全文获取类型
收费全文 | 8608篇 |
免费 | 254篇 |
国内免费 | 62篇 |
专业分类
化学 | 5780篇 |
晶体学 | 83篇 |
力学 | 255篇 |
综合类 | 1篇 |
数学 | 1346篇 |
物理学 | 1459篇 |
出版年
2021年 | 80篇 |
2020年 | 155篇 |
2019年 | 112篇 |
2018年 | 85篇 |
2017年 | 97篇 |
2016年 | 189篇 |
2015年 | 152篇 |
2014年 | 195篇 |
2013年 | 398篇 |
2012年 | 461篇 |
2011年 | 557篇 |
2010年 | 285篇 |
2009年 | 225篇 |
2008年 | 495篇 |
2007年 | 457篇 |
2006年 | 443篇 |
2005年 | 450篇 |
2004年 | 431篇 |
2003年 | 289篇 |
2002年 | 311篇 |
2001年 | 96篇 |
2000年 | 87篇 |
1999年 | 66篇 |
1998年 | 85篇 |
1997年 | 106篇 |
1996年 | 143篇 |
1995年 | 91篇 |
1994年 | 75篇 |
1993年 | 97篇 |
1992年 | 71篇 |
1991年 | 76篇 |
1990年 | 72篇 |
1989年 | 61篇 |
1988年 | 71篇 |
1987年 | 61篇 |
1986年 | 54篇 |
1985年 | 114篇 |
1984年 | 132篇 |
1983年 | 92篇 |
1982年 | 115篇 |
1981年 | 112篇 |
1980年 | 92篇 |
1979年 | 83篇 |
1978年 | 113篇 |
1977年 | 94篇 |
1976年 | 83篇 |
1975年 | 69篇 |
1974年 | 69篇 |
1973年 | 61篇 |
1971年 | 43篇 |
排序方式: 共有8924条查询结果,搜索用时 265 毫秒
121.
Piotr Kujawa Annie Audibert‐Hayet Joseph Selb Franoise Candau 《Journal of Polymer Science.Polymer Physics》2004,42(9):1640-1655
Multisticker associative polyelectrolytes of acrylamide (≈86 mol %) and sodium 2‐acrylamido‐2‐methylpropanesulfonate (≈12 mol %), hydrophobically modified with N,N‐dihexylacrylamide groups (≈2 mol %), were prepared with a micellar radical polymerization technique. This process led to multiblock polymers in which the length of the hydrophobic blocks could be controlled through variations in the surfactant‐to‐hydrophobe molar ratio, that is, the number of hydrophobes per micelle (NH). The rheological behavior of aqueous solutions of polymers with the same molecular weight and the same composition but with two different hydrophobic block lengths (NH = 7 or 3 monomer units per block) was investigated as a function of the polymer concentration with steady‐flow, creep, and oscillatory experiments. The critical concentration at the onset of the viscosity enhancement decreased as the length of the hydrophobic segments in the polymers increased. Also, an increase in the NH value significantly enhanced the thickening ability of the polymers and affected the structure of the transient network. In the semidilute unentangled regime, the behavior of the polymer with long hydrophobic segments (NH = 7) was studied in detail. The results were well explained by the sticky Rouse theory of associative polymer dynamics. Finally, the viscosity decreased with an increase in the temperature, mainly because of a lowering of the sample relaxation time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1640–1655, 2004 相似文献
122.
The novel phosphinite PPh(2)(3-OCH(2)C(5)H(4)N) (1) has been synthesized, and its coordination properties to Ag(I) have been studied. When reacted in a 1:1 ratio with Ag(I), coordination polymers with different coordination numbers about the Ag are found depending on the anion. For PPh(2)(3-OCH(2)C(5)H(4)N)AgBF(4) (2), a two-coordinate Ag is observed with a P-Ag-N angle of 167 degrees. Mixed three and four coordination about Ag is observed for PPh(2)(3-OCH(2)C(5)H(4)N)AgOTf (3), and for the trifluoroacetate derivative, PPh(2)(3-OCH(2)C(5)H(4)N)Agtfa (4), only a four-coordinate Ag is produced. X-ray crystal-structure determinations for compounds 2-4 have been carried out. The X-ray structures show a wide range of Ag-Ag distances in the polymers, which are dependent on the conformation of the bridging ligand. 相似文献
123.
Huang KW Han JH Cole AP Musgrave CB Waymouth RM 《Journal of the American Chemical Society》2005,127(11):3807-3816
Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium. 相似文献
124.
Berkowitz SA Zhong H Berardino M Sosic Z Siemiatkoski J Krull IS Mhatre R 《Journal of chromatography. A》2005,1079(1-2):254-265
A simple high-resolution capillary zone electrophoresis (CZE) method capable of rapidly assessing the micro-heterogeneity of a 24 kDa molecular weight glycoprotein, has been developed. Separation is carried out using a bare silica capillary at a pH of 2.5 in a commercially available electrophoresis buffer system composed of triethanolamine and phosphoric acid. Over 30 peaks were detected within a run time of 15 min using a 27 cm capillary and approximately 60 peaks were detected using a 77 cm capillary. Although most of the peaks arise from differences in the oligosaccharide structures present on the one glycosylation site on this molecule, other forms of micro-heterogeneity due to the presence of the nonglycosylated form of this glycoprotein and various types of chemical degradation, e.g., deamidation, are also responsible for the multitude of peaks observed. Although the exact chemical identity of each peak in the resulting electropherogram of this glycoprotein is not known, useful information can be obtained for assessing comparability, stability, and batch consistency. Factors impacting the resolution, precision, accuracy, and robustness of the assay are also discussed along with inherent advantages and limitations associated with measuring the micro-heterogeneity of intact glycoproteins. 相似文献
125.
Cvetovich RJ Chartrain M Hartner FW Roberge C Amato JS Grabowski EJ 《The Journal of organic chemistry》1996,61(19):6575-6580
A convergent synthesis of [S-(R,S)]-2-[4-[(4-methylpiperazin-1-yl)carbonyl]phenoxy]-3,3-diethyl-N-[1-[3,4-(methylenedioxy)phenyl]butyl]-4-oxo-1-azetidinecarboxamide (L-694,458, 1), a potent human leukocyte elastase inhibitor, was achieved via chiral synthesis of key intermediates: (S)-3,3-diethyl-4-[4'-[(N-methylpiperazin-1-yl)carbonylphenoxy]-2-azetidinone (2) and (R)-alpha-propylpiperonyl isocyanate (3). Synthesis of beta-lactam 2 was achieved by a novel enantioselective lipase hydrolysis of ester 5 to produce (S)-3,3-diethyl-4-(4'-carboxyphenoxy)-2-azetidinone (6) (60% yield, three cycles, 93% ee) with isolation, epimerization, and recycling of the undesired (R)-ester 5. Isocyanate 3 was prepared by chiral addition of Zn(n-Pr)(2) to piperonal (98% yield, 99.2% ee), azide displacement and reduction to (R)-alpha-propylpiperonylamine (11) (58% yield, 85% ee), crystallization as the D-pyroglutamic acid salt (92% yield, 98.2% ee), and isocyanate formation (98% yield) with phosgene. 相似文献
126.
Paula R. Cable Mark Parker R. Kenneth Marcus Joseph M. Pochkowski 《Journal of the American Society for Mass Spectrometry》1995,6(2):140-147
The use of a two-dimensional charge injection device (CID) to directly image the spatial profile of impingent positively charged ions is described. By this approach, no prior conversion from an ion beam to a photon image is required. Because of the positive response of the device to plasma photons, ions that emanated from the radiofrequency glow discharge source were diverted around a photon stop and focused onto the CID. The resultant ion images were digitized via an external image processor and corrected for dark current contributions. Two-dimensional ion images and single pixel line profiles are presented. 相似文献
127.
Gunnlaugsson T Leonard JP Sénéchal K Harte AJ 《Chemical communications (Cambridge, England)》2004,(7):782-783
The cationic cyclen based Eu(III)-phen conjugated 1.Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)-Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1.Eu conjugates. 相似文献
128.
Gajewski JJ Bocian W Brichford NL Henderson JL 《The Journal of organic chemistry》2002,67(12):4236-4240
The competitive kinetics of additions of allyl to benzaldehyde-h and -d from allyltributyl tin, from diisopropyltartrylallyl boronate, and from allyl bromide and zinc dust in aqueous tetrahydrofuran have inverse secondary deuterium kinetic isotope effects, SDKIEs. These inverse SDKIEs are in contrast to the normal SDKIEs that were obtained with allyl lithium and allyl Grignard, suggesting rate-determining single-electron transfer in these cases. By various MO calculations the transition state for addition of allyl boronate occurs with substantial B-O bond formation and little C-C bond formation. The magnitudes of the SDKIEs with the other two allylating reagents, when compared with reasonable equilibrium isotope effects for the addition, suggest transition states with substantial C-C bond formation. 相似文献
129.
Joseph A. Connor Jose A. Martinho-Simoes Henry A. Skinner Mohammad T. Zafarani-Moattar 《Journal of organometallic chemistry》1979,179(3):331-356
Microcalorimetric measurements at elevated temperatures of the heats of thermal decomposition and iodination have led to values of the standard enthalpies of formation of the following crystalline compounds (values given in kJ mol?1) at 298K: [Cr(η6-1,3,5-C6H3(CH3)3)2] = (63±12); [Cr(η6-C6(CH3)6)2] : -(88±12); [Cr(1,2,3,4,4a,8a-η-C10H8)2] = (407±11); [Cr(CO)3(1,2,3,4,4a,8a-η-C10H8)] = -(258±8). Separate measurements by the vacuum sublimation microcalorimetric technique gave the following values for the enthalpy of sublimation at 298K (kJ mol?1) : [Cr(η6-1,3,5-C6H3(CH3)3)2] = (104±1); [Cr(η6-C6(CH3)6)2] = (119±4); [Cr(CO)3(1,2,3,4,4a,8a-η-C10H8)] = (107±3). From these and other data, the bond enthalpy contributions of the metal-ligand bonds in the gaseous metal complexes were evaluated as follows: [(η6-C6(CH3)6)-Cr] (155±7); [(η6-C6H3(CH3)3)-Cr] (151±6); [(1,2,3,4,4a, 8a-η-C10H8)-Cr](145±6) kJ mol?1]The question of the transferability of the enthalpy contributions of chromium—ligand bonds between organochronium complexes is discussed with aid of information from structural and spectroscopic investigation. The limitations of the procedure are defined.The thermodynamic data are used to discuss various substitution, redistribution and exchange reaction of Cr(η-arene)2 and [Cr(CO)3(η-arene)] compounds. 相似文献
130.
Keiper JS Behles JA Bucholz TL Simhan R DeSimone JM Lynn GW Wignall GD Melnichenko YB Frielinghaus H 《Langmuir : the ACS journal of surfaces and colloids》2004,20(4):1065-1072
Anionic phosphodiester surfactants, possessing either two fluorinated chains (F/F) or one hydrocarbon chain and one fluorinated chain (H/F), were synthesized and evaluated for solubility and self-assembly in liquid and supercritical carbon dioxide. Several surfactants, of both F/F and H/F types and having varied counterions, were found to be capable of solubilizing water-in-CO2 (W/C), via the formation of microemulsions, expanding upon the family of phosphate fluorosurfactants already found to stabilize W/C microemulsions. Small-angle neutron scatteringwas used to directly characterize the microemulsion particles at varied temperatures, pressures, and water loadings, revealing behavior consistent with previous results on W/C microemulsions. 相似文献