全文获取类型
收费全文 | 2596篇 |
免费 | 89篇 |
国内免费 | 9篇 |
专业分类
化学 | 1913篇 |
晶体学 | 7篇 |
力学 | 28篇 |
数学 | 405篇 |
物理学 | 341篇 |
出版年
2022年 | 17篇 |
2021年 | 20篇 |
2020年 | 41篇 |
2019年 | 44篇 |
2018年 | 21篇 |
2016年 | 62篇 |
2015年 | 63篇 |
2014年 | 75篇 |
2013年 | 89篇 |
2012年 | 122篇 |
2011年 | 136篇 |
2010年 | 89篇 |
2009年 | 61篇 |
2008年 | 100篇 |
2007年 | 127篇 |
2006年 | 133篇 |
2005年 | 119篇 |
2004年 | 82篇 |
2003年 | 75篇 |
2002年 | 84篇 |
2001年 | 55篇 |
2000年 | 36篇 |
1999年 | 40篇 |
1998年 | 38篇 |
1997年 | 29篇 |
1996年 | 24篇 |
1995年 | 32篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 45篇 |
1991年 | 26篇 |
1990年 | 26篇 |
1989年 | 29篇 |
1988年 | 35篇 |
1987年 | 39篇 |
1986年 | 31篇 |
1985年 | 43篇 |
1984年 | 31篇 |
1983年 | 21篇 |
1982年 | 21篇 |
1981年 | 29篇 |
1980年 | 33篇 |
1979年 | 22篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 28篇 |
1975年 | 32篇 |
1974年 | 34篇 |
1973年 | 32篇 |
1971年 | 19篇 |
排序方式: 共有2694条查询结果,搜索用时 15 毫秒
991.
Trapp N Scherer H Hayes SA Berger RJ Kütt A Mitzel NW Saame J Krossing I 《Physical chemistry chemical physics : PCCP》2011,13(13):6184-6191
The syntheses of the perfluorinated alcohols (F(5)C(6))(F(3)C)(2)COH (1) and (F(5)C(6))(C(5)F(10))COH (2) are described. Both compounds were prepared in reasonable yields (1: 65%, 2: 85%) by reacting the corresponding ketone with C(6)F(5)MgBr, followed by acidic work-up. The alcohols were characterized by NMR, vibrational spectroscopy, single-crystal X-ray diffraction, acidity measurements and gas-phase electron diffraction. A combination of appropriate 2D NMR experiments allowed the unambiguous assignment of all signals in the (19)F spin systems, of which that of 2 was especially complex. High acidity of the alcohols is indicated by acidity measurements as well as the calculated gas phase acidities. It is also supported by the crystal structure of 2, which exhibits only a single weak intermolecular hydrogen bridge with an O...O distance of 301 pm. This shows the low donor strength of the oxygen atom in the compound, which is partly compensated through formation of two intramolecular CF...H contacts of 220 and 232 pm length to the proton not involved in the hydrogen bridge. The pK(a) values in acetonitrile are 22.2 for 1 and 22.0 for 2; their calculated gas phase acidities are 1367 and 1343 kJ mol(-1) (MP2/TZVPP level). 相似文献
992.
Lehner AJ Trapp N Scherer H Krossing I 《Dalton transactions (Cambridge, England : 2003)》2011,40(7):1448-1452
The CX3(+) salts [CCl(3)](+)[Al(OR(F))(4)](-)1, [CCl(3)](+)[(R(F)O)(3)Al-F-Al(OR(F))(3)](-)2, [CBr(3)](+)[Al(OR(F))(4)](-)3, [CBr(3)](+)[(R(F)O)(3)Al-F-Al(OR(F))(3)](-)4 (R(F) = C(CF(3))(3)) were prepared in 56 to 85% yield from CX(4) (X = Cl, Br) and the corresponding silver salts (weight balance, NMR, IR, X-ray structure of 1). The most convenient solvent for the preparation of 1 and 2 is SO(2)ClF but for 3 and 4 it is SO(2). The reactions are complete after about three days stirring at -30 to -40 °C. The salts are stable for weeks in solution at -40 °C and stable for a few hours at RT in the solid state. In SO(2)ClF (1, 2) or SO(2) (3, 4) solution they decompose slowly at -20 °C and within several hours at RT; in general the CBr3(+) salts are more stable than the CCl3(+) homologues. The decomposition products were assigned as CCl(3)F and primarily CBr(2)F(2) (which likely forms as a Lewis acid induced disproportionation product of the initial CBr(3)F). The C-X vibrations of the salts were found in the expected range and the assignments were made based on experimental and calculated data. The IR spectrum of a CBr3(+) salt is for the first time reported here. 相似文献
993.
The microwave spectrum of 1-vinylimidazole has been investigated in the 21-80 GHz spectral region. The spectra of two conformers have been assigned. One of these forms is planar, while the other is nonplanar with the imidazole ring and the vinyl group forming an angle of 15(4)° from coplanarity. The planar form is found to be 5.7(7) kJ/mol more stable than the nonplanar rotamer by relative intensity measurements. The spectra of 10 vibrationally excited states of the planar form and one excited-state spectrum of the nonplanar form were assigned. The vibrational frequencies of several of these states were determined by relative intensity measurements. The microwave work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ, MP2/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory. The B3LYP calculations predict erroneously that both forms of 1-vinylimidazole are planar, whereas the MP2 and CCSD calculations correctly predict the existence of a planar and a nonplanar conformer of this compound. 相似文献
994.
The microwave spectrum of (2-chloroethyl)amine, ClCH(2)CH(2)NH(2), has been investigated in the 22-120 GHz region. Five rotameric forms are possible for this compound. In two of these conformers, denoted I and II, the Cl-C-C-N chain of atoms is antiperiplanar, with different orientations of the amino group. The link of the said atoms is synclinal in the three remaining forms, III-V, which differ with respect to the orientation of the amino group. The microwave spectra of four of these conformers, I-IV, have been assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner that rare and weak five-membered N-H···Cl intramolecular hydrogen bonds are formed. The geometries of conformers I and II preclude a stabilization by this interaction. The energy differences between the conformers were obtained from relative intensity measurements of spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum. This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III, 4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311++G(3df,3pd) levels of theory. The CCSD rotational constants and energy differences are in good agreement with their experimental counterparts. 相似文献
995.
Govor EV Lysenko AB Quiñonero D Rusanov EB Chernega AN Moellmer J Staudt R Krautscheid H Frontera A Domasevitch KV 《Chemical communications (Cambridge, England)》2011,47(6):1764-1766
Chloride-centered hexanuclear hydroxopyrazolate reveals potential as a receptor of halomethane and halometallate species and as a molecular building block for coordination polymers. 相似文献
996.
Besenius P van den Hout KP Albers HM de Greef TF Olijve LL Hermans TM de Waal BF Bomans PH Sommerdijk NA Portale G Palmans AR van Genderen MH Vekemans JA Meijer EW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(18):5193-5203
The supramolecular oligomerization of three water-soluble C(3)-symmetrical discotic molecules is reported. The compounds all possess benzene-1,3,5-tricarboxamide cores and peripheral Gd(III)-DTPA (diethylene triamine pentaacetic acid) moieties, but differ in their linker units and thus in their propensity to undergo secondary interactions in H(2)O. The self-assembly behavior of these molecules was studied in solution using circular dichroism, UV/Vis spectroscopy, nuclear magnetic resonance, and cryogenic transmission electron microscopy. The aggregation concentration of these molecules depends on the number of secondary interactions and on the solvophobic character of the polymerizing moieties. Hydrophobic shielding of the hydrogen-bonding motif in the core of the discotic is of paramount importance for yielding stable, helical aggregates that are designed to be restricted in size through anti-cooperative, electrostatic, repulsive interactions. 相似文献
997.
Vojteer N Sagawe V Stauffer J Schroeder M Hillebrecht H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(11):3128-3135
We present synthesis, crystal structure, hardness, and IR/Raman and UV/Vis spectra of a new compound with the mean composition LiB12PC. Transparent single crystals were synthesised from Ga, Li, B, red phosphorus and C at 1500 °C in boron nitride crucibles welded in Ta ampoules. Depending on the type of boron used for the synthesis we obtained colourless, brown and red single crystals with slightly different P/C ratios. Colourless LiB12PC crystallizes orthorhombic in the space group Imma (No. 74) with a=10.188(2) Å, b=5.7689(11) Å, c=8.127(2) Å and Z=4. Brown LiB12P0.89C1.11 is very similar, but with a lower P content. Red single crystals of LiB12P1.13C0.87 have a larger unit cell with a=10.4097(18) Å, b=5.9029(7) Å, c=8.2044(12) Å. EDX measurements confirm that the red crystals contain more phosphorus than the other ones. The crystal structure is characterized by a covalent network of B12 icosahedra connected by exohedral B? B bonds and P? P, P? C or C? C units. Li atoms are located in interstitials. The structure is closely related to MgB7, LiB13C2 and ScB13C. LiB12PC fulfils the electron counting rules of Wade and also Longuet‐Higgins. Measurements of Vickers micro‐hardness (HV=27 GPa) revealed that LiB12PC is a hard material. The optical band gaps obtained from UV/Vis spectra match the colours of the crystals. Furthermore we report on the IR and Raman spectra. 相似文献
998.
Naumann I Klein BC Bartel SJ Darsow KH Buchholz R Lange HA 《Rapid communications in mass spectrometry : RCM》2011,25(17):2517-2523
The pharmaceutical industry is interested in identifying novel target compounds. Due to their versatile pharmacological activities (e.g. antiviral, anti-carcinogen and immunosuppressive) sulfoquinovosyldiacylglycerides (SQDGs) are potential drug candidates. The present publication deals with the purification and structural characterization of SQDGs from three different strains of Phaeodactylum tricornutum. Besides detection of SQDGs (sn-1: C16:1/sn-2: C16:0 and sn-1: C20:5/sn-2: C16:0), two novel 2'-O-acylsulfoquinovosyldiacylglyerides (Ac-SQDGs, sn-1: C16:0/ sn-2: C16:0/2' C20:5 and sn-1: C20:5/sn-2: C16:0/2' C20:5) were identified by using matrix-assisted laser desorption/ionization (MALDI) QTrap time-of-flight (ToF) hybrid mass spectrometry (MS) with multistage MS(n). The analytical method enables the sn-position verification of fatty acids (MS(2)) as well as the confirmation of the regioposition of eicospentanoic acid at the sulfoquinovose (MS(3)). 相似文献
999.
We have determined the concentration–depth profiles of sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CDS) in their
pure solutions, by which the surface structure of those solutions are characterized. With the identical bulk concentration,
more Cs ions than sodium ions are present at the topmost layer and they penetrate deeper than sodium ions into the layer formed
by the heads of the anions, shielding the electrostatic repulsion among those negatively charged anions more efficiently.
The distributions of the charge at the surface of each studied solution were determined from those concentration–depth profiles
of surfactant ions. The charge density varies more drastically in SDS solutions than in CDS solutions when their bulk concentrations
are identical. These charge density profiles exhibit a visible and direct insight into the electric charge structure of the
surface of ionic surfactant solutions. The experimental findings might be helpful to the investigations on the surface structures
of aqueous solutions of ionic surfactants. 相似文献
1000.
Hoffmann R Obloh H Tokuda N Yang N Nebel CE 《Langmuir : the ACS journal of surfaces and colloids》2012,28(1):47-50
The crystalline form of sp(3)-hybridized carbon, diamond, offers various electrolyte-stable surface terminations. The H-termination-selective attachment of nitrophenyl diazonium, imaged by AFM, shows that electrochemical oxidation can control the fractional hydrogen/oxygen surface termination of diamond on the nanometer scale. This is of particular interest for all applications relying on interfacial electrochemistry, especially for biointerfaces. 相似文献