全文获取类型
收费全文 | 24599篇 |
免费 | 617篇 |
国内免费 | 142篇 |
专业分类
化学 | 16624篇 |
晶体学 | 237篇 |
力学 | 544篇 |
综合类 | 1篇 |
数学 | 3872篇 |
物理学 | 4080篇 |
出版年
2022年 | 178篇 |
2021年 | 223篇 |
2020年 | 334篇 |
2019年 | 329篇 |
2018年 | 214篇 |
2017年 | 211篇 |
2016年 | 467篇 |
2015年 | 412篇 |
2014年 | 519篇 |
2013年 | 1143篇 |
2012年 | 1160篇 |
2011年 | 1509篇 |
2010年 | 728篇 |
2009年 | 621篇 |
2008年 | 1212篇 |
2007年 | 1281篇 |
2006年 | 1267篇 |
2005年 | 1237篇 |
2004年 | 1030篇 |
2003年 | 882篇 |
2002年 | 867篇 |
2001年 | 382篇 |
2000年 | 365篇 |
1999年 | 303篇 |
1998年 | 271篇 |
1997年 | 322篇 |
1996年 | 364篇 |
1995年 | 300篇 |
1994年 | 310篇 |
1993年 | 347篇 |
1992年 | 294篇 |
1991年 | 268篇 |
1990年 | 237篇 |
1989年 | 224篇 |
1988年 | 198篇 |
1987年 | 230篇 |
1986年 | 193篇 |
1985年 | 301篇 |
1984年 | 328篇 |
1983年 | 226篇 |
1982年 | 292篇 |
1981年 | 309篇 |
1980年 | 239篇 |
1979年 | 247篇 |
1978年 | 282篇 |
1977年 | 215篇 |
1976年 | 223篇 |
1975年 | 213篇 |
1974年 | 234篇 |
1973年 | 239篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
Let {ø n (dμ)} be a system of orthonormal polynomials on the unit circle with respect to a measuredμ. Szegö's theory is concerned with the asymptotic behavior ofø n (dμ) when logμ′∈L 1. In what follows we will discuss the asymptotic behavior of the ratioø n (dμ 2)/ø n (dμ 1) on the unit circle whendμ 1 anddμ 2 are close in a sense (e.g.,dμ 2=gdμ 1, where g≥0 is such thatQ(e it )g(t) andQ(e it )/g(t) are bounded for a suitable polynomialQ) and μ 1 ′ >0 almost everywhere or (a somewhat weaker requirement) lim n→∞Φ n (dμ 1,0)=0 for the monic polynomial Φ n . The asymptotic behavior of the same fraction outside the unit circle was discussed in an earlier paper. 相似文献
103.
104.
105.
Let be an additive permutation of a finite integral base. It is shown that ifB is symmetric, then there is a unique additive permutation ofB which is compatible with in the sense that
–1 is also an additive permutation; and that, further, ifB is asymmetric, then there is no additive permutation ofB which is compatible with.
Thus, in the symmetric case, there are no additively compatible sets (of permutations) forB of size greater than 3. This contrasts with the situation for completely compatible sets (equivalently, additive sequences of permutations) where for certainB compatible sets of size (resp. length) 4 or less are known, but where nothing is known of sets of greater size (resp. length). It is also noted how this result restricts the possibility of a useful multiplication theorem for the additive analogue of perfect systems of difference sets and graceful graphs. 相似文献
106.
Roy Edward Bruns Paul Martin Kuznesof James Elton Moore 《Journal of Molecular Structure》1975,29(2):211-223
CNDO molecular orbital calculations have been performed to analyze donor—acceptor interactions between molecular chlorine and benzene, toluene, mesitylene and hexamethylbenzene and the, as yet, unreported chlorine—hexafluorobenzene and carbon disulfide—benzene pairs. The stabilization energy and the dipole moment and its derivative (?p/?RCICI) calculated for the benzene—chlorine complex are in good agreement with the estimated experimental values. The trends in the experimental stabilization energies and the Cl-Cl vibrational frequencies with increasing methyl substitution appear to be well reproduced by the calculations. The charge transferred from the benzene donor is polarized toward the outer chlorine atom or sulfur atom. For hexafluorobenzene-chlorine the direction of electronic charge polarization is reversed from that of the benzene and methylbenzene complexes. The calculated results are discussed within the framework of Muliiken's simplified resonance theory for complexes. 相似文献
107.
Ab initio molecular orbital structures and energies of B2F4, B2Cl4, N2O4, and C2O have been calculated for both perpendicular D2d and planar D2h rotamers. The experimental trend toward greater preference for the D2d forms in going from B2F4 to B2Cl4 is reproduced. N2O4 favors the planar conformation, although the rotation barrier is overestimated at the theoretical levels used. The oxalate dianion is calculated to be more stable in the D2d conformation; the experimental planar arrangement in the solid may be due to crystal packing forces. The preferences for one conformation over another are small; analysis indicates that different effects may predominate in each case: π stabilization for B2F4, hyperconjugation for B2Cl4, lone-pair interactions for N2O4, and electrostatic repulsions for C2O. 相似文献
108.
The results of allowing tetrahydro-as-triazine-3(2H)-thiones to react with various α,β-diiunctional compounds, such as, α-bromoethyl p-toluenesulfonate, chloroacetaldehyde, α-bromophenylacet-uldehyde, phenaeyl bromide, chloroacetonitrile, α-bromophenylaeelonitrile, and α-cyanobenzyl p-toluenesulfonate are discussed. These condensations give either a 5H-thiazolo[3,2-b]-as-triazine or a 2H-thiazolo[2,3-c]-as-triazine. 相似文献
109.
Several aromatic compounds containing one or two C6F5S groups have been prepared by nucleophilic displacement reactions using CuSC6F5 in DMF solution. Aromatic iodine or bromine, rather than chlorine of fluorine is replaced by the SC6F5 group using CuSC6F5. A mechanism is postulated. New compounds prepared include -(C6F6F5S)2C6H4, - and -(C6F5S)2C6F4 and XC6H4SC6F5(X=C1, NO2, I, CH3, CO2C2H5). 相似文献
110.
Paul S. Pregosin S. N. Sze Piero Salvadori Raffaello Lazzaroni 《Helvetica chimica acta》1977,60(8):2514-2521
13C- and 195Pt-NMR. spectra for the complexes trans-PtCl2(amine)(CH2?CH2) have been measured. For amine = (S)-N-methyl-α-methylbenzylamine the two diastereomers present may be distinguished from the values 3J(Pt,C). The 195Pt-chemical shift is shown to be sensitive to subtle differences stemming from intramolecular non-bonded interactions. 相似文献