全文获取类型
收费全文 | 84篇 |
免费 | 6篇 |
专业分类
化学 | 64篇 |
力学 | 1篇 |
数学 | 5篇 |
物理学 | 20篇 |
出版年
2022年 | 1篇 |
2021年 | 1篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 4篇 |
2014年 | 2篇 |
2013年 | 8篇 |
2012年 | 5篇 |
2011年 | 10篇 |
2010年 | 3篇 |
2009年 | 2篇 |
2008年 | 8篇 |
2007年 | 3篇 |
2006年 | 4篇 |
2005年 | 3篇 |
2004年 | 4篇 |
2003年 | 2篇 |
2002年 | 1篇 |
2001年 | 4篇 |
2000年 | 1篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1936年 | 2篇 |
1928年 | 1篇 |
1926年 | 2篇 |
1925年 | 1篇 |
排序方式: 共有90条查询结果,搜索用时 15 毫秒
1.
The molecular structure and properties of 7-azaindole in its first four singlet states were studied with a view to improving current understanding of the photophysical behavior of its C(2h) dimer. This dimer, which exhibits a double proton transfer via its two hydrogen bonds upon electronic excitation, has for 35 years been used as a model for the photophysical behavior of DNA base pairs. Electronic excitation of 7-azaindole simultaneously increases its acidity and basicity; these changes facilitate a concerted mechanism for the double proton transfer in the dimer. In this work, we found the acidity and basicity changes to occur only in its first pi,pi(*) excited singlet state. 相似文献
2.
M. J. Medina-Hernández M. Catalá-Icardo M. C. García-Alvarez-Coque 《Chromatographia》1995,41(5-6):455-461
Summary Hydrophobic character is usually expressed in terms of the partition coefficient in 1-octanol-water (log Po/w). However, measurement of this coefficient is often problematic. Retention in micellar liquid chromatography is mainly due
to hydrophobic interactions and can also be used as an index of hydrophobicity. A hydrophobicity scale was established with
retention data foro-phthalaldehyde (OPA)-N-acetyl-L-cysteine (NAC) amino acid derivatives, using the glycine derivative as reference. Since the
OPA-NAC derivatives only differ in the nature of R1 in the amino acid (R1CH (COOH)NH2), in the absence of electrostatic interactions the hydrophobic character of the substituent was responsible for retention.
Linear relationships were obtained between log of the ratiok′ of amino acid derivatives:k′ of the glycine derivative for a given mobile phase, and logP
o/w for the R1 substituent. Good correlations were also found for phenylthiohydantoin amino acid derivatives. 相似文献
3.
X. Cahís A. Benavent-Climent A. Catalán Goñi 《Revista Internacional de Métodos Numéricos para Cálculo y Dise?o en Ingeniería》2013,29(4):248-259
Waffle-flat-plate structures have been widely used for residential and office buildings in the South of Europe between the seventies and nineties of the last century. These buildings were designed with old seismic codes and its vulnerability needs to be evaluated to determine whether they need to be seismic retrofitted or not. This article proposes a simple model that represents the moment transfer between plate and column, for existing exterior connections loaded in the direction normal to the building façade. The model takes into account singularities of existing waffle-flat-plate structures such as the concentration of reinforcement in the direction of the joints, and the existence of punching shear reinforcement. The model consists of frame elements for the columns and shell elements for the slabs, both connected with flexural and torsion hinges. The strength of the flexural hinges is obtained by estimating the portion of longitudinal reinforcement of the plate that transfers directly the unbalanced moment to the column by flexure. The yielding deformations of the flexural and torsion hinges are estimated with simple expressions. The ductility of the flexural hinges is taken from FEMA 356, while infinite ductility is adopted for the torsional hinges in accordance with previous experimental studies. It is shown that the proposed model provides a good approximation of the experimental response of the connection under monotonically increasing lateral displacements, in terms of stiffness, strength and ductility. 相似文献
4.
Sergio Català‐Clariana Fernando Benavente Estela Giménez José Barbosa Victoria Sanz‐Nebot 《Electrophoresis》2013,34(13):1886-1894
Biologically active peptides derived from complex bovine milk protein hydrolysates are of particular interest in food science and nutrition because they have been shown to play different physiological roles, providing benefits in human health. In this study, we used CE‐TOF‐MS for separation and identification of bioactive peptides in three hypoallergenic infant milk formulas. An appropriate sample cleanup using a citrate buffer with DTT and urea followed by SPE with Sep‐Pack® C18 and StrataXTM cartridges allowed the detection of a large number of low molecular mass bioactive peptides. This preliminary identification was solely based on the measured experimental monoisotopic molecular mass values (Mexp). Later, we evaluated the classical semiempirical relationships between electrophoretic mobility and charge‐to‐mass ratio (me vs. q/Mα, α = 1/2 for the classical polymer model) to describe their migration behavior. The assistance of migration prediction proved to be useful to improve reliability of the identification, avoiding misinterpretations and solving some identity conflicts. After revision, the identity of 24, 30, and 38 bioactive peptides was confirmed in each of the three infant milk formulas. A significant number of these peptides were reported as inhibitors of angiotensin‐converting enzyme, however, the presence of sequences with other biological activities such as antihypertensive, antithrombotic, hypocholesterolemic, immunomodulation, cytotoxicity, antioxidant, antimicrobial, antigenic, or opioid was also confirmed. 相似文献
5.
M. Carmen Pérez Morales Dr. Julieta V. Catalán Dr. Victoriano Domingo Prof. Martín Jaraíz Dr. M. Mar Herrador Dr. José F. Quílez del Moral Prof. José‐Luis López‐Pérez Prof. Alejandro F. Barrero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(21):6598-6612
Treatment of germacrone ( 1 ) with different electrophiles, and of its epoxy derivatives germacrone‐4,5‐epoxide ( 2 ), germacrone‐1,10‐epoxide ( 3 ) and isogermacrone‐4,5‐epoxide ( 4 ) with Brönsted/Lewis acids and TiIII, gives rise to a great structural diversity. Thus, by using a maximum of two steps, the production of more than 40 compounds corresponding to 14 skeletons is described. Computational calculations rationalizing the structural divergence produced are also described. Finally, since some of the compounds generated are bioactive natural sesquiterpenes, the mechanisms of formation of these substances will provide new insights in their biosynthesis. 相似文献
6.
7.
Raquel Múrtula M. Adela Yáñez Elena Soria Vicente Catalán 《Accreditation and quality assurance》2010,15(4):217-221
The standard methodology used for the detection of bacteria in environmental samples and food is primarily based on bacterial
growth on specific culture media and confirmation by biochemical and/or immunological tests of all presumptive colonies. However,
this methodology presents a number of drawbacks, such as low sensitivity and specificity, and the long time needed to obtain
results. For this reason, the implementation of molecular methods in diagnostic laboratories has increased over the past several
years. Nevertheless, most of these newer methods have not been included in current legislation, and, in most of cases, they
have not yet been normalized. In this sense, the availability of appropriate reference materials (RMs) can help to overcome
these deficiencies. The aim of this study was to develop and validate, following ISO Guide 34, a new RM, in a tablet format,
for the quantification of Legionella pneumophila and Salmonella spp. by quantitative PCR (qPCR). This new RM can be used as a work reference sample in internal quality control, in the organization
of proficiency testing schemes (PTS), as well as for the validation of molecular methods based on qPCR. 相似文献
8.
Alejandro Madrid Villegas Katy Díaz Peralta César González Tapia Karen Catalán Marín 《Natural product research》2015,29(6):586-588
The resinous exudate, three meroterpenes, namely bakuchiol (1), 3-hydroxybakuchiol (2), 12-hydroxyisobakuchiol (3), and one furanocoumarin, psoralen (4), were isolated from the leaves of culen (Psoralea glandulosa). In addition to these, two semi-synthetic derivatives, bakuchiol acetate (5) and bakuchiol methyl eter (6), were obtained from 1, and were subsequently evaluated in vitro for the inhibitory effect of resin and compounds on the mycelial growth of Botrytis cinerea Pers.: Fr. and Phytophthora cinnamomi Rands. The resinous exudate inhibited the mycelial growth of both the pathogens, while bakuchiol (1) exhibited an inhibitory effect on the mycelial growth of B. cinerea up to 94% at a concentration of 150 mg/L and psoralen (4) reduced the mycelial growth of P. cinnamomi up to 80% at a concentration of 150 mg/L. These compounds have the ability of blocking the development of mycelial growth and may be used as a potential biopesticide in the agricultural sector once the in vivo test results have been validated. 相似文献
9.
Asymmetric Synthesis of Fluorinated Isoindolinones through Palladium‐Catalyzed Carbonylative Amination of Enantioenriched Benzylic Carbamates 下载免费PDF全文
Dr. Pablo Barrio Ignacio Ibáñez Lidia Herrera Dr. Raquel Román Dr. Silvia Catalán Prof. Dr. Santos Fustero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(32):11579-11584
The asymmetric synthesis of fluorinated isoindolinones has been achieved by a palladium‐catalyzed aminocarbonylation reaction of the corresponding α‐fluoroalkyl o‐iodobenzylamines. A base‐mediated anti β‐hydride elimination process was suggested to explain the partial erosion of the optical purity observed in some cases. This mechanistic rationale enabled the minimization of this partial racemization by fine‐tuning the pKa of the base. 相似文献
10.
In this work, we substantiate the change in the emitting state of indole caused by the dipolarity increase in the solvent 1-chlorobutane, observed on lowering the temperature from 293 to 133 K, accompanied by no significant changes in the corresponding excitation and absorption spectra. No similar changes in indole emission were observed over the temperature range 293-133 K for solutions of indole in 2-methylbutane in the presence and absence of 0.5 M 1-chlorobutane. The solvatochromism of indole in 1-chlorobutane at temperatures from 293 to 133 K allowed us to estimate the dipole moment and polarizability of the emission state of the chromophore and to detect two states (S(1) and ): one, the S(1), involving no significant change and the other, the , exhibiting a substantial change in the dipole moment of the chromophore upon electronic excitation (viz. μ(S(1)) = 2.5 and vs. μ(S(0)) = 2.13 D). The former state, S(1), is the major contributor to the structured emission of indole at temperatures from 293 to 193 K, as is the latter, S'(1) , to its structureless, red-shifted emission over the range 193-133 K. Although the emission changes of indole, dissolved in 1-chlorobutane at temperatures from 293 to 133 K, are seemingly consistent with the widely accepted photophysical model for inversion of its (1)L(b) and (1)L(a) states as the polarity of the medium is increased, below 133 K the emission becomes structured and blue-shifted, two typical features of indole above 193 K. Also, below 123 K is not feasible to photo-select the (1)L(a) state in spite of this state being the first excited electronic state of indole under large dipolarity conditions. Therefore, the established photophysical model cannot hold under these conditions and a new one accounting for these experimental facts is proposed instead. 相似文献