全文获取类型
收费全文 | 29394篇 |
免费 | 806篇 |
国内免费 | 194篇 |
专业分类
化学 | 19131篇 |
晶体学 | 214篇 |
力学 | 593篇 |
综合类 | 4篇 |
数学 | 5219篇 |
物理学 | 5233篇 |
出版年
2023年 | 177篇 |
2022年 | 477篇 |
2021年 | 566篇 |
2020年 | 476篇 |
2019年 | 537篇 |
2018年 | 504篇 |
2017年 | 477篇 |
2016年 | 974篇 |
2015年 | 820篇 |
2014年 | 911篇 |
2013年 | 1685篇 |
2012年 | 1783篇 |
2011年 | 2532篇 |
2010年 | 1224篇 |
2009年 | 1220篇 |
2008年 | 1760篇 |
2007年 | 1667篇 |
2006年 | 1525篇 |
2005年 | 1417篇 |
2004年 | 1176篇 |
2003年 | 972篇 |
2002年 | 801篇 |
2001年 | 552篇 |
2000年 | 477篇 |
1999年 | 398篇 |
1998年 | 318篇 |
1997年 | 317篇 |
1996年 | 390篇 |
1995年 | 265篇 |
1994年 | 281篇 |
1993年 | 257篇 |
1992年 | 262篇 |
1991年 | 209篇 |
1990年 | 204篇 |
1989年 | 160篇 |
1988年 | 151篇 |
1987年 | 165篇 |
1986年 | 154篇 |
1985年 | 197篇 |
1984年 | 200篇 |
1983年 | 144篇 |
1982年 | 164篇 |
1981年 | 127篇 |
1980年 | 134篇 |
1979年 | 98篇 |
1978年 | 94篇 |
1977年 | 88篇 |
1976年 | 82篇 |
1974年 | 57篇 |
1973年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Rayón VM Redondo P Barrientos C Largo A 《The journal of physical chemistry. A》2007,111(28):6345-6353
A theoretical study of the first-row transition metal dicarbide cations MC2+ (M=Sc-Zn) has been carried out. Predictions for different molecular properties that could help in their eventual experimental detection have been made. Most MC2+ compounds prefer a C2v symmetric arrangement over the linear geometry. In particular, the C2v isomer is specially favored for early transition metals. Only for CuC2+ is the linear isomer predicted to be the global minimum, although by only 1 kcal/mol. In all cases the isomerization barrier between cyclic and linear species seems to be very small (below 2 kcal/mol). The topological analysis of the electronic density shows that most C2v isomers are T-shaped structures. In general, MC2+ compounds for early transition metals have larger dissociation energies than those formed by late transition metals. In most cases the dissociation energies for MC2+ compounds are much smaller than those obtained for their neutral analogues. An analysis of the bonding in MC2+ compounds in terms of the interactions between the valence orbitals of the fragments helps to interpret their main features. 相似文献
952.
Although very bulky ligands e.g.(o-MeC6H4)3E or (μ-C10H7)3E (E = P or As) are inert, the normal photochemical or thermal reaction of tertiary phosphines or arsines, L, with [Mn2(CO)10] is CO substitution with the formation of [Mn2(CO)8(L)2] derivatives (I). At elevated temperatures some triarylarsines, R3As, undergo Lambert's reaction with ligand fragmentation to give [Mn2(CO)8(μ-AsR2)2] complexes (II) (R = Ph, p-MeOC6H4, p-FC6H4, or p-CIC6H4) even though, in the absence of [Mn2(CO)10] R3As are stable under the same conditions. Exceptional behaviour is exhibited by (p-Me2NC6H4)3- As which forms a product of type I; by some HN(C6H4)2AsR which give a product of type II as a result of loss of the non-aryl groups R = PhCH2, cyclo-C6H11, or MeO; and by Ph(α-C10H72P which is the only phosphine to form a product of type II, albeit in trace amounts only. The thermal decomposition of a n-butanol solution of [Mn2(CO)8(AsPh3)2] in a sealed tube gives C6H6 and [Mn2(CO)8(α-AsPh2)2], whilst in an open system in the presence of various tertiary phosphines, L, [Mn(H)(CO)3(L)2] are obtained. It is suggested that Lambert's reaction is a thermal fragmentation of [Mn(CO)4(AsR3]* radicals, the first to be recognised. They lose the radical R* which abstracts hydrogen from the solvent. The resulting [Mn(CO)4(AsR2)] moiety dimerises to [Mn2(CO)8-(α-AsR2)2]. the reaction is facilitated by the stability of the departing radical (e.g. PhCH2 or MeO) and, as the crowding about As is relieved, by its size (e.g. Ph, cyclo-C6H11, o-MeC6H4, or α-C10H7). In general, phosphine-substituted radicals [Mn(CO)4(PR)3]* do not undergo this decomposition, probably because the PC bonds are much stronger than AsC. 相似文献
953.
Bla Ivn Joseph P. Kennedy Tibor Kelen Ferenc Tüdos 《Journal of polymer science. Part A, Polymer chemistry》1981,19(3):679-685
Colorless poly(vinyl chloride)s (PVC) containing up to 1.6 allylic chlorines per molecule have been prepared by controlled random dehydrochlorination with a strong base. The effect of temperature in the ?50 to +24°C range on the number of allylic chlorines and color of PVC has been investigated by ozonization experiments and UV–visible spectroscopy. A two-parameter kinetic model has been developed which quantitatively accounts for the observations and points the way for further research. 相似文献
954.
Fondo M García-Deibe AM Corbella M Ruiz E Tercero J Sanmartín J Bermejo MR 《Inorganic chemistry》2005,44(14):5011-5020
The new tetranuclear carbonate complex [Cu2L)2(CO3)] x 8H2O (1 x 8H2O) (H3L = (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) has been obtained by two different synthetic routes and fully characterized. Recrystallization of 1 x 8H2O in methanol yields single crystals of {[(Cu2L)2(CO3)]}2 x 12H2O (1 x 6H2O), suitable for X-ray diffraction studies. The crystal structure of 1 x 6H2O shows two crystallographically different tetranuclear molecules in the asymmetric unit, 1a and 1b. Both molecules can be understood as self-assembled from two dinuclear [Cu2L]+ cations, joined by a mu4-eta(2):eta(1):eta(1) carbonate ligand. The copper atoms of each crystallographically different [(Cu2L)2(CO3)] molecule present miscellaneous coordination polyhedra: in both 1a and 1b, two metal centers are in square pyramidal environments, one displays a square planar chromophore and the other one has a geometry that can be considered as an intermediate between square pyramid and trigonal bipyramid. Magnetic studies reveal net intramolecular ferromagnetic coupling between the metal atoms. Density functional calculations allow the assignment of the different magnetic coupling constants and explain the unexpected ferromagnetic behavior, because of the presence of an unusual NCN bridging moiety and countercomplementarity of the phenoxo (or carbonate) and NCN bridges. 相似文献
955.
Indachlorins: Nonplanar Indanone‐Annulated Chlorin Analogues with Panchromatic Absorption Spectra between 300 and 900 nm 下载免费PDF全文
Dr. Lalith P. Samankumara Dr. Sarina J. Dorazio Dr. Joshua Akhigbe Ruoshi Li Dr. Arunpatcha Nimthong‐Roldán Dr. Matthias Zeller Prof. Dr. Christian Brückner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(31):11118-11128
Indaphyrins, pyrrole‐modified porphyrins containing a cleaved pyrrole β,β′‐bond and two annulated indanone moieties, possess unusually broadened and redshifted UV/Vis spectra because of their π‐expanded chromophores. The parent free base indaphyrin has been crystallographically characterized, highlighting its strongly ruffled conformation incorporating a helimeric twist. It was shown to be susceptible to regiospecific derivatizations at the opposite side of the ring‐cleaved pyrrole (dihydroxylation, followed by functional group transformations of the resulting diol functionality), generating indaphyrin‐based chlorin analogues, indachlorins, that incorporate a dihydroxypyrroline, pyrrolindione, oxazolone, or a morpholine moiety. Structural modifications resulted in further broadening and hyper‐ and bathochromic shifts of the optical spectra, some of which possess a nearly panchromatic absorption between 300 to well above 900 nm. The extents to which these modifications affect their solid‐state conformations were analyzed. 相似文献
956.
Dr. Stefano Fedeli Dr. Paolo Paoli Prof. Alberto Brandi Dr. Lorenzo Venturini Dr. Giuliano Giambastiani Dr. Giulia Tuci Prof. Stefano Cicchi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(43):15349-15353
A series of azido‐dyes were synthesized through Knoevenagel reactions of an azido‐BODIPY with aromatic aldehydes. The nature of the substituents allowed the fine tuning of their spectroscopic properties. The dyes were used to decorate oxidized multiwalled carbon nanotubes (ox‐MWCNTs), bearing terminal triple bond groups, by CuAAC reactions, affording fluorescent materials. This decoration allowed the efficient determination of the internalization of the ox‐MWCNT derivatives by different model cancer cells, such as MCF7. 相似文献
957.
The correlation energy in the direct random phase approximation (dRPA) can be written, among other possibilities, either in terms of the interaction strength averaged correlation density matrix, or in terms of the coupled cluster doubles amplitudes obtained in the direct ring approximation (drCCD). Although the corresponding dRPA correlation density matrix on the one hand, and the drCCD amplitude matrix on the other hand, differ significantly, they yield identical energies. Similarly, the analogous RPA and rCCD correlation energies calculated from antisymmetrized two-electron integrals are identical to each other despite very different underlying working equations. In the present communication, a direct correspondence between amplitudes and densities is established and investigated with perturbation theory arguments. Our analysis also sheds some light on the properties of recently proposed RPA/rCCD variants which use antisymmetrized integrals in part of the equations and nonantisymmetrized integrals in others. 相似文献
958.
Christopher M. Burba Roger Frech Agneta Seidel Lennart Häggström Anton Nytén John O. Thomas 《Journal of Solid State Electrochemistry》2009,13(8):1267-1272
The first discharge of the Li+ ion anode material LiSn2(PO4)3 was investigated with Mössbauer spectroscopy and electrochemical techniques. Mössbauer spectroscopy provided insight into the structure of the tin atoms of the fully discharged anode materials. Spectra consist of overlapping peaks, which are assigned to noncrystalline β-Sn and Li–Sn alloy domains. An analysis of the relative intensities of the Mössbauer spectra shows the relative abundance of β-Sn increases at the expense of the Li–Sn alloy as the discharge rate increases. Cell polarization occurs at higher discharge rates, leading to inefficient electrode utilization and poor cycling performance. Sluggish Li+ ion diffusion through the amorphous Li3PO4 network that is formed early in the discharge process might be responsible for the poor electrochemical performance and the accumulation of unalloyed tin. 相似文献
959.
Elena Sanna Eduardo C. Escudero-Adán Antonio Bauzá Pablo Ballester Antonio Frontera Carmen Rotger Antonio Costa 《Chemical science》2015,6(10):5466-5472
A crystalline porous material showing one-dimensional (1-D) rectangular micropores (12 × 9 Å2) has been assembled from a semirigid macrocyclic tetraimine and EtOAc as the templating agent. The 1-D nature of the material is intrinsic to the conformationally rigid structure of a macrocyclic sub-unit bearing four cyclohexylidene residues. The multiple dispersive forces established among the aliphatic residues glutted the 1-D channels and provided thermal stability to the material at temperatures below 160 °C. Upon removal of the template, the structure of the empty solid exhibited permanent microporosity (S
BET = 342 m2 g–1). Being a true molecular sponge, the channel framework of this material allowed the inclusion of a variety of molecular sample guests without compromising its crystalline nature. Remarkably, this crystalline material enabled the structure determination by X-ray diffraction of the included molecules. Theoretical studies demonstrated the vital role played by the dispersive forces in the overall stabilization of the crystal packing. 相似文献
960.
Irene Ortín 《Tetrahedron》2009,65(48):9944-6277
To avoid the epimerization of the C(11a)-stereocenter previously observed in 6,11a-cis-pyrazino[1,2-b]isoquinolin-1,4-diones, we present in this paper the C(3)-alkylation of 1-methoxy-pyrazino[1,2-b]isoquinolin-4-ones to obtain all-cis derivatives through a very reliable protocol. The success of the acid-promoted cyclization to get pentacyclic (R3=arylmethyl) or tetracyclic (R3=2-bromo-2-propenyl) compounds is dependent on the nature of the C(3)-unsaturated chain and of the N-substituent, but these limitations have been overcome by using trifluoromethanesulfonic as a superacid catalyst. The C-(3)-alkylation of pyrazino[1,2-b]isoquinolin-4-one is also studied. 相似文献