全文获取类型
收费全文 | 33407篇 |
免费 | 1110篇 |
国内免费 | 249篇 |
专业分类
化学 | 23035篇 |
晶体学 | 232篇 |
力学 | 789篇 |
数学 | 5513篇 |
物理学 | 5197篇 |
出版年
2022年 | 201篇 |
2021年 | 426篇 |
2020年 | 523篇 |
2019年 | 491篇 |
2018年 | 381篇 |
2017年 | 370篇 |
2016年 | 842篇 |
2015年 | 746篇 |
2014年 | 823篇 |
2013年 | 1793篇 |
2012年 | 1899篇 |
2011年 | 2393篇 |
2010年 | 1163篇 |
2009年 | 1016篇 |
2008年 | 2048篇 |
2007年 | 2051篇 |
2006年 | 2037篇 |
2005年 | 1902篇 |
2004年 | 1626篇 |
2003年 | 1414篇 |
2002年 | 1316篇 |
2001年 | 429篇 |
2000年 | 412篇 |
1999年 | 366篇 |
1998年 | 345篇 |
1997年 | 368篇 |
1996年 | 502篇 |
1995年 | 359篇 |
1994年 | 310篇 |
1993年 | 283篇 |
1992年 | 290篇 |
1991年 | 247篇 |
1990年 | 210篇 |
1989年 | 211篇 |
1988年 | 240篇 |
1987年 | 214篇 |
1986年 | 187篇 |
1985年 | 361篇 |
1984年 | 353篇 |
1983年 | 253篇 |
1982年 | 332篇 |
1981年 | 332篇 |
1980年 | 307篇 |
1979年 | 290篇 |
1978年 | 275篇 |
1977年 | 236篇 |
1976年 | 220篇 |
1975年 | 218篇 |
1974年 | 214篇 |
1973年 | 218篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
Arrhenius parameters for the reaction of hydrogen atoms with azide and thiocyanate in aqueous solution have been determined using electron pulse radiolysis and electron paramagnetic resonance free induction decay attenuation measurements. Absolute values for SCN-, N3(-), and HN3 were well-described over the temperature range of 9-81 degrees C by the equations log k5 = (12.03 +/- 0.12) - [(21.05 +/- 0.66 kJ mol(-1))/2.303RT], log k10 = (12.75 +/- 0.21) - [(18.43 +/- 1.22 kJ mol(-1))/2.303RT], and log k15 = (11.59 +/- 0.12) - [(21.44 +/- 0.69 kJ mol(-1))/2.303RT], corresponding to room temperature (22 degrees C) rate constants of (2.07 +/- 0.03) x 10(8), (3.15 +/- 0.08) x 10(9), and (6.31 +/- 0.05) x 10(7) M(-1) s(-1) and activation energies for these chemicals of 21.05 +/- 0.66, 18.4 +/- 1.2, and 21.44 +/- 0.69 kJ mol(-1), respectively. The similarity of these three measured activation energies, taken together with the available information on reaction products, suggests a similar reaction mechanism, which is proposed to be an initial hydrogen atom adduct formation in these molecules, followed by single bond breakage. 相似文献
982.
Bernard Alpert David M. Jameson Ricardo Lopez-Delgado Ralph Schooley 《Photochemistry and photobiology》1979,30(4):479-481
Abstract— –Fluorescence decay times of aqueous dilute solutions (?20 µM) of L-tryptophan have been determined using the phase shift technique as well as single photon-counting coupled with synchrotron radiation (ACO at Orsay and SPEAR at Stanford). Decay times were obtained as a function of the excitation wavelength (in the spectral region 220–320 nm) monitoring emission of λ> 320 nm (in certain specified cases, λ> 360 nm). We have found that, at neutral pH and 20°C. fluorescence decays are single exponentials and independent of the excitation wavelength; under these conditions we find τ= 3.1 ± 0.1 ns. 相似文献
983.
Zubida Sahraoui Jean-Claude David Jean-Maurice Vergnaud 《Journal of Analytical and Applied Pyrolysis》1980,2(2):141-152
The pyrolysis of poly-α-acetoxystyrene was investigated by using two methods thermogravimetry, and a reactor working under dynamic conditions coupled with a chromatograph.Thermogravimetry was used firstly with a constant heating rate and secondly under isothermal conditions. Two different degradation steps for the polymer were found, the first between 140 and 200°C and the second above 220°C. The kinetics of the overall pyrolysis were studied for the first thermal degradation step. The degradation order is zero when the weight loss is less than 10%, and unity when the weight loss is between 10 and 32%. The activation energy is about 40 kcal/mole for either order. The results obtained by using dynamic and isothermal thermogravimetry are in good agreement.Volatile products were identified and analysed by using a reactor working under dynamic conditions coupled with a chromatograph. The decomposition is similar to that of poly(vinyl acetate) and different to that of known substituted polystyrenes. Acetic acid was the main component of the volatile products (90–95%), the others being the monomer and acetophenone. 相似文献
984.
985.
Chin JW Santoro SW Martin AB King DS Wang L Schultz PG 《Journal of the American Chemical Society》2002,124(31):9026-9027
We report the selection of a new orthogonal aminoacyl tRNA synthetase/tRNA pair for the in vivo incorporation of a photocrosslinker, p-azido-l-phenylalanine, into proteins in response to the amber codon, TAG. The amino acid is incorporated in good yield with high fidelity and can be used to crosslink interacting proteins. 相似文献
986.
Hiddessen AL Weitz DA Hammer DA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(16):6788-6795
The selectivity and range of energies offered by specific biological interactions serve as valuable tools for engineering the assembly of colloidal particles into novel materials. In this investigation, high affinity biological interactions between biotin-coated "A" particles (RA = 0.475 microm) and streptavidin-coated "B" particles (RB = 2.75 microm) drive the self-assembly of a series of binary colloidal structures, from colloidal micelles (a large B particle coated by smaller A particles) to elongated chain microstructures (alternating A and B particles), as the relative number of small (A) to large (B) particles (2 < or = NA/NB < or = 200) is decreased at a low total volume fraction (10(-4) < or = phiT < or = 10(-3)). At a significantly higher total volume fraction (phiT > or = 10(-1)) and a low number ratio (NA/NB = 2), the rheological behavior of volume-filling particle networks connected by streptavidin-biotin bonds is characterized. The apparent viscosity (eta) as a function of the shear rate gamma, measured for networks at phiT = 0.1 and 0.2, exhibits shear-rate-dependent flow behavior, and both the apparent viscosity and the extent of shear thinning increase upon an increase of a factor of 2 in the total volume fraction. Micrographs taken before and after shearing show a structural breakdown of the flocculated binary particle network into smaller flocs, and ultimately a fluidlike suspension, with increasing shear rate. Rheological measurements provide further proof that suspension microstructure is governed by specific biomolecular interactions, as control experiments in which the streptavidin molecules on particles were blocked displayed Newtonian flow behavior. This investigation represents the first attempt at measuring the rheology of colloidal suspensions where assembly is driven by biomolecular cross-linking. 相似文献
987.
The spectrum of CD2HF was measured by high-resolution interferometric Fourier-transform IR (FTIR) spectroscopy (apodised instrumental band with:0.004 cm−1 fwhm) between 800 and 1200 cm−1 covering the four lowest fundamentals. A complete rotational analysis using a semi-automatic assignment procedure yields accurate band centres (ν9: 912.2028 cm−1, ν6:964.4994 cm−1, ν5: 1050.5104 cm−1, ν4: 1093.8632 cm−1) and a complete set of first-order Coriolis coupling constants. The most important couplings occur between ν9 and ν6 (ξa= 1.069 cm−1, ξc= −0.3535 cm−1) and between ν5 and ν4 (ξb= −0.80606 cm−1). The analysis was guided by and compared with results from our ab initio calculations for Coriolis constants and transition moments using CADPAC at TZP/MP2 level. 相似文献
988.
Feedings of [1-13C]- and [1,2-13C2]acetate Aspergillus terreus gave quadrone and terrecyclic acid which were analyzed by 13C NMR. The pattern of 13C-enrichments and couplings is consistent with the formation of 1 and 2 by cyclization of farnesyl pyrophosphate. 相似文献
989.
Schultz J Gottlieb DM Petersen M Nesic L Jacobsen S Søndergaard I 《Electrophoresis》2004,25(3):502-511
Methods for classification of two-dimensional (2-DE) electrophoresis gels based on multivariate data analysis are demonstrated. Two-dimensional gels of ten wheat varieties are analyzed and it is demonstrated how to classify the wheat varieties in two qualities and a method for initial screening of gels is presented. First, an approach is demonstrated in which no prior knowledge of the separated proteins is used. Alignment of the gels followed by a simple transformation of data makes it possible to analyze the gels in an automated explorative manner by principal component analysis, to determine if the gels should be further analyzed. A more detailed approach is done by analyzing spot volume lists by principal components analysis and partial least square regression. The use of spot volume data offers a mean to investigate the spot pattern and link the classified protein patterns to distinct spots on the gels for further investigation. The explorative approach in analysis of 2-D gels makes it possible, in a fast and convenient way, to screen many gels in order to determine the protein patterns that form clusters and could be selected for further examination. 相似文献
990.
Moriarty RM Rani N Enache LA Rao MS Batra H Guo L Penmasta RA Staszewski JP Tuladhar SM Prakash O Crich D Hirtopeanu A Gilardi R 《The Journal of organic chemistry》2004,69(6):1890-1902
A general and novel solution to the synthesis of biologically important stable analogues of prostacyclin PGI(2), namely benzindene prostacyclins, has been achieved via the stereoselective intramolecular Pauson-Khand cyclization (PKC). This work illustrates for the first time the synthetic utility and reliability of the asymmetric PKC route for synthesis and subsequent manufacture of a complex drug substance on a multikilogram scale. The synthetic route surmounts issues of individual step stereoselectivity and scalability. The key step in the synthesis involves efficient stereoselection effected in the PKC of a benzoenyne under the agency of the benzylic OTBDMS group, which serves as a temporary stereodirecting group that is conveniently removed via benzylic hydrogenolysis concomitantly with the catalytic hydrogenation of the enone PKC product. Thus the benzylic chiral center dictates the subsequent stereochemistry of the stereogenic centers at three carbon atoms (C(3a), C(9a), and C(1)). 相似文献