全文获取类型
收费全文 | 3554篇 |
免费 | 86篇 |
国内免费 | 15篇 |
专业分类
化学 | 2200篇 |
晶体学 | 15篇 |
力学 | 195篇 |
数学 | 744篇 |
物理学 | 501篇 |
出版年
2023年 | 19篇 |
2022年 | 52篇 |
2021年 | 98篇 |
2020年 | 73篇 |
2019年 | 51篇 |
2018年 | 65篇 |
2017年 | 57篇 |
2016年 | 111篇 |
2015年 | 104篇 |
2014年 | 80篇 |
2013年 | 187篇 |
2012年 | 206篇 |
2011年 | 248篇 |
2010年 | 146篇 |
2009年 | 121篇 |
2008年 | 201篇 |
2007年 | 220篇 |
2006年 | 163篇 |
2005年 | 158篇 |
2004年 | 145篇 |
2003年 | 118篇 |
2002年 | 98篇 |
2001年 | 43篇 |
2000年 | 25篇 |
1999年 | 37篇 |
1998年 | 35篇 |
1997年 | 34篇 |
1996年 | 36篇 |
1995年 | 26篇 |
1994年 | 30篇 |
1993年 | 47篇 |
1992年 | 18篇 |
1991年 | 29篇 |
1990年 | 31篇 |
1989年 | 18篇 |
1988年 | 27篇 |
1987年 | 24篇 |
1986年 | 23篇 |
1985年 | 47篇 |
1984年 | 42篇 |
1983年 | 23篇 |
1982年 | 25篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 22篇 |
1978年 | 26篇 |
1977年 | 23篇 |
1976年 | 22篇 |
1975年 | 13篇 |
1974年 | 21篇 |
排序方式: 共有3655条查询结果,搜索用时 15 毫秒
11.
Dore Augusto Clemente Armando Marzotto Giovanni Valle 《Journal of chemical crystallography》1988,18(2):147-156
The crystal and molecular structure of thiamine monochloride (C12H17N4OSCl) was determined by X-ray diffraction and refined to a finalR value of 0.042. The compound crystallizes in the monoclinic system, space groupP21/a, with cell constantsa=18.929(4),b=11.663(2),c=6.376(2) Å and=96.72(8)°. The thiamine molecule is anhydrous and unprotonated, and the torsion angles at the methylene carbon show that it possesses anF conformation. The dihedral angle value of 84.16(6)° between thiazolium and pyrimidine rings is in the normal range found for the thiamine withF conformation, protonated or not, hydrated or not. Thiamine therefore has the same conformation notwithstanding protonation or hydration, with the rings similarly oriented in all the crystal structures containing thiamine. 相似文献
12.
Domenico Spinelli Giovanni Consiglio Renato Noto 《Journal of heterocyclic chemistry》1977,14(8):1325-1329
The rates of piperidino substitution of some 2-L-3-nitrothiophenes (I) and 2-L-5-nitrothiophenes (II) (L = Cl, Br, I, OC6H4NO2-p, and SO2Ph) have been measured in methanol and in benzene at various piperidine concentrations. The reactivity of compounds (I) is not affected by the piperidine concentration in both methanol and benzene, except for the case of L = I (Ic). Probably due to association effects, the reactivity of Ic in benzene decreases as the piperidine concentration is increased. The reactions of compounds II follow overall second order kinetics in methanol while in benzene a different behaviour is observed as a function of the nature of the leaving group. In fact, the piperidino substitutions of IIa-c (L = Cl, Br, I) are mildly accelerated at high piperidine concentrations (a moderate solvent effect); on the contrary the reactivity of IId and e shows a strong dependence on the piperidine concentration, pointing out a genuine base catalysis. 相似文献
13.
Apparent molal heat capacities
of some piperidine, morpholine, and piperazine derivatives in aqueous solution have been determined by adiabatic calorimetry
in the temperature range 20–55°C and in the molality range 0.2–1m. Comparison of experimental
values with those calculated through group contributions, found for monofunctional compounds, indicates strong interactions
between the hydrophilic centers. An interpretation is given of the possible mechanism of this interaction. Also, values of
ΔC
p
for the addition reaction of proton to nitrogen centers of mono- and bifunctional organic compounds are examined. 相似文献
14.
P. Rodighiero A. Chilin G. Pastorini A. Guiotto P. Manzini 《Journal of heterocyclic chemistry》1990,27(4):1153-1158
A number of new methyltriazolocoumarins with a linear psoralen-like structure or an angular angelicin-like structure were synthetized. The syntheses were performed starting from the appropriate methylated 7-aminocoumarins which were nitrated, reduced and successively diazotized with concomitant cyclization to form the condensed triazolo ring. The new methyltriazolocoumarins show promising spectroscopic properties which are potentially predictive of their photochemical and photobiological behaviour. 相似文献
15.
The geometrical isomers of 2,3-difluoro-2,3-dihydrobenzofuran have been prepared and their structure assigned on the basis of the nmr spectra. Some suggestions on the conformational situation of two adducts are also reported. 相似文献
16.
Marina Brustolon Carlo Corvaja Giovanni Giacometti 《Theoretical chemistry accounts》1971,22(1):90-104
The proton hyperfine splitting constants of a large number of positive, negative, and neutral radicals, have been examined in term of the Heller-McConnell relation a
H=Bc cos2
whose validity is discussed. B is taken as a function of the energy of the singly occupied orbital and values are calculated by first order perturbation theory for the cases of a methyl, methylene, and dimethylene group attached to the system. Substantial agreement is found between theory and experiment indicating the correctness of the postulated cause of the B behaviour.
Zusammenfassung Die Konstante der Hyperfeinaufspaltung für -Protonen einer großen Anzahl positiver, negativer und neutraler -Radikale wurde mit der Beziehung von Heller und McConnell als Grundlage untersucht; die Gültigkeit dieser Beziehung wird diskutiert. B wird als von der Energie des einfach besetzten Orbitals abhängig angenommen und seine Werte mit Hilfe der Störungstheorie erster Ordnung für die Fälle von Methyl-, Methylen- und Dimethylengruppe am -System berechnet. Aus der guten Übereinstimmung zwischen Theorie und Experiment wird auf die Richtigkeit der postulierten Ursachen für das Verhalten von B geschlossen.相似文献
17.
NMR data have been used to assign the stereochemistry to some new (E)- and (Z)-α-phenyl-β-[2-(N-methyl)nitropyrrolyl]acrylic acids. The (E)-molecules are biased in the s-cis conformation showing NMR spectroscopic features strictly depending on the conformation.The analysis of the NMR spectra reveals that the (Z)-isomers exist also in the s-cis conformation. 相似文献
18.
Giovanni Monegato 《Numerical Algorithms》1996,11(1):271-283
We consider the numerical solution of second kind integral equations of the form $$u(y) - \int\limits_0^1 {k(y/x)\frac{{u(x)}}{x}dx = f(y), 0 \le y \le 1,} $$ for some given kernelk(t). These equations, usually indicated as of Mellin type, arise in a variety of applications. In particular, we examine a Nyström interpolant based on the following product quadrature rule: $$\int\limits_0^1 {k(y/x)\frac{{u(x)}}{x}dx \approx \sum\limits_{i = 0}^n {w_{ni} (y)u(x_{mi} ).} } $$ This rule is obtained by interpolatingu(x) by the Lagrange polynomial associated with the set of Gauss-Radau nodes {x ni}. Under certain assumptions on the kernelk(t), we are able to prove the stability of our interpolant and derive convergence estimates. 相似文献
19.
A (k,k)-current T on an open subset of CN is plurisubharmonic if is positive. Positive plurisubharmonic currents admit Lelong numbers; we prove here that they are independent on the coordinates system. Moreover, if Y is an analytic subset of pure dimension, then the Lelong numbers of T on Y are given by a non negative weakly plurisubharmonic function. 相似文献
20.
Giovanni Gaiffi 《manuscripta mathematica》1996,91(1):83-94
Let
be the complexified Coxeter arrangement of hyperplanes of typeA
n−1. In this paper we construct anS
n+1 extension of the naturalS
n action on the complex cohomology ring of the complement ofA
n−1. Recurrence formulas connecting characters with respect to theS
n and theS
n+1 action are given. 相似文献