全文获取类型
收费全文 | 1359篇 |
免费 | 40篇 |
国内免费 | 8篇 |
专业分类
化学 | 772篇 |
晶体学 | 8篇 |
力学 | 63篇 |
数学 | 84篇 |
物理学 | 480篇 |
出版年
2023年 | 22篇 |
2022年 | 30篇 |
2021年 | 47篇 |
2020年 | 35篇 |
2019年 | 36篇 |
2018年 | 36篇 |
2017年 | 36篇 |
2016年 | 47篇 |
2015年 | 35篇 |
2014年 | 46篇 |
2013年 | 108篇 |
2012年 | 92篇 |
2011年 | 120篇 |
2010年 | 53篇 |
2009年 | 44篇 |
2008年 | 67篇 |
2007年 | 68篇 |
2006年 | 69篇 |
2005年 | 54篇 |
2004年 | 41篇 |
2003年 | 33篇 |
2002年 | 29篇 |
2001年 | 17篇 |
2000年 | 14篇 |
1999年 | 9篇 |
1998年 | 13篇 |
1997年 | 14篇 |
1996年 | 21篇 |
1995年 | 17篇 |
1994年 | 17篇 |
1993年 | 9篇 |
1992年 | 14篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1969年 | 2篇 |
排序方式: 共有1407条查询结果,搜索用时 15 毫秒
31.
Abachi S Abbott B Abolins M Acharya BS Adam I Adams DL Adams M Ahn S Aihara H Alitti J Álvarez G Alves GA Amidi E Amos N Anderson EW Aronson SH Astur R Avery RE Baden A Balamurali V Balderston J Baldin B Bantly J Bartlett JF Bazizi K Bendich J Beri SB Bertram I Bezzubov VA Bhat PC Bhatnagar V Bhattacharjee M Bischoff A Biswas N Blazey G Blessing S Bloom P Boehnlein A Bojko NI Borcherding F Borders J Boswell C Brandt A Brock R Bross A Buchholz D Burtovoi VS Butler JM Carvalho W Casey D 《Physical review letters》1996,76(18):3271-3276
32.
In catalytic concentrations (10?5?10?4 mol l?1) sulphur dioxide induces polymerization of MMA, particularly on photoactivation. The effective initiating species appears to be the monomer-SO2 complex rather than free SO2. A mechanism involving biradical initiation by decomposition of the initiating species, linear propagation in two directions, and significant termination of growing chains by chain transfer with initiating species has been suggested. The initiator transfer constant is 1.6 at 40°. 相似文献
33.
Biswas P Panja SS Manogaran S Chakraborty T 《The journal of physical chemistry. A》2005,109(14):3225-3234
Laser-induced fluorescence excitation and resolved fluorescence spectra following excitations of the single vibronic levels (SVL) of p-vinyltoluene (p-VT) and p-vinylfluorobenzene (p-VFB) have been measured in a seeded supersonic free-jet expansion. A complete vibronic assignment of the fluorescence spectrum measured following excitation of the 0(0)0-band of p-VT has been presented. Normal vibrational modes in the S0 and S1 states of the molecule have been calculated by CASSCF method, and the correlation between the two set of modes is made by expressing the excited-state normal modes in terms of those of the ground state. The calculations predict that in the excited state methyl and vinyl torsional motions of p-VT are extensively mixed with many of the out-of-plane modes of the aromatic ring. Our resolved fluorescence spectral data measured following SVL excitations essentially agree with such predictions. In the excited state, the molecule exhibits a dramatically low threshold for the rotor-induced IVR in a supersonic jet expansion. Several mechanisms have been discussed to explain the phenomenon. 相似文献
34.
Chatterjee S Basu S Ghosh N Chakrabarty M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(8):1887-1891
In fluorescence quenching study via electron transfer (ET), the quenching rate constant (k(q)) values generally decrease with lowering of quencher concentration, since smaller concentration of quencher always leads to a red shift in the donor-acceptor (D-A) distance in ET [M. Tachiya, S. Murata, J. Phys. Chem. 96 (1992) 8441; S. Murata, M. Tachiya, J. Phys. Chem. 100 (1996) 4064; L. Burel, M. Mastafavi, S. Murata, M. Tachiya, J. Phys. Chem. A 103 (1999) 5882]. However, while doing a comparative study with different carbazole (CZ) derivatives-1,4-dicyanobenzene (DCB) systems in benzene (BZ), we observed a deviation from that normal behaviour. It was found that for all of them with lower quencher (DCB) concentration, k(q) values actually increase instead of the expected reduction. Exceptionally, for simple CZ (C12H9N) with decrease in concentration of DCB, k(q) values can even reach the order of energy transfer (10(11) s(-1)). Interestingly, it is not observed when toluene (TL) or xylene (XY) is used as solvent. To explain this unique observation, a sandwich type of molecular structure is predicted, where BZ sliding in between CZ and DCB brings them closer enough, imparting more through bond character to CZ-DCB interaction and hence a higher rate of ET (k(q)) is observed [L. Burel, M. Mastafavi, S. Murata, M. Tachiya, J. Phys. Chem. A. 103 (1999) 5882]. 相似文献
35.
Interpolation methods such as the nudged elastic band and string methods are widely used for calculating minimum energy pathways and transition states for chemical reactions. Both methods require an initial guess for the reaction pathway. A poorly chosen initial guess can cause slow convergence, convergence to an incorrect pathway, or even failed electronic structure force calculations along the guessed pathway. This paper presents a growing string method that can find minimum energy pathways and transition states without the requirement of an initial guess for the pathway. The growing string begins as two string fragments, one associated with the reactants and the other with the products. Each string fragment is grown separately until the fragments converge. Once the two fragments join, the full string moves toward the minimum energy pathway according to the algorithm for the string method. This paper compares the growing string method to the string method and to the nudged elastic band method using the alanine dipeptide rearrangement as an example. In this example, for which the linearly interpolated guess is far from the minimum energy pathway, the growing string method finds the saddle point with significantly fewer electronic structure force calculations than the string method or the nudged elastic band method. 相似文献
36.
Brajagopal Samanta Joy Chakraborty C. R. Choudhury S. K. Dey D. K. Dey S. R. Batten P. Jensen Glenn P. A. Yap Samiran Mitra 《Structural chemistry》2007,18(1):33-41
We have reported herein the synthesis of three new Cu(II) complexes of tri- and tetradentate Schiff base ligands containing
N3 or N4 donor set along with terminal NNN− or SCN− ligands: [L1Cu(NCS)]ClO4 (1), [L2Cu(NCS)2] (2) and [L3Cu(NNN)]ClO4 (3) [L1 = NC5H4C(CH3)=N(CH2)3N=C(CH3)C5H4N, L2= Me2N–(CH2)3–N=C(CH3)C5H4N and L3 = NC5H4CH=N–(CH2)4–N=CHC5H4N]. The complexes have been systematically characterised by elemental, spectroscopic and electrochemical techniques. Antimicrobial
activities of the Schiff base ligands and their metal complexes have been studied using the disc diffusion method on the strains
of Candida tropicalis and Bacillus megaterium. Structures of all the complexes have been unequivocally established from single crystal X-ray diffraction analyses that
show the monomeric units containing a five-coordinated copper center in highly distorted square pyramidal geometry with thiocyanate
or azide anion coordinated as terminal ligand. The complexes 1 and 3 crystallise in monoclinic (P21/c) and 2 in triclinic (P-1) space group, respectively. 相似文献
37.
A.K. Chakraborty 《Journal of Sol-Gel Science and Technology》2003,28(1):87-95
Diphasic Al2O3-SiO2 gels have been synthesized by hydrolyzing ethyl orthosilicate at three different pH ranges in presence of colloidal boehmite. Result shows that dominant mullitization process are sensitive to pH of the gelation process. At highly acidic pH range, solid state reaction between corundum and cristobalite occurs, and develops liquid phase for mullite nucleation. At moderately acidic pH, nucleation and crystallization are most operative for nucleation in aluminosilicate matrix. In highly basic pH region, Al-Si spinel phase develops by incorporation of Si in aluminous phase as intermediary phase. Polymorphic transformation of it may be the cause of sudden mullitization. The changes in crystallization sequence in three distinct processes may be due to variation in the nature and size of silicic acid particle formed by hydrolysis and condensation of TEOS at different pH. 相似文献
38.
Attempts have been made to determine the enthalpy and entropy of transfer of H+ ion from water to mixed solvents using the calorimetric data of earlier experiments. The results are in qualitative agreement with the facts that ΔH t 0 (H+) passes through an exothermic maximum andTΔS t 0 passes through a minimum at about 20 to 30 wt% of organic solvent indicating the initial structure formation and the ultimate breakdown of the solvent structure with the addition of organic solvent. 相似文献
39.
Chakraborty A Wu A Witt D Lagona J Fettinger JC Isaacs L 《Journal of the American Chemical Society》2002,124(28):8297-8306
Cucurbit[6]uril (CB[6]) is a macrocyclic compound, prepared in one pot from glycoluril and formaldehyde, whose molecular recognition properties have made it the object of intense study. Studies of the mechanism of CB[n] formation, which might provide insights that allow the tailor-made synthesis of CB[n] homologues and derivatives, have been hampered by the complex structure of CB[n]. By reducing the complexity of the reaction to the formation of S-shaped (12S-18S) and C-shaped (12C-18C) methylene bridged glycoluril dimers, we have been able to probe the fundamental steps of the mechanism of CB[n] synthesis to a level that has not been possible previously. For example, we present strong evidence that the mechanism of CB[n] synthesis proceeds via the intermediacy of both S-shaped and C-shaped dimers. The first experimental determination of the relative free energies of the S-shaped and C-shaped dimers indicates a thermodynamic preference (1.55-3.25 kcal mol(-)(1)) for the C-shaped diastereomer. This thermodynamic preference is not because of self-association, solvation, or template effects. Furthermore, labeling experiments have allowed us to elucidate the mechanism of this acid-catalyzed equilibrium between the S-shaped and C-shaped diastereomers. The equilibration is an intramolecular process that proceeds with high diastereoselectivity and retention of configuration. On the basis of the broad implications of these results for CB[n] synthesis, we suggest new synthetic strategies that may allow for the improved preparation of CB[n] (n > 8) and CB[n] derivatives from functionalized glycolurils. 相似文献
40.
A variety of structurally different aldehydes undergo chemoselectire reduction over ketones with zinc borohydride in tetrahydrofuran at −10°C to the corresponding alcohols. 相似文献