全文获取类型
收费全文 | 3838篇 |
免费 | 137篇 |
国内免费 | 20篇 |
专业分类
化学 | 2932篇 |
晶体学 | 21篇 |
力学 | 61篇 |
数学 | 509篇 |
物理学 | 472篇 |
出版年
2023年 | 22篇 |
2022年 | 30篇 |
2021年 | 31篇 |
2020年 | 77篇 |
2019年 | 50篇 |
2018年 | 33篇 |
2017年 | 44篇 |
2016年 | 81篇 |
2015年 | 113篇 |
2014年 | 117篇 |
2013年 | 197篇 |
2012年 | 230篇 |
2011年 | 246篇 |
2010年 | 176篇 |
2009年 | 140篇 |
2008年 | 246篇 |
2007年 | 240篇 |
2006年 | 246篇 |
2005年 | 224篇 |
2004年 | 173篇 |
2003年 | 155篇 |
2002年 | 184篇 |
2001年 | 93篇 |
2000年 | 69篇 |
1999年 | 72篇 |
1998年 | 59篇 |
1997年 | 61篇 |
1996年 | 75篇 |
1995年 | 60篇 |
1994年 | 46篇 |
1993年 | 36篇 |
1992年 | 39篇 |
1991年 | 22篇 |
1990年 | 25篇 |
1989年 | 27篇 |
1988年 | 12篇 |
1987年 | 21篇 |
1986年 | 19篇 |
1985年 | 20篇 |
1984年 | 20篇 |
1983年 | 12篇 |
1982年 | 21篇 |
1981年 | 17篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 8篇 |
排序方式: 共有3995条查询结果,搜索用时 31 毫秒
71.
[reaction: see text] A novel one-pot procedure for the three-component coupling reaction of isocyanates, aldehydes, and dienophiles (IAD reaction) has been developed. Condensation of isocyanates and aldehydes and subsequent Diels-Alder reactions with electron-deficient dienophiles furnishes endo-selective amino-substituted cyclohexenes in good yield. 相似文献
72.
Summary A global LSER model that relates HPLC retention to mobile phase composition and pH is tested for a varied group of solutes,
both neutral and ionizable, in a polymeric column and methanol-water mobile phases. It is compared to the local LSER model
developed only for a given mobile phase, i.e., a fixed organic modifier content, and to the global LSER model set only for
neutral solutes. The global LSER model for neutral and ionizable solutes requires a few supplementary parameters over the
other models tested, but it accounts for retention under any experimental conditions for a given column and methanol-water
mobile phases, describing properly the interactions established in the HPLC system (hydrophobicity, hydrogen-bond acidity
and basicity, dipolarity/polarizability…).
This paper is number 13 of a series with the same general title: “Retention of Ionizable Compounds on HPLC” published in various
journals. 相似文献
73.
Self-assembly of amphiphilic hexapyridinium cations at the air/water interface and on HOPG surfaces.
Haofei Gong Dr. Björn Bredenkötter Dr. Christoph Meier Carola Hoffmann‐ Richter Dr. Ulrich Ziener Dr. Dirk G. Kurth Dr. Dirk Volkmer Prof. Dr. 《Chemphyschem》2007,8(16):2354-2362
Mono- and multilayers of a novel amphiphilic hexapyridinium cation with six eicosyl chains (3) are spread at the air/water interface as well as on highly ordered pyrolytic graphite (HOPG). On water, the monolayer of 3 is investigated by recording surface pressure/area and surface potential/area isotherms, and by Brewster angle microscopy (BAM). Self-organized tubular micelles with an internal edge-on orientation of molecules form at the air/water interface at low surface pressure whereas multilayers are present at high surface pressure, after a phase transition. Packing motifs suggesting a tubular arrangement of the constituting molecules were gleaned from atomic force microscopy (AFM) investigations of Langmuir-Blodgett (LB) monolayers being transferred on HOPG at different surface pressures. These LB film structures are compared to the self-assembled monolayer (SAM) of 3 formed via adsorption from a supersaturated solution, which is studied by scanning tunnelling microscopy (STM). On HOPG the SAM of 3 consists of nanorods with a highly ordered edge-on packing of the aromatic rings and an arrangement of alkyl chains which resembles the packing of molecules at the air/water interface at low surface pressure. Additional details of the molecular packing were gleaned from single-crystal X-ray structure analysis of the hexapyridinium model compound 2b, which possesses methyl instead of eicosyl residues. 相似文献
74.
F. Bosch Reig V. Peris Martinez F. Bosch Mossi J. V. Gimeno Adelantado 《Fresenius' Journal of Analytical Chemistry》1992,342(4-5):295-298
Summary An algorithm for quantifying interelemental effects in X-ray fluorescence techniques is developed. By applying an addition process, the ratio between the mass absorption coefficients of the analyte and the unknown sample (
i
*
/
s
*
) is calculated to correct the fluorescence intensity of the element to be determined and linearize the I-c calibration plot. This coefficient can be calculated graphically and numerically. The method is applied to the determination of tin in lead alloys with good results over wide concentration ranges.Notation used Q
Constant of proportionality in Eq. (4)
X-ray fluorescence intensity of the I
i
o
standard
- I
i
s
unknown sample
- I
i
ns
dilute unknown sample
- I
i
ms
unknown sample after addition of i analyte
Corrected fluorescence intensity of the I
i
cs
unknown sample
- I
i
msc
unknown sample after addition of i analyte
Relationship of fluorescence intensity between Ri
sample and standard
- R
i
dilute sample and standard
Factor of fm
addition
- fx
addition equivalent to the mass fraction of the i analyte in the unknown sample
Mass absorption coefficient of
i
*
analyte
-
s
*
unknown sample
-
ms
*
unknown sample after addition
Mass fraction of c
i
s
analyte
- c
i
ms
unknown sample after addition 相似文献
75.
Binuclear Nickel(0) Alkyne Coordination Compounds – Correlation between Ligand Periphery and Supramolecular Structure Reaction of Ni(cdt: 1,5,9-cyclododecatriene) with functionalized alkynes and subsequent reaction with ethylenediamines gives binuclear compounds of the type (diamine)Ni(μ-alkyne)Ni(alkyne). Compounds with alkyne-diols (N?N)Ni2(HOR1R2C? C?C? CR1R2OH)2 show supramolecular structures in which two identical intramolecular and one intermolecular hydrogen bonds are realized. 1 and 2 (chelate ligand in each case N,N,N′,N′-tetramethylethylenediamine, TMEDA, in 1 R1 = R2 = Me, in 2 R1 = R2 = Et) polymer-like chains are built up by connecting the binuclear units. Via two intermolecular hydrogen bonds per organometallic unit in 1 and via one intermoleculare hydrogen bond in 2 the chains are connected to give double chains. By substitution of one methyl group of TMEDA by hydrogen ( 3 : R1 = R2 = Me) a polymerlike network is produced by connecting the polymer-like chains. In compound 4 in which one of the methyl groups of TMEDA is substituted by CH2CH2NMe2 the polymer-like chains remain unconnected. In 5 (diamine = TMEDA, alkyne = (CH3)3C? C?C? CMe2OH) one intermolecular hydrogen bond per organometallic unit is observed forming again polymer-like chains that are independent of each other. 相似文献
76.
Dirk Claus Benny Geypens Yvo Ghoos Paul Rutgeerts Jenny Ghyselen Keika Hoshi Geertrui Delanghe 《Journal of separation science》1997,20(2):94-98
A method is described for the analysis of volatile organic compounds in saliva and tongue coating samples. The techniue is based on an off-line preconcentration step by means of a closed-loop trapping system followed by gas chromatography-ion trap detection. With the closed-loop technique, the volatile organic compounds(VOCs) are released from the matrix and trapped on an adsorbent without interference of water. The VOCs are released from the adsorbent into the gas chromatograph by thermdesorption. After separation, identification of the compounds is performed by ion trap technology. By this technique 82 compounds could be demonstrated in saliva and tongue coating samples. The technique is also used to demonstrate the formation of volatile bacterial fermentation compounds when a protein substrate is added to tongue coating samples. It is considered a very promising tool in further research on oral malodor. 相似文献
77.
T. López P. Bosch R. Gómez E. Basaldella A. Kikot E. Pereyra 《Reaction Kinetics and Catalysis Letters》1991,43(2):461-466
The impregnation preparation of H-zeolite over amorphous aluminosilicate solids (cracking catalysts) is compared with new synthesis procedures based on sol-gel process. The characterization results show that the H-zeolite reacts with the intermediates of the amorphous aluminosilicate.
H- ( ) , -. , H- .相似文献
78.
Dirk Steinborn Olaf Gravenhorst Clemens Bruhn Dusan Miklo Michal Dunaj-Jur
o Alfred Kolbe 《无机化学与普通化学杂志》1997,623(12):1954-1958
Synthesis and Characterization of Aquapentachloroplatinates(IV) – Structure of [K(18-crown-6)][PtCl5(H2O)] The crown ether complex of the aquapentachloroplatinic acid of the composition [H13O6][PtCl5(H4O2)] · 2(18-cr-6) ( 2 ) reacts with K2CO3 and [NnBu4]OH in aqueous solution to give [K(18-cr-6)][PtCl5(H2O)] ( 5 a ) and [NnBu4][PtCl5(H2O)] · 1/2 (18-cr-6) · H2O ( 5 b ), respectively. Both compounds were characterized by microanalysis, vibrational (IR, Raman) and NMR (1H, 13C, 195Pt) spectroscopy. The X-ray structure analysis of 5 a (orthorhombic, pnma; a = 16,550(4), b = 18,044(3), c = 7,415(1) Å; Z = 4; R1 = 0,0183; wR2 = 0,0414) reveals that the crystal is threaded by chains built up of [PtCl5(H2O)]? and [K(18-cr-6)]+ units. There are tight K …? Cl contacts (d(K? Cl1)) = 3,0881(9) Å and OW? H? Ocr hydrogen bridges (d(O1 …? O2) = 2,806(3) Å) between these units. The coordination polyhedron [PtCl5O] has approximately C4v symmetry. 相似文献
79.
The potassium hydroxide-induced (Stevens) rearrangement of 1,3,4-trimethyl-1-(3,4,5-trimethoxybenzyl)-1,2,5,6-tetrahydropyridinium chloride (I) gives the desired 1,3,4-trimethyl-2-(3,4,5-trimethoxybenzyl)-1,2,5,6-tetrahydropyridine (III) and the Hofmann elimination product, N-methyl-N-(3,4,5-trimethoxybenzyl)-2,3-dimethyl-2,4-pentadienamine (II). In the presence of ethereal phenyllithium, the salt I undergoes rearrangement giving the expected tetrahydropyridine III in about 17% yield and four other products, N-(3,4,5-trimethoxybenzyl)methylamine (VI), 1,3,4-trimethyl-2-(6-methyl-2,3,4-trimethoxyphenyl)-1,2,5,6-tetrahydropyridine (IV), 1,3,3-trirnethyl-2-(3,4,5-trimethoxyphenyl)-4-rnethylenepiperidine (V) and 1,3,4-trimethyl-4-(3,4,5-trimethoxybenzyl)-1,4,5,6-tetrahydropyridine (VII), the latter being the 1,4-Stevens rearrangement product which cyclizes easily to β-2′,3′,4′-trimethoxy-2,5,9-trimethyl-7,8-benzomorphan (VIII). Their structures have been proved both by analytical and spectral data. A possible route for VIII and its stereochemical aspects are discussed. 相似文献
80.
Dirk Zahn 《Journal of solid state chemistry》2004,177(10):3590-3594
The pressure induced transformation of rubidium bromide from the NaCl (B1) to the CsCl (B2) type structure is elucidated by means of molecular dynamics simulations. Two different approaches were followed. The “conventional” procedure of applying pressures, which are increased successively, leads to a phase transformation at a critical pressure of 80-85 kbar. This is 16-17 times the experimental value. On the other hand, the phase transition is studied by path sampling molecular dynamics simulations. This approach allows investigating the process at 5 kbar, i.e. it does not require over-driving. At this pressure the system takes pathways related to the route proposed by Bürger, exclusively. In the runs in which an over-pressurization of 80 kbar is applied, we instead observe both the Bürger mechanism and the route proposed by Watanabe et al. 相似文献