全文获取类型
收费全文 | 6147篇 |
免费 | 1018篇 |
国内免费 | 846篇 |
专业分类
化学 | 4797篇 |
晶体学 | 103篇 |
力学 | 324篇 |
综合类 | 34篇 |
数学 | 697篇 |
物理学 | 2056篇 |
出版年
2024年 | 31篇 |
2023年 | 116篇 |
2022年 | 221篇 |
2021年 | 234篇 |
2020年 | 280篇 |
2019年 | 289篇 |
2018年 | 241篇 |
2017年 | 213篇 |
2016年 | 341篇 |
2015年 | 352篇 |
2014年 | 383篇 |
2013年 | 500篇 |
2012年 | 536篇 |
2011年 | 569篇 |
2010年 | 397篇 |
2009年 | 365篇 |
2008年 | 378篇 |
2007年 | 313篇 |
2006年 | 322篇 |
2005年 | 242篇 |
2004年 | 241篇 |
2003年 | 180篇 |
2002年 | 232篇 |
2001年 | 171篇 |
2000年 | 117篇 |
1999年 | 108篇 |
1998年 | 68篇 |
1997年 | 67篇 |
1996年 | 48篇 |
1995年 | 36篇 |
1994年 | 51篇 |
1993年 | 31篇 |
1992年 | 27篇 |
1991年 | 36篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 32篇 |
1987年 | 14篇 |
1986年 | 15篇 |
1985年 | 20篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1974年 | 6篇 |
排序方式: 共有8011条查询结果,搜索用时 0 毫秒
991.
He L Chen XH Wang DN Luo SW Zhang WQ Yu J Ren L Gong LZ 《Journal of the American Chemical Society》2011,133(34):13504-13518
A variety of chiral bisphosphoric acids derived from binaphthols have been evaluated for enantioselective 1,3-dipolar cycloaddition reactions, revealing that the feature of the linker in the catalysts exerted great impact on the stereoselectivity. Among them, the oxygen-linked bisphosphoric acid 1a provided the highest level of stereoselectivity for the 1,3-dipolar cycloaddition reaction tolerating a wide range of substrates including azomethine ylides, generated in situ from a broad scope of aldehydes and α-amino esters, and various electron-deficient dipolarophiles such as maleates, fumarates, vinyl ketones, and esters. This reaction actually represents one of the most enantioselective catalytic approaches to access structurally diverse pyrrolidines with excellent optical purity. Theoretical calculations with DFT method on the formation of azomethine ylides and on the transition states of the 1,3-dipolar cycloaddition step showed that the dipole and dipolarophile were simultaneously activated by the bifunctional chiral bisphosphoric acids through the formation of hydrogen bonds. The effect of the bisphosphoric acids on reactivity and stereochemistry of the three-component 1,3-dipolar cycloaddition reaction was also theoretically rationalized. The bisphosphoric acid catalyst 1a may take on a half-moon shape with the two phosphoric acid groups forming two intramolecular hydrogen bonds. In the case of maleates, one phosphate acts as a base to activate the 1,3-dipole, and simultaneously, the two hydroxyl groups in the catalyst 1a may respectively form two hydrogen bonds with the two ester groups of maleate to make it more electronically deficient as a much stronger dipolarophile to participate in a concerted 1,3-dipolar cycloaddition with azomethine ylide. However, in the cases involving acrylate and fumarate dipolarophiles, only one hydroxyl group forms a hydrogen bond with the ester functional group to lower the LUMO of the C-C double bond and another one is remained to adjust the acidity and basicity of two phosphoric acids to activate the dipole and dipolarophile more effectively. 相似文献
992.
A novel and facile hybrid analytical method coupling electrochemical “adsorption–desorption” and colorimetric analyses was developed to detect heavy metal ions in turbid water samples. The target metal ions were deposited onto an electrode inserted into the original sample, which was referred to as the “adsorption” process. After changing the medium, the concentrated target metal ions were dissolved in a new, clean buffer (blank buffer), which was referred to as the “desorption” process. The concentrations of the target metal ions were measured by colorimetric analyses after the addition of specific indicator amounts. We demonstrated the applicability of this method by detecting Cd2+, Pb2+ and Cu2+ with co-depositing Bi3+ on portable screen-printed electrodes (SPEs). A good correlation (correlation coefficient of R = 0.997) was observed between concentrations ranging from 1 to 200 μM and absorbance values. After the multiple “desorption” process, the even better detection limits as low as 10, 10 and 100 nM were achieved for Cd2+, Pb2+ and Cu2+, respectively. The practicality of this hybrid method was confirmed by the detection of Cd2+, Pb2+ and Cu2+ in wastewater samples, and these results were in agreement with inductively coupled plasma atomic emission spectroscopy (ICP-AES). Overall, this hybrid method provides a simple, selective and effective technique for environmental pollutant analyses. 相似文献
993.
Nguyen HT Tran HQ Nguyen TT Chau VM Bui KA Pham QL Nguyen MC Kim YH 《Chemical & pharmaceutical bulletin》2011,59(11):1417-1420
Ten oleanane-type saponins (1-10), including three new compounds, namely bifinosides A-C (1-3), were isolated from the roots of Panax bipinnatifidus SEEM. Their structures were elucidated on the basis of chemical and spectroscopic methods. 相似文献
994.
Abulimiti B Zhu R Long J Xu Y Liu Y Ghazal AY Yang M Zhang B 《The Journal of chemical physics》2011,134(23):234301
The dynamics of electronically excited states in 2-picoline is studied using femtosecond time-resolved photoelectron imaging spectroscopy. The internal conversion from the S(2) state to the vibrationally excited S(1) state is observed in real time. The secondarily populated high vibronic S(1) state deactivates further to the S(0) state. Photoelectron energy and angular distributions reveal the feature of ionization from the singlet 3p Rydberg states. In addition, variation of time-dependent anisotropy parameters indicates the rotational coherence of the molecule. 相似文献
995.
Vanadate(V)-templated Dawson-type capsules {V(IV)M(VI)(17)(VO(4))(2)} (M = Mo, W; 1-2) have been synthesized and investigated by electrochemical methods in aqueous and organic media using spectroscopic techniques, EPR, UV-vis/NIR, IR, and CSI-MS (cryospray ionization mass spec.), and the clusters have been examined in the solid state by magnetic studies. The collision-induced dissociation (CID-MS) studies confirmed the solution structures as well as helped pinpoint the position of the vanadium ion on the {VM(17)}-type shell, which was corroborated by EPR and theoretical studies. 相似文献
996.
Two nanotubular metal-organic frameworks (MOFs), {Cu(L1)·2H(2)O·1.5DMF}(∞) (1) and {Cu(2)(L2)(2)(H(2)O)(2)·7H(2)O·3DMF}(∞) (2), with novel topologies have been constructed based on Cu(2+), 5-(pyridin-4-yl)isophthalic acid (L1) and 5-(pyridin-3-yl)isophthalic acid (L2), respectively. Two MOFs were characterized by IR spectroscopy, thermogravimetry, single-crystal, and powder X-ray diffraction methods. Network analysis reveals a two-nodal (3,6)-connected (4·6(2))(2)(4(2)·6(10)·8(3)) net and a three-nodal (3,4)-connected (4·8(2))(4)(4(2)·8(2)·10(2))(2)(8(4)·12(2)) net. Interpenetration is inherently prevented by both of the topologies of the frameworks. The porosity of MOF 1 was confirmed by N(2) and CO(2) gas adsorption investigations. MOF 1 exhibits remarkable hydrogen sorption hysteresis at low pressure and a H(2) uptake capacity of 1.05 wt% at 77 K and 1 atm. 相似文献
997.
Mass spectra were recorded for (2 + n) resonance enhanced multiphoton ionization (REMPI) of HCl as a function of resonance excitation energy in the 88865-89285 cm(-1) region to obtain two-dimensional REMPI data. Band spectra due to two-photon resonance transitions to number of Rydberg states (Ω' = 0, 1, and 2) and the ion-pair state V((1)Σ(+)(Ω' = 0)) for H(35)Cl and H(37)Cl were identified, assigned, and analyzed with respect to Rydberg to ion-pair interactions. Perturbations show as line-, hence energy level-, shifts, as well as ion signal intensity variations with rotational quantum numbers, J', which, together, allowed determination of parameters relevant to the nature and strength of the state interactions as well as dissociation and ionization processes. Whereas near-resonance, level-to-level, interactions are found to be dominant in heterogeneous state interactions (ΔΩ ≠ 0) significant off-resonance interactions are observed in homogeneous interactions (ΔΩ = 0). The alterations in Cl(+) and HCl(+) signal intensities prove to be very useful for spectra assignments. Data relevant to excitations to the j(3)Σ(0(+)) Rydberg states and comparison with (3 + n) REMPI spectra allowed reassignment of corresponding spectra peaks. A band previously assigned to an Ω = 0 Rydberg state was reassigned to an Ω = 2 state (ν(0) = 88957.6 cm(-1)). 相似文献
998.
The nature of the lowest energy optical transition for the complexes (η(6)-naphthalene)Cr(CO)(3) and (η(6)-phenanthrene)Cr(CO)(3) in the solid state has been investigated by Raman spectroscopy using a range of different excitation wavelengths progressively approaching the resonant condition. Examination of the resonantly enhanced Raman modes confirms that the first absorption is attributed predominantly to a metal-to-arene charge transfer transition for both complexes. A notable difference in the photochemistry of the two complexes was observed. In the case of the phenanthrene complex, population of the lowest energy excited state leads to a photochemical process which resulted in the loss of the arene ligand and formation of Cr(CO)(6). 相似文献
999.
Zheng H Wang Q Long Y Zhang H Huang X Zhu R 《Chemical communications (Cambridge, England)》2011,47(38):10650-10652
Blue luminescent reduced state carbon dots were prepared by reducing carbon dots with NaBH(4). The quantum yield of the reduced state carbon dots increased from 2% to 24% and the maximum emission wavelength shifted from 520 to 450 nm. This offers a simple pathway to enhance the luminescence of carbon dots. 相似文献
1000.
Cheng X Jia H Long T Feng J Qin J Li Z 《Chemical communications (Cambridge, England)》2011,47(43):11978-11980
A new design strategy for the development of fluorescent turn-on chemodosimeters toward OCl(-) was proposed by the removal, but not the general inhibition, of the C=N isomerization. Accordingly, Flu-1 was prepared, which, as shown in the fluorescent picture, exhibited the off-on response for OCl(-), sensitively and selectively. 相似文献