全文获取类型
收费全文 | 1453篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
化学 | 901篇 |
晶体学 | 16篇 |
力学 | 15篇 |
数学 | 89篇 |
物理学 | 446篇 |
出版年
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2017年 | 15篇 |
2016年 | 18篇 |
2015年 | 8篇 |
2014年 | 18篇 |
2013年 | 54篇 |
2012年 | 42篇 |
2011年 | 47篇 |
2010年 | 26篇 |
2009年 | 23篇 |
2008年 | 48篇 |
2007年 | 71篇 |
2006年 | 80篇 |
2005年 | 74篇 |
2004年 | 79篇 |
2003年 | 57篇 |
2002年 | 69篇 |
2001年 | 35篇 |
2000年 | 38篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 33篇 |
1995年 | 21篇 |
1994年 | 32篇 |
1993年 | 27篇 |
1992年 | 30篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 27篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 28篇 |
1984年 | 24篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 23篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 25篇 |
1977年 | 18篇 |
1976年 | 13篇 |
1975年 | 10篇 |
1974年 | 21篇 |
1973年 | 24篇 |
1972年 | 12篇 |
1971年 | 8篇 |
排序方式: 共有1467条查询结果,搜索用时 31 毫秒
81.
The development of a sensitive and specific method of determining individually the five tetraalkyllead compounds normally present in ambient air is described. The method is based on collection of the analytes on a porous polymer using a prefilter to destroy ozone and prevent decomposition of the sample during collection and storage. Two-stage thermal desorption, separation by gas chromatography and detection by modified flame atomic absorption spectrometry gives detection limits (3σ) of ca. 0.25 ng Pb m?3 for tetramethyllead and 0.37 ng Pb m?3 for tetraethyllead in an air sample of 80 dm3 collected over a 3–24 h period. Environmental sampling by this method in paralle with a wet chemical (iodine monochloride) method was used to validate the method. During the comparative study, higher organic lead levels were consistently found by the iodine monochloride method; it is postulated that this indicates the presence of vapour-phase tri-(or di)-alkyllead in excess of 1 ng Pb m?3 in rural air. The versatility of the method is demonstrated by the results of atmospheric sampling at two locations, one rural and one at the kerbside in a city centre. 相似文献
82.
Electron spin resonance (ESR) and magnetic-susceptibility measurements on the Li1+xTi2?xO4 spinel system () indicate the presence of two types of localized moments in this material. In both cases, an unpaired electron is trapped as a Ti3+ ion in a crystal field that is predominantly octahedral, but with a strong tetragonal component. This type of crystal field cannot arise in the stoichiometric spinel. We propose two types of defect in the title spinel system: an oxygen vacancy and a hydroxyl ion. Unpaired electrons are trapped as Ti3+ ions adjacent to these defects, and it is argued that the strong tetragonal field is associated with the formation of a static (TiO)+ ion by a displacement of the titanium ion from the defect. Spin relaxation occurs via a thermal ionization of the trapped electron that appears to be associated with a static-dynamic transition in the titanium-ion displacement. 相似文献
83.
Sabatino Nacson Alex. G. Harrison William R. Davidson 《Journal of mass spectrometry : JMS》1986,21(6):317-319
The collision-induced dissociation (CID) mass spectra of protonated cocaine and protonated heroin have been measured using a triple quadrupole mass spectrometer at 50 eV ion/neutral collision energy for protonated molecules prepared by different protonating agents. The CID mass spectra of protonated cocaine using H+(H2O)n, H+(NH3)n and H+((CH3)2NH)n as protonating agents are essentially identical and it is concluded that, regardless of the initial site of protonation, the fragmentation reactions occurring on collisional activation are identical. By contrast, protonated heorin prepared with H+(H2O)n and H+(NH3)n as protonating agents show substantial differences. That formed by reaction of H+(H2O)n shows a much more abundant peak corresponding to loss of CH3CO2H. From a comparison with model compounds, and from a consideration of the three-dimensional structure of heroin, it is concluded that with H+(H2O)n as protonating agent significant protonation occurs at the acetate group attached to the alicyclic ring, leading to acetic acid loss on collisional activation, but that reaction of H+(NH3)n leads to protonation at the nitrogen function. The proton attached to nitrogen cannot interact with the acetate group and, consequently, the probability of loss of acetic acid on collislional activation is greatly reduced. 相似文献
84.
D.J. Buttrey H.R. Harrison J.M. Honig R.R. Schartman 《Journal of solid state chemistry》1984,54(3):407-413
Growth of large single crystals of quasi-two-dimensional Ln2NiO4 (Ln = La, Pr, Nd) has been accomplished using the radio frequency technique of skull melting under controlled oxygen partial pressures. No evidence of intergrowth of other phases such as Ln3Ni2O7 or higher homologs was observed. Determination of the Ni3+ content indicates that the crystals are cation deficient. 相似文献
85.
Alex. G. Harrison R. Krishna Mohan Rao Kallury 《Journal of mass spectrometry : JMS》1980,15(5):249-256
A study of the electron impact and chemical ionization (H2, CH4, and iso-C4H10) mass spectra of stereoisomeric benzoin oximes and phenylhydrazones indicates that while the former can be distinguished only by their chemical ionization mass spectra the latter are readily distinguishable by both their electron impact and chemical ionization mass spectra. The electron impact mass spectra of the isomeric oximes are practically identical; however, the chemical ionization spectra show that the E isomer forms more stable [MH]+ and [MH? H2O]+ ions than the Z isomer for which both the [MH]+ and [MH? H2O]+ ions are relatively unstable. In electron impact the Z-phenylhydrazone shows a lower [M]+˙ ion abundance and more facile loss of H2O than does the E isomer. This more facile H2O loss also is observed for the [MH]+ ion of the Z isomer under chemical ionization conditions. 相似文献
86.
The preparation and properties of cyanobiphenyl liquid crystal droplets encapsulated by the polymerizable lecithin 1,2-bis(10,12-tricosadiynoyl)-sn-glyero-3-phosphocholine (DC8,9PC) are described. Under a wide variety of preparation conditions the droplets obtain a diameter of approximately 10 mum. These droplets are stable for periods of over one year at room temperature. Furthermore, they are stable upon temperature cycling between the nematic and isotropic phases and between the smectic A to nematic to isotropic phase transitions. 相似文献
87.
In this work we show through an analytic solution of the set of two coupled dynamic equations for the slowly varying amplitudes of the Stokes optical field and material density in stimulated Brillouin scattering (SBS), its analysis and a set of experiments, that the SBS-induced delay of an output Stokes pulse which is amplified by the SBS interaction is due exclusively to the inertia of the acoustic wave excitation. 相似文献
88.
árpád Somogyi Alex G. Harrison Béla Paizs 《Journal of the American Society for Mass Spectrometry》2012,23(12):2055-2058
Middle-sized b n (n????5) fragments of protonated peptides undergo selective complex formation with ammonia under experimental conditions typically used to probe hydrogen?Cdeuterium exchange in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Other usual peptide fragments like y, a, a*, etc., and small b n (n????4) fragments do not form stable ammonia adducts. We propose that complex formation of b n ions with ammonia is characteristic to macrocyclic isomers of these fragments. Experiments on a protonated cyclic peptide and N-terminal acetylated peptides fully support this hypothesis; the protonated cyclic peptide does form ammonia adducts while linear b n ions of acetylated peptides do not undergo complexation. Density functional theory (DFT) calculations on the proton-bound dimers of all-Ala b 4 , b 5 , and b 7 ions and ammonia indicate that the ionizing proton initially located on the peptide fragment transfers to ammonia upon adduct formation. The ammonium ion is then solvated by N+-H??O H-bonds; this stabilization is much stronger for macrocyclic b n isomers due to the stable cage-like structure formed and entropy effects. The present study demonstrates that gas-phase guest?Chost chemistry can be used to selectively probe structural features (i.e., macrocyclic or linear) of fragments of protonated peptides. Stable ammonia adducts of b 9 , b 9 -A, and b 9 -2A of A8YA, and b 13 of A20YVFL are observed indicating that even these large b-type ions form macrocyclic structures. 相似文献
89.
Alex G. Harrison 《Journal of the American Society for Mass Spectrometry》2013,24(10):1555-1564
The fragmentation reactions of the MH+ ions as well as the b7, a7, and a7* ions derived therefrom have been studied in detail for the octapeptides MAAAAAAA, AAMAAAAA, AAAAMAAA, and AAAAAAMA. Ionization was by electrospray using a QqToF mass spectrometer, which allowed a study of the evolution of the fragmentation channels as a function of the collision energy. Not surprisingly, the product ion mass spectra for the b7 ions are independent of the original precursor sequence, indicating macrocyclization and reopening to the same mixture of protonated oxazolones prior to fragmentation. The results show that this sequence scrambling results in a distinct preference to place the Met residue in the C-terminal position of the protonated oxazolones. The a7 and a7* ions also produce product ion mass spectra independent of the original peptide sequence. The results for the a7 ions indicate that fragmentation occurs primarily from an amide structure analogous to that observed for a4 ions (Bythell et al. in J Am Chem Soc 132:14766–14779, 2010). Clearly, the rearrangement reaction they have proposed applies equally well to an ions as large as a7. The major fragmentation modes of the MH+ ions at low collision energies produce b7, b6, and b5 ions. As the collision energy is increased further fragmentation of these primary products produces, in part, non-direct sequence ions, which become prominent at lower m/z values, particularly for the peptides with the Met residue near the N-terminus. Figure
? 相似文献
90.
Alex G. Harrison Cagdas Tasoglu Talat Yalcin 《Journal of the American Society for Mass Spectrometry》2013,24(10):1565-1572
The fragmentation reactions of the MH+ ions of Leu-enkephalin amide and a variety of heptapeptide amides have been studied in detail as a function of collision energy using a QqToF beam type mass spectrometer. The initial fragmentation of the protonated amides involves primarily formation of bn ions, including significant loss of NH3 from the MH+ ions. Further fragmentation of these bn ions occurs following macrocyclization/ring opening leading in many cases to bn ions with permuted sequences and, thus, to formation of non-direct sequence ions. The importance of these non-direct sequence ions increases markedly with increasing collision energy, making peptide sequence determination difficult, if not impossible, at higher collision energies. Figure
? 相似文献