全文获取类型
收费全文 | 6588篇 |
免费 | 517篇 |
国内免费 | 33篇 |
专业分类
化学 | 5154篇 |
晶体学 | 29篇 |
力学 | 159篇 |
综合类 | 1篇 |
数学 | 1075篇 |
物理学 | 720篇 |
出版年
2021年 | 53篇 |
2020年 | 65篇 |
2019年 | 109篇 |
2018年 | 66篇 |
2017年 | 66篇 |
2016年 | 215篇 |
2015年 | 241篇 |
2014年 | 237篇 |
2013年 | 395篇 |
2012年 | 307篇 |
2011年 | 329篇 |
2010年 | 290篇 |
2009年 | 295篇 |
2008年 | 337篇 |
2007年 | 289篇 |
2006年 | 275篇 |
2005年 | 255篇 |
2004年 | 290篇 |
2003年 | 225篇 |
2002年 | 295篇 |
2001年 | 155篇 |
2000年 | 144篇 |
1999年 | 78篇 |
1998年 | 67篇 |
1997年 | 79篇 |
1996年 | 90篇 |
1995年 | 67篇 |
1994年 | 76篇 |
1993年 | 75篇 |
1992年 | 67篇 |
1991年 | 43篇 |
1990年 | 55篇 |
1989年 | 52篇 |
1988年 | 45篇 |
1987年 | 44篇 |
1986年 | 40篇 |
1985年 | 52篇 |
1984年 | 70篇 |
1983年 | 59篇 |
1982年 | 73篇 |
1981年 | 77篇 |
1980年 | 76篇 |
1979年 | 88篇 |
1978年 | 84篇 |
1977年 | 94篇 |
1976年 | 84篇 |
1975年 | 84篇 |
1974年 | 92篇 |
1973年 | 64篇 |
1972年 | 21篇 |
排序方式: 共有7138条查询结果,搜索用时 15 毫秒
991.
Soulard P Asselin P Cuisset A Aviles Moreno JR Huet TR Petitprez D Demaison J Freedman TB Cao X Nafie LA Crassous J 《Physical chemistry chemical physics : PCCP》2006,8(1):79-92
CHFClI is among the more favorable molecules for parity violation (PV) measurements in molecules. Despite the fact that calculated PV effects are two orders of magnitude smaller than in some organometallic compounds, CHFClI displays interesting features which could make possible a new experimental PV test on this molecule. Indeed, ultrahigh resolution spectroscopy using an ultrastable CO(2) laser is favored by several intrinsic properties of this molecule. For example, the high vapor pressure of CHFClI allows investigation by supersonic beam spectroscopy. Indeed, the spectroscopic constants have been accurately determined by microwave and millimetre wave spectroscopy. This is important for the subsequent selection of an appropriate absorption band of CHFClI that could be brought to co?ncide with the absorption of CO(2). Partially resolved (+)- and (-)-CHFClI enantiomers with respectively 63.3 and 20.5% ee's have been recently prepared and analyzed by molecular recognition using chiral hosts called cryptophanes. Finally, the S-(+)/R-(-) absolute configuration was ascertained by vibrational circular dichro?sm (VCD) in the gas phase. 相似文献
992.
An intrastrand cross-link lesion, in which two neighboring nucleobases are covalently tethered, has been site-specifically synthesized into defined sequence oligonucleotides in order to perform in vitro replication studies using either bacterial replicative or translesional synthesis polymerases. The investigated tandem base lesion that involves a cross-link between the methylene group of thymine and the C8 of an adjacent guanine residue has been prepared by UV-photolysis under anaerobic condition of the photolabile precursor 5-(phenylthiomethyl)-2'-deoxyuridine that has been site-specifically incorporated into a 9-mer oligonucleotide. After ligation, the lesion-containing modified oligonucleotide was used as a DNA template in primer extension reactions catalyzed by several DNA polymerases including the fragment Klenow exo-(Kf-) of E. coli polymerase I, the Thermus aquaticus polymerase (Taq pol) and the E. coli translesional DNA polymerase Pol IV (dinB). It was found that the primer extension reaction was stopped after the incorporation of the correct nucleotide dAMP opposite the 3'-thymine residue of guanine(C8-CH2) thymine lesion by Kf- and Pol IV; however it was noted that the efficiency of the nucleotide incorporation was reduced. In contrast, the Taq polymerase was totally blocked at the nucleotide preceding the tandem lesion. These results are strongly suggestive that the present intrastrand cross-link lesion, if not repaired, would constitute a blocking lesion for prokaryotic DNA polymerases, being likely lethal for the cell. 相似文献
993.
An injection molding process for the fabrication of disposable plastic microfluidic chips with a cycle time of 2 min has been designed, developed, and implemented. Of the sixteen commercially available grades of cyclo-olefin copolymer (COC) that were screened for autofluorescence and transparency to ultraviolet (UV) light, Topas 8007 x 10 was identified as the most suitable for production. A robust solid metal mold insert defining the microfluidic channels was rapidly microfabricated using a process that significantly reduces the time required for electroplating. No wear of the insert was observed even after over 1000 cycles. The chips were bonded by thermal fusion using different bonding conditions. Each condition was tested and its suitability evaluated by burst pressure measurements. The COC microfluidic chips feature novel, integrated, reversible, standardized, ready-to-use interconnects that enable operation at pressures up to 15.6 MPa, the highest value reported to date. The suitability of these UV transparent, high pressure-resistant, disposable devices was demonstrated by in situ preparation of a high surface area porous polymer monolith within the channels. 相似文献
994.
Tessonnier JP Louis B Walspurger S Sommer J Ledoux MJ Pham-Huu C 《The journal of physical chemistry. B》2006,110(21):10390-10395
On the basis of our previous H/D exchange studies devoted to the quantification of the number of Br?nsted acid sites in solid acids, we report here an innovative approach to determine both the amount and the localization of Mo atoms inside the Mo/ZSM-5 catalyst, commonly used for the methane dehydroaromatization reaction. The influence of Mo introduction in the MFI framework was studied by means of BET, X-ray diffraction, 27Al magic angle spinning NMR, NH3 temperature-programmed desorption, and H/D isotopic exchange techniques. A dependence was found between the decrease of acidic OH groups and the Mo content. Depending on the Si/Al ratio of the zeolite, i.e., the proximity of two Br?nsted acid sites, the Mo atoms substitute a different number of OH groups. Consequently, a chemical structure was proposed to describe the geometry of the Mo complex in the channels of the ZSM-5 zeolite. 相似文献
995.
Firley D Courcot B Gillet JM Fraisse B Zouhiri F Desmaële D d'Angelo J Ghermani NE 《The journal of physical chemistry. B》2006,110(1):537-547
We have established that polyhydroxylated styrylquinolines are potent inhibitors of HIV-1 integrase (IN). Among them, we have identified (E)-8-hydroxy-2-[2-(4,5-dihydroxy-3-methoxyphenyl)-ethenyl]-7-quinolinecarboxylic acid (1) as a promising lead. Previous molecular dynamics simulations and docking procedures have shown that the inhibitory activity involves one or two metal cations (Mg2+), which are present in the vicinity of the active center of the enzyme. However, such methods are generally based on a force-field approach and still remain not as reliable as ab initio calculations with extended basis sets on the whole system. To go further in this area, the aim of the present study was to evaluate the predictive ability of the electron density and electrostatic properties in the structure-activity relationships of this class of HIV-1 antiviral drugs. The electron properties of the two chemical progenitors of 1 were derived from both high-resolution X-ray diffraction experiments and ab initio calculations. The twinning phenomenon and solvent disorder were observed during the crystal structure determination of 1. Molecule 1 exhibits a planar s-trans conformation, and a zwitterionic form in the crystalline state is obtained. This geometry was used for ab initio calculations, which were performed to characterize the electronic properties of 1. The electron densities, electrostatic potentials, and atomic charges of 1 and its progenitors are here compared and analyzed. The experimental and theoretical deformation density bond peaks are very comparable for the two progenitors. However, the experimental electrostatic potential is strongly affected by the crystal field and cannot straightforwardly be used as a predictive index. The weak difference in the theoretical electron densities between 1 and its progenitors reveals that each component of 1 conserves its intrinsic properties, an assumption reinforced by a 13C NMR study. This is also shown through an excellent correlation of the atomic charges for the common fragments. The electrostatic potential minima in zwitterionic and nonzwitterionic forms of 1 are discussed in relation with the localization of possible metal chelation sites. 相似文献
996.
Lamoureux M Patard L Hernandez B Couesnon T Santini GP Cognet JA Gouyette C Cordier C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,65(1):84-94
Successive investigations over the last decade have revealed and confirmed a stable loop closure in a family of d-[GTAC-5Pur6N7N-GTAC] hairpins, where 5Pur6N7N is a AAA, GAG and AXC loop (X being any nucleotide). The trinucleotide loop is characterized by a well defined 5Pur-7N mispairing mode, and by upfield chemical shifts for three sugar protons of the apical nucleotide 6N. The GTTC-ACA-GAAC DNA hairpin, of interest for its likely involvement in Vibrio cholerae genome mutations, has now been investigated. The GTAC-ACA-GTAC DNA hairpin has also been studied because it is intermediate between the other structures, as it contains the loop of the hairpin under consideration and the stem of the above family. The two hairpins with the ACA loop are stable. They show the same mispairing mode and similar upfield shifts as the previous family, but GTTC-ACA-GAAC seems to be slightly less compact than any other. GTTC-ACA-GAAC is remarkable in that it exhibits a B(II) character for the phosphate-ester conformation at 8Gp9A, together with a swing of the upper hairpin into the major groove that, in particular, brings 6CH1' roughly as close to 7AH2 as to 6CH6. These unexpected structural features are qualitatively deduced from (1)H and (31)P NMR spectra, and confirmed by Raman spectroscopy. This comparative study shows that not only the loop sequence but also the stem sequence may control hairpin structures. 相似文献
997.
Cravino A Roquet S Leriche P Alévêque O Frère P Roncali J 《Chemical communications (Cambridge, England)》2006,(13):1416-1418
Introduction of dicyanovinyl groups on a triphenylamine-based conjugated system leads to an intramolecular charge transfer which extends the spectral response and raises the open-circuit voltage of the resulting hetero-junction solar cells. 相似文献
998.
Figueira-Duarte TM Clifford J Amendola V Gégout A Olivier J Cardinali F Meneghetti M Armaroli N Nierengarten JF 《Chemical communications (Cambridge, England)》2006,(19):2054-2056
The synthesis and excited state properties of a compound assembling C60 with a new multi-photon absorption chromophore are reported. 相似文献
999.
Edder C Armstrong PB Prado KB Fréchet JM 《Chemical communications (Cambridge, England)》2006,(18):1965-1967
We report the design, synthesis and characterization of new benzothiadiazole- and pyrrole-based copolymers whose solubility and bandgap drastically change after thermal treatment of their thin films. 相似文献
1000.
Mass-selected peptide ions produced by electrospray ionization were deposited as ions by soft-landing (SL) onto fluorinated and hydrogenated self-assembled monolayer (FSAM and HSAM) surfaces using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially designed for studying collisions of large ions with surfaces. Analysis of modified surfaces was performed in situ by combining 2 keV Cs(+) secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR-SIMS). Similar SIMS spectra obtained following SL at different collision energies indicate that peptide fragmentation occurred in the analysis step (SIMS) rather than during ion deposition. The effect of the surface on SL was studied by comparing the efficiencies of SL on gold, FSAM, HSAM, and COOH-terminated SAM surfaces. It was found that FSAM surfaces are more efficient in retaining ions than their HSAM analogues, consistent with their larger polarizability. The efficiency of soft-landing of different peptides on the FSAM surface increases with the charge state of the ion, also consistent with an ion-polarizable molecule model for the initial stage of soft-landing on SAM surfaces. The gradual decrease of peptide ion deposition efficiency with an increase in collision energy found experimentally was quantitatively rationalized using the hard-cube model. 相似文献