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181.
Dommergue A Bahlmann E Ebinghaus R Ferrari C Boutron C 《Analytical and bioanalytical chemistry》2007,388(2):319-327
Snow surfaces play an important role in the biogeochemical cycle of mercury in high-latitude regions. Snowpacks act both as
sources and sinks for gaseous compounds. Surprisingly, the roles of each environmental parameter that can govern the air–surface
exchange over snow are not well understood owing to the lack of systematic studies. A laboratory system called the laboratory
flux measurement system was used to study the emission of gaseous elemental mercury from a natural snowpack under controlled
conditions. The first results from three snowpacks originating from alpine, urban and polar areas are presented. Consistent
with observations in the field, we were able to reproduce gaseous mercury emissions and showed that they are mainly driven
by solar radiation and especially UV-B radiation. From these laboratory experiments, we derived kinetic constants which show
that divalent mercury can have a short natural lifetime of about 4–6 h in snow. 相似文献
182.
The amino acid derivative Boc-Asp-OBzl (Boc=N-butyloxycarbonyl; Asp=aspartic acid; Bzl=benzyl) was functionalized by coupling its carboxylate side chain to dipicolylamine. This yielded the tridentate nitrogen donor ligand Boc-Asp(Dpa)-OBzl (-OBzl). The compound -OBzl contains three different carbonyl groups: a tertiary amide linkage between Asp and Dpa, a C-terminal benzyl ester function, and an N-terminal urethane protecting group. NMR spectra were used to compare the reactivity of these moieties. The Boc protecting group gives rise to two isomers, (E, 9%) and (Z, 91%). Coordination of Cd(NO3)2 and Zn(NO3)2 yielded the complexes and. These compounds have significantly reduced barriers to rotation about the tertiary amide C-N bond compared with the free ligand (-OBzl:18.5 kcal mol-1 in CDBr3;: 12.9 kcal mol-1 in (CD3)2CO;: 13.8 kcal mol-1 in (CD3)2CO). Both complexes readily undergo transesterification in methanol or CD3OD. Experimental pseudo-first order rate constants were determined in CD3OD and (CD3)2CO:CD3OD (3:1;). It was found that the zinc complex (k=(2.28+/-0.02)x10(-4) s-1) is significantly more reactive than the cadmium complex (k=(1.41+/-0.03)x10(-6) s-1). In order to study their tertiary amide cis-trans isomerization, the cadmium complex [(-OCH3)Cd(NO3)2] was synthesized, and the zinc complex [(-OCD3)Zn(NO3)2] was generated in situ in (CD3)2CO:CD3OD (3:1). The barriers to rotation were determined (:14.1 kcal mol-1 in CD3OD;: 13.4 kcal mol-1 in (CD3)2CO:CD3OD (3:1)). Our results show that the stronger Lewis-acid zinc(II) is significantly more active than cadmium(II) in the acceleration of the transesterification. This is in marked contrast to the tertiary amide bond rotation which is comparably fast with both metal ions. 相似文献
183.
Soft single-photon ionisation (SPI)–time-of-flight mass spectrometry (TOFMS) has been used to investigate the effect of different
cigarette-lighting devices on the chemical composition of the mainstream smoke from the first cigarette puff. Lighting devices
examined were a Borgwaldt electric lighter, a propane/butane gas lighter, a match, a candle, and the burning zone of another
cigarette. To eliminate the effects of the different masses of tobacco burnt by use of the different lighting methods a normalisation
procedure was performed which enabled investigation of changes in the chemical patterns of the resulting smoke. When another
cigarette was used as the lighting device, elevated levels of ammonia and other nitrogen-containing substances were observed.
These are high in the sidestream smoke of the cigarette used for lighting and would be drawn into the mainstream smoke of
the cigarette being lit. In contrast, smoke from the cigarette lit by the electric lighter contained slightly higher normalised
amounts of isoprene. Lighting the cigarette by use of a candle resulted in larger amounts of substances, e.g. benzene, which
most probably originated from thermal decomposition of wax. The composition of the first puff of smoke obtained by use of
the three lighting methods with open flames (gas lighter, match, and candle) was usually similar whereas the composition of
the smoke produced by use of the electric lighter and the cigarette as the lighter were more unique. The chemical patterns
generated by the different lighting devices could, however, be separated by principal-component analyses. Two additional test
series were also studied. In the first the cigarette was lit with an electric lighter, then extinguished, the ash was cut
off, and the cigarette was re-lit. In the second the cigarette was heated in an oven to 80 °C for 5 min before being lit.
These treatments did not result in changes in the chemical composition compared with cigarettes lit in the ordinary way.
Figure Time-of-flight mass spectrometry (TOFMS) has been used to investigate the effect of different cigarette-lighting devices on
the chemical composition of the mainstream smoke from the first cigarette puff 相似文献
184.
Stapleton HM Keller JM Schantz MM Kucklick JR Leigh SD Wise SA 《Analytical and bioanalytical chemistry》2007,387(7):2365-2379
Standard reference materials (SRMs) are valuable tools in developing and validating analytical methods to improve quality
assurance standards. The National Institute of Standards and Technology (NIST) has a long history of providing environmental
SRMs with certified concentrations of organic and inorganic contaminants. Here we report on new certified and reference concentrations
for 27 polybrominated diphenyl ether (PBDE) congeners in seven different SRMs: cod-liver oil, whale blubber, fish tissue (two
materials), mussel tissue and sediment (two materials). PBDEs were measured in these SRMs, with the lowest concentrations
measured in mussel tissue (SRM 1974b) and the highest in sediment collected from the New York/New Jersey Waterway (SRM 1944).
Comparing the relative PBDE congener concentrations within the samples, we found the biota SRMs contained primarily tetrabrominated
and pentabrominated diphenyl ethers, whereas the sediment SRMs contained primarily decabromodiphenyl ether (BDE 209). The
cod-liver oil (SRM 1588b) and whale blubber (SRM 1945) materials were also found to contain measurable concentrations of two
methoxylated PBDEs (MeO-BDEs). Certified and reference concentrations are reported for 12 PBDE congeners measured in the biota
SRMs and reference values are available for two MeO-BDEs. Results from a sediment interlaboratory comparison PBDE exercise
are available for the two sediment SRMs (1941b and 1944). 相似文献
185.
Two-dimensional electrophoresis of membrane proteins 总被引:1,自引:0,他引:1
One third of all genes of various organisms encode membrane proteins, emphasizing their crucial cellular role. However, due
to their high hydrophobicity, membrane proteins demonstrate low solubility and a high tendency for aggregation. Indeed, conventional
two-dimensional gel electrophoresis (2-DE), a powerful electrophoretic method for the separation of complex protein samples
that applies isoelectric focusing (IEF) in the first dimension and sodium dodecyl sulfate polyacrylamide gel electrophoresis
(SDS-PAGE) in the second dimension, has a strong bias against membrane proteins. This review describes two-dimensional electrophoretic
techniques that can be used to separate membrane proteins. Alternative methods for performing conventional 2-DE are highlighted;
these involve replacing the IEF with electrophoresis using cationic detergents, namely 16-benzyldimethyl-n-hexadecylammonium chloride (16-BAC) and cetyl trimethyl ammonium bromide (CTAB), or the anionic detergent SDS. Finally, the
separation of native membrane protein complexes through the application of blue and clear native gel electrophoresis (BN/CN-PAGE)
is reviewed, as well as the free-flow electrophoresis (FFE) of membranes. 相似文献
186.
187.
188.
According to ab initio MO calculations at the G2(MP2) level of theory, branched isomers of dialkyl triselenides, R-Se(=Se)-Se-R (1; R = Me, Et), are less stable by more than 60 kJ mol(-1) than the isomeric unbranched chains R-Se-Se-Se-R (2). Therefore, species 1 cannot be generated in substantial concentrations under equilibrium conditions at moderate temperatures, as has recently been claimed by Meja and Caruso (Inorg. Chem. 2004, 43, 7486). Alternatively, the isomeric CH3-Se-CH2-Se-Se-Et (3) can be considered to explain the reported gas chromatograms and mass spectra previously assigned to Et-Se(=Se)-Se-Et (1b). However, the isomerization 2b --> 3 is also endothermic, by deltaG(o)298 = 63 kJ mol(-1). The isomeric selenols HSe-C2H4-Se-Se-Et (4) and CH3-CH(SeH)-Se-Se-Et (5) are also less stable than 2b (by ca. 56 kJ mol(-1)), but 4 is another candidate to explain the mass spectrum formerly assigned to 1b. The calculated structures of 1-5 are reported. 相似文献
189.
190.
Here we report the first experimental detection of the C(s) symmetric nitroformyl radical, OCNO(X 2A') in a nitrogen-carbon dioxide matrix at 10 K using a Fourier transform infrared spectrometer (FTIR). The nu1 vibrational frequency was observed at 2113 cm(-1). This assignment was confirmed by follow-up experiments using isotopically labeled reactant molecules (15N, 18O, 13C). To synthesize this radical, we irradiated solid nitrogen-carbon dioxide ice mixtures with energetic electrons at 10 K. Suprathermal nitrogen atoms in their electronic ground and/or first electronically excited state were generated via the radiation induced degradation of molecular nitrogen; these atoms could then react with carbon dioxide to eventually yield the nitroformyl radical. We also investigated the kinetics of the formation of the nitroformyl radical and support the arguments with computations on the doublet and quartet OCNO potential energy surfaces (PESs). 相似文献