全文获取类型
收费全文 | 186769篇 |
免费 | 1990篇 |
国内免费 | 793篇 |
专业分类
化学 | 90011篇 |
晶体学 | 2114篇 |
力学 | 7028篇 |
综合类 | 36篇 |
数学 | 17035篇 |
物理学 | 51442篇 |
无线电 | 21886篇 |
出版年
2020年 | 1399篇 |
2019年 | 1448篇 |
2018年 | 1759篇 |
2017年 | 1725篇 |
2016年 | 2967篇 |
2015年 | 1954篇 |
2014年 | 3010篇 |
2013年 | 7404篇 |
2012年 | 5808篇 |
2011年 | 7285篇 |
2010年 | 5134篇 |
2009年 | 5396篇 |
2008年 | 7142篇 |
2007年 | 7403篇 |
2006年 | 6881篇 |
2005年 | 6507篇 |
2004年 | 5907篇 |
2003年 | 5275篇 |
2002年 | 5340篇 |
2001年 | 5643篇 |
2000年 | 4508篇 |
1999年 | 3622篇 |
1998年 | 3200篇 |
1997年 | 3104篇 |
1996年 | 2810篇 |
1995年 | 2721篇 |
1994年 | 2655篇 |
1993年 | 2428篇 |
1992年 | 2664篇 |
1991年 | 2761篇 |
1990年 | 2544篇 |
1989年 | 2490篇 |
1988年 | 2411篇 |
1987年 | 2063篇 |
1986年 | 2005篇 |
1985年 | 2568篇 |
1984年 | 2687篇 |
1983年 | 2226篇 |
1982年 | 2371篇 |
1981年 | 2237篇 |
1980年 | 2163篇 |
1979年 | 2261篇 |
1978年 | 2394篇 |
1977年 | 2321篇 |
1976年 | 2350篇 |
1975年 | 2214篇 |
1974年 | 2243篇 |
1973年 | 2246篇 |
1972年 | 1485篇 |
1971年 | 1322篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The crystal structure of the title compound has been determined from X-ray diffractometer data by direct methods, and refined by full matrix least squares techniques to R = 0·057 for 1231 reflections. The crystals are monoclinic, space group C2/C, cell dimensions a = 1665, b = 987, c = 1443 pm, β = 107·37° and Z = 8. The conformation of the tetrahydro-1,2-oxazine ring is a chair with the N-substituent equatorial. There is evidence of significantly greater torsional angles around the N and O atoms than around the ring C atoms, showing the ring to be more puckered than cyclohexane. The hydrogen bond is between the acid group on one molecule and the ring nitrogen on its neighbour. 相似文献
992.
Carboxy-terminated polybutadiene (CTPB) was oxidised as a coating on various metal oxides by heating in air on a thermobalance. The oxidation was found to be free from diffusion control and was catalysed by certain of the oxides. The catalysis was shown to be independent of semiconductor type and to be associated with those oxides for which the width of the forbidden zone between the valence and conduction bands (U) is less than 1.9 eV. In general, low values of U result in high catalytic activity. It is proposed that catalysis occurs by a redox mechanism, viz. RO2H + p → RO2. + H+, RO2H + e → RO. + OH?. 相似文献
993.
K. Seshadri F. A. Williams 《Journal of polymer science. Part A, Polymer chemistry》1978,16(7):1755-1778
Since poly(methyl methacrylate) is known to depolymerize largely to its monomer when heated, the chemical kinetics in the gaseous diffusion flame produced by this polymer in a fire may coincide with that of burning liquid methyl methacrylate. To test this hypothesis, flat diffusion flames were probed and extinguished adjacent to surfaces of each of these fuels. Profiles of temperature and of concentrations of stable chemical species are reported, as are gas velocities of approach flow required to produce extinction for various oxygen/nitrogen ratios of the stream. Results revealed structural differences attributable to differing thermal properties of the fuels. Many fuel species were observed in the gas phase, their profiles being partially rationalized on the basis of a suggested decomposition mechanism for gaseous methyl methacrylate. Overall kinetic parameters for gasphase combustion, obtained by use of extinction results in a previously developed theory, are nearly the same for the polymer and monomer but appear to differ by amounts exceeding experimental uncertainties. It is suggested that this may be traced to small differences in fuel species leaving the condensed phase which, for the polymer, is covered by a thin, two-phase region. 相似文献
994.
A. V. Kazymov L. P. Shchelkina N. G. Kabirova A. F. Vompe 《Chemistry of Heterocyclic Compounds》1971,7(11):1452-1455
Several new polymethine dyes of the cyanine and merocyanine series with 1-methyl-1H-imidazo [4,5-b]pyridine residues were synthesized, and their colors were studied. Replacement of the benzimidazole residue in the cyanine dyes by a l-methyl-1H-imidazo[4,5-b]-pyridine residue leads to deepening of the dye color.See [1] for communication II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1561–1565, November, 1971. 相似文献
995.
V. F. Pozdnev 《Chemistry of Natural Compounds》1980,16(3):286-290
The acylation of histidine with di-tert-butyl and di-tert-amyl pyrocarbonates has given the corresponding N,Nim-di-tert-alkoxycarbonyl derivatives. The N,Nim-di-tert-butoxycarbonyl derivative was obtained in the crystalline form by crystallization from benzene or carbon tetrachloride, or in the form of salts with cyclohexylamine, dicyclohexylamine, and diethylamine. N,Nim-di-tert-amyloxycarbonyl-histidine was characterized in the form of the salt with dicyclohexylamine.Institute of Biological and Medicinal Chemistry, Academy of Medical Sciences of the USSR, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 379–383, May–June, 1980. 相似文献
996.
The major product from 2,3-naphthoquinodimethane formed by cyclisation of -dipropadienylbenzene was found to be the dimer 5 containing an eight-membered ring, for which the inversion barrier was determined by dynamic 1H NMR spectrometry, ΔG3 = 18 kcal/mole. 相似文献
997.
F. Ruttens R. oedeweeck F. Lopez-Arbeloa F. C. De Schryver 《Photochemistry and photobiology》1985,42(4):341-346
Abstract— The synthesis, absorption and emission properties of erythro (e) and threo (t) Na-acetyl-1-pyrenylalanyl-1-methyltryptophan methylester (APTE) are reported. From the dependency of the exciplex emission maximum on the solvent polarity, the exciplex dipole moment of erythro and threo APTE were calculated. The evolution of the ratio of the quantum yield of exciplex emission and the quantum yield of emission from the locally excited state is correlated with solvent polarity and with the tendency of the solvent to interact with the peptide chain through hydrogen bonding. It is shown that solvents, inert towards the peptide function, shift the equilibrium between the two ground state conformations towards C7 , in which an exciplex geometry can be reached. Hydrogen accepting solvents shift the conformational equilibrium towards C5 , which cannot form an exciplex directly within the lifetime of excited pyrene. 相似文献
998.
M. Loudet M. Grimaud F. Metras G. Pfister-Guillouzo 《Journal of Molecular Structure》1976,35(2):213-222
The photoelectronic spectra of cis-2-chloro, 4-tert-butylcyclohexanone and cis-2-methyl-2-chlorocyclohexanone have been analysed. These analyses have been used in an investigation into the nature of the orbital interactions in these molecules, and into the dependence of these interactions on the axial or equatorial position of the halogen atom. The preferential conformations of 2-chlorocyclohexanone and 4,4-dimethyl-2-chlorocyclo-hexanone have thus been obtained. 相似文献
999.
Vibrational chemiluminescence in the Δν1 = Δν3 = ?1 band of NO2 is observed both in the O + NO and O3 + NO reactions and shown to be emitted by molecules with up to 11 000 cm?1 of vibrational energy. Quenching rate constants of NO23 are estimated ranging from about 6 × 10?14 for Ar to about 3 × 10?12 cm3 s?1 for NO2. The ratio of vibrational to electronic emission is 0.06 ± 0.03 for O + NO and 5.3 ± 1.0 for O3 + NO. It is suggested that vibrationally excited NO2 is a major product of that channel of the O3 + NO reaction which forms ground-state NO2(2A1) directly. 相似文献
1000.
An universal function for non-bonded interactions, which takes into account the relative orientation of the bonds is considered in calculating the conformational energies of cycloalkenes and cycloalkadienes. A comparison is made with previous results obtained by using usual 6-exp functions for non-bonded interactions. 相似文献