全文获取类型
收费全文 | 29108篇 |
免费 | 814篇 |
国内免费 | 228篇 |
专业分类
化学 | 19594篇 |
晶体学 | 130篇 |
力学 | 658篇 |
综合类 | 1篇 |
数学 | 5444篇 |
物理学 | 4323篇 |
出版年
2022年 | 156篇 |
2021年 | 278篇 |
2020年 | 348篇 |
2019年 | 316篇 |
2018年 | 281篇 |
2017年 | 294篇 |
2016年 | 604篇 |
2015年 | 629篇 |
2014年 | 662篇 |
2013年 | 1420篇 |
2012年 | 1434篇 |
2011年 | 1744篇 |
2010年 | 971篇 |
2009年 | 829篇 |
2008年 | 1499篇 |
2007年 | 1502篇 |
2006年 | 1585篇 |
2005年 | 1440篇 |
2004年 | 1356篇 |
2003年 | 1124篇 |
2002年 | 1069篇 |
2001年 | 473篇 |
2000年 | 423篇 |
1999年 | 412篇 |
1998年 | 389篇 |
1997年 | 440篇 |
1996年 | 407篇 |
1995年 | 374篇 |
1994年 | 411篇 |
1993年 | 362篇 |
1992年 | 352篇 |
1991年 | 335篇 |
1990年 | 302篇 |
1989年 | 304篇 |
1988年 | 286篇 |
1987年 | 234篇 |
1986年 | 261篇 |
1985年 | 387篇 |
1984年 | 382篇 |
1983年 | 327篇 |
1982年 | 355篇 |
1981年 | 332篇 |
1980年 | 346篇 |
1979年 | 278篇 |
1978年 | 303篇 |
1977年 | 290篇 |
1976年 | 238篇 |
1975年 | 197篇 |
1974年 | 204篇 |
1973年 | 172篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Reed M. Izatt Gypzy C. Lindh Ronald L. Bruening Peter Huszthy John D. Lamb Jerald S. Bradshaw James J. Christensen 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(6):739-745
The macrocycle-mediated fluxes of several alkali metal cations have been determined in a H2O-CH2Cl2-H2O liquid membrane system. Water-insoluble proton-ionizable macrocycles of the pyridono type were used. The proton-ionizable feature allows the coupling of cation transport to reverse H+ transport. This feature offers promise for the effective separation and/or concentration of alkali metal ions with the metal transport being driven by a pH gradient. A counter anion in the source phase is not co-transported. The desired separation of a particular metal ion involves its selective complexation with the macrocycle, subsequent extraction from the aqueous phase to the organic phase, and exchange for H+ at the organic phase-receiving phase interface. Factors affecting transport which were studied include ring size, source phase pH, and receiving phase pH. Lithium was transported at a rate higher than that of the other alkali metals in both single and competitive systems using a 15-crown-5 pyridono carrier. 相似文献
962.
Leslie M. Connaughton J. Peter Hershey Frank J. Millero 《Journal of solution chemistry》1986,15(12):989-1002
The densities of the major sea salts (NaCl, Na2SO4, MgCl2, and MgSO4) have been measured from 25 to 95°C and to saturation. These results have been combined with literature data and fitted to equations of the form $$\Delta d = Am{\text{ }} + {\text{ }}Bm^{3/2} {\text{ }} + {\text{ }}Cm^2 {\text{ }} + {\text{ }}Dm^{5/2} $$ where Δd=d?do (do is the density of water) and A, B, and C, etc., are polynomial functions of temperature. The standard deviations of the fits were better than ±50×10?6 g-cm?3 for all the salts from 0 to 95°C and to saturation. The apparent molal volumes V? of the salts have been fitted to the equations of Pitzer. The infinite dilution values of V? were in good agreement with literature data, provided the results were not overfit. The large deviations of V? for MgSO4 from additivity as a function of concentration were attributed to the formation of MgSO4 ion pairs. 相似文献
963.
The thermal expansion of VN1?x was determined from measurements of the lattice parameters in the temperature range of 298–1000 K and in the composition range of VN0.707–VN0.996. Within the accuracy of the results the expansion of the lattice parameter with temperature is not dependent on the composition. The lattice parameter as a function of composition ([N]/[V]=0.707?0.996) and temperature (298–1000 K) is given by $$\begin{gathered} a([N]/[V],T) = 0.38872 + 0.02488([N]/[V]) - \hfill \\ - (1.083 \pm 0.021) \cdot 10^{ - 4} T^{1/2} + (6.2 \pm 0.1) \cdot 10^{ - 6} T. \hfill \\ \end{gathered} $$ . The coefficient of linear thermal expansion as a function of temperature (in the same range) is given by $$\alpha (T) = a([N]/[V],T)^{ - 1} [( - 5.04 \pm 0.01) \cdot 10^5 T^{ - 1/2} + (6.2 \pm 0.1) \cdot 10^{ - 6} ].$$ . The average linear thermal expansion coefficient is $$\alpha _{av} = 9.70 \pm 0.15 \cdot 10^{ - 6} K^{ - 1} (298 - 1 000K).$$ . The data are compared with those of several fcc transition metal nitrides collected and evaluated from the literature. 相似文献
964.
The phenomenon of enthalpy-entropy compensation in gas chromatography is examined. Using 53 probe solutes that span a wide range in size (dispersive interaction), dipolarity, hydrogen-bond-donor and hydrogen-bond-acceptor strength, enthalpy-entropy compensation is not observed, while for probe solutes within a homologous series enthalpy-entropy compensation is observed as predicted by the linear salvation energy relationship methodology. 相似文献
965.
Mitchell RH Ward TR Chen Y Wang Y Weerawarna SA Dibble PW Marsella MJ Almutairi A Wang ZQ 《Journal of the American Chemical Society》2003,125(10):2974-2988
The syntheses of several new simple negative, a simple positive, and multiple negative photochromes containing the dihydropyrene-cyclophanediene photochromic system are described. The photo-openings of the negative photochromes, the [e]-annelated benzo (7), naphtho (9), anthro (11), furano (19), and triphenyleno (15) derivatives of the parent 2,7-di-tert-butyl-trans-10b,10c-dimethyl-dihydropyrene (5), as well as its 4,5-dibromo derivative (13), are described to give the corresponding cyclophanedienes, as well as their photoclosures and thermal closures back to the dihydropyrenes. These are compared to the results obtained for the positive photochrome dibenzo[e,l]dihydropyrene (21) and to the bis(dihydropyreno)chrysene (44) and the (dihydropyrenobenzo)(benzo)metacyclophanediene (47) photochromes, which have more than one photochromic switch present and thus have more than a simple "on-off" state. Thermodynamic data are obtained for the thermal closing reactions. The anthrodihydropyrene (12) has the fastest thermal closing (tau(1/2) = 20 min), while the furanodihydropyrene (19') has the slowest (tau(1/2) = 63 h) at 46 degrees C. An electrochemical readout of the state of the switch is demonstrated for the benzodihydropyrene (7). 相似文献
966.
Ring strain energies (RSEs) are predicted using homodesmotic reactions at the B3LYP/6-31G* level of theory. Substituents are conserved in the acyclic reference and any difference in energy between the ring and the acyclic reference corresponds exclusively to RSE. Small rings are stabilized by alkyl substituents and this stabilization decreases as the size of the ring increases. There is a destabilization of medium sized rings. Greater stabilization is found upon alkyl substitution at a double bond in an unsaturated ring and this stabilization decreases as ring size increases. The effects of cis-1,2-disubstitution on RSEs have been evaluated and indicate stabilization for both small and medium sized rings. RSEs of saturated and unsaturated polycyclic systems agree well with the RSEs derived from experimental thermochemical data. RSEs are reported for substituted norbornanes, norbornenes, and norbornadienes to complement experimental studies. 相似文献
967.
Background
Bullfrog myosin-1c contains three previously recognized calmodulin-binding IQ domains (IQ1, IQ2, and IQ3) in its neck region; we identified a fourth IQ domain (IQ4), located immediately adjacent to IQ3. How calmodulin binds to these IQ domains is the subject of this report. 相似文献968.
[reaction: see text] A facile and efficient methodology of the synthesis of 6-(hydroxymethyl)purine derivatives (bases and nucleosides) was developed based on Pd-catalyzed cross-coupling reactions of 6-halopurines with acyloxymethylzinc iodides followed by deprotection. Several title compounds are inhibitors of adenosine deaminase and exert cytostatic activity. 相似文献
969.
The mechanism for signal transduction from the LOV-domains toward the kinase region of phototropin is still not well understood. We have performed molecular dynamics (MD) simulations and CONCOORD calculations on the LOV2 domain of Adiantum capillus-veneris, with the goal to detect possible differences between the two forms of the LOV domain which may not show up in the static crystal structures. Since no such clear differences are found in the MD simulations also, we suggest that the real, biologically active conformation of the LOV domain within the whole phototropin is different from the crystal structure of the isolated LOV domains. The MD simulations do offer, however, insight into details of the dynamics of the dark and illuminated LOV domains, which are discussed in the light of recent experiments. 相似文献
970.