全文获取类型
收费全文 | 1132篇 |
免费 | 57篇 |
国内免费 | 3篇 |
专业分类
化学 | 871篇 |
晶体学 | 3篇 |
力学 | 9篇 |
数学 | 139篇 |
物理学 | 170篇 |
出版年
2023年 | 8篇 |
2021年 | 9篇 |
2020年 | 14篇 |
2019年 | 18篇 |
2017年 | 11篇 |
2016年 | 37篇 |
2015年 | 36篇 |
2014年 | 39篇 |
2013年 | 50篇 |
2012年 | 44篇 |
2011年 | 49篇 |
2010年 | 34篇 |
2009年 | 31篇 |
2008年 | 41篇 |
2007年 | 46篇 |
2006年 | 45篇 |
2005年 | 36篇 |
2004年 | 27篇 |
2003年 | 24篇 |
2002年 | 26篇 |
2001年 | 23篇 |
2000年 | 14篇 |
1999年 | 16篇 |
1998年 | 25篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 30篇 |
1994年 | 26篇 |
1993年 | 16篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 27篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1982年 | 25篇 |
1981年 | 19篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1978年 | 17篇 |
1977年 | 12篇 |
1976年 | 12篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 16篇 |
1971年 | 12篇 |
1970年 | 9篇 |
排序方式: 共有1192条查询结果,搜索用时 15 毫秒
41.
Studies on the Coordinative Behaviour of Potentially Four and Fivedentate Bis(salicylidene amino)-Ligands with Ions of the 3d-Elements. Fourdentate bis(salicylidene amino)-ligands (?ONΦCH2ΦNO? and ?ONΦOΦNO?, meaning of the abbreviations see text) with the centres of donor atoms connected by rigid, bifunctional residues, form polymeric complexes MeII(ONΦCH2ΦNO) and MeII(ONΦOΦNO) with cobalt(II), nickel(II), and copper(II). Among the cobalt(II) chelates the structure is uniformly tetrahedral, but the nickel(II) and copper(II) complexes contain square planar central atoms and such with a higher coordination number side by side. The potential fivedentate ligands ~ON3PPh3NO?, ?ON2O2NO?, and ?ON2S2NO? differ as to their coordinative behaviour. The coordination number of the complexes Ni(ON3PPh3NO) and Co(ON3PPh3NO) is five. On the other hand the compounds Co(ON2S2NO) and Co(ON2O2NO) are tetrahedral. No coordination of the ether or sulfide group is observed. Ni(ON2O2NO) has an octahedral structure which is produced by coordination polymerisation and possibly by coordination of the ether group. The new complexes are compared with the transition metal complexes of simple salicylaldimines. 相似文献
42.
Ulrich Englich Ingo Prass Karin Ruhlandt-Senge Thorsten Schollmeier Frank Uhlig 《Monatshefte für Chemie / Chemical Monthly》2002,133(7):931-943
Summary. Hydrido substituted stannasilanes of the type or (Z = H, Me, Ph; R, R′ = alkyl, Ph) are accessible by reaction of either alkali metal stannides (MSn(Z)R
2; M = Li, Na) with halogen substituted silanes (; X = F, Cl) or chlorostannanes (R
2SnCl2, Ph3SnCl) and fluorosilanes in the presence of magnesium. Stannasilanes with halogen substituents at the silicon as well as the tin atom are formed by
treatment of the hydrido substituted stannasilanes with CHCl3 or CCl4. The hydrido substituted stannasilanes decompose in contact with air to distannanes and siloxanes or to the linear (
t
Bu2Sn(–O–
t
Bu2Si–OH)2) and cyclic ((–
t
Bu2Sn–O–
i
Pr2Si–O–)2) stannasiloxanes.
Received November 29, 2001. Accepted (revised) January 16, 2002 相似文献
43.
In the solid state complexes of the type MiL2X2 (L = N-substituted β-aminoethyl-pyridine; X = Cl, Br, J) have a cis-octahedral (X = Cl, Br) or a distorted trigonal bipyramidal structure (X = J). In solutions in acetone a partial dissociation occurs with the formation of NiLX2, L, NiL2X+, and X?. Using a spectrophotometric method stability constants K2S of the complexes NiL2X2 are determined. A correlation exists between log K2s and the pK- values of the quarternary ammonium ions derived from the ligands L. Sterical factors cause the exeptional position of the chelates of β-methylaminoethylpyridine-(2). 相似文献
44.
George L Sander W 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(13):3225-3232
The infrared spectra of the formic acid-water complexes isolated in argon matrices are reported. Both supersonic jet expansion and a conventional effusive source followed by trapping in solid argon at 10K are used to obtain the matrices. The experimental IR spectra are compared to the data obtained from high level ab initio (MP2) and DFT (B3LYP) calculations with 6-311++G(d,p) and aug-cc-pVTZ basis sets. The complex formation results in red shifts in the C=O and O-H stretching vibrations and a blue shift in the C-O stretching vibration of formic acid. The O-H stretching modes of water also exhibit pronounced red shifts. Both the MP2 and B3LYP calculations located three minima corresponding to cyclic HCOOH...H2O complexes with two hydrogen bond interactions. The binding energies are -10.3, -5.1, and -3.5 kcal mol(-1), respectively, for the three complexes at the MP2/ aug-cc-pVTZ level, corrected for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise scheme. Comparison of the calculated frequencies of the three complexes with the matrix IR spectrum reveals that the lowest energy complex is formed. In addition, a complex of formic acid with two water molecules is observed. 相似文献
45.
Niamh NicDaéid Saravana Jayamana William J. Kerr Wolfram Meier-Augenstein Helen F. Kemp 《Analytical and bioanalytical chemistry》2013,405(9):2931-2941
A number of methods of clandestine manufacture of methylamphetamine involve the extraction and subsequent reaction of pseudoephedrine hydrochloride with other essential chemicals. The precursor can be easily extracted from over-the-counter medication widely available in the UK and elsewhere. Essential chemicals such as iodine and red phosphorous are also readily available and can be extracted from iodine tinctures and matchboxes, respectively. This work reports the repetitive preparation of methylamphetamine using two popular routes (the Moscow and Hypophosphorous synthesis). The focus was on the extraction solvent used for isolation of the precursor chemical and any consequential isotopic variation which may arise in the final product. Six batches of methylamphetamine were prepared under precisely controlled conditions for each synthetic route and for each of three different precursor extraction solvents. Synthesis of the final product from laboratory grade precursor using the synthetic methods described was used as a template for comparison. The resultant IRMS data from all 48 prepared samples suggests some underlying trends in the identification of the synthetic route which may aid in the interpretation of IRMS data derived from clandestine samples. 相似文献
46.
Prof. Dr. Axel G. Griesbeck Dr. Johannes Uhlig Dr. Thomas Sottmann Dr. Lhoussaine Belkoura Prof. Dr. Reinhard Strey 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(50):16161-16165
Pluronic F‐127 hydrogels are highly efficient microenvironments for photochemical reactions, as demonstrated for singlet oxygen reactions of monoalkenes. Nonpolar substrates are localized in the nanosized polymer compartment, which can be visualized by neutron scattering. The efficiency of 1O2 reactions is strongly increased for tiglate derivatives and the regioselectivity of the ene reaction of trisubstituted alkenes is completely switched in comparison with solution phase and inverted in comparison with intrazeolite photo‐oxygenations. 相似文献
47.
A confidence interval for the probability of detection across laboratories (LPOD) for qualitative methods, described in the AOAC INTERNATIONAL guidelines for validation of microbiological methods for food and environmental surfaces, is considered. It is demonstrated that under certain conditions, the observed confidence of this confidence interval can be rather low, so that statistical minimum requirements are not fulfilled. A new profile likelihood confidence interval based on a latent random laboratory effect approach is proposed. Observed confidence levels for this confidence interval demonstrate its applicability already for a small number of laboratories. 相似文献
48.
Frank Uhlig 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6):iii-iv
49.
Dr. Tim Schleif Dr. Jörg Tatchen Julien F. Rowen Frederike Beyer Prof. Dr. Elsa Sanchez-Garcia Prof. Dr. Wolfram Sander 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(46):10452-10458
The Cope rearrangement of selectively deuterated isotopomers of 1,5-dimethylsemibullvalene 2 a and 3,7-dicyano-1,5-dimethylsemibullvalene 2 b were studied in cryogenic matrices. In both semibullvalenes the Cope rearrangement is governed by heavy-atom tunneling. The driving force for the rearrangements is the small difference in the zero-point vibrational energies of the isotopomers. To evaluate the effect of the driving force on the tunneling probability in 2 a and 2 b , two different pairs of isotopomers were studied for each of the semibullvalenes. The reaction rates for the rearrangement of 2 b in cryogenic matrices were found to be smaller than the ones of 2 a under similar conditions, whereas differences in the driving force do not influence the rates. Small curvature tunneling (SCT) calculations suggest that the reduced tunneling rate of 2 b compared to that of 2 a results from a change in the shape of the potential energy barrier. The tunneling probability of the semibullvalenes strongly depends on the matrix environment; however, for 2 a in a qualitatively different way than for 2 b . 相似文献
50.
Sergei V. Chapyshev Denis V. Korchagin Paulo Costa Wolfram Sander 《Magnetic resonance in chemistry : MRC》2022,60(8):829-835
The first X-band EPR spectrum containing only non-overlapping signals of septet pyridyl-2,4,6-trinitrene and triplet pyridylnitrenes is reported. This spectrum was recorded after photolysis of 2,4,6-triazidopyridine in solid argon at 5 K. The zero-field splitting (ZFS) parameters of this trinitrene as well as of intermediate triplet mononitrenes and quintet dinitrenes formed at early stages of the photolysis were determined using the combination of modern computer line-shape spectral simulations and density functional theory (DFT) calculations. It was found that septet pyridyl-2,4,6-trinitrene has the record negative parameter DS = −0.1031 cm−1 among all known to date septet pyridyl-2,4,6-trinitrenes and may be of interest as a model multi-qubit spin system for investigations of quantum computation processing. 相似文献