全文获取类型
收费全文 | 956篇 |
免费 | 31篇 |
国内免费 | 1篇 |
专业分类
化学 | 609篇 |
晶体学 | 7篇 |
力学 | 52篇 |
数学 | 83篇 |
物理学 | 237篇 |
出版年
2022年 | 12篇 |
2021年 | 16篇 |
2020年 | 12篇 |
2019年 | 6篇 |
2018年 | 6篇 |
2017年 | 9篇 |
2016年 | 26篇 |
2015年 | 17篇 |
2014年 | 18篇 |
2013年 | 46篇 |
2012年 | 52篇 |
2011年 | 63篇 |
2010年 | 15篇 |
2009年 | 20篇 |
2008年 | 33篇 |
2007年 | 35篇 |
2006年 | 45篇 |
2005年 | 32篇 |
2004年 | 29篇 |
2003年 | 17篇 |
2002年 | 31篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 23篇 |
1992年 | 14篇 |
1991年 | 20篇 |
1990年 | 18篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 18篇 |
1984年 | 12篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1979年 | 7篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 16篇 |
1972年 | 8篇 |
1969年 | 6篇 |
1862年 | 9篇 |
排序方式: 共有988条查询结果,搜索用时 15 毫秒
31.
John R. Van Camp Tish Young Rosemarie F. Hartman Seth D. Rose 《Photochemistry and photobiology》1987,45(3):365-370
Abstract— Photosensitized pyrimidine dimer splitting characterizes the enzymatic process of DNA repair by the DNA photolyases. Possible pathways for the enzymatic reaction include photoinduced electron transfer to or from the dimer. To study the mechanistic photochemistry of splitting by a sensitizer representative of excited state electron donors, a compound in which an indole is covalently linked to a pyrimidine dimer has been synthesized. This compound allowed the quantitative measurement of the quantum efficiency of dimer splitting to be made without uncertainties resulting from lack of extensive preassociation of the unlinked dimer and sensitizer free in solution. Irradiation of the compound with light at wavelengths absorbed only by the indolyl group (approximately 280 nm) resulted in splitting of the attached dimer. The quantum yield of splitting of the linked system dissolved in N2 0-saturated aqueous solution was found to be 0.04 ± 0.01. The fluorescence typical of indoles was almost totally quenched by the attached dimer. A splitting mechanism in which an electron is efficiently transferred intramolecularly from photoexcited indole to ground state dimer has been formulated. The surprisingly low quantum yield of splitting has been attributed to inefficient splitting of the resulting dimer radical anion. Insights gained from this study have important mechanistic implications for the analogous reaction effected by the DNA photolyases. 相似文献
32.
Simulations of diffusion-limited cluster-cluster aggregation (DLCA) with no restructuring, full restructuring, and partial restructuring have been performed. The scattering patterns produced from these aggregates have been simulated using the Rayleigh-Gans-Debye approximation. Pure DLCA aggregates produced a scattering pattern with the slope of the fractal region being about -1.8. In contrast, the slope of the fractal region of the scattering pattern for fully restructured aggregates was about -2.1, indicating an increase in fractal dimension. Partial restructuring at large length scales produced an upward turn in the scattering pattern at low qr(o), while at high qr(o) the fractal section of the pure DLCA aggregate was retained. This last result was expected and is consistent with the results and postulations of several other workers. This simulation shows that the type of scattering pattern often obtained from orthokinetic or sheared aggregation can be produced by restructuring of aggregates at large length scales. Copyright 2001 Academic Press. 相似文献
33.
Several new methods for the synthesis of differently substituted 2-amidofurans are described. The thermolysis of furan-2-carbonyl azide results in a Curtius rearrangement and the resulting furanyl isocyanate was trapped with various organometallic reagents. A second method consists of a C-N cross-coupling reaction of a bromo-substituted furan with various amides, carbamates, and lactams. The CuI-catalyzed cross-coupling reaction between furanyl bromides and amides furnished 2- and 3-substituted amidofurans in 45-95% yield. The third protocol used involves the reaction of cyclic carbinol amides with triflic anhydride. The reaction proceeds under very mild conditions to provide alpha-(trifluoromethyl)sulfonamido-substituted furans in high yield. The resulting iminium ion derived from the reaction of the hydroxy pyrrolidinone with Tf(2)O undergoes a facile ring opening as a consequence of the adjacent hydroxyl group to produce an imino triflate intermediate. Subsequent cyclization of this highly electrophilic imine with the oxygen atom of the adjacent carbonyl group leads to an imino dihydrofuran that reacts further with another equivalent of Tf(2)O to give the observed product. 相似文献
34.
Two new methods of capillary zone electrophoresis based on aqueous phosphate running buffers with UV spectrophotometric detection were developed and optimized for the determination of eight carboxylates as copper complexes. Metalcomplexes are negatively charged, so measurements were made as anion analyses with flow reversal in the capillary. Two flow modifiers were used: a common tetradecyltrimethylammonium bromide (TTAB) and a new ionic liquid dimethyldinonylammonium bromide (DMDNAB). The methods were compared to each other. Better separation was achieved with DMDNAB as the flow modifier. Method development was done using a fused silica capillary (61 cm x 50 microm i.d.).Optimization was done using 95 mmol L(-1) phosphate buffer with TTAB or DMDNAB in the concentration 0.5 mmol L(-1) at pH 7.1. A -20 kV voltage and direct UV detection at 254 nm was used in measurements. In both CE methods all the peaks in the electropherograms were properly separated, the calibration plots gave good correlation coefficients and all eight carboxylates were detected in less than 7.5 min. The two methods were tested with natural water samples and a paper mill sample, and proved to be feasible. 相似文献
35.
36.
Rose A Scheller FW Wollenberger U Pfeiffer D 《Fresenius' Journal of Analytical Chemistry》2001,369(2):145-152
The use of thick-film electrodes as basic transducers for highly sensitive amperometric biosensors using PQQ (pyrroloquinoline quinone) dependent glucose dehydrogenase (GDH) with short response times is described. The enzyme is embedded in a polyurethane matrix on top of a platinum based thick film electrode and its ability to reduce oxidized phenolic compounds is exploited. The electrochemical amplification is based on the oxidation of the analyte on the surface of the electrode followed by its enzymatic reduction. Different parameters of the glucose dehydrogenase electrode system using dopamine as a model analyte were optimized, e.g., membrane thickness, pH value, buffer system, flow rate and storage conditions. Using optimized parameters the sensitivity and detection limits for various phenolic compounds were evaluated. The comparison of electrodes from the identical as well as from different batches shows the ability to produce a number of well reproducible sensors showing remarkably small differences with respect to parameters as sensitivity, response times and measuring range. 相似文献
37.
Petra J. W. Pouwels Robert Kaptein Rosemarie F. Hartman Seth D. Rose 《Photochemistry and photobiology》1995,61(6):575-583
A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor Nα-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1′-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product. 相似文献
38.
Anthraquinone-2-sulfonate (AQS) photosensitizes pyrimidine dimer splitting. Electron abstraction from the dimer is thought to induce dimer splitting, but direct evidence for the existence and intermediacy of dimer radical cations has been lacking. By employing photochemically induced dynamic nuclear polarization, we have found emission signals in the NMR spectra of dimers upon photolysis of dimers in the presence of anthraquinone-2-sulfonate. The two dimers employed were cis, syn-thymine dimer in which the N(1)-positions were linked by a three-carbon bridge and the N(3), N(3')-dimethyl derivative of that compound. The anthraquinone-2-sulfonate sensitized photochemically induced dynamic nuclear polarization spectrum of the methylated derivative exhibited an emission signal from the dimer-C(6) hydrogens. This result implied the existence of a dimer radical cation (mD+.) formed by electron abstraction by excited anthraquinone-2-sulfonate and nuclear spin sorting within a solvent caged radical ion pair [mD+. AQS-.]. Product pyrimidine photochemically induced dynamic nuclear polarization signals were also seen [enhanced absorption by C(6)-hydrogens and emission by C(5)-methyl groups]. Nuclear spin polarization in the product resulted from spin sorting in one or more of its precursors, including mD+. The results support the conclusion that dimer radical cations not only exist but are intermediates in the photosensitized splitting of pyrimidine dimers by anthraquinonesulfonate. 相似文献
39.
Carl M. Bender Fred Cooper Gerald Guralnik Harvey A. Rose David H. Sharp 《Journal of statistical physics》1980,22(6):647-660
We discuss the simple, randomly driven systemdx/dt = –x –x3 +f(t), wheref(t) is a Gaussian random function or stirring force with f(t)f(t) = (t – t). We show how to obtain approximately the coefficients of the expansion of the equal-time Green's functions as power series in (1/R)n, whereR is the internal Reynolds number ()1/2/, by using a new expansion for the path integral representation of the generating functional for the correlation functions. Exploiting the fact that the action for the randomly driven system is related to that of a quantum mechanical anharmonic oscillator with Hamiltonianp
2/2 +m
2
x
2/2 +vx
4 +x
6/2, we evaluate the path integral on a lattice by assuming that thex
6 term dominates the action. This gives an expansion of the lattice theory Green's functions as power series in 1/(a)1/3, wherea is the lattice spacing. Using Padé approximants to extrapolate toa = 0, we obtain the desired large-Reynolds-number expansion of the two-point function.Supported financially by the National Science Foundation and the U.S. Department of Energy. 相似文献
40.
Let An(ω) be the nxn matrix An(ω)=(aij with aij=ωij, 0?i,j?n?1, ωn=1. For n=rs we show =(Ar?Is)Tsr(Ir?As). When r and s are relatively prime this identity implies a wide class of identities of the form PAn(ω)QT=Ar(ωαs)?As(ωβr). The matrices Psr, Prs, P, and Q are permutation matrices corresponding to the “data shuffling” required in a computer implementation of the FFT, and Tsr is a diagonal matrix whose nonzeros are called “twiddle factors.” We establish these identities and discuss their algorithmic significance. 相似文献