全文获取类型
收费全文 | 2393篇 |
免费 | 56篇 |
国内免费 | 8篇 |
专业分类
化学 | 1381篇 |
晶体学 | 10篇 |
力学 | 57篇 |
综合类 | 2篇 |
数学 | 325篇 |
物理学 | 682篇 |
出版年
2022年 | 23篇 |
2021年 | 29篇 |
2020年 | 30篇 |
2019年 | 35篇 |
2017年 | 25篇 |
2016年 | 48篇 |
2015年 | 31篇 |
2014年 | 47篇 |
2013年 | 107篇 |
2012年 | 104篇 |
2011年 | 122篇 |
2010年 | 61篇 |
2009年 | 44篇 |
2008年 | 92篇 |
2007年 | 101篇 |
2006年 | 89篇 |
2005年 | 112篇 |
2004年 | 75篇 |
2003年 | 80篇 |
2002年 | 62篇 |
2001年 | 62篇 |
2000年 | 59篇 |
1999年 | 34篇 |
1998年 | 23篇 |
1997年 | 38篇 |
1996年 | 33篇 |
1995年 | 50篇 |
1994年 | 48篇 |
1993年 | 47篇 |
1992年 | 42篇 |
1991年 | 34篇 |
1990年 | 37篇 |
1989年 | 24篇 |
1988年 | 25篇 |
1987年 | 22篇 |
1986年 | 17篇 |
1985年 | 33篇 |
1984年 | 26篇 |
1983年 | 17篇 |
1982年 | 26篇 |
1981年 | 33篇 |
1980年 | 21篇 |
1979年 | 34篇 |
1978年 | 28篇 |
1977年 | 31篇 |
1976年 | 36篇 |
1975年 | 31篇 |
1974年 | 26篇 |
1973年 | 37篇 |
1972年 | 26篇 |
排序方式: 共有2457条查询结果,搜索用时 15 毫秒
81.
Studies of dispersion patterns in nonsegmented streams, flowing through narrow open tubes, show that it is possible to obtain highly reproducible concentration gradients within a sample zone injected into the moving stream. By varying the geometry of the flow path, low, medium and high dispersion patterns can be achieved; the high dispersion pattern forms the basis for a new approach to continuous flow titrimetry. In this type of titration, discrete samples are passed through a gradient device and are then mixed with a continuously flowing stream of titrant of fixed concentration. The new technique has been tested for potentiometric as well as spectrophotometric end-point indication. A simple one-channel system allows titrations to be performed automatically in less than 1 min. 相似文献
82.
We report the first application of pulsed resonance Raman spectroscopy to the study of short-lived free radicals produced by pulse radiolysis. A single pulse from a flash-lamp pumped tunable dye laser is used to excite the resonance Raman spectrum of the p-terphenyl anion radical with an initial concentration of 4 × 10?5 moles per liter and a half life of 2 μs. The spectrum is recorded by an optical multichannel system consisting of an image intensifier coupled to a TV-camera. 相似文献
83.
Peptidase-catalyzed formation of macrocyclic lactams on solid phase identifies ring systems that are favorably bound in the enzyme active site. We evaluated several cyclic peptide motifs linked by ester bonds between the P2 and P1' or the P1 and P2' side chains. The depsipeptide represented by structure 5 was readily generated by a variety of peptidases from precursor omega-amino acids or omega-amino esters. This strategy for identifying ring systems for potential macrocyclic transition state analogues was demonstrated with the serine peptidases trypsin and chymotrypsin, with the aspartic peptidase pepsin, and with the zinc peptidase thermolysin. 相似文献
84.
The electronic structure and spectrum of pyrrole have been studied using the semiempirical LCAO SCF MO method. With configuration interaction included, low excited singlet states are calculated to occur at 5.98 ev, 6,74 ev, 7,33 ev, and 8,20 ev, in good agreement with the experimental values of about 5.88 ev, 6.77 ev, and 7,21 ev. The dipole moment of the molecule is calculated to 1.84 D, to be compared with the experimental value 1.80 D. Tables of coreattraction integrals for combinations of carbon and nitrogen atoms are presented.
Zusammenfassung Elektronenstruktur und -spektrum des Pyrrols wurden nach, der halbempirischen LCAO SCF MO-Methode untersucht. Unter Einschluß der Konfigurationenwechselwirkung wurden niedrigliegende angeregte Singulettniveaus bei 5,98, 6,74, 7,33 und 8,20 eV (über dem Grundzustand) errechnet, in guter Übereinstimmung mit den experimentellen Werten von etwa 5,88, 6,77 und 7,21 eV. Das Dipolmoment des Moleküls ergibt sich zu 1,84 D, bei einem experimentellen Wert von 1,80 D.Tabellen von (sphärischen) Rumpf-Elektron-Integralen für alle vier Kombinationen von C und N werden angegeben.
Résumé La structure et le spectre électroniques du pyrrole ont été étudiés à l'aide de la méthode sémiempirique LCAO SCF MO. L'interaction de configurations inclue, le calcul donne des états excités à 5,98, 6,74, 7,33 et 8,20 eV (au-dessus de l'état fondamental), en bon accord avec les valeurs expérimentales de 5,88, 6,77 et 7,21 eV. Le moment dipolaire calculé de la molécule est 1,84 D, l'expérience donnant 1,80 D. Des tables d'intégrales d'attraction entre un coeur sphérique et un électron d'un autre atome sont données pour les quatre combinaisons d'atomes C et N.相似文献
85.
Jianhua WangElo Harald Hansen 《Analytica chimica acta》2002,456(2):283-292
An automated sequential injection (SI) on-line solvent extraction-back extraction separation/preconcentration procedure is described. Demonstrated for the assay of cadmium by electrothermal atomic absorption spectrometry (ETAAS), the analyte is initially complexed with ammonium pyrrolidinedithiocarbamate (APDC) in citrate buffer and the chelate is extracted into isobutyl methyl ketone (IBMK), which is separated from the aqueous phase by means of a newly designed dual-conical gravitational phase separator. A metered amount of the organic eluate is aspirated and stored in the PTFE holding coil (HC) of the SI-system. Afterwards, it is dispensed and mixed with an aqueous back extractant of dilute nitric acid containing Hg(II) ions as stripping agent, thereby facilitating a rapid metal-exchange reaction with the APDC ligand and transfer of the Cd into the aqueous phase. The aqueous phase is separated in a second dual-conical gravitational phase separator, and 30 μl of it is entrapped and metered in a sample loop (SL) and subsequently introduced via air segmentation into the graphite tube for analyte quantification. The ETAAS determination is performed in parallel with the separation/preconcentration process of the ensuing sample. An enrichment factor of 21.4, a detection limit of 2.7 ng l−1, along with a sampling frequency of 13 h−1 were obtained at a sample flow rate of 6.0 ml min−1. The precision (R.S.D.) at the 0.4 μg l−1 level was 1.8% as compared to 3.2% when quantifying the organic extractant directly. The applicability of the procedure is demonstrated for the determination of trace levels of cadmium in three certified reference materials. 相似文献
86.
Margiolaki I Margadonna S Prassides K Hansen T Ishii K Suematsu H 《Journal of the American Chemical Society》2002,124(38):11288-11289
The powder neutron diffraction technique has been used for the direct observation of magnetic scattering below a Curie temperature of approximately 14 K in the fullerene-based molecular ferromagnet Eu6C60. Europium is in the divalent state with a magnetic moment of 7.1(3) muB per atom, and the configurational symmetry of the magnetic structure is body-centered cubic. Close contacts between Eu2+ and neighboring C60 units provide the signature of orbital hybridization, which can evidently account for the conducting and magnetic properties of the material. 相似文献
87.
Bjrn E. Berg Ellen M. Hansen Synnve Gjrven Tyge Greibrokk 《Journal of separation science》1993,16(6):358-363
Supercritical carbon dioxide can be utilized both as an introduction solvent in capillary SFC and as a reaction medium; both possibilities are illustrated in this study. An off-line SFE unit was modified for on-line SFE-SFC. To facilitate rapid depressurization of the extraction cell and to prevent memory effects, a 6-port valve was installed at the outlet line of the extraction unit. An increased background signal was obtained when PEEK polymer was used in the construction of the extraction vessel; when stainless steel was used, the blanks improved. The synthesis of methyl esters and butyl esters of fatty acids from triglycerides in edible fat was examined using an immobilized lipase as a catalyst in on-line SFE-SFC. As a result of 30 minutes reaction – extraction time, high yields of fatty acid esters were obtained at a pressure of 150 bar and a temperature of 50°C. 相似文献
88.
Jan Becher Klaus Brsndum Poul Hansen Jens Peter Jan Jacobsen 《Journal of heterocyclic chemistry》1988,25(3):795-798
The anions of 1,4-diaryl-3-buten-2-ones 1 reacts with arylisothiocyanates, yielding intermediates 4 which can ring close to 5,6-dihydro-4H-thiopyran-4-ones 5 . Under similar reaction conditions ethyl 3-oxo-4-pentenoates 7 gives the 6-spiropyrans 9 . Methylation of 3 gives the S-methylated open form 6 . 相似文献
89.
A rapid, robust, sensitive and selective time-based flow injection (FI) on-line solvent extraction system interfaced with electrothermal atomic absorption spectrometry (ETAAS) is described for analyzing ultra-trace amounts of Cr(VI). The sample is initially mixed on-line with isobutyl methyl ketone (IBMK). The Cr(VI) is complexed by reaction with ammonium pyrrolidine dithiocarbamate (APDC), and the non-charged Cr(VI)–PDC chelate formed is extracted into IBMK in a knotted reactor made from PTFE tubing. The organic extractant is separated from the aqueous phase by a gravity phase separator with a small conical cavity and delivered into a collector tube, from which 55 μl organic concentrate is subsequently introduced via an air flow into the graphite tube of the ETAAS instrument. The operations of the FI-system and the ETAAS detector are synchronously coupled. A significant advantage of the approach is that matrix constituents, such as high salt contents, effectively are eliminated. The extraction procedure was optimized by a simplex approach. A central composite design was subsequently employed to verify the estimated operational optimum. An 18-fold enhancement in sensitivity of Cr(VI) was achieved after preconcentration for 99 s at a sample flow rate of 5.5 ml min−1, as compared to direct introduction of 55 μl of sample, yielding a detection limit (3σ) of 3.3 ng l−1. The sampling frequency was 24.2 samples h−1. The proposed method was successfully evaluated by analyzing a NIST Cr(VI)-reference material, synthetic seawater and waste waters, and waste water samples from an incineration plant and a desulphurization plant, respectively. 相似文献
90.
Robert M. Smith Ping Yuan David P. Weiner Caryn R. Dutton David E. Hansen 《Applied biochemistry and biotechnology》1994,47(2-3):329-343
We describe here a novel strategy for the isolation of antibodies with sequence-specific protease activity: the synthesis
of dipeptide haptens in which the targeted peptide bond has been replaced by a ring-strained or torsionally strained hydroxyethylene
transition-state analog. Thus, the analogs mimic both a peptide bond in a distorted, reactive conformation and the transition
state for peptide bond hydrolysis. In order to obtain sequence-specific antibody proteases, these analogs have been flanked
with additional amino acid residues in preparation for immunization. In particular, we have synthesized peptides containing
analogs such as 2-cis-amino-3-cis-hydroxycyclobutane carboxylic acid andendo-(3-amino-2-hydroxy)bicyclo[2.2.1]heptane-7-anti-carboxylic acid. We have also prepared a series of peptide derivatives containing
analogs, such as 2-[3-amino-2-oxo-1-azetidinyl]-3-methylbutanoic acid, in which the targeted peptide bond has been incorporated
into a β-lactam ring. Since the “peptide bond” has been left intact, these species mimic only a distorted ground state. At
present, antibodies are being elicited against a number of the above peptide derivatives. 相似文献