D), which are either free to slide or pinned at the confining walls. We have numerically obtained w* for both free and pinned 2D and 3D clusters. Furthermore, we have calculated the buckled configurations of 2D twin bubbles, either free or pinned, and of 3D free twin bubbles, whose energy is independent of w and equal to the minimum energy E* of the unbuckled state. Finally, we have also predicted the critical wt* at which the terminal configurations under extension of 2D and 3D single and twin bubbles are realised. Experimental illustrations of these transitions under compression and extension are presented. Our results, together with others from the literature, suggest that a bubble cluster bounded by two parallel walls is stable only if the normal force it exerts on the walls is attractive, i.e., if dE/dw > 0; clusters that cause repulsion between the walls are unstable. We correlate this with the distribution of film orientations: films in a stable cluster cannot be too parallel to the confining walls; rather, their average tilt must be larger than for a random distribution of film orientations. 相似文献
999.
Auriemma F De Rosa C Esposito S Coates GW Fujita M 《Journal of the American Chemical Society》2005,127(9):2850-2851
Crystals of alternating isotactic ethylene-cyclopentene copolymer include conformational disorder of cyclopentene rings and packing disorder of chains. 相似文献
1000.
We investigate theoretically the nonequilibrium transport properties of carbon nanotube quantum dots. Owing to the two-dimensional band structure of graphene, a double orbital degeneracy plays the role of a pseudospin, which is entangled with the spin. Quantum fluctuations between these 4 degrees of freedom result in an SU(4) Kondo effect at low temperatures. This exotic Kondo effect manifests as a four-peak splitting in the nonlinear conductance when an axial magnetic field is applied. 相似文献