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991.
Møllendal H Demaison J Petitprez D Wlodarczak G Guillemin JC 《The journal of physical chemistry. A》2005,109(1):115-121
1,2-Propadienylphosphine (allenylphosphine), H(2)C=C=CHPH(2), has been investigated by Stark and Fourier transform microwave spectroscopy. Two rotameric forms denoted syn and gauche have been assigned. The syn form has a symmetry plane (C(s)() symmetry) where the lone electron pair of phosphorus points toward the double bonds. The phosphino group is rotated roughly 120 degrees from this position in the gauche rotamer. The dipole moment of syn was determined to be mu(a) = 1.613(23), mu(b) = 2.347(24), mu(c) = 0 (for symmetry reasons), and mu(tot) = 2.848(28) x 10(-30) C m [0.854(8) D]. The energy difference between the two forms was found to be 2.1(4) kJ/mol from relative intensity measurements with syn as the more stable conformer. Extensive quantum chemical calculations have been carried out and accurate equilibrium structures have been determined for these two rotamers, as well as for the corresponding two conformers of vinylphosphine (H(2)C=CHPH(2)). 相似文献
992.
We have studied the potential-energy curves and the spectroscopic constants of the ground and low-lying excited states of NbC by employing the complete active space self-consistent field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified 23 low-lying electronic states of NbC with different spin multiplicities and spatial symmetries within 40,000 cm(-1). At the multireference single and double configuration interaction level of theory the 2sigma+ and 2delta states are nearly degenerated, with the 2delta state located 187 cm(-1) lower than the 2sigma+ state. The estimated spin-orbit splitting for the 2delta state results in a 2delta(3/2) ground state and A 2sigma+ which is placed 650 cm(-1) above the ground state, in reasonable agreement with the experimental result, 831 cm(-1). Our computed spectroscopic constants are in good agreement with experimental values although our results differ from those of a previous density-functional investigation of the excited states of NbC, mainly due to the strong multiconfigurational character of NbC. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally. 相似文献
993.
Zumailan A. Denis G. Dargent E. Saiter J. M. Grenet J. 《Journal of Thermal Analysis and Calorimetry》2002,68(1):5-13
PET films uniaxially drawn in hot water are studied by means of conventional DSC and modulated DSC (MDSC).Glass transition
is studied by MDSC which allows to access the glass transition temperature T
g and the variations of ΔC
p=C
p1−C
pg (difference between thermal capacity in the liquid-like and glassy states at T=T
g). Variations of T
g with the water content (which act as plasticizer) and with the drawing (which rigidifies the amorphous phase) are discussed
with regard to the structure engaged in these materials. The increments of ΔC
p at T
g are also interpreted using a three phases model and the 'strong-fragile’ glass former liquid concept. We show that the ‘fragility’
of the medium increases due to the conjugated effects of deformation and water sorption as soon as a strain induced crystalline
phase is obtained. Then, ‘fragility’ decreases drastically with the occurring rigid amorphous phase.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
994.
The reaction of K[ReH6(PPh3)2] with [RhCl(CO)L2] [L= PPh3, 1,2,5-triphenylphosphole (TPP), or P(OMe)3] leads to the new electronically unsaturated heterobimetallic polyhydride complexes [(CO)(PPh3)2HRe(μ-H)3RhL2] in moderate-to-good yields. The structures of these complexes have been established on the basis of spectroscopic data, especially 1H and 31P NMR. The bridging hydride ligands are fluxional but there is either a slow or nonexistent exchange between terminal and bridging hydrides. For L = PPh3 or TPP, protonation with tetrafluoroboric acid affords quantitatively the cationic complexes [(CO)(PPh3)2HRe(μ-H)3RhHL2]+, isolated as the BF4− or the BPh4− salts. 相似文献
995.
Oger JM Morel C Helesbeux JJ Litaudon M Séraphin D Dartiguelongue C Larcher G Richomme P Duval O 《Natural product research》2003,17(3):195-199
Two new 2-hydroxy-3-methylbut-3-enyl substituted xanthones, (+/-)-caledol 1 and (+/-)-dicaledol 2 were isolated from a dichloromethane extract of the leaves of Calophyllum caledonicum (Clusiaceae). Compounds 1 and 2 are the first 2-hydroxy-3-methylbut-3-enyl substituted xanthones isolated from natural source. Their structures were elucidated by means of combined analytical methods including HRFABMS, 1D and 2D NMR spectroscopies and also confirmed by total synthesis using biomimetic ortho-prenylphenols photooxygenation (1O2) as a key step. The antifungal activity against Aspergillus fumigatus is reported. 相似文献
996.
It is known that for every finite subgroup G of SL(2,), the invariant subring [X,Y]G is a hyper-surface. In this note we treat finite subgroups of SL(3,) and give complete classification of the finite subgroups of SL(3,) whose invariant subrings are complete intersections. 相似文献
997.
Li2CoTi3O8 has an ordered Li2BB′3O8 spinel structure, space group P4332, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order-disorder phase transition in Li2CoTi3O8 which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li,B cations. 相似文献
998.
1,3-Anti amino alcohols 5(a)-18(a) are obtained with high diastereoselectivity by use of diorganozinc reagents in additions to amino aldehydes 2a and 2b. The corresponding Grignard reagents exhibit low to modest diastereoselectivity. The highly diastereoselective zinc-based method makes available a wide range of 4,4-disubstituted cyclohexenone derivatives containing contiguous stereocenters. 相似文献
999.
Fagerquist CK Hellerstein MK Faubert D Bertrand MJ 《Journal of the American Society for Mass Spectrometry》2001,12(6):754-761
An important problem in mass isotopomer abundance mass spectrometry (MIAMS) is the dependence of measured mass isotopomer abundances on sample concentration. We have evaluated the role of ionization energy on mass isotopomer abundance ratios of methyl palmitate as a function of sample concentration. Ionization energy was varied using electron impact ionization (EI) and metastable atom bombardment (MAB). The latter generates a beam of metastable species capable of ionizing analyte molecules by Penning ionization. We observed that ionization of methyl palmitate by EI (70 eV) showed the greatest molecular ion fragmentation and also showed the greatest dependence of relative isotopomer abundance ratios on sample concentration. Ionization using the 3P2 and 3P0 states of metastable krypton (9.92 and 10.56 eV, respectively) resulted in almost no molecular ion fragmentation, and the isotopomer abundances quantified were essentially independent of sample concentration. Ionization using the 3P2 and 3P0 states of metastable argon (11.55 and 11.72 eV, respectively) showed molecular ion fragmentation intermediate between that of EI and MAB(Kr) and showed an isotopomer concentration dependence which was less severe than that observed with EI but more severe than that observed with MAB(Kr). The observed decrease in the dependence of isotopomer abundance on sample concentration with a decrease in molecular ion fragmentation is consistent with the hypothesis that proton transfer from a fragment cation to a neutral molecule is the gas phase reaction mechanism responsible for the concentration dependence. Alternative explanations, e.g., hydrogen abstraction from a neutral molecule to a molecular cation, is not supported by these results. Moreover, the MAB ionization technique shows potential for eliminating one source of error in MIAMS measurements of methyl palmitate, in particular, and of fatty acids methyl esters, in general. 相似文献
1000.
The effects of protonation on the charge distributions of pyridine and cytosine are investigated with the help of the Iterative Extended Hückel technique. The polarization of the -electrons is then included explicitly in a Pariser-Parr-Pople computation of the -
* transition energies of the pyridinium and cytosinium ions.
This work was supported by grant n GM-12229 of the U.S. Public Health Service (National Institute of General Medical Sciences). 相似文献
Zusammenfassung Mit Hilfe der IEH-Methode wird die Auswirkung der Protonierung auf die Ladungsverteilung bei Pyridin und Cytosin untersucht. Die Polarisierung der -Elektronen wird dann in eine PPP-Rechnung der - *-übergangsenergie beim Pyridinium- und Cytosiniumion übernommen.
Résumé On étudie, à l'aide de la méthode de Hückel étendue iterative, les effets de la protonation sur la répartition des électrons dans les molécules de pyridine et de cytosine. On introduit ensuite explicitement la polarisation des électrons dans le calcul des énergies de transition - * des ions pyridinium et cytosinium par la méthode de Pariser, Parr et Pople.
This work was supported by grant n GM-12229 of the U.S. Public Health Service (National Institute of General Medical Sciences). 相似文献