全文获取类型
收费全文 | 694篇 |
免费 | 16篇 |
国内免费 | 3篇 |
专业分类
化学 | 454篇 |
晶体学 | 5篇 |
力学 | 27篇 |
数学 | 91篇 |
物理学 | 136篇 |
出版年
2023年 | 7篇 |
2022年 | 20篇 |
2021年 | 22篇 |
2020年 | 15篇 |
2019年 | 12篇 |
2018年 | 16篇 |
2017年 | 5篇 |
2016年 | 25篇 |
2015年 | 25篇 |
2014年 | 23篇 |
2013年 | 35篇 |
2012年 | 57篇 |
2011年 | 62篇 |
2010年 | 38篇 |
2009年 | 34篇 |
2008年 | 46篇 |
2007年 | 51篇 |
2006年 | 38篇 |
2005年 | 49篇 |
2004年 | 27篇 |
2003年 | 25篇 |
2002年 | 30篇 |
2001年 | 3篇 |
2000年 | 6篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1915年 | 1篇 |
排序方式: 共有713条查询结果,搜索用时 132 毫秒
131.
Serena Staccioli Maria Altamura Gianluca Giorgi Antonio Triolo 《Journal of mass spectrometry : JMS》2009,44(7):1081-1086
Di(hydroxybutyl) ether (DHBE), a liver protecting drug, is composed of a mixture of three regioisomers: 4‐(3‐hydroxybutoxy)‐2‐butanol (1), 3‐(4‐hydroxy‐2‐butoxy)‐1‐butanol (2), and 3‐(3‐hydroxybutoxy)‐1‐butanol (3). Unequivocal differentiation of each regioisomer of DHBE was rapidly obtained without isolation of the single components, using GC‐MS with electron ionization (EI). The mass spectrum of 1 showed a rearrangement ion at m/z 118, characteristic of the 3‐hydroxybutyl chain, deriving from loss of acetaldehyde from the molecular ion, whereas 2 and 3 were characterized by the ion at m/z 117, expected from α‐cleavage of the 4‐hydroxy‐2‐butyl chain. The species at m/z 118, in turn, loses a water molecule via a mechanism involving both alcohol hydrogens, as shown by deuterium exchange experiments. Both this finding and theoretical calculations support a mechanism in which the loss of acetaldehyde in 1 occurs via a cyclic intermediate, stabilized by a strong hydrogen bond between the alcohol oxygen bearing the charge and the other alcohol oxygen, and involves initial hydrogen transfer from the former to the latter. The EI spectrum of 2, having two 4‐hydroxy‐2‐butyl chains, showed the fragmentations expected from classical fragmentation rules of aliphatic ethers and alcohols, whereas the EI spectrum of 3, bearing one 4‐hydroxy‐2‐butyl and one 3‐hydroxybutyl chain, showed essentially the characteristic fragments of both chains. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
132.
Austin Nevin Daniela Comelli Iacopo Osticioli Lucia Toniolo Gianluca Valentini Rinaldo Cubeddu 《Analytical and bioanalytical chemistry》2009,395(7):2139-2149
The assessment of the influence of natural and artificial ageing on the spectrofluorescence of triterpenoid varnishes dammar
and mastic is the focus of this work. Both Fourier transform infrared (FTIR) microscopy using attenuated total reflectance
and Raman spectroscopy have been employed for complementary molecular analysis of samples. Synchronous fluorescence spectroscopy,
excitation emission spectroscopy, and statistical analysis of data have been used to monitor changes in the optical properties
of varnish samples. Assessment of naturally and artificially aged samples using excitation emission spectroscopy suggests
that extensive exposure to visible light does not lead to easily appreciable differences in the fluorescence of mastic and
dammar; cluster analysis has been used to assess changes, which occur with artificial ageing under visible light, indicating
that differences in the fluorescence spectra of aged triterpenoids may be insufficient for their discrimination. The results
highlight significant differences between the initial fluorescence of films of dammar and mastic and the fluorescence, which
develops with ageing and oxidation, and specific markers, which change with ageing in FTIR and Raman spectra, have been identified. 相似文献
133.
Buscemi S Pace A Piccionello AP Pibiri I Vivona N Giorgi G Mazzanti A Spinelli D 《The Journal of organic chemistry》2006,71(21):8106-8113
The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine or methylhydrazine as bidentate nucleophiles has been investigated. The reaction occurred through the addition of the bidentate nucleophile to the C(5)-N(4) double bond of the 1,2,4-oxadiazole followed by ring-opening and ring-closure (ANRORC) involving the second nucleophilic site of the reagent. This ring-closure step could involve either the original C(3) of the 1,2,4-oxadiazole (giving a five-to-five membered ring rearrangement) or an additional electrophilic center linked to it (exploiting a five-to-six membered ring rearrangement). An alternative initial nucleophilic attack may involve the additional electrophilic center linked at C(3), that is the carbonyl group, leading to the formation of the hydrazones which undergo the Boulton-Katritzky rearrangement (BKR). The chosen reaction path is a function of the used nucleophile and of the nature of the substituent at C(3). At variance with previous hypotheses, when methylhydrazine was used, the observed regiochemistry always showed the preferred initial attack by the less hindered NH(2) end of the nucleophile on C(5). Moreover, new spectroscopic evidence allowed the assignment of correct structures to the products formed by reaction of 5-perfluoroalkyl-3-phenyl-1,2,4-oxadiazoles with methylhydrazine. 相似文献
134.
Absolute frequency stabilization of an extended-cavity diode laser at 0.94 μm is reported. The diode laser was frequency locked against rovibrational absorption lines of water vapour by using the frequency modulation spectroscopy technique. The stabilized oscillator shows a short-term frequency stability level of 40 kHz for integration times of 1 s and a long-term frequency drift lower than 10 MHz for observation times longer than 103 s. The frequency-stabilized oscillator system is mounted on a compact breadboard (75 cm×50 cm) and constitutes the seed laser system for the injection of a high-energy DIAL laser transmitter operating in the 0.94-μm spectral region. 相似文献
135.
Carla Gasbarri Maurizio Ronci Antonio Aceto Roshan Vasani Gianluca Iezzi Tullio Florio Federica Barbieri Guido Angelini Luca Scotti 《Molecules (Basel, Switzerland)》2021,26(17)
The aim of this work was to deeply investigate the structure and properties of electrochemically synthesized silver nanoparticles (AgNPs) through high-resolution techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), Zeta Potential measurements, and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). Strong brightness, tendency to generate nanoclusters containing an odd number of atoms, and absence of the free silver ions in solution were observed. The research also highlighted that the chemical and physical properties of the AgNPs seemed to be related to their peculiar oxidative state as suggested by X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRPD) analyses. Finally, the MTT assay tested the low cytotoxicity of the investigated AgNPs. 相似文献
136.
Calcagni Gianluca Modesto Leonardo Nicolini Piero 《The European Physical Journal C - Particles and Fields》2014,74(8):1-13
Application of the effective action approach to amplitudes with loop integration is studied for collisions on two and three centers with possible gluon emission. A rule is formulated for the integration around pole singularities in the induced vertices which brings the results in agreement with the QCD. It is demonstrated that the amplitudes can be restored from the purely transverse picture by introducing the standard Feynman propagators for intermediate gluons and quarks. 相似文献
137.
We investigate the asymptotic decrease of the Wannier functions for the valence and conduction band of graphene, both in the monolayer and the multilayer case. Since the decrease of the Wannier functions is characterised by the structure of the Bloch eigenspaces around the Dirac points, we introduce a geometric invariant of the family of eigenspaces, baptised eigenspace vorticity. We compare it with the pseudospin winding number. For every value $n \in \mathbb {Z}$ of the eigenspace vorticity, we exhibit a canonical model for the local topology of the eigenspaces. With the help of these canonical models, we show that the single band Wannier function $w$ satisfies $|w(x)| \le {\mathrm {const}} \cdot |x|^{-2}$ as $|x| \rightarrow \infty $ , both in monolayer and bilayer graphene. 相似文献
138.
Armentano D De Munno G Di Donna L Sindona G Giorgi G Salvini L Napoli A 《Journal of the American Society for Mass Spectrometry》2004,15(2):268-279
Electrospray tandem mass spectrometry (ESI-MS/MS) is used to evaluate the assembling of cytosine and thymine nucleosides in the gas phase, through the formation of hydrogen bonded supermolecules. Mixtures of cytidine analogues and homologues deliver in the gas phase proton-bound heterodimers stabilized by multiple interactions, as proven by the kinetics of their dissociation into the corresponding protonated monomers. Theoretical calculations, performed on initial structures of methylcytosine homodimers available in the literature, converged to a minimized structure whereby the two pyrimidine rings interact through the formation of three hydrogen bonds of similar energy. The crystallographic data here reported show the equivalency of the two interacting pyrimidines which is attributable to the presence of an inversion center. Thymine and uracil pyrimidyl nucleosides form, by ESI, gaseous proton-bound dimers. The kinetic of their dissociation into the related protonated monomers shows that the nucleobases are weekly interacting through a single hydrogen bond. The minimized structure of the protonated heterodimer formed by thymine and N-1-methylthymine confirmed the existence of mainly one hydrogen bond which links the two nucleobases through the O4 oxygens. No crystallographic data exists on thymine proton-bound species, nor have we been able to obtain these aggregates in the solid phase. The gaseous phase, under high vacuum conditions, seems therefore a suitable environment where vanishing structures produced by ESI can be studied with a good degree of approximation. 相似文献
139.
C Chirivì G Fontana D Monti G Ottolina S Riva B Danieli 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(33):10355-10361
Laccase-catalysed oxidation of ergot alkaloids in the absence of chemical mediators allowed the unexpected isolation of the mono-hydroxylated derivatives of compounds 2-7. Structure determination by NMR techniques clearly indicated that hydroxylation took place at the C-4 benzylic position. Quite notably, the proposed protocol allowed, for the first time, functionalisation at the C-4 position of the ergoline skeleton. Depending on the absence or on the presence of a C-10 α-methoxy substituent, hydroxylation was either stereoselective (furnishing C-4α OH derivatives) or gave rise to a C-4α/C-4β OH mixture in a 2:1 ratio, respectively. 相似文献