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排序方式: 共有231条查询结果,搜索用时 140 毫秒
81.
82.
Prof. Dr. Liudmil Antonov Dr. Vera Deneva Dr. Svilen Simeonov Prof. Dr. Vanya Kurteva Dr. Aurelien Crochet Prof. Dr. Katharina M. Fromm Dr. Boris Shivachev Prof. Dr. Rositsa Nikolova Dr. Marika Savarese Prof. Dr. Carlo Adamo 《Chemphyschem》2015,16(3):649-657
A series of new tautomeric azonaphthols are synthesized and the possibilities for molecular switching are investigated using molecular spectroscopy, X‐ray analysis and density functional theory quantum chemical calculations. Two opposite effects that influence switching are studied: attaching a piperidine sidearm, and adding substituents to the phenyl ring. On the one hand, the attached piperidine moiety stabilizes the enol form leading to a controlled shift of the equilibrium upon protonation. On the other hand, the relative stability of the azonaphthol tautomers strongly depends on the effects of the substituents on the phenyl ring: electron donors tend to stabilize the enol tautomer, whereas electron acceptors lead to stabilization of the keto form. However, these effects do not shift fully the equilibrium towards either of the tautomers. Nevertheless, the effect of the substituents can be an additional tool to affect the switching between “on” and “off” states. Electron‐withdrawing substituents stabilize the keto form and impede switching to the off state, whereas electron donors stabilize the enol form. The effect of the piperidine unit is dominant overall, and with strongly electron‐withdrawing substituents at the phenyl ring, the enol form exists as a zwitterion. 相似文献
83.
Anisimov Yu. A. Nikolova O. N. Anisimov Yu. N. 《Russian Journal of Applied Chemistry》2011,84(5):902-905
The kinetics of low-temperature graft copolymerization of triethylene glycol dimethacrylate with Krasol LBH-3000 oligobutadiene
rubber was studied in relation to the initial mixture composition at 20°C. The experimental kinetic data were compared with
the results of calculation by the suggested equation. The copolymer compositions were calculated from the extraction data
in relation to the initial mixture compositions. The binary copolymerization constants were calculated by the analytical and
graphical methods. 相似文献
84.
Angelova LV Terech P Natali I Dei L Carretti E Weiss RG 《Langmuir : the ACS journal of surfaces and colloids》2011,27(18):11671-11682
A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. (11)B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions. 相似文献
85.
Nikolova EN Gottardo FL Al-Hashimi HM 《Journal of the American Chemical Society》2012,134(8):3667-3670
Nucleic acids transiently morph into alternative conformations that can be difficult to characterize at the atomic level by conventional methods because they exist for too little time and in too little abundance. We recently reported evidence for transient Hoogsteen (HG) base pairs in canonical B-DNA based on NMR carbon relaxation dispersion. While the carbon chemical shifts measured for the transient state were consistent with a syn orientation for the purine base, as expected for A(syn)?T(anti) and G(syn)?C(+)(anti) HG base pairing, HG type hydrogen bonding could only be inferred indirectly. Here, we develop two independent approaches for directly probing transient changes in N-H···N hydrogen bonds and apply them to the characterization of transient Hoogsteen type hydrogen bonds in canonical duplex DNA. The first approach takes advantage of the strong dependence of the imino nitrogen chemical shift on hydrogen bonding and involves measurement of R(1ρ) relaxation dispersion for the hydrogen-bond donor imino nitrogens in G and T residues. In the second approach, we assess the consequence of substituting the hydrogen-bond acceptor nitrogen (N7) with a carbon (C7H7) on both carbon and nitrogen relaxation dispersion data. Together, these data allow us to obtain direct evidence for transient Hoogsteen base pairs that are stabilized by N-H···N type hydrogen bonds in canonical duplex DNA. The methods introduced here greatly expand the utility of NMR in the structural characterization of transient states in nucleic acids. 相似文献
86.
Tsvetanka K. Nedeltcheva Andriana R. Surleva Liliya G. Nikolova Rahila G. Borissova Stela Iv. Georgieva 《Central European Journal of Chemistry》2012,10(6):1875-1881
Spectrophotometric study of competitive complex formation equilibria involving overlapped spectral responding species applying a simple and versatile algorithm was carried out. The algorithm involves multivariable regression for calculation of equilibrium concentrations from multiwavelength data and mass action law for the stability constant calculation. The used regression functions are part of common statistical software. Stability constants and complex stoichiometry of competing equilibria were simultaneously determined. The species concentration profiles at several spectral overlapping and ??-coefficient of competing reaction were obtained. Non-absorbing bismuth ?? pyrophosphate (PPh) system was studied as a competitive reaction of bismuth ?? 4-(2-Pyridylazo) resorcinol (PAR) complex. The formation of Bi-PPh complex with 1:1 stoichiometry was proved in the studied concentration region (CBi = 1×10?5 mol L?1; CPPh = 5×10?6 ? 1×10?4 mol L?1). The stability constant of the complex at pH 1 and ?? = 1.0 have been determined: log?? = 4.2±0.2. 相似文献
87.
B. B. Koleva R. Nikolova S. Zareva T. Kolev R. W. Seidel H. Mayer‐Figge W. S. Sheldrick 《Journal of Physical Organic Chemistry》2009,22(8):726-734
The novel pyridyl‐substituted coumarin ( 1 ) and its perchlorate salt ( 2 ) have been synthesized and their structure and properties elucidated in detail spectroscopically, thermally and structurally, using single crystal X‐ray diffraction for 2 , linear‐polarized solid state IR‐spectroscopy, UV‐spectroscopy, TGA, DSC, DTA, and positive and negative ESI MS. Quantum chemical calculations were used to obtain the electronic structure, vibrational data and electronic spectra. The studied compound crystallizes in the centrosymmetric space group P‐1 and exhibits an infinite layered structure with the ions linked by means of the intermolecular N+H…OClO3 (2.795 Å) interactions. The cations are disposed in a manner leading to a significant π‐stacking effect with a distance of 2.980 Å. The effects of Npy protonation on the optical and magnetic properties are elucidated by comparing the data of the protonated and neutral compounds. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
88.
We consider the approximation of fluctuation induced almost invariant sets arising from stochastic dynamical systems. The dynamical evolution of densities is derived from the stochastic Frobenius-Perron operator. Given a stochastic kernel with a known distribution, approximate almost invariant sets are found by translating the problem into an eigenvalue problem derived from reversible Markov processes. Analytic and computational examples of the methods are used to illustrate the technique, and are shown to reveal the probability transport between almost invariant sets in nonlinear stochastic systems. Both small and large noise cases are considered. 相似文献
89.
Previous studies have shown that T2(dagger)-weighted magnetic resonance images acquired using localization by adiabatic selective refocusing (LASER) can provide early tissue contrast following ischemia, possibly due to alterations in microscopic susceptibility within the tissue. The purpose of this study was to make a direct in vivo comparison of T2-, T2(dagger)- and diffusion-weighted image contrast during acute ischemia. Acute middle cerebral artery (MCA) occlusion was attempted in 14 rats using a modified Tamura approach incorporating electrocoagulation of the left MCA. T2(dagger)-weighted LASER images (Echo Time [TE]=108 ms), T2-weighted Carr-Purcell-Meiboom-Gill (CPMG) images (TE=110 ms) and diffusion-weighted images (b value=105 s/mm(2)) were acquired at 4 T within 1.5 h of ischemia onset. Tissue contrast in the MCA territory was quantified for histologically verified ischemic tissue (n=6) and in sham controls (n=4). T2(dagger)-weighted LASER images demonstrated greater contrast compared to the T2-weighted CPMG images, and more focal contrast compared to the diffusion-weighted images, suggesting different contrast mechanisms were involved. 相似文献
90.