全文获取类型
收费全文 | 3777篇 |
免费 | 87篇 |
国内免费 | 44篇 |
专业分类
化学 | 2684篇 |
晶体学 | 25篇 |
力学 | 52篇 |
数学 | 669篇 |
物理学 | 478篇 |
出版年
2022年 | 18篇 |
2021年 | 34篇 |
2020年 | 21篇 |
2019年 | 44篇 |
2018年 | 22篇 |
2017年 | 31篇 |
2016年 | 73篇 |
2015年 | 52篇 |
2014年 | 61篇 |
2013年 | 189篇 |
2012年 | 169篇 |
2011年 | 195篇 |
2010年 | 99篇 |
2009年 | 80篇 |
2008年 | 237篇 |
2007年 | 255篇 |
2006年 | 261篇 |
2005年 | 262篇 |
2004年 | 203篇 |
2003年 | 138篇 |
2002年 | 134篇 |
2001年 | 47篇 |
2000年 | 59篇 |
1999年 | 53篇 |
1998年 | 39篇 |
1997年 | 47篇 |
1996年 | 72篇 |
1995年 | 49篇 |
1994年 | 53篇 |
1993年 | 38篇 |
1992年 | 40篇 |
1991年 | 30篇 |
1990年 | 32篇 |
1989年 | 26篇 |
1988年 | 32篇 |
1987年 | 35篇 |
1986年 | 46篇 |
1985年 | 60篇 |
1984年 | 63篇 |
1983年 | 51篇 |
1982年 | 43篇 |
1981年 | 47篇 |
1980年 | 53篇 |
1979年 | 33篇 |
1978年 | 46篇 |
1977年 | 37篇 |
1976年 | 30篇 |
1975年 | 26篇 |
1974年 | 24篇 |
1973年 | 31篇 |
排序方式: 共有3908条查询结果,搜索用时 31 毫秒
991.
Nozière B Voisin D Longfellow CA Friedli H Henry BE Hanson DR 《The journal of physical chemistry. A》2006,110(7):2387-2395
To investigate the link between molecular structure, reactivity, and partitioning of oxygenated organic compounds in acidic aerosols, the uptake of three compounds found in the atmosphere, methyl vinyl ketone (MVK), methacrolein (MACR), and 2-methyl-3-butene-2-ol (MBO), by sulfuric acid solutions has been measured using a rotated wetted-wall reactor (RWW) coupled to a chemical ionization mass spectrometer (CIMS). MVK was found to partition reversibly into 20-75 wt % H(2)SO(4) solutions, and we report Henry's law coefficients between 20 and 7000 M atm(-1) over this range. A chemical reaction for MVK was likely responsible for the uptake observed for 80-96 wt % H(2)SO(4) solutions. We derive an upper limit to the aldol self-reaction rate coefficient for MVK in 80 wt % solution of approximately 3 M(-1) s(-1). MACR partitioned reversibly over most of the acidity range, and in contrast to that for MVK, the Henry's law coefficient was relatively independent of H(2)SO(4) content. These differences indicate that the increase of the coefficient with acidity is likely due to the ability of the carbonyl molecule to form an enol. These results indicate that aldol condensation can be facile in concentrated sulfuric acid solutions, but it should be negligibly slow in dilute acid solutions such as tropospheric aerosols. MBO uptake could be explained by a Henry's law coefficient that decreases slightly as acid content varies from 20 to 55 wt % H(2)SO(4); we also measured the value in water, 70 M atm(-1) at 298 K. A steady-state uptake of MBO was observed onto 40-80 wt % H(2)SO(4) solutions, a reaction product was observed, and the reaction was tentatively identified as Pinacol rearrangement. Similar rearrangements could be at the origin of some substituted oxygenated species found in atmospheric aerosols. 相似文献
992.
The reactions of Mo2Oy- suboxide clusters with both methane and ethane have been studied with a combination of mass spectrometry, anion photoelectron spectroscopy, and density functional theory calculations. Reactions were carried out under "gentle" and "violent" conditions. For methane, a number of products appeared under the gentler source conditions that were more logically attributed to dissociation of Mo2Oy- clusters upon reacting with methane to form MoCH2-, Mo(O)CH2-, and HMo(O2)CH3-. With ethane, products observed under the same gentle conditions were Mo(O)C2H2-, Mo(O)C2H4-, Mo(O2)C2H4-, and Mo(O2)(C2H5)2-. As expected, more products were observed when the reactions were carried out under violent conditions. The photoelectron spectra obtained for these species were compared to calculated adiabatic and vertical electron affinities and vibrational frequencies, leading to definitive structural assignments for several of the products. 相似文献
993.
This paper begins an exploration of the use of the combination of DFT computations with experimental inelastic neutron scattering (INS) spectra as a method for establishing what conformation is present in a molecular crystal at low temperature. Presented here are INS spectra of a series of medium-sized cycloalkanes: C6H12, C7H14, C8H16, C10H20, C12H24, and C14H28. Optimized geometries and normal mode calculations were performed at B3LYP/6-311G(d,p) on the lowest energy conformations (i.e., those thermally accessible at the experimental temperature of 30 K). The calculated and observed spectra were analyzed for the best fit from each set of conformers, allowing a prediction of the dominant conformation in a conformationally rich system. For each cycloalkane, the calculated spectrum for the lowest energy conformer shows good agreement with experiment while the higher energy conformations have a much poorer fit. With little ambiguity, the lowest energy conformer is therefore predicted to be the dominant conformation, consistent with the diffraction data available for C6H12, C10H20, C12H24, and C14H28. These results indicate that INS spectroscopy may be a useful tool in determining the dominant conformation in a crystal lattice in cases such as this in which the intermolecular interactions are weak and the different conformers are calculated to have distinguishable spectra. Such an analysis is applied to the cases of C7H14 and C8H16 for which no low-temperature X-ray analysis is available. Clear structure predictions result, and the conformer observed is that computed to be of lowest energy for the molecule in isolation. 相似文献
994.
A sensitivity analysis of bulk water thermodynamics is presented in an effort to understand the relation between qualitative features of molecular potentials and properties that they predict. The analysis is incorporated in molecular dynamics simulations and investigates the sensitivity of the Helmholtz free energy, internal energy, entropy, heat capacity, pressure, thermal pressure coefficient, and static dielectric constant to components of the potential rather than the parameters of a given functional form. The sensitivities of the properties are calculated with respect to the van der Waals repulsive and the attractive parts, plus short- and long-range Coulomb parts of three four site empirical water potentials: TIP4P, Dang-Chang and TTM2R. The polarization sensitivity is calculated for the polarizable Dang-Chang and TTM2R potentials. This new type of analysis allows direct comparisons of the sensitivities for different potentials that use different functional forms. The analysis indicates that all investigated properties are most sensitive to the van der Waals repulsive, the short-range Coulomb and the polarization components of the potentials. When polarization is included in the potentials, the magnitude of the sensitivity of the Helmholtz free energy, internal energy, and entropy with respect to this part of the potential is comparable in magnitude to the other electrostatic components. In addition similarities in trends of observed sensitivities for nonpolarizable and polarizable potentials lead to the conclusion that the complexity of the model is not of critical importance for the calculation of these thermodynamic properties for bulk water. The van der Waals attractive and the long-range Coulomb sensitivities are relatively small for the entropy, heat capacity, thermal pressure coefficient and the static dielectric constant, while small changes in any of the potential contributions will significantly affect the pressure. The analysis suggests a procedure for modification of the potentials to improve predictions of thermodynamic properties and we demonstrate this general approach for modifying potentials for one of the potentials. 相似文献
995.
This paper examines geometric scaling models for field flow fractionation systems to understand how channel dimensions affect resolution and retention. Specifically, the changing contribution of the instrumental plate height during miniaturization of field flow fractionation (FFF) systems is reported. The work is directed towards determining the optimal geometrical parameters for miniaturization of field flow fractionation systems. The experimental relationship between channel height in FFF systems and instrumental plate heights is reported. FFF scaling models are modified to: (i) better clarify the dependence of plate height and resolution on channel height in FFF and (ii) include a more complete geometrical scaling analysis and model comparison in the low retention regime. Electrical field flow fractionation has been shown to benefit from miniaturization, so this paper focuses on that subtype, but surprisingly, the results also indicate the possibility of improvement in performance with miniaturization of other field flow fractionation systems including general FFF subtypes in which the applied field does not vary with channel height. This paper also discusses the potential role of more powerful microscale field flow fractionation systems as a new class of sample preparation units for micro-total-analysis systems (mu-TAS). 相似文献
996.
Synthesis,properties and solid state structure of 5‐diphenylphosphino‐2‐hydroxy‐1,3‐xylyl‐18‐crown‐5
Rex A. Corbin Todd W. Crabill Bruce N. Storhoff John C. Huffman 《Journal of heterocyclic chemistry》2006,43(4):997-1001
997.
Michael I. Bruce Marcus L. Cole Brian W. Skelton 《Journal of organometallic chemistry》2006,691(22):4601-4614
Complexes M(CCCSiMe3)(CO)2Tp′ (Tp′ = Tp [HB(pz)3], M = Mo 2, W 4; Tp′ = Tp∗ [HB(dmpz)3], M = Mo 3) are obtained from M(CCCSiMe3)(O2CCF3)(CO)2(tmeda) (1) and K[Tp′].Reactions of 2 or 4 with AuCl(PPh3)/K2CO3 in MeOH afforded M{CCCAu(PPh3)}(CO)2Tp′ (M = Mo 5, W 6) containing C3 chains linking the Group 6 metal and gold centres.In turn, the gold complexes react with Co3(μ3-CBr)(μ-dppm)(CO)7 to give the C4-bridged {Tp(OC)2M}CCCC{Co3(μ-dppm)(CO)7} (M = Mo 7, W 8), while Mo(CBr)(CO)2Tp∗ and Co3{μ3-C(CC)2Au(PPh3)}(μ-dppm)(CO)7 give {Tp∗(OC)2Mo}C(CC)2C{Co3(μ-dppm)(CO)7} (9) via a phosphine-gold(I) halide elimination reaction. The C3 complexes Tp′(OC)2MCCCRu(dppe)Cp∗ (Tp′ = Tp, M = Mo 10, W 11; Tp′ = Tp∗, M = Mo 12) were obtained from 2-4 and RuCl(dppe)Cp∗ via KF-induced metalla-desilylation reactions. Reactions between Mo(CBr)(CO)2Tp∗ and Ru{(CC)nAu(PPh3)}(dppe)Cp∗ (n = 2, 3) afforded {Tp∗(OC)2Mo}C(CC)n{Ru(dppe)Cp∗} (n = 2 13, 3 14), containing C5 and C7 chains, respectively. Single-crystal X-ray structure determinations of 1, 2, 7, 8, 9 and 12 are reported. 相似文献
998.
Yu G Durduran T Zhou C Zhu TC Finlay JC Busch TM Malkowicz SB Hahn SM Yodh AG 《Photochemistry and photobiology》2006,82(5):1279-1284
Photodynamic therapy (PDT) requires oxygen to cause cellular and vascular tumor damage. Tissue oxygen concentration, in turn, is influenced by blood flow and blood oxygenation. Real-time clinical measurement of these hemodynamic quantities, however, is rare. This paper reports the development and application of a probe, combining diffuse reflectance spectroscopy (DRS) for measurement of tumor blood oxygenation and diffuse correlation spectroscopy (DCS) for measurement of tumor blood flow. The instrument was adapted for clinical use during interstitial prostate PDT. Three patients with locally recurrent prostate cancer received 2 mg/ kg motexafin lutetium (MLu) 3 h before illumination and a total light dose of 100 J/cm(2) at 150 mW/cm. Prostrate blood oxygen saturation (StO2) decreased only slightly (approximately 3%) after treatment. On the other hand, prostate blood flow and total hemoglobin concentration over the course of PDT decreased by 50% and 15%, respectively, suggesting MLu-mediated PDT has an anti-vascular effect. While it is certainly impossible to draw definite conclusions from measurements of only three patients, the observed differences in tumor blood flow and blood oxygenation responses during PDT can, in principle, be used to choose among tissue oxygen consumption models and therefore emphasize the potential clinical value for simultaneous monitoring of both parameters. 相似文献
999.
A nonavalent glycodendrimer bearing terminal alpha-d-mannopyranoside units has been synthesized with a convergent approach. Terminal trivalent mannoside dendrons bearing p-halophenyl ethers were prepared by glycosylation of pentaerythritol derivatives having three 2-hydroxyethyl ether substituents. Two efficient routes were developed for the synthesis of the pentaerythritol-based core (17), which has three terminal propargyl ethers. Conditions were found under which the triple Sonogashira coupling reaction of the dendron and the tri-O-propargyl ether (17) proceeded efficiently. The product was deprotected and it and precursors were fully characterized by NMR spectroscopy and FT-ICR mass spectrometry. 相似文献
1000.
Zhang X Li Q Ingels JB Simmonett AC Wheeler SE Xie Y King RB Schaefer HF Cotton FA 《Chemical communications (Cambridge, England)》2006,(7):758-760
The optimised structures, electron affinities, and vibrational frequencies of the simplest benzenoid cyanocarbons, namely hexacyanobenzene C6(CN)6, octacyanonaphthalene C10(CN)8, and decacyanoanthracene C14(CN)10, have been studied using carefully calibrated density functional methods (Chem. Rev., 2002, 102, 231-282); the predicted adiabatic electron affinities are 3.53 eV for C6(CN)6, 4.35 eV for C10(CN)8 and 5.02 eV for C14(CN)10, which are significantly larger than those of the analogous benzenoid fluorocarbons as well as tetracyanoethane and tetracyanoquinodimethane. 相似文献