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981.
Sachiko Yoshihashi-Suzuki Izuru Sato Kunio Awazu 《International journal of mass spectrometry》2008,270(3):134-138
Matrix-assisted laser desorption and ionization by infrared laser (IR-MALDI) is expected to be an effective methods for soft-ionization of high-molecular weight proteins and intracellular proteins. IR-MALDI is not widely used because its low sensitivity, complexity, high cost, and as it does not work well on commercial MALDI time-of-flight mass spectrometers (TOFMSs). We employed a tunable mid-infrared (MIR) laser as a light source for MALDI to investigate the IR-MALDI. The laser wavelength can be tuned within a range from 5.5 to 10.0 μm, and included several biomaterial group vibration modes. We evaluated the wavelength dependence of ionization in IR-MALDI for four matrices: succinic acid, urea, 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid) and 2,5-dihydroxybenzoic acid (DHB). These matrices contained various groups of vibration modes, and absorbed an infrared (IR) energy at a specific wavelength. The mass spectra of angiotensin II was obtained at a specific wavelength corresponding to the CO stretching and benzene ring vibration mode. In IR-MALDI, we considered the strong molecular bond attracting an electron from a neighboring hydrogen atom, possibly protonating the hydrogen atom. 相似文献
982.
Mark D. Gould Christopher Taylor Stephen K. Wolff Graham S. Chandler Dylan Jayatilaka 《Theoretical chemistry accounts》2008,119(1-3):275-290
Formulae for hermitian operators representing covalent, ionic, and total bond indices are derived. The eigenstates of these
operators come in pairs, and can be considered as bonding, anti-bonding and lone-pair orbitals. The form of these operators
is derived by generalising the rule that the bond order be defined as the net number of bonding electron pairs. The percentage
of covalency and ionicity of a chemical bond may be obtained, and bond indices can also be defined between groups of atoms.
The calculation of the bond indices depends only on the electron density operator, and certain projection operators used to
represent each atom in the molecule. Bond indices are presented for a series of first and second row hydrides and fluorides,
hydrocarbons, a metal complex, a Diels–Alder reaction and a dissociative reaction. In general the agreement between the bond
indices is in accord with chemical intuition. The bond indices are shown to be stable to basis set expansion. 相似文献
983.
984.
The geometry of Re2Cl82− has been optimized for the eclipsed (D
4h
) equilibrium conformation and for the staggered (D
4d
) conformation at BP86/TZ2P. The nature of the Re–Re bond which has a formal bond order four has been studied with an energy
decomposition analysis (EDA). The EDA investigation indicates that the contribution of the b
2 (δ
xy
) orbitals to the Re–Re bond in the ground state is negligibly small. The vertical excitation of one and two electrons from the bonding δ orbital into the antibonding δ* orbitals yielding the singly and doubly excited states and gives a destabilization of 17.5 and 36.1 kcal/mol, respectively, which is nearly the same as the total excitation energies.
The preference for the D
4h
geometry with eclipsing Re–Cl bonds is explained in terms of hyperconjugation rather than δ bonding. This is supported by the calculation of the triply bonded Re2Cl8 which also has an eclipsed energy minimum structure. The calculations also suggest that the Re–Re triple bond in Re2Cl8 is stronger than the Re–Re quadruple bond in Re2Cl82−. A negligible contribution of the δ orbital to the metal–metal bond strength is also calculated for Os2Cl8 which is isoelectronic with Re2Cl82−.
Contribution of the Mark S. Gordon 65th Birthday Festschrift Issue.
Theoretical Studies of Inorganic Compounds. 38. Part 37 (2006) Bessac F, Frenking G, Inorg Chem 45:6956. 相似文献
985.
986.
Hao-Tao Niu 《Tetrahedron》2008,64(27):6300-6306
Cyclic and acyclic ferrocene derivatives bearing two imidazolium rings have been synthesized and characterized by NMR, elemental analysis, mass spectra, and X-ray crystallography. Electrochemical measurements revealed that all the receptors displayed a significant anodic shift response for F−. In addition, for receptors 1, 2, and 4, addition of HSO4− induced quite different electrochemical behavior with dramatic cathodic peak current increase on CV. 1H NMR titrations demonstrated that receptors 1, 2, and 4 showed selectivity for AcO− while receptor 3 exhibited high affinity toward Cl− among the anions investigated. 相似文献
987.
Fabio Bellina 《Tetrahedron》2008,64(26):6060-6072
Highly selective, practical, and efficient protocols for the preparation of 4(5)-aryl-1H-imidazoles 2, 2,4(5)-diaryl-1H-imidazoles 3, and 4,5-diaryl-1H-imidazoles 1 are described. A key step of these protocols is the regioselective synthesis of 5-aryl-1-benzyl-1H-imidazoles 9 by Pd-catalyzed direct C-5 arylation of commercially available 1-benzyl-1H-imidazole (8) with aryl halides. The three-step synthesis of compounds 3 from 8 also involves the Pd-catalyzed and Cu-mediated direct C-2 arylation of imidazoles 9 with aryl halides under base-free and ligandless conditions. On the other hand, the four-step synthesis of imidazoles 1 from 8 also involves the regioselective bromination of compounds 9 and a Suzuki reaction of the resulting 5-aryl-1-benzyl-4-bromo-1H-imidazoles 11 with arylboronic acids 5 under phase-transfer conditions, followed by N-debenzylation. 相似文献
988.
J.S. Yadav B.V. Subba Reddy Swapan Kumar Biswas Sandip Sengupta P. Vishnumurthy 《Tetrahedron letters》2008,49(6):1034-1036
Phenacyl bromides undergo smooth vic-diallylation and dipropargylation with allyl and propargyl indium reagents generated in situ from metallic indium and allyl or propargyl bromide to produce 4-arylocta-1,7-dien-4-ol derivatives in good yields. Phenacyl chloride and azide also participated effectively in bis-allylation. Similar results are also obtained from in situ generated allyl or propargyl zinc bromide. 相似文献
989.
990.
Xinrui Zhang Xianqiang Pei Bo Mu Qihua Wang 《Surface and interface analysis : SIA》2008,40(5):961-965
Pitch‐based short carbon fibers (CFs) were treated by air oxidation and cryogenic nitrogen, respectively. Thereafter the treated and untreated CFs were incorporated into polyimide (PI) matrix to form composites. The CFs before and after treatment were examined by XPS and SEM.The flexural strength of the specimen was determined in a three‐point test machine and the tribological properties of PI composites sliding against GCr15 steel rings were evaluated on an M‐2000 model ring‐on‐block test rig. The results show that the surface of the treated CFs became rougher. Lots of active groups formed on the CF surface after air oxidation.The treatment can effectively improve the mechanical and tribological properties in their PI composites due to the enhanced fiber‐matrix interfacial bonding. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献