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51.
The first discrete, selenium-centered tetranuclear zinc cluster [Zn4(mu4-Se)[Se2P(OPr)2]6] was isolated and characterized. The cluster consists of six edge-bridged dsep ligands with four zinc atoms in a slightly distorted tetrahedron and a mu4-Se atom in the center. In addition, 12 mu2-bridging selenium atoms form a Se12 icosahedron. From variable-temperature 31P NMR studies, it was observed that the cluster [Zn4(Se)[Se2P(OPr)2]6] is partly decomposed to [Zn[Se2P(OPr)2]2] and the monomeric species [Zn[Se2P(OPr)2]2] is further in equilibrium with its dimer [Zn[Se2P(OPr)2]2]2. 相似文献
52.
The concept of hydrophobicity has been addressed by researchers in all aspects of science, particularly in the fields of biology and chemistry. Over the past several decades, the study of the hydrophobicity of biomolecules, particularly amino acids has resulted in the development of a variety of hydrophobicity scales. In this review, we discuss the various methods of measuring amino acid hydrophobicity and provide explanations for the wide range of rankings that exist among these published scales. A discussion of the literature on amino acid interactions is also presented. Only a surprisingly small number of papers exist in this rather important area of research; measuring pairwise amino acid interactions will aid in understanding structural aspects of proteins. 相似文献
53.
Prathima N Harini M Rai N Chandrashekara RH Ayappa KG Sampath S Biswas SK 《Langmuir : the ACS journal of surfaces and colloids》2005,21(6):2364-2374
The thermal stability of short alkanethiol CH(3)(CH(2))(7)SH (C(8)) and long C(18) self-assembled monolayers (SAMs) is investigated using grazing angle reflection-absorption infrared spectroscopy, cyclic voltammetry, and molecular dynamics simulation. We track the disordering of SAM by untilting and gauche defect accumulation with increasing temperature in the 300-440 K range, a range of interest to tribology. Molecular dynamics simulation with both fully covered and partially covered C(6), C(8), and C(18) monolayers brings out the morphological changes in the SAM, which may be associated with the observed thermal stability characteristics. The molecular dynamics simulations reveal that short-chain C(6) and C(8) alkanethiols are more defective at lower temperature than the long-chain C(18) alkanethiol. With increasing temperature disorder in the SAM, as reflected in both untilting and gauche defect accumulation, tends to saturate at temperatures below 360 K for short-chain SAMs such that any further increase in temperature, until desorption, does not lead to any significant change in conformational order. In contrast the disorder in the long-chain C(18) SAM increases monotonically with temperature beyond 360 K. Thus, in a practical range of temperature, the ability of a SAM to retain order with increasing thermal perturbations is governed by the state of disorder prior to heat treatment. This deduction derived from molecular dynamics simulation helps to rationalize the significant difference we have observed experimentally between the thermal response of short- and long-chain thiol molecules. 相似文献
54.
Subhendu?Biswas Kamala?Mitra Bibhutosh?AdhikaryEmail author C.?Robert.?Lucas 《Transition Metal Chemistry》2005,30(5):586-592
Synthesis of six hydroxo-bridged binuclear manganese(III) complexes of formulae [MnL-X-MnL](ClO4) [X = OH (1–6)] along with a mononuclear manganese(III) complex (7) [Mn(L)(L′)(MeOH)2] [HL′ = 2-(2-hydroxy-phen-yl)benzimidazole] and two carboxylate-bridged binuclear manganese(III) complexes (8) and (9) are described. The complexes have been characterized by the combination of i.r., u.v.-vis spectroscopy, magnetic moments and by their redox properties. The electronic spectra of all the complexes exhibit almost identical features consisting of two d–d bands at ca. 550 and 600 nm, one MLCT band at ca.400 nm, together with two π–π* intra-ligand transitions at ca. 250 nm and ca.300 nm. Room temperature magnetic data range from μ = 2.7–3.0 BM indicates some super-exchange between the binuclear metal centers via bridging hydroxo/carboxylato groups. The X-ray crystal structure of the binuclear complex (5) revealed that it has a symmetric MnIIIN2O2 core bridged by a hydroxyl group. The X-ray analysis of the mononuclear complex (7) showed that the manganese-center possesses a distorted octahedral geometry. Electrochemical properties of hydroxo-bridged manganese(III) complexes (1–6) show identical features consisting of an irreversible and a quasi-reversible reduction corresponding to the Mn2III → MnIIMnIII → MnIIMnII couples in the voltammogram. It was found that electron withdrawing substituents on the ligand result in easier reduction. Complex (7) displays an irreversible reduction at 0.08 V and a reversible oxidation at 0.45V assignable to the MnIII → MnII reduction and MnIII → MnIV oxidation, respectively. The carboxylate-bridged compound (8) exhibits two irreversible oxidations at 0.4 and 0.6 V, probably due to Mn2III → MnIIIMnIV → MnIVMnIV oxidations and shows a quasi-reversible reductive wave at −0.85 V, tentatively assigned to Mn2III → MnIIMnIII reduction. 相似文献
55.
An efficient construction of a 24-membered macrocyclic hexaoxazole derivative pertinent to the synthesis of analogues of the important natural product telomestatin was developed, which featured a convergent union of two trisoxazole units. 相似文献
56.
Premamoy Ghosh Samir Biswas Utpal Niyogi 《Journal of polymer science. Part A, Polymer chemistry》1986,24(5):1053-1063
Photopolymerization of methyl methacrylate (MMA) in visible light was studied at 40°C using Rhodamine 6G—Benzoyl peroxide combination as photoinitiator. The photopolymerization proceeds by a free radical mechanism and the radical generation process occurs by an initial complexation reaction between the initiator components. Kinetic data indicated a lower-order dependence of Rp on initiator concentrations (initiator exponent < 0.5). Initiator-dependent chain termination was significant along with the bimolecular mode of chain termination. 相似文献
57.
Virendra Singh Anita Rampal S. N. Biswas K. Datta 《Letters in Mathematical Physics》1980,4(2):131-134
The solution of a difference equation in the form of an infinite continued fraction is used to obtain a class of exact solutions for the eigenfunctions and eigenvalues of doubly anharmonic oscillators described by potentials of the type (1/2)2x2+(1/4)x4+(1/6)x6, n>0, provided certain constraints on the couplings are satisfied. The class is denumerably infinite but not complete. 相似文献
58.
The polymerization of N-vinylcarbazole (NVC) initiated by PhMgBr in benzene was studied at 32°C. Rp is second order with respect to PhMgBr concentration but increases with NVC concentration (up to 0.06 M) and falls thereafter. Rp and P n are depressed by the addition of thiophene and water. Modifiers such as benzaldehyde, butanone, and ethylene glycol practically inhibit the polymerization. Carbon tetrachloride and carbon dioxide, when passed through the NVC solution first, enhance the Rp and P n increases with increasing PhMgBr and NVC concentrations, respectively. Rp increases with temperature, but P n shows a maxium at a certain temperature. A cationic mechanism has been proposed where the polymerization is initiated by RMg+ cations produced from the ionization of PhMgBr by the Ashby and Smith mechanism. 相似文献
59.
Njardarson JT Biswas K Danishefsky SJ 《Chemical communications (Cambridge, England)》2002,(23):2759-2761
A total synthesis of Epothilone 490 and a synthesis of 11-hydroxy dEpoB utilizing a vinyl-boronate cross-metathesis followed by a Suzuki macrocyclization. A mild route to reach aldehydes from terminal olefins, anticipating Nozaki-Kishi macrocyclization is described. 相似文献
60.
A Sarkar N Biswas S Kapoor H S Mahal C K K Nair T Mukherjee 《Research on Chemical Intermediates》2005,31(9):857-866
The oxidation of flavonoids is of great interest because of their action as antioxidants with the ability to scavenge radicals
by means of electron-transfer processes. The redox reactions of the flavonoid derivative troxerutin, (2-[3,4-bis-(2-hydroxyethoxy)
phenyl]-3[[6-deoxy-α-L-manno-pyranosyl)-β-(D-glucopyranosyl]-oxy]-5-hydroxy-7-(2-hydroxyethoxy)-4H-1-benzo-pyran-4-one), were investigated over a wide range of conditions,
using pulse radiolysis and cyclic voltammetry. The oxidation mechanism proceeds in sequential steps. One-electron redox potentials
for troxerutin were found to be +1.196, +0.846 and −0.634 V vs. NHE. 相似文献