全文获取类型
收费全文 | 2119篇 |
免费 | 83篇 |
国内免费 | 3篇 |
专业分类
化学 | 1645篇 |
晶体学 | 25篇 |
力学 | 27篇 |
数学 | 88篇 |
物理学 | 420篇 |
出版年
2023年 | 14篇 |
2022年 | 15篇 |
2021年 | 23篇 |
2020年 | 35篇 |
2019年 | 49篇 |
2018年 | 26篇 |
2017年 | 23篇 |
2016年 | 55篇 |
2015年 | 50篇 |
2014年 | 55篇 |
2013年 | 101篇 |
2012年 | 132篇 |
2011年 | 132篇 |
2010年 | 62篇 |
2009年 | 85篇 |
2008年 | 113篇 |
2007年 | 124篇 |
2006年 | 133篇 |
2005年 | 106篇 |
2004年 | 99篇 |
2003年 | 90篇 |
2002年 | 71篇 |
2001年 | 51篇 |
2000年 | 46篇 |
1999年 | 31篇 |
1998年 | 15篇 |
1997年 | 22篇 |
1996年 | 17篇 |
1995年 | 17篇 |
1994年 | 28篇 |
1993年 | 14篇 |
1992年 | 21篇 |
1991年 | 24篇 |
1990年 | 21篇 |
1989年 | 12篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 32篇 |
1984年 | 31篇 |
1983年 | 12篇 |
1982年 | 19篇 |
1981年 | 13篇 |
1980年 | 16篇 |
1979年 | 23篇 |
1978年 | 19篇 |
1977年 | 19篇 |
1976年 | 9篇 |
1975年 | 12篇 |
1974年 | 10篇 |
排序方式: 共有2205条查询结果,搜索用时 31 毫秒
61.
Akai S Kawashita N Satoh H Wada Y Kakiguchi K Kuriwaki I Kita Y 《Organic letters》2004,6(21):3793-3796
[reaction: see text] The reactions of (phenylsulfinyl)furans or -thiophenes with carbon nucleophiles in the presence of trifluoroacetic anhydride allowed the nucleophilic installation of carbon functional groups on the furan and thiophene nuclei with complete regioselectivity. 相似文献
62.
Summary The retention and selectivity of flavonoids (baicalin, baicalein, wogonin, oroxylin A) inScutellariae radix have been studies by high-performance thin-layer chromatography on phenyldimethylethoxysilane-treated silica plates. The silica plates treated with phenyl groups were used for physical and chemical analysis. From elemental carbon analysis, the maximum number of bonded phenyl surface groups per gram was calculated to be 0.467×1021 (Oginal silica plate: Merck Art. 15109, Silica gel 100 F254). With methanol-1/15 M phosphate buffer (pH 6.2) mixtures as mobile phase, baicalin, baicalein, wogonin, and oroxylin A inScutellariae radix were separated. It has been shown that phenyl-treated plates are more suitable for selective separation of baicalin, baicalein, wogonin, and oroxylin A than octadecyl-treated plates. 相似文献
63.
H. Ohta K. Jinno Y. Saito J. C. Fetzer W. R. Biggs J. J. Pesek M. T. Matyska Y. -L. Chen 《Chromatographia》1996,42(1-2):56-62
Summary The temperature-dependency of the separation of fullerenes in liquid chromatography (LC) has been examined using various alkyl bonded stationary phases. It has been found that a maximum retention temperature exists with long alkyl bonded stationary phases, whereas there is no similar effect with the newly synthesized alkyl bonded phases which have two phenyl groups at the base of the bonded phase. The interpretation of the retention behavior of fullerenes in the low temperature region on alkyl bonded stationary phases is discussed using information obtained by CP-MAS solid-state NMR spectroscopy and LC. 相似文献
64.
Summary A new method for the determination of the mobile phase volume (V
m) in liquid chromatography is presented based on the model regarding the retention of ionic solutes in the presence of eluent
electrolytes. TheV
m value can be determined by measuring the retention volumes of two ions that have the same charge in two eluent electrolyte
systems. Compared with the methods using isotopically labelled eluent components or inorganic salts asV
m markers, the method presented is proved to give more reasonableV
m values for both normal and reversed-phase liquid chromatography. As well as in binary mixed solvent systems, theV
m values in single solvent systems can be determined by this method. 相似文献
65.
Light-stable micelles with azo dyes were prepared by micelle formation of a nonamphiphilic diblock copolymer containing azobenzene and UV absorbent at ca. 1 mol% as the unit ratios. The nonamphiphilic block copolymer consists of two different kinds of random copolymer blocks: poly[4-(phenylazophenoxymethyl)styrene-co-vinylphenol] (P(AS-co-VPh)) and poly[4-(2-hydroxybenzophenoxymethyl)styrene-co-styrene] (P(HBS-co-St)). This random block copolymer, P(AS-co-VPh)-b-P(HBS-co-St) formed the micelles in the presence of 1,4-butanediamine (BDA) through hydrogen bond cross-linking between the VPh units via BDA. The micelles had the azobenzene moieties at the cores and the UV absorbents at the coronas. The micelles showed a small color difference in color fading experiments, in comparison with the unimers and with micelles having no UV absorbent at the coronas. It is significant that the diblock copolymer forms the micelles and has the UV absorbents at the coronas to suppress the color fading. Furthermore, the chain length of ,-diamines had no effect on the hydrodynamic radius of the micelles, but affected the aggregation number and the cmc. 相似文献
66.
67.
Takashi Yokoyama Kazuhiro Ikemoto Takatsugu Kihara Kouhei Teramura Keisuke Uryu Kohji Shimizu Motoki Bitoh Kohji Matsuzaki Michio Zenki 《Journal of solution chemistry》2006,35(5):639-657
Coordination equilibrium constants (K
NiS) of some donor solvent molecules to 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecanenickel(II) ([Ni(Me4[12]aneN4)]2+) were determined in nitrobenzene (a noncoordinating bulk solvent). The first (K
NiS1) and second stepwise coordination equilibrium constants (K
NiS2) for 1,4,7,10-tetraazacyclododecanenickel(II) ([Ni([12]aneN4)]2+), 1,4,8,11-tetraazac yclotetradecane- nickel(II) ([Ni([14] aneN4)]2+), 1,4,8,11-tetrathiacyclotetra-decanenickel(II) ([Ni([14]aneS4)]2+) were also reinvestigated. The K
NiS values for [Ni(Me4[12]aneN4)]2+ were compared to those of [Ni([12]aneN4)]2+, (1R,4S, 8R,11S)-1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) (R,S,R,S-[Ni(Me4[14]aneN4)]2+), R,R,S,S-[Ni(Me4[14]aneN4)]2+, [Ni([14]aneN4)]2+, and [Ni([14]aneS4)]2+. Coordination of pyridine (Py), N,N,N′,N′-tetramethylurea (TMU), and N,N-dimethylacetamide (DMA) to [Ni(Me4[12]aneN4)]2+ was observed, although these donor solvent molecules did not coordinate to R,S,R,S-[Ni(Me4[14]aneN4)]2+. The K
NiS values for Py, TMU, and DMA are 7.9, 2.8, and 9.0 dm3⋅mol−1, respectively. Some hydrogen-bonding waters were coordinated to R,S,R,S-[Ni(Me4[14]aneN4)]2+, but such waters did not coordinate to [Ni(Me4[12] aneN4)]2+. Also, the K
NiS2 values were larger than the corresponding K
NiS1 values for [Ni([14]aneS4)]2+. Furthermore, the K
NiS1 values for [Ni([12]aneN4)]2+ were the largest among these nickel(II) complex cations. The K
NiS, K
NiS1, and K
NiS2 values are discussed in terms of properties of the donor solvents and steric strains of these nickel(II) complex cations. 相似文献
68.
T. Nakamura S. Kojima T. Ohta M. Nishida A. Rakowski A. Ikeda H. Oda E. Niu 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(2):327-332
14C variations of atmospheric CO2 as well as carbonaceous fraction of living materials, such as collagen from tooth and bone, tissue, skin, hair, nail, etc.,
of modern humans are influenced by 14C produced artificially by nuclear bomb tests in the atmosphere from late 1950s to early 1960s. By careful investigation of
14C concentration of tree rings and human body samples formed in this time intervals, we can establish a relationship of their
14C concentrations with calendar year. By applying this relation to a sample whose 14C concentration can be measured, we can estimate the formation age of the sample. In addition, sources of the chemicals that
were used in some criminal cases can be possibly identified, by their carbon isotope ratios (13C/12C and 14C/12C). This method of age determination has been applied to a forensic study, i.e., two criminal cases of murder. For each case,
by comparing the measured 14C abundances of several pieces of hair and one tooth (the third molar) from the body with the annual change on concentrations
of bomb-produced 14C, the time of death of the body and the age of the victim were estimated. The estimated values were consistent with the real
ones that were revealed by the confession of the real murderers. 相似文献
69.
Tomohisa?Yamaguchi Keisuke?Kimura Akira?Tsuchida Tsuneo?OkuboEmail author Mitsuhiro?Matsumoto 《Colloid and polymer science》2005,283(10):1123-1130
Macroscopic and microscopic dissipative structural patterns formed in the course of drying the fractionated and monodisperse bentonite particles (plate-like in their shape) in aqueous deionized suspension and in the presence of NaCl have been studied on a cover glass. The patterns coexisted with the broad ring of the hill accumulated with the particles and with the round hills are formed around the outside edges of the film and in the center, respectively, in the macroscopic scale. By the addition of NaCl the pattern shifts from the broad ring to the round hill in the center. The spoke-like cracks, which have been observed for the suspensions of the spherical particles so often hitherto, are not observed at all for the bentonite suspensions. The characteristic convection flow of the particles and the interactions among the particles and substrate are important for the macroscopic pattern formation. Wrinkled, branch-like and/or star-like fractal patterns are observed in the microscopic scale. These patterns are determined mainly by the electrostatic and polar interactions between the particles and/or between the particle and the substrate in the course of drying. 相似文献
70.
Pinakoulaki E Ohta T Soulimane T Kitagawa T Varotsis C 《Journal of the American Chemical Society》2005,127(43):15161-15167
Reaction pathways in the enzymatic formation and cleavage of the N-N and N-O bonds, respectively, are difficult to verify without the structure of the intermediates, but we now have such information on the heme a(3)(2+)-NO species formed in the reaction of ba(3)-oxidase with NO from resonance Raman spectroscopy. We have identified the His-heme a(3)(2+)-NO/Cu(B)(1+) species by its characteristic Fe-NO and N-O stretching frequencies at 539 and 1620 cm(-)(1), respectively. The Fe-NO and N-O frequencies in ba(3)-oxidase are 21 and 7 cm(-)(1) lower and higher, respectively, than those observed in Mb-NO. From these results and earlier Raman and FTIR measurements, we demonstrate that the protein environment of the proximal His384 that is part of the Q-proton pathway controls the strength of the Fe-His384 bond upon ligand (CO vs NO) binding. We also show by time-resolved FTIR spectroscopy that Cu(B)(1+) has a much lower affinity for NO than for CO. We suggest that the reduction of NO to N(2)O by ba(3)-oxidase proceeds by the fast binding of the first NO molecule to heme a(3) with high-affinity, and the second NO molecule binds to Cu(B) with low-affinity, producing the temporal co-presence of two NO molecules in the heme-copper center. The low-affinity of Cu(B) for NO binding also explains the NO reductase activity of the ba(3)-oxidase as opposed to other heme-copper oxidases. With the identification of the His-heme a(3)(2+)-NO/Cu(B)(1+) species, the structure of the binuclear heme a(3)-Cu(B)(1+) center in the initial step of the NO reduction mechanism is known. 相似文献