全文获取类型
收费全文 | 1526篇 |
免费 | 37篇 |
国内免费 | 5篇 |
专业分类
化学 | 1140篇 |
晶体学 | 13篇 |
力学 | 19篇 |
数学 | 215篇 |
物理学 | 181篇 |
出版年
2022年 | 11篇 |
2021年 | 16篇 |
2020年 | 25篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 25篇 |
2014年 | 35篇 |
2013年 | 60篇 |
2012年 | 84篇 |
2011年 | 88篇 |
2010年 | 57篇 |
2009年 | 38篇 |
2008年 | 70篇 |
2007年 | 63篇 |
2006年 | 72篇 |
2005年 | 84篇 |
2004年 | 70篇 |
2003年 | 57篇 |
2002年 | 55篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 16篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 11篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 31篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 19篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1984年 | 13篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 19篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 15篇 |
1975年 | 10篇 |
1974年 | 15篇 |
1973年 | 16篇 |
1970年 | 9篇 |
1882年 | 7篇 |
排序方式: 共有1568条查询结果,搜索用时 15 毫秒
61.
Elettra L. Piacentino Edwin Rodriguez Kevin Parker Thomas M. Gilbert Richard A. J. O'Hair Victor Ryzhov 《Journal of mass spectrometry : JMS》2019,54(6):520-526
Gas‐phase C―C coupling reactions mediated by Ni (II) complexes were studied using a linear quadrupole ion trap mass spectrometer. Ternary nickel cationic carboxylate complexes, [(phen)Ni (OOCR1)]+ (where phen = 1,10‐phenanthroline), were formed by electrospray ionization. Upon collision‐induced dissociation (CID), they extrude CO2 forming the organometallic cation [(phen)Ni(R1)]+, which undergoes gas‐phase ion‐molecule reactions (IMR) with acetate esters CH3COOR2 to yield the acetate complex [(phen)Ni (OOCCH3)]+ and a C―C coupling product R1‐R2. These Ni(II)/phenanthroline‐mediated coupling reactions can be performed with a variety of carbon substituents R1 and R2 (sp3, sp2, or aromatic), some of them functionalized. Reaction rates do not seem to be strongly dependent on the nature of the substituents, as sp3‐sp3 or sp2‐sp2 coupling reactions proceed rapidly. Experimental results are supported by density functional theory calculations, which provide insights into the energetics associated with the C―C bond coupling step. 相似文献
62.
The dynamic processes of N(1s) core-hole excitation in gas-phase CH3CN molecule have been studied at both Hartree-Fock and hybrid density-functional theory levels. The vibrational structure is analyzed for fully optimized core-excited states. Frank-Condon factors are obtained using the linear coupling model for various potential surfaces. It is found that the vibrational profile of the N-K absorption can be largely described by a summation of two vibrational progressions: a structure-rich profile of nu(CN) stretching mode and a large envelope of congestioned vibrational levels related to the strong (-C-CN) terminal bending bond. Excellent agreement between theoretical and experimental spectra is obtained. 相似文献
63.
Computer least-squares methods are used to conduct an automatic reverse search of a library of complete mass spectra for the analysis of the output data matrix from a g.c.—m.s. system. The search is limited to those mass spectra falling within a narrow retention index window about the retention index corresponding to each scan in the datamatrix. The least-squares estimated parameters obtained are proportional to the concentrations of the substances present at each scan and are displayed graphically as the profiles of resolved overlapping g.c. peaks. The precision of the parameters is estimated. The root mean square Values of the residuals of the least-squares fits are used successfully to indicate the presence of substances not in the library. Examples are given of the results produced with data from this laboratory. Possible elaborations and improvements of this computer program are discussed. 相似文献
64.
65.
M. Donath D. Scholl H. C. Siegmann E. Kay 《Applied Physics A: Materials Science & Processing》1991,52(3):206-209
A surface probing depth of only 2 layers for low energy cascade electrons excited with a 3.2 keV primary electron beam is clearly demonstrated by model experiments with non-magnetic overlayers of Ta on magnetic substrates of Fe/Ni80Fe20. This result establishes a short probing depth of low energy electrons in transition metals generalizing the previously observed short magnetic probing depth for spin-polarized electrons in ferromagnets. The short probing depth sheds new light on a number of spectroscopic observations on ferromagnetic transition metals, and has important implications concerning surface magnetic properties and scattering processes of hot electrons in transition metals. 相似文献
66.
Li Pang Edwin A. C. Lucken Gérald Bernardinelli 《Journal of inclusion phenomena and macrocyclic chemistry》1992,13(1):63-76
NQR spectroscopy and X-ray crystallography have been jointly applied to the study of the small guest molecules CH2Cl2, CHCl3, CCl4, CBr2Cl2, C(CH3)2Cl2 etc. in the Werner host complex Ni(SCN)2(3-methylpyridine)4. Specific host-guest dipole-dipole interactions play an important role in order-disorder effects for guest molecules in the cavities of orthorhombic system,Fddd. Thus a weakly polar molecule such as CBr2Cl2 exhibits a disordered alternation, both the bromine and the chlorine atoms occupying the two possible sites in theC
2 cavity, whereas for the polar molecule C(CH3)2Cl2, an ordered structure is observed. In addition, two new types of crystal structures,C2/c andP
, are reported here for dichloromethane and chloroform clathrates. 相似文献
67.
Ingeborg Csöregh Thomas Brehmer Silke I. Nitsche Wilhelm Seichter Edwin Weber 《Journal of inclusion phenomena and macrocyclic chemistry》2003,47(3-4):113-121
X-ray crystal structures are reported of a free host compound 1, comprising two diphenylmethanol terminal groups attached to a central 9,10-ethynyl substituted anthracene unit, and of three inclusion compounds of a fluoren-9-ol substituted analogous host 2 with acetone, dimethyl sulphoxide (DMSO) and dimethylformamide (DMF) as guest, respectively. Despite the presence of two hydroxyl groups in 1, there is no O–H...O hydrogen bond between the molecules in the guest free crystal – only weaker C–H...O interactions and van der Waals' type connections. In the inclusion compounds of 2, H-bonded 1:2 host–guest associates are formed, where each of the host hydroxyl groups binds to a guest oxygen atom. The orientations of the host–guest connections in these complexes vary, being E for acetone and Z for both DMSO and DMF guests, relative to the host anthracene unit. The DMSO and DMF inclusion compounds of 2 proved to be isostructural. 相似文献
68.
Dixit AA Lei Y Lee KW Quiñones E Houston PL 《The journal of physical chemistry. A》2005,109(9):1770-1775
Multiphoton excitation and dissociation of SO(2) have been investigated in the wavelength range from 224 to 232 nm. Strong evidence is found for two-photon excitation to the H Rydberg state, followed by dissociation to SO + O and ionization of the SO product by absorption of a third photon. The two-photon excitation is resonantly enhanced via the C (1)B(2) intermediate state, and the two-photon yield spectrum thus bears a strong resemblance to the spectrum of this intermediate. Imaging of the O((3)P(2)), S((1)D(2)), and SO products suggests that, following dissociation of SO(2) from the H state, SO is produced in the A and B electronic states. S((1)D(2)) is produced both from two-photon dissociation of SO(2) to give S((1)D(2)) + O(2) and by single-photon dissociation of SO(+). In the former process, the O(2) is likely formed in all of its lowest three electronic states. 相似文献
69.
Treatment of 11a with methyllithium affords the destannylated product 12 together with a small amount of tetracyclic product derived from intramolecular Michael addition. The same procedure from the deuterated analogue 11b gives the tetracyclic 18 as the major product, the result of a substantial kinetic deuterium isotope effect that favors formation of 16 and 17 by suppressing indole ring lithiation to the undesired 15. When the product mixture is quenched with phenylselenenyl chloride, 17 is converted into the aziridinomitosene 19 in 80% yield. Conversion into the aziridinomitosene alcohol 21 and the deprotected aziridine 20 is also demonstrated. 相似文献
70.