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941.
Adam H. Steeves Anthony J. Merer Nami Yamakita Robert W. Field 《Journal of Molecular Spectroscopy》2009,256(2):256-11943
Rotational analyses are reported for a number of newly-discovered vibrational levels of the S1-trans (Ã1Au) state of C2H2. These levels are combinations where the Franck-Condon active and vibrational modes are excited together with the low-lying bending vibrations, and . The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling-Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of (trans bend, ag) and (in-plane cis bend, bu). This anharmonicity presumably represents the approach of the molecule to the trans-cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of and . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling-Dennison resonance starts to break down. The effective Darling-Dennison parameter, K4466, is found to increase rapidly with excitation of , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2131B1 and 31B3 polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S1-trans state in order to search for possible S1-cis (1A2) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm−1. 相似文献
942.
Solving the frequency equation and plotting the dispersion curves in problems of wave propagation in cylinders and plates, particularly when the material is anisotropic, are complicated tasks. The traditional numerical methods are usually based on determination of the zeros of the frequency equation by using an iterative find-root algorithm. In this paper, an alternative method is proposed which extracts the solution of the frequency equation in the form of dispersion curves from the three-dimensional illustration of the frequency equation. For this purpose, a three-dimensional representation of the real roots of the frequency equation is first plotted. The dispersion curves, which are the numerical solutions of the frequency equation, are then obtained by a suitable cut in the velocity-frequency plane. The advantages of this method include simplicity, high speed, low possibility of numerical error, and presentation of the results in a graphical form that promotes ease of interpretation. This method is not directly applicable to problems which incorporate high damping or leaky waves. However, if the damping is not very high, it could be a good estimate of the true dispersion curves. 相似文献
943.
Grzegorz Mlosto Agnieszka Majchrzak Marzena Rutkowska Marta Woznicka Anthony Linden Heinz Heimgartner 《Helvetica chimica acta》2005,88(10):2624-2636
The 3,3‐dichloro‐2,2,4,4‐tetramethylcyclobutanethione ( 4b ) was prepared from the parent diketone by successive reaction with PCl5 and Lawesson reagent in pyridine. This new thioketone 4b was transformed into 1‐chlorocyclobutanesulfanyl chloride 5 and chloro 1‐chlorocyclobutyl disulfide 9 by treatment with PCl5 and SCl2, respectively, in chlorinated solvents (Schemes 1 and 2). These products reacted with S‐ and P‐nucleophiles by substitution of Cl− at the S‐atom; e.g., the reaction with 4b yielded the di‐ and trisulfides 6b and 11 , respectively. Surprisingly, only pentasulfide 12 was formed in the reaction of 9 with thiobenzophenone (Scheme 3). In contrast to 5 and 9 , the corresponding chloro 1‐chlorocyclobutyl trisulfide 13 could not be detected, but reacted immediately with the starting thioketone 4b to give the tetrasulfide 14 (Scheme 4). Oxidation of 4b with 3‐chloroperbenzoic acid (mCPBA) yielded the corresponding thione oxides (= sulfine) 15 , which underwent 1,3‐dipolar cycloadditions with thioketones 3a and 4b (Scheme 5). Furthermore, 4b was shown to be a good dipolarophile in reactions with thiocarbonylium methanides (Scheme 6) and iminium ylides (= azomethine ylides; Scheme 7). In the case of phenyl azide, the reaction with 4b gave the symmetrical trithiolane 25 (Scheme 8). 相似文献
944.
Anthony J. Guttmann Jesper L. Jacobsen Iwan Jensen Sanjay Kumar 《Journal of mathematical chemistry》2009,45(1):223-237
We study the conformations of polymer chains in a poor solvent, with and without bending rigidity, by means of a simple statistical
mechanics model. This model can be exactly solved for chains of length up to N = 55 using exact enumeration techniques. We analyze in detail the differences between the constant force and constant distance
ensembles for large but finite N. At low temperatures, and in the constant force ensemble, the force–extension curve shows multiple plateaus (intermediate
states), in contrast with the abrupt transition to an extended state prevailing in the N → ∞ limit. In the constant distance ensemble, the same curve provides a unified response to pulling and compressing forces,
and agrees qualitatively with recent experimental results. We identify a cross-over length, proportional to N, below which the critical force of unfolding decreases with temperature, while above, it increases with temperature. Finally,
the force–extension curve for stiff chains exhibits “saw-tooth” like behavior, as observed in protein unfolding experiments. 相似文献
945.
Isci A Himmelsbach JN Strohl J Pometto AL Raman DR Anex RP 《Applied biochemistry and biotechnology》2009,157(3):453-462
Aqueous-ammonia-steeped switchgrass was subject to simultaneous saccharification and fermentation (SSF) in two pilot-scale
bioreactors (50- and 350-L working volume). Switchgrass was pretreated by soaking in ammonium hydroxide (30%) with solid to
liquid ratio of 5 L ammonium hydroxide per kilogram dry switchgrass for 5 days in 75-L steeping vessels without agitation
at ambient temperatures (15 to 33 °C). SSF of the pretreated biomass was carried out using Saccharomyces cerevisiae (D5A) at approximately 2% glucan and 77 filter paper units per gram cellulose enzyme loading (Spezyme CP). The 50-L fermentation
was carried out aseptically, whereas the 350-L fermentation was semiaseptic. The percentage of maximum theoretical ethanol
yields achieved was 73% in the 50-L reactor and 52–74% in the 350-L reactor due to the difference in asepsis. The 350-L fermentation
was contaminated by acid-producing bacteria (lactic and acetic acid concentrations approaching 10 g/L), and this resulted
in lower ethanol production. Despite this problem, the pilot-scale SSF of aqueous-ammonia-pretreated switchgrass has shown
promising results similar to laboratory-scale experiments. This work demonstrates challenges in pilot-scale fermentations
with material handling, aseptic conditions, and bacterial contamination for cellulosic fermentations to biofuels. 相似文献
946.
Shigekazu Ito Dr. Joji Miura Noboru Morita Masaaki Yoshifuji Anthony J. Arduengo III 《无机化学与普通化学杂志》2009,635(3):488-495
Two and three stable 1‐sec‐butyl‐2,4‐bis(2,4,6‐tri‐tert‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl units were catenated to construct multi‐biradical derivatives by utilizing 1,3‐di‐, 1,4‐di‐, and 1,3,5‐trimethylenebenzenes as bridging groups, respectively. UV/Vis spectroscopic and cyclovoltammetric (CV) properties of the multi‐biradicals indicate a non‐conjugative interaction between the concatenated biradical units. 相似文献
947.
Mohammed K. Abdel-Hamid Jonathan Coates Celia Miländer Brian W. Skelton Anthony C. Willis 《Tetrahedron letters》2009,50(50):6947-8550
The allylation of indigo results in the one-step synthesis of two unique complex heterocyclic systems: a spiroindoline-pyridoindolone arising from the addition of three allyl moieties and a fused pyridoindolo-azepinoindolone generated from the addition and subsequent cyclisation of two allyl moieties. The structures of these novel heterocycles are assigned unambiguously using extensive NMR experiments and by X-ray crystallographic analysis. The distribution of the products is influenced by the use of thermal versus microwave heating. 相似文献
948.
Thanh V. Nguyen David J. Sinclair Dr. Anthony C. Willis Dr. Michael S. Sherburn Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(24):5892-5895
Guest‐love‐ometer : The equilibrium inside–outside atropisomeric ratio of ortho‐substituted phenyl cavitands shows a strong solvent dependence. The competition between the ortho‐substituent and the solvent guest for the bowl cavity leads to a sensitive system for the measurement of relative guest binding affinities.
949.
Anthony Midey Itzhak Dotan J.V. Seeley A.A. Viggiano 《International journal of mass spectrometry》2009,280(1-3):6
The rate constants and product ion branching ratios were measured for the reactions of various small negative ions with O2(X 3Σg−) and O2(a 1Δg) in a selected ion flow tube (SIFT). Only NH2− and CH3O− were found to react with O2(X) and both reactions were slow. CH3O− reacted by hydride transfer, both with and without electron detachment. NH2− formed both OH−, as observed previously, and O2−, the latter via endothermic charge transfer. A temperature study revealed a negative temperature dependence for the former channel and Arrhenius behavior for the endothermic channel, resulting in an overall rate constant with a minimum at 500 K. SF6−, SF4−, SO3− and CO3− were found to react with O2(a 1Δg) with rate constants less than 10−11 cm3 s−1. NH2− reacted rapidly with O2(a 1Δg) by charge transfer. The reactions of HO2− and SO2− proceeded moderately with competition between Penning detachment and charge transfer. SO2− produced a SO4− cluster product in 2% of reactions and HO2− produced O3− in 13% of the reactions. CH3O− proceeded essentially at the collision rate by hydride transfer, again both with and without electron detachment. These results show that charge transfer to O2(a 1Δg) occurs readily if the there are no restrictions on the ion beyond the reaction thermodynamics. The SO2− and HO2− reactions with O2(a) are the only known reactions involving Penning detachment besides the reaction with O2− studied previously [R.S. Berry, Phys. Chem. Chem. Phys., 7 (2005) 289–290]. 相似文献
950.
K. C. Nicolaou Prof. Dr. Jason S. Chen David J. Edmonds Dr. Anthony A. Estrada 《Angewandte Chemie (International ed. in English)》2009,48(4):660-719
Ever since the world‐shaping discovery of penicillin, nature's molecular diversity has been extensively screened for new medications and lead compounds in drug discovery. The search for agents intended to combat infectious diseases has been of particular interest and has enjoyed a high degree of success. Indeed, the history of antibiotics is marked with impressive discoveries and drug‐development stories, the overwhelming majority of which have their origin in natural products. Chemistry, and in particular chemical synthesis, has played a major role in bringing naturally occurring antibiotics and their derivatives to the clinic, and no doubt these disciplines will continue to be key enabling technologies. In this review article, we highlight a number of recent discoveries and advances in the chemistry, biology, and medicine of naturally occurring antibiotics, with particular emphasis on total synthesis, analogue design, and biological evaluation of molecules with novel mechanisms of action. 相似文献