全文获取类型
收费全文 | 1283篇 |
免费 | 35篇 |
国内免费 | 9篇 |
专业分类
化学 | 808篇 |
晶体学 | 7篇 |
力学 | 88篇 |
数学 | 148篇 |
物理学 | 276篇 |
出版年
2022年 | 26篇 |
2021年 | 19篇 |
2020年 | 7篇 |
2019年 | 17篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 25篇 |
2015年 | 15篇 |
2014年 | 33篇 |
2013年 | 54篇 |
2012年 | 72篇 |
2011年 | 92篇 |
2010年 | 39篇 |
2009年 | 62篇 |
2008年 | 79篇 |
2007年 | 61篇 |
2006年 | 71篇 |
2005年 | 67篇 |
2004年 | 35篇 |
2003年 | 44篇 |
2002年 | 39篇 |
2001年 | 27篇 |
2000年 | 31篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 15篇 |
1996年 | 33篇 |
1995年 | 17篇 |
1994年 | 15篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 23篇 |
1990年 | 15篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1978年 | 7篇 |
1977年 | 24篇 |
1976年 | 11篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1972年 | 5篇 |
排序方式: 共有1327条查询结果,搜索用时 0 毫秒
41.
The photodissociation of methyl iodide in various matrices at low temperature was studied. The observed Raman spectra excited by 514.5 nm laser radiation showed that there were two different photolytically produced iodine species isolated in the matrices after illumination by a medium pressure mercury lamp. One species which was dominant at lower iodine concentrations and exhibited a progression with an ωe of 201 cm?1, belonged to the matrix isolated iodine monomer (I2). The other species, which was dominant at higher iodine concentrations with an ωe of approximately 180 cm?1, belonged to the iodine aggregate ((I2)n). Five progressions of resonance Raman or resonance fluorescence of these two species were also observed in the other matrices. The iodine aggregate in the methyl iodide matrix at 77 K was formed in a crystalline structure, while the photolytically generated iodine aggregate from CH3I/Ar (2/3) matrix at 10 K, after illumination with a mercury lamp, was in amorphous form. The rearrangement of photolytically produced iodine aggregate in methyl iodide matrix was observed as a function of the duration of illumination. Local heating effects of the laser radiation might induce the iodine monomer to aggregate in matrices. The photodissociation mechanism of methyl iodide in matrices is also proposed. 相似文献
42.
B. K. Wagner P. E. Garrett Minfang Yeh S. W. Yates 《Journal of Radioanalytical and Nuclear Chemistry》1997,219(2):217-220
The first excited state of137Ba has been excited by the inelastic scattering of accelerator-produced neutrons. The energy of this state at 283.5 keV is not in agreement with the generally accepted value of 279.2 keV, but is in accord with other recent measurements. No evidence for a doublet of states near this energy is found. 相似文献
43.
Jwu-Ting Chen Yu-Sung Yeh Wen-Hwa Tzeng Tsang-Miao Huang Ming-Chu Cheng Geen-Hsiang Lee Yu Wang 《中国化学会会志》1991,38(6):573-580
Treatment of trans-Pt(COCOPh)(Cl)(PPh3)2 (1a) with AgBF4in THF led to the formation of a metastatic complex trans-[Pt(COCOPh)(THF)(PPh3)2](BF4) (2) which readily underwent ligand substitution to give a cationic aqua complex trans-[Pt(COCOPh)(OH2)(PPh3)2](BF4) (5a). Complex 5a has been characterized spectroscopically and crystallographically. Analogous reaction of trans-Pt(COCOOMe)(Cl)(PPh3)2 (1b) with Ag(CF3SO3) in dried CH2C12 was found first to yield a methoxyoxalyl triflato complextrans-Pt(COCOOMe)(OTf)(PPh3)2 (6). Attempts to crystallize the triflato product in CH2-cl2hexane under ambient conditions also afforded an aqua complex of the triflate salt f/wu-[Pt(COCOOMe)(OH2)(PPhj)2](CF3SO3) (5b). Complex 5a in a noncoordinating solvent such as CH2C12 or CHCl3 suffered spontaneous decarbonylation to form first cis-[Pt(COPh)(CO)(PPh3)2l(BF4) (3a) then the thermodynamically stable isomer trans-[Pt(COPh)(CO)(PPh3)2](BF4) (3b). Crystallization of complex 3b under ambient conditions resulted in an aqua benzoyl complex trans-[Pt(COPh)(OH2)(PPh3)2](BF4) (7). The replacement of the H2O ligand in complex 7 by CO was done simply by bubbling CO into the solution of 7. The single crystal structures of 5b and 7 have been determined by X-ray diffraction. The distances of the Pt-O bonds in 5a, 5b, and 7 support that the aqua ligand is a weak donor in such cationic aquaorganoplatinum(lI) complexes, in agreement with their lability to the substitution reactions. 相似文献
44.
A new series of quinolinolate osmium carbonyl complexes were synthesized and characterized by spectroscopic methods. Single-crystal X-ray diffraction studies indicate that these complexes consist of an octahedral ligand arrangement with one chelating quinolinolate, one tfa or halide ligand, and three mutually orthogonal terminal CO ligands. Variation of the substituents on quinolinolate ligands imposes obvious electronic or structural effects, while changing the tfa ligand to an electron-donating iodide slightly increases the charge density on the central osmium atom. These Os(II) complexes show salient dual emissions consisting of fluorescence and phosphorescence, the spectral properties and relaxation dynamics of which have been studied comprehensively. The results, in combination with the theoretical approaches, lead us to propose that the emission mainly originates from the quinolinolate pi pi* state. Both experimental and theoretical approaches generalize various types of intersystem crossing versus those of the tris(quinolinolate) iridium Ir(Q)3, and their relative efficiencies were accessed on the basis of the associated frontier orbital configurations. Our results suggest that [1d(pi)pi* absolute value(H(so))3 pi pi*] (or [3d(pi)pi* absolute value(H(so))1 pi pi*]) in combination with a smaller deltaE(S1-T1) gap (i.e., increasing the MLCT (d(pi)pi*) character) is the main driving force to induce the ultrafast S1 --> T1 intersystem crossing in the third-row transition metal complexes, giving the strong phosphorescent emission. 相似文献
45.
Saalfrank RW Glaser H Demleitner B Hampel F Chowdhry MM Schünemann V Trautwein AX Vaughan GB Yeh R Davis AV Raymond KN 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(2):493-497
In a one-pot reaction, the tetranuclear iron chelate complex [Fe4(L4)4] 6 was generated from benzene-1,3,5-tricarboxylic acid trichloride (4), bis-tert-butyl malonate (5a), methyllithium, and iron(II) dichloride under aerobic conditions. Alternatively, hexanuclear iron chelate complex [Fe(L5)6] 7 was formed starting from bis-para-tolyl malonate (5b) by employing identical reaction conditions to those applied for the synthesis of 6. The clusters 6 and 7 are present as racemic mixtures of homoconfigurational (delta,delta,delta,delta)/(lambda,lambda,lambda,lambda)-fac or (delta,delta,delta,delta,delta,delta)/(lambda,lambda,lambda,lambda,lambda,lambda)-fac stereoisomers. The structures of 6 and 7 were unequivocally resolved by single-crystal X-ray analyses. The all-iron(III) character of 6 and 7 was determined by M?ssbauer spectroscopy. 相似文献
46.
Neil Bartlett Sam Yeh Kostantinos Kourtakis Tom Mallouk 《Journal of fluorine chemistry》1984,26(1):97-116
Iridium hexafluoride oxidizes ReF6 (via an ReF6+ salt) and at room temperatures IrF6, ReF6, ReF7 and (IrF5)4 are each present in the equilibrium mixture. From these and related findings: ΔH°(ReF6 → ReF6+ + e?) 1092 ± 27 kj mole?1(261 ± 6 kcal mole?1), and thermodynamic data are selected to yield ΔH°(ReF7(g) → ReF6+(g) + F?(g))=893 ± 33 kj mole?1(213 ± 8 kcal mole?1). From observations on the stability of IF6+BF4? and the lattice enthalpy evaluation for the salt, ΔH°(IF7(g) → IF6+(g) + F?(g))= 870 ± 24 kj mole?1(208 ± 6 kcal mole?1). 相似文献
47.
Chih-Hsin Yeh Chia-Yi Chou Chin-Sheng Wu Lee-Ping Chu Wei-Juan Huang Hsin-Chun Chen 《Molecules (Basel, Switzerland)》2022,27(14)
To establish the analytic conditions for examining the aroma quality of vanilla pods, we compared different extraction methods and identified a suitable option. We utilized headspace solid-phase microextraction (HS-SPME), steam distillation (SD), simultaneous steam distillation (SDE) and alcoholic extraction combined with gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS) to identify volatile components of vanilla pods. A total of 84 volatile compounds were identified in this experiment, of which SDE could identify the most volatile compounds, with a total of 51 species, followed by HS-SPME, with a total of 28 species. Ten volatile compounds were identified by extraction with a minimum of 35% alcohol. HS-SPME extraction provided the highest total aroma peak areas, and the peak areas of aldehydes, furans, alcohols, monoterpenes and phenols compounds were several times higher than those of the other extraction methods. The results showed that the two technologies, SDE and HS-SPME, could be used together to facilitate analysis of vanilla pod aroma. 相似文献
48.
A differential radial distribution function (DRDF) of molten natural rubber (NR) was derived from its wide-angle x-ray scattering (WAXS) data. Three peaks with Bragg spacings 4.85, 2.31, and 1.23 Å corresponding, respectively, to interchain, C1-C3 and C1 -C2 distances are present in the corrected WAXS curve. The derived DRDF, which is shown to be greatly improved than the ones published so far, contains six peaks located at 1.51, 2.48, 3.98, 5.68, 10.65, and 15.25 Å. Maximum contributions of intramolecular atomic distances to the radial distribution peaks were estimated and compared with the experimental results. The comparison shows that the first three peaks are intramolecular in origin and the remaining peaks are predominantly due to intermolecular regularities. A periodicity of an approximately constant interval of 5.26 Å is found between the intermolecular peaks and is attributable to the organizations of more or less parallel chain segments in the material. This finding provided additional evidence for the presence of local lateral ordering to the extent of about 30 Å in molten-state natural rubber. 相似文献
49.
Dot-matrix holograms created by two-beam writers contain many grating dots. Because the phase difference between two laser beams for interference cannot be controlled accurately, the fringe positions of grating dots are randomly determined. Therefore, fringe positions are a good kind of tool to identify dot-matrix holograms. In this paper, a number difference between two special fringes of a target grating dot is used to identify a dot-matrix hologram. The two special fringes are determined by three grating dots with parallel fringes. The first special fringe is corresponding to a fringe pair with the best matching for the fringes of the target grating dot and the fringes of the second grating dot. The second special fringe is corresponding to a fringe pair with the best matching for the fringes of the target grating dot and the fringes of the third grating dot. An experiment has proved the proposed method practical and feasible. Because reproducing a grating dot with a specified fringe number difference is difficult, the proposed method is excellent for anti-counterfeiting. 相似文献
50.