全文获取类型
收费全文 | 1043篇 |
免费 | 16篇 |
国内免费 | 11篇 |
专业分类
化学 | 670篇 |
晶体学 | 3篇 |
力学 | 29篇 |
数学 | 190篇 |
物理学 | 178篇 |
出版年
2021年 | 6篇 |
2019年 | 5篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 12篇 |
2014年 | 16篇 |
2013年 | 55篇 |
2012年 | 42篇 |
2011年 | 45篇 |
2010年 | 29篇 |
2009年 | 30篇 |
2008年 | 40篇 |
2007年 | 39篇 |
2006年 | 49篇 |
2005年 | 55篇 |
2004年 | 45篇 |
2003年 | 32篇 |
2002年 | 45篇 |
2001年 | 22篇 |
2000年 | 10篇 |
1999年 | 21篇 |
1998年 | 21篇 |
1997年 | 21篇 |
1996年 | 20篇 |
1995年 | 14篇 |
1994年 | 21篇 |
1993年 | 21篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 17篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 15篇 |
1984年 | 22篇 |
1983年 | 24篇 |
1982年 | 12篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 21篇 |
1977年 | 22篇 |
1976年 | 14篇 |
1975年 | 16篇 |
1974年 | 18篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1968年 | 7篇 |
1967年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有1070条查询结果,搜索用时 15 毫秒
41.
Wayne Carpenter Younus M. Sheikh A. M. Duffield Carl Djerassi 《Journal of mass spectrometry : JMS》1968,1(1):3-12
In spite of previous extensive studies on the mass spectrometry of alkyl cyanides, no good mechanistic insight has as yet been gained into the behavior of straight chain alky1 cyanides. For this reason the possible origins for the principal ions in the mass spectrum of a typical alkyl cyanide (n-hexyl cyanide) have been determined using high resolution mass spectrometry anddeuterium labeling of every carbon atom. The results lead to the conclusion that most of the typical fragmentation processes of n-alkyl cyanides proceed through cyclic intermediates. 相似文献
42.
CH-stretching overtone spectra of 3-methylpentane are measured for both liquid and low temperature (77 K) solid phases. A local-mode analysis is used to extract the diagonal local-mode anharmonicity constant. The results show that the local vibrational potential is more harmonic at high viscosity. 相似文献
43.
Wayne W. Barrett 《Journal of statistical physics》1977,16(4):371-394
A one-parameter family of partition functions is considered which for zero value of the parameter reduces to the spherical model of a ferromagnet. The model for > 0 is closer to the usual discrete lattice spin model of a ferromagnet than is the spherical model. The first four terms in of the limiting value of the partition function are calculated above and below the critical temperature for arbitrary interactions using the saddle point method to calculate certain correlation functions for the spherical model. These calculations indicate that the critical temperature is independent of for small and certain interactions.Part of this research appeared in the author's doctoral thesis.(3) 相似文献
44.
James H. Looker Duane L. Shaneyfelt Wayne Halfar 《Journal of heterocyclic chemistry》1979,16(6):1281-1282
Condensation reactions of comenaldehyde methyl ether (I) with malonic acid, ethyl cyanoacetate, and cyanoacetamide to give β-(5-methoxy-4H-pyran-4-on-2-yl)acrylic acid (II), ethyl 2-cyano-3-(5-methoxy-4H-pyran-4-on-2-yl)propenoate (III), and 2-cyano-3-(5-methoxy-4H-pyran-4-on-2-yl)propenamide (IV), respectively, are described. Ultraviolet absorption spectra for 2-hydroxymethyl-5-methoxy-4H-pyran-4-one, I and II are presented. 相似文献
45.
Time-of-flight (TOF) and electrostatic sector analysis (ESA) have been used to measure particles scattered and sputtered by direct recoils and surface recoils during 3 keV Ar+ and 6 keV Ar2+ bombardment of CsBr at forward and backscattering angles. Charge fractions of scattered argon and recoiling surface atoms are obtained. Hydrogen and oxygen surface impurities are detected predominantly as directly recoiled neutrals. 相似文献
46.
C. S. Lee Wayne W. Y. Lau S. Y. Lee S. H. Goh 《Journal of polymer science. Part A, Polymer chemistry》1992,30(6):983-988
Copolymers of 2,2,6,6-tetramethylpiperidinyl methacrylate (TPMA) with styrene (S) and with methyl methacrylate (MMA) were synthesized using AIBN as initiator. S–TPMA copolymers from feed ranging from 0.10–0.80 mole fractions TPMA and MMA-TPMA copolymers from feed of 0.04–0.85 mole fractions TPMA were used in the determination of monomer reactivity ratios r1, r2. Four different methods were employed in the calculations of r1 and r2 and all calculated results were in good agreement with each other. The structure of S–TPMA copolymers was inferred to be of an alternating nature while that of MMA–TPMA copolymers was random. Both copolymers are potential hindered amine light stabilizers (HALS) and are expected to be less extractable from, and more compatible with, polystyrene and poly(methyl methacrylate) base polymers. 相似文献
47.
The title compounds were synthesized from 3-[bis(2-hydroxyethyl)amino]quinolin-2(1H)-one 11a and 3-[bis(2-hydroxyethyl)amino]pyridin-2(1H)-one 18 respectively. The preparation involved a tandem chlorination/cyclization reaction. 相似文献
48.
Grange AH Genicola FA Sovocool GW 《Rapid communications in mass spectrometry : RCM》2002,16(24):2356-2369
Concentration factors of 1000 and more reveal dozens of compounds in extracts of water supplies. Library mass spectra for most of these compounds are not available, and alternative means of identification are needed. Determination of the elemental compositions of the ions in mass spectra makes feasible searches of commercial and chemical literature that often lead to compound identification. Instrumental capabilities that constrain the utility of a mass spectrometer for determining ion compositions for compounds that elute from a chromatographic column are scan speed, mass accuracy, linear dynamic range, and resolving power. Mass peak profiling from selected ion recording data (MPPSIRD) performed with a double-focusing mass spectrometer provides the best combination of these capabilities. This technique provides unique ion compositions for ions of higher mass from compounds eluting from a gas chromatograph than can be obtained by orthogonal acceleration time-of-flight (oa-TOF) or Fourier transform ion cyclotron resonance mass spectrometry. Multiple compositions are usually possible for an ion with a mass exceeding 150 Da within the error limits of the mass measurement. The correct composition is selected based on measured exact masses of the mass peak profiles resulting from isotopic ions higher in mass by 1 and 2 Da and accurate measurement of the summed abundances of these isotopic ions relative to the monoisotopic ion. A profile generation model (PGM) automatically determines which compositions are consistent with measured exact masses and relative abundances. The utility of oa-TOF and double-focusing mass spectrometry using ion composition elucidation (MPPSIRD plus the PGM) are considered for determining ion compositions of two compounds found in drinking water extracts and a third compound from a monitoring well at a landfill. Published in 2002 by John Wiley & Sons, Ltd. 相似文献
49.
Self-assembled monolayers (SAMs) obtained from 1-(10-acetylsulfanyldecyl)-4-[2-(4-dimethylaminophenyl)vinyl]quinolinium iodide exhibit asymmetric current-voltage (I-V) characteristics. The rectification may be reversibly switched: it is suppressed when the film is exposed to HCl vapor, the intramolecular charge-transfer axis being inhibited by protonation, but restored when exposed to NH(3). The behavior is intrinsic to the donor-(pi-bridge)-acceptor moiety, and ambiguity in the assignment has been excluded by matching the alkyl tails on the substrate and contacting STM tip to locate the chromophore midway between the electrodes: Au-S-C(10)H(21)//D-pi-A-C(10)H(20)-S-Au. Films contacted by gold tips exhibit rectification ratios of ca. 18 at +/-1 V, whereas those contacted by pentanethiolate (Au-S-C(5)H(11))- and decanethiolate (Au-S-C(10)H(21))-coated tips have corresponding ratios of ca. 11 and 5, respectively. The I-V curves are different, but when adjusted for thickness the current versus electric field dependence is indistinguishable. Seven dyes are reported: SAMs with sterically hindered D-pi-A moieties, in which the donor and acceptor are twisted out of plane, exhibit rectification, whereas those that are planar or have a weak donor-acceptor combination do not. 相似文献
50.
Mass-selected beams of N+ and N2+ in the energy range 5–50 eV react with molybdenum to produce a surface nitride. The relative reaction cross section for N+ reaction is higher than that of N2+ in the range 5–25 eV and N2+ exhibits a reaction threshold near 7 eV. The N2+ threshold suggests collisional dissociation prior to reaction. 相似文献