全文获取类型
收费全文 | 597篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
化学 | 459篇 |
晶体学 | 2篇 |
力学 | 16篇 |
数学 | 53篇 |
物理学 | 83篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 8篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2017年 | 4篇 |
2016年 | 13篇 |
2015年 | 13篇 |
2014年 | 10篇 |
2013年 | 40篇 |
2012年 | 42篇 |
2011年 | 59篇 |
2010年 | 27篇 |
2009年 | 23篇 |
2008年 | 40篇 |
2007年 | 31篇 |
2006年 | 33篇 |
2005年 | 34篇 |
2004年 | 37篇 |
2003年 | 34篇 |
2002年 | 24篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 12篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1964年 | 1篇 |
1957年 | 1篇 |
1956年 | 1篇 |
1948年 | 1篇 |
1939年 | 1篇 |
排序方式: 共有613条查询结果,搜索用时 15 毫秒
121.
A series of bis-iminonitroxide diradical derivatives of different lengths and geometry have been prepared that incorporate a conjugated phenylene-ethynylene bridge as a rigid spacer. This paper describes the synthesis of these new components and their main characterizations. An unexpected singlet ground state and substituent effects on the singlet-triplet gap have been found for substituted "m-phenylene"-based diradicals. The effects of the pi-conjugation on the intramolecular through-bond spin coupling have been investigated by changing the length of the spacer within linear derivatives. The EPR studies demonstrate the intramolecular magnetic coupling between the radical spins within all compounds. This result is very attractive and unusual, given the large distance between the radicals from 15 A in the dimer to 36 A in the pentamer. 相似文献
122.
123.
Federico Cisnetti Dr. Christelle Gateau Dr. Colette Lebrun Pascale Delangle Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(30):7456-7469
Unnatural metal‐chelating amino acids bearing aminodiacetate side‐chains have been introduced into two hexapeptides to obtain efficient lanthanide‐binding peptides. The synthesis of the enantiopure Fmoc‐Adan(tBu)2‐OH synthons is described with overall yields of 32 and 50 % for n=2 and n=3 side‐chain carbon atoms, respectively. The two peptides AcWAdanPGAdanGNH2 ( P n ) were synthesized from the protected synthons by standard solid‐phase peptide synthesis. Studies of the lanthanide complexes of the two peptides P n by luminescence titrations, mass spectrometry, circular dichroism, and solution NMR spectroscopy demonstrate that the Adan chain length has a dramatic effect on the complexation properties. Indeed, the flexible compound P3 forms a mononuclear complex of moderate stability (β11=109.9), which tends to transform into a binuclear species in the presence of excess of the metal ion. Interestingly, the more compact peptide P2 provides stable Ln3+ complexes with the exclusive formation of the mononuclear Ln P2 adduct. The stability constant of Tb P2 is two orders of magnitude higher (β11=1012.1) than that measured for P3 . The 800 MHz NMR spectrum of the La3+ complex of P2 evidences a well‐defined type II β‐turn as well as a hydrophobic Trp(indole)–Pro interaction. These interactions exemplify the non‐innocent character of the peptide spacer in the complex La P2 as well as the role of a peptide secondary structure in the stabilization of metal complexes. 相似文献
124.
Baudron SA Avarvari N Batail P Coulon C Clérac R Canadell E Auban-Senzier P 《Journal of the American Chemical Society》2003,125(38):11583-11590
The deliberate design of a series of single crystals of conducting two-dimensional radical cation salts of o-bis(amide)-appended ethylenedithiotetrathiafulvalene, beta'-[EDT-TTF-(CONH(2))(2)](2)X (X = HSO(4)(-), ClO(4)(-), ReO(4)(-), or AsF(6)(-)) and of their parent monocomponent solid EDT-TTF-(CONH(2))(2) is demonstrated and allows us to reach a level of prediction of the structure of molecular conductors. Their conductivity is activated with a gap of 1650 K and a sizable room-temperature conductivity of 0.15 S.cm(-)(1) (for X = ClO(4)(-)) and a singular spin susceptibility for a beta'-type salt that, in addition, changes very remarkably with the anion. The key design element is that of a recurrent, puckered ribbon constructed out of self-complementary, hydrogen-bonded amide...amide ring motifs whose minute modulations of curvature and shape throughout the series have been shown to correlate to very remarkable differences in the intrastack beta(HOMO)(-)(HOMO) interaction energies and changes in the density of states at the Fermi level and on to important differences of spin susceptibility behavior in a system where electron correlations are significant. The coupled activation of structure, electron interactions, and magnetic susceptibility discovered and discussed throughout the paper is unprecedented and is seen as a genuine expression of interfacial hydrogen-bond interactions onto the collective electronic properties. 相似文献
125.
Francis Canon Franck Paté Emmanuelle Meudec Thérèse Marlin Véronique Cheynier Alexandre Giuliani Pascale Sarni-Manchado 《Analytical and bioanalytical chemistry》2009,395(8):2535-2545
Numerous protein–polyphenol interactions occur in biological and food domains particularly involving proline-rich proteins,
which are representative of the intrinsically unstructured protein group (IUP). Noncovalent protein–ligand complexes are readily
detected by electrospray ionization mass spectrometry (ESI-MS), which also gives access to ligand binding stoichiometry. Surprisingly,
the study of interactions between polyphenolic molecules and proteins is still an area where ESI-MS has poorly benefited,
whereas it has been extensively applied to the detection of noncovalent complexes. Electrospray ionization mass spectrometry
has been applied to the detection and the characterization of the complexes formed between tannins and a human salivary proline-rich
protein (PRP), namely IB5. The study of the complex stability was achieved by low-energy collision-induced dissociation (CID)
measurements, which are commonly implemented using triple quadrupole, hybrid quadrupole time-of-flight, or ion trap instruments.
Complexes composed of IB5 bound to a model polyphenol EgCG have been detected by ESI-MS and further analyzed by MS/MS. Mild
ESI interface conditions allowed us to observe intact noncovalent PRP–tannin complexes with stoichiometries ranging from 1:1
to 1:5. Thus, ESI-MS shows its efficiency for (1) the study of PRP–tannin interactions, (2) the determination of stoichiometry,
and (3) the study of complex stability. We were able to establish unambiguously both their stoichiometries and their overall
subunit architecture via tandem mass spectrometry and solution disruption experiments. Our results prove that IB5·EgCG complexes
are maintained intact in the gas phase.
相似文献
126.
The kinetics of the tetrabutoxyzirconium-catalyzed esterifications between 11-dodecylamideundecanoic acid and 1-dodecanol, 2-tridecanol, a-dodecyl-ω-hydroxypolyoxyethylene are studied and compared to that of α, ω-dicarboxypolyamide-l 1 and α, ω-dihydroxypolyoxyethylene. The reaction with 2-tridecanol is characterized by an autoacceleration phenomenon. In the other cases the reaction rate is not greatly affected by catalyst concentration. An explanation involving the formation of catalyst condensates at the very beginning of the reaction is put forward. 相似文献
127.
Efficient Synthesis of Differentiated syn‐1,2‐Diol Derivatives by Asymmetric Transfer Hydrogenation–Dynamic Kinetic Resolution of α‐Alkoxy‐Substituted β‐Ketoesters 下载免费PDF全文
Dr. Laure Monnereau Dr. Damien Cartigny Dr. Michelangelo Scalone Dr. Tahar Ayad Dr. Virginie Ratovelomanana‐Vidal 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(33):11799-11806
Asymmetric transfer hydrogenation was applied to a wide range of racemic aryl α‐alkoxy‐β‐ketoesters in the presence of well‐defined, commercially available, chiral catalyst RuII–(N‐p‐toluenesulfonyl‐1,2‐diphenylethylenediamine) and a 5:2 mixture of formic acid and triethylamine as the hydrogen source. Under these conditions, dynamic kinetic resolution was efficiently promoted to provide the corresponding syn α‐alkoxy‐β‐hydroxyesters derived from substituted aromatic and heteroaromatic aldehydes with a high level of diastereoselectivity (diastereomeric ratio (d.r.)>99:1) and an almost perfect enantioselectivity (enantiomeric excess (ee)>99 %). Additionally, after extensive screening of the reaction conditions, the use of RuII‐ and RhIII‐tethered precatalysts extended this process to more‐challenging substrates that bore alkenyl‐, alkynyl‐, and alkyl substituents to provide the corresponding syn α‐alkoxy‐β‐hydroxyesters with excellent enantiocontrol (up to 99 % ee) and good to perfect diastereocontrol (d.r.>99:1). Lastly, the synthetic utility of the present protocol was demonstrated by application to the asymmetric synthesis of chiral ester ethyl (2S)‐2‐ethoxy‐3‐(4‐hydroxyphenyl)‐propanoate, which is an important pharmacophore in a number of peroxisome proliferator‐activated receptor α/γ dual agonist advanced drug candidates used for the treatment of type‐II diabetes. 相似文献
128.
Under aqueous conditions, 4-azidouracil/tetrazolo[1,5-c]pyrimidin-5(6H)-one nucleosides undergo a very efficient photochemical nitrogen elimination and ring expansion to 1,3,5-triazepin-2,4-dione nucleosides whose structure has been confirmed by X-ray crystallography. In contrast, when the photolysis was attempted under anhydrous conditions in the presence of a nucleophile, a ring contraction reaction occurred, affording 2-oxoimidazolone nucleosides. A mechanism to account for the formation of ring expansion and contraction reactions and involving a carbodiimide intermediate is proposed which is reminiscent of the known photochemical behavior of 2-azidopyridines/tetrazolo[1,5-a]pyridines. 相似文献
129.
Espagne A Changenet-Barret P Plaza P Martin MM 《The journal of physical chemistry. A》2006,110(10):3393-3404
We previously reported that two analogues of the Photoactive Yellow Protein chromophore, trans-p-hydroxycinnamic acid (pCA(2-)) and its amide derivative (pCM-) in their deprotonated forms, undergo a trans-cis photoisomerization whereas the thioester derivative, trans-p-hydroxythiophenyl cinnamate (pCT-), does not. pCT- is also the only one to exhibit a short-lived intermediate on its excited-state deactivation pathway. We here further stress the existence of two different relaxation mechanisms for these molecules and examine the reaction coordinates involved. We looked at the effect of the solvent properties (viscosity, polarity, solvation dynamics) on their excited-state relaxation dynamics, probed by ultrafast transient absorption spectroscopy. Sensitivity to the solvent properties is found to be larger for pCT- than for pCA(2-) and pCM-. This difference is considered to reveal that either the relaxation pathway or the reaction coordinate is different for these two classes of analogues. It is also found to be correlated to the electron donor-acceptor character of the molecule. We attribute the excited-state deactivation of analogues bearing a weaker acceptor group, pCA(2-) and pCM-, to a stilbene-like photoisomerization mechanism with the concerted rotation of the ethylenic bond and one adjacent single bond. For pCT-, which contains a stronger acceptor group, we consider a photoisomerization mechanism mainly involving the single torsion of the ethylenic bond. The excited-state deactivation of pCT- would lead to the formation of a ground-state intermediate at the "perp" geometry, which would return to the initial trans conformation without net isomerization. 相似文献
130.
A series of tris(2-aminoethylamine) (tren) capped iron(II) porphyrins has been synthesized and characterized and their affinities for dioxygen and carbon monoxide measured. The X-ray structure of the basic scaffold with nickel inserted in the porphyrin is also reported. All the ligands differ by the nature of the group(s) attached to the secondary amine functions of the cap. These various substitutions were introduced to probe if a hydrogen bond with these secondary amine groups acting as the donor could rationalize the high affinity of these myoglobin models. This work clearly indicates that the cage structure of the tren predominates over all the other appended groups with the exception of p-nitrophenol. 相似文献