全文获取类型
收费全文 | 5862篇 |
免费 | 110篇 |
国内免费 | 19篇 |
专业分类
化学 | 3708篇 |
晶体学 | 77篇 |
力学 | 231篇 |
数学 | 512篇 |
物理学 | 1463篇 |
出版年
2022年 | 33篇 |
2021年 | 34篇 |
2020年 | 43篇 |
2019年 | 46篇 |
2018年 | 47篇 |
2016年 | 89篇 |
2015年 | 99篇 |
2014年 | 77篇 |
2013年 | 278篇 |
2012年 | 225篇 |
2011年 | 258篇 |
2010年 | 149篇 |
2009年 | 123篇 |
2008年 | 221篇 |
2007年 | 234篇 |
2006年 | 251篇 |
2005年 | 224篇 |
2004年 | 190篇 |
2003年 | 171篇 |
2002年 | 190篇 |
2001年 | 132篇 |
2000年 | 128篇 |
1999年 | 91篇 |
1998年 | 84篇 |
1997年 | 125篇 |
1996年 | 128篇 |
1995年 | 102篇 |
1994年 | 118篇 |
1993年 | 138篇 |
1992年 | 125篇 |
1991年 | 72篇 |
1990年 | 67篇 |
1989年 | 98篇 |
1988年 | 85篇 |
1987年 | 80篇 |
1986年 | 78篇 |
1985年 | 93篇 |
1984年 | 102篇 |
1983年 | 78篇 |
1982年 | 78篇 |
1981年 | 79篇 |
1980年 | 70篇 |
1979年 | 72篇 |
1978年 | 69篇 |
1977年 | 69篇 |
1976年 | 57篇 |
1975年 | 79篇 |
1974年 | 72篇 |
1973年 | 59篇 |
1972年 | 31篇 |
排序方式: 共有5991条查询结果,搜索用时 15 毫秒
171.
Gel filtration chromatography using a TSKgel G2000 SW column coupled with differential refractive index detection was used to analyse five grades of polyvinyl alcohol. Limits of detection and quantification for the assay were 0.14 mg mL–1, 0.47 mg mL–1 respectively. The inter and intra-day co-efficient of variance were both <7%. There was a significant difference (p<0.05, n=5) between the calibration curves across the five grades of PVA due to a refractive index range of 13.0790 –1.3181 (n=3). The assay accuracy was 98.99% ± 8.97% (n=5) and 90.60% ± 7.87% (n=5) of a spiked PVA sample was recovered from a commercial formulation. 相似文献
172.
Kohtani M Jones TC Schneider JE Jarrold MF 《Journal of the American Chemical Society》2004,126(24):7420-7421
High-temperature ion mobility measurements have been performed for alpha-helical Ac-A15K+H+ and globular Ac-KA15+H+ peptides. The alpha-helical and globular conformations do not melt into random coils as the temperature is raised. Instead, both conformations survive to the point where the peptide signals vanishes due to fragmentation. This occurs at 600 K for the globular Ac-KA15+H+ peptide and at 725 K for the alpha-helical Ac-A15K+H+. For the helical Ac-A15K+H+ peptide it appears that fragmentation is triggered by disruption of the helical conformation. 相似文献
173.
Frank J. Manganiello Larry W. Christensen W.M. Jones 《Journal of organometallic chemistry》1982,235(3):327-334
Contrary to earlier reports [1,2] cyclopropyliron σ complexes can be conveniently prepared in fair to excellent yields by photodecarbonylation of the corresponding acyl complexes. 相似文献
174.
The reaction of the cycloheptatrienylzirconium half-sandwich complex [(η(7)-C(7)H(7))ZrCl(tmeda)] (1) (tmeda = N,N,N',N'-tetramethylethylenediamine) with Li(Im(Dipp)N), generated from bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH) with methyllithium, yields the imidazolin-2-iminato complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(tmeda)] (2). The corresponding tmeda-free complex [(η(7)-C(7)H(7))Zr(Im(Dipp)N)] (5) can be synthesized via the 1,3-bis(trimethylsilyl)allyl complex [(η(7)-C(7)H(7))Zr{η(3)-C(3)H(3)(TMS)(2)}(THF)] (3; TMS = SiMe(3)), which undergoes an acid-base reaction with Im(Dipp)NH to form 5 and 1,3-bis(trimethylsilyl)propene. 5 exhibits an unusual one-legged piano stool ("pogo stick") geometry with a particularly short Zr-N bond of 1.997(2) ?. Addition of 2,6-dimethylphenyl or tert-butyl isocyanide affords the complexes [(η(7)-C(7)H(7))Zr(Im(Dipp)N)(CNR)] (R = o-Xy, 6; R = t-Bu, 7), while the reaction with 2,6-dimethylphenyl isocyanate results in a [2 + 2] cycloaddition to form the ureato(1-) complex [(η(7)-C(7)H(7))Zr{Im(Dipp)N(C═O)N-o-Xy}] (8). 5 can also act as an initiator for the ring-opening polymerization of ε-caprolactone. These reactivity patterns together with density functional theory calculations reveal a marked similarity of the bonding in imidazolin-2-iminato and conventional imido transition-metal complexes. 相似文献
175.
Bellm SM Builth-Williams JD Jones DB Chaluvadi H Madison DH Ning CG Wang F Ma XG Lohmann B Brunger MJ 《The Journal of chemical physics》2012,136(24):244301
Cross section data for electron scattering from DNA are important for modelling radiation damage in biological systems. Triply differential cross sections for the electron impact ionization of the highest occupied outer valence orbital of tetrahydrofurfuryl alcohol, which can be considered as an analogue to the deoxyribose backbone molecule in DNA, have been measured using the (e,2e) technique. The measurements have been performed with coplanar asymmetric kinematics at an incident electron energy of 250 eV, an ejected electron energy of 20 eV, and at scattered electron angles of -5°, -10°, and -15°. Experimental results are compared with corresponding theoretical calculations performed using the molecular 3-body distorted wave model. Some important differences are observed between the experiment and calculations. 相似文献
176.
The field of proteomics, the large-scale analysis of proteins, has undergone a huge expansion over the past decade. Mass spectrometry-based proteomics relies on the dissociation of peptide and/or protein ions to provide information on primary sequence and sites of post-translational modifications. Fragmentation techniques include collision-induced dissociation, electron capture dissociation and electron transfer dissociation. Here, we describe each of these techniques and their use in proteomics. The principles, advantages, limitations, and applications are discussed. 相似文献
177.
A diiron(II) complex containing two μ-1,3-(κN:κO)-amidate linkages has been synthesized using the 2,2',2'-tris(isobutyrylamido)triphenylamine (H(3)L(iPr)) ligand. The resulting diiron complex, 1, reacts with dioxygen (or iodosylbenzene) to effect intramolecular C-H bond activation at the methine position of the ligand isopropyl group. The ligand-activated product, 2, has been isolated and characterized by a variety of methods including X-ray crystallography. Electrospray ionization mass spectroscopy of 2 prepared from(18)O(2) was used to confirm that the oxygen atom incorporated into the ligand framework is derived from molecular oxygen. 相似文献
178.
Alexander Kreft Alexander Lücht Jrg Grunenberg Peter G. Jones Daniel B. Werz 《Angewandte Chemie (International ed. in English)》2019,58(7):1955-1959
The kinetics of (3+2) cycloaddition reactions of 18 different donor–acceptor cyclopropanes with the same aldehyde were studied by in situ NMR spectroscopy. Increasing the electron density of the donor residue accelerates the reaction by a factor of up to 50 compared to the standard system (donor group=phenyl), whereas electron‐withdrawing substituents slow down the reaction by a factor up to 660. This behavior is in agreement with the Hammett substituent parameter σ. The obtained rate constants from the (3+2) cycloadditions correlate well with data from additionally studied (3+n) cycloadditions with a nitrone (n=3) and an isobenzofuran (n=4). A comparison of the kinetic data with the bond lengths in the cyclopropane (obtained by X‐ray diffraction and computation), or the 1H and 13C NMR shifts, revealed no correlation. However, the computed relaxed force constants of donor–acceptor cyclopropanes proved to be a good indicator for the reactivity of the three‐membered ring. 相似文献
179.
Rowland SJ West CE Scarlett AG Jones D 《Rapid communications in mass spectrometry : RCM》2011,25(12):1741-1751
The identification of most individual members of the complex mixtures of carboxylic acids found in petroleum ('naphthenic acids') has eluded chemists for over a century; they remain unresolved by conventional gas chromatographic methods. Recently, however, we successfully used two-dimensional comprehensive gas chromatography/mass spectrometry to identify numerous individual diamondoid acids in the naphthenic acids of oil sands process water (OSPW). We have now applied the same methods to a study of a mixture of commercially available naphthenic acids originally refined from petroleum. The results confirm that OSPW and refined petroleum contain very different distributions of acids, as noted previously, although some of the diamondoid acids recently identified in OSPW were detectable in both. Rather, two-dimensional comprehensive gas chromatography/time-of-flight mass spectrometry (GCxGC/ToF-MS) of the methyl esters of the petroleum acids and of numerous acids synthesised for comparison showed that the former comprised mainly C(8-18) straight-chain, methyl-branched, acyclic isoprenoid, cyclohexyl and isomeric octahydropentalene, perhydroindane and perhydronaphthalene (decalin) acids. Some of the latter bicyclic acids occurred as both the non-alkyl-substituted isomers and the bicyclic ethanoic and propanoic acids. Also present in minor quantities was a range of phenyl carboxylic and substituted phenyl alkanoic acids, and traces of non-acids, including trimethylnaphthalenes, again identified by comparison with the synthesised compounds. These results represent some of the first identifications of multiple individual naphthenic acids in commercial mixtures originating from petroleum and provide a basis for future studies of the petroleum geochemistry, toxicities and environmental impacts of the acids. Furthermore, characterisation of the acids will be important for improving the understanding of the role of naphthenic acids in petroleum engineering, particularly for oil pipeline deposition problems. 相似文献
180.
Fluorescence quantum yields and lifetimes of coumarin dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect is interpreted in terms of relaxation of excited dye from an initial planar conformation to a twisted zwitterionic state. 相似文献