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61.
The ultrafast vibrational phase relaxation of O–H stretch in bulk water is investigated in molecular dynamics simulations. The dephasing time (T2) of the O–H stretch in bulk water calculated from the frequency fluctuation time correlation function (Cω(t)) is in the range of 70–80 femtosecond (fs), which is comparable to the characteristic timescale obtained from the vibrational echo peak shift measurements using infrared photon echo [W.P. de Boeij, M.S. Pshenichnikov, D.A. Wiersma, Ann. Rev. Phys. Chem. 49 (1998) 99]. The ultrafast decay of Cω(t) is found to be responsible for the ultrashort T2 in bulk water. Careful analysis reveals the following two interesting reasons for the ultrafast decay of Cω(t). (A) The large amplitude angular jumps of water molecules (within 30–40 fs time duration) provide a large scale contribution to the mean square vibrational frequency fluctuation and gives rise to the rapid spectral diffusion on 100 fs time scale. (B) The projected force, due to all the atoms of the solvent molecules on the oxygen (FO(t)) and hydrogen (FH(t)) atom of the O–H bond exhibit a large negative cross-correlation (NCC). We further find that this NCC is partly responsible for a weak, non-Arrhenius temperature dependence of the dephasing rate. 相似文献
62.
Two oxovanadium phosphonate cage compounds have been synthesized in an organic solvent, and their characterization has been done by single-crystal X-ray analysis, IR spectroscopy, and bond valence sum calculations. The simple reaction of a mixed-valent closed V12 cage system produced another quasi-closed system composed of two V6 bowl-type cages. 相似文献
63.
Ramanathan R Zhong R Blumenkrantz N Chowdhury SK Alton KB 《Journal of the American Society for Mass Spectrometry》2007,18(10):1891-1899
The widely different LC-MS response observed for many structurally different compounds limits the use of LC-MS in full scan detection mode for quantitative determination of drugs and metabolites without using reference standard. The recently introduced nanospray ionization (NSI) technique shows comparable MS response for some compounds under non-LC-MS conditions. However, in the presence of numerous endogenous compounds commonly associated with biological samples such as urine, plasma, and bile, LC-MS is required to separate, detect, identify, and measure individual analytes. An LC-NSI-MS system was devised and the MS response obtained in this system for a variety of pharmaceutical drugs and their metabolites. The set-up involves two high-performance liquid chromatography (HPLC) systems, a chip-based NSI source and a quadrupole-time-of-flight (Q-TOF) mass spectrometer. Herein this is referred to as the response normalized-liquid chromatography NSI-MS (RNLC-NSI-MS) system. One HPLC unit performs the analytical separation, while the other unit adds solvent post-column with an exact reverse of the mobile phase composition such that the final composition entering the NSI source is isocratic throughout the entire HPLC run. The data obtained from four different structural classes of compounds [vicriviroc (VCV), desloratadine (DL), tolbutamide, and cocaine] and their metabolites indicate that by maintaining the solvent composition unchanged across the HPLC run, the influence of the solvent environment on the ionization efficiency is minimized. In comparison to responses obtained from radiochromatograms, responses from conventional LC-ESI-MS overestimated the VCV and DL responses, respectively, by 6- and 20-fold. Although VCV and DL responses obtained using LC-NSI-MS are within 2- to 6-fold from the respective radiochromatographic responses, the response normalization modification results in nearly uniform LC-NSI-MS response for all compounds evaluated. 相似文献
64.
The hole and electron mobilities of the polymorphs of benzene and naphthalene crystals are estimated through quantum chemical calculations. The reorganization energy (lambda) and the charge-transfer matrix elements (Hmn) calculated for the two molecules reveal that these crystals can be used for dual applications, for both hole and electron conductance. The electron mobilities are five to eight times more than the hole mobilities for benzene while for naphthalene, the hole mobilities are almost an order magnitude more than the electron mobilities. The transfer matrices for both hole and electron conductance decrease monotonically with increase in the intermolecular distances. Calculations for various unique stacked dimers as determined from the radial distribution functions in both the crystals for the two molecules show strong dependence on the orientations of the rings and for similar intermolecular separations; Hmnhole is larger than Hmnelectron. The crystal mobilities are calculated from the weighted average over all the unique pair of molecules. The overall preference in a crystal for hole or electron mobility depends on the mutual competition of lambdahole/lambdaelectron and Hmnhole/Hmnelectron. From our microscopic understanding of essential parameters, specific dimers are identified from the crystalline solids of the two polymorphs and experimental strategies are suggested to enrich such pairs in aggregates for enhancing mobilities for these organic solids. 相似文献
65.
Y. Anusooya Aparna Chakrabarti Swapan K. Pati S. Ramasesha 《International journal of quantum chemistry》1998,70(3):503-513
We have studied magnetism and aromaticity of polycyclic ring systems by analyzing ring currents for different circulations in these molecules. The technique employed for calculating ring currents uses correction vectors which implicitly includes all the eigenstates of the Hamiltonian in the space of the chosen configurations. We have employed the Pariser–Parr–Pople Hamiltonian and have carried out full configuration interaction (CI) calculations for small systems and approximate CI calculations for large systems. The systems studied include polyacenes, nonaromatic ring systems including the C60 fragments pyracylene, fluoranthene, and corannulene, and heteroatomic systems with upto two six-membered rings. We find that in polyacenes, the aromaticity of the extreme phenyl rings reduces with increasing number of phenyl rings in the system, and it saturates at ≈⅔ the benzene value. In systems containing nonaromatic rings, we find paramagnetic or diamagnetic behavior for different circulations depending upon the number of atoms in the chosen ring cycle, in agreement with the 4n+2 rule. In corannulene, the largest fragment of C60 we have studied, the five-membered ring is weakly diamagnetic while the six-membered ring is more diamagnetic, although much less than in isolated benzene. The ring structures with heteroatoms studied are pyridine, pyrimidine, and its isomers, s-triazine, quinoline and its isomer, and quinazoline and its isomers. All these have similar ring currents as in their purely carbon counterparts, although ions of these molecules show interesting behavior. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 503–513, 1998 相似文献
66.
We have studied the variation of hardness, polarizability, and electronic and nuclear repulsion energy components of molecules in different nonequilibrium situations obtained by bond distortion and also by placing external point charges on the symmetry axis of the molecules. The above quantities are calculated through Kohn-Sham version of spin-polarized density functional theory with nonlocal exchange correlation functional. Interesting correlations have been found to exist between different energy components, hardness, and polarizability. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 917–926, 1997 相似文献
67.
Three lanthanide‐based two‐dimensional (2D) coordination polymers (CPs), [Ln(L)(H2O)2]n, {H3L=(HO)2P(O)CH2CO2H; Ln=Dy3+ (CP 1 ), Er3+ (CP 2 )} and [{Gd2(L)2(H2O)3}.H2O]n, (CP 3 ) were hydrothermally synthesized using phosphonoacetic acid as a linker. Structural features revealed that the dinuclear Ln3+ nodes were present in the 2D sheet of CP 1 and CP 2 while in the case of CP 3 , nodes were further connected to each other forming a chain‐type arrangement throughout the network. The magnetic studies show field‐induced slow magnetic relaxation property in CP 1 and CP 2 with Ueff values of 72 K (relaxation time, τ0=3.05×10?7 s) and 38.42 K (relaxation time, τ0=4.60×10?8 s) respectively. Ab‐initio calculations suggest that the g tensor of Kramers doublet of the lanthanide ion (Dy3+ and Er3+) is strongly axial in nature which reflects in the slow magnetic relaxation behavior of both CPs. CP 3 exhibits a significant magnetocaloric effect with ?ΔSm=49.29 J kg?1 K?1, one of the highest value among the reported 2D CPs. Moreover, impedance analysis of all the CPs show high proton conductivity with values of 1.13×10?6 S cm?1, 2.73×10?3 S cm?1 and 2, 6.27×10?6 S cm?1 for CPs 1 – 3 , respectively, at high temperature (>75 °C) and maximum 95 % relative humidity (RH). 相似文献
68.
Possible formation of stable Au atomic wire on the hydrogen terminated Si(0 0 1): 3×1 surface is investigated under the density functional formalism. The hydrogen terminated Si(0 0 1): 3×1 surface is patterned in two different ways by removing selective hydrogen atoms from the surface. The adsorption of Au on such surfaces is studied at different sub-monolayer coverages. At 4/9 monolayer (ML) coverage, zigzag continuous Au chains are found to be stable on the patterned hydrogen terminated Si(0 0 1): 3×1 surface. The reason for the stability of the wire structures at 4/9 ML coverage is explained. It is to be noted that beyond 4/9 ML coverage, the additional Au atoms may introduce clusters on the surface. The continuous atomic gold chains on the substrate may be useful for the fabrication of atomic scale devices. 相似文献
69.
As is well-known to those familiar with the Gillman-Jerison text on the ring C(X) of real-valued continuous functions on a topological space (which we may assume is a Tychonoff space), every prime ideal is contained in a unique maximal ideal. In case X is compact, each maximal ideal is of the form M p for some ${p \in X}$ and consists of all ${f \in C(X)}$ such that f(p) = 0, while the intersection of all minimal prime ideals contained in M p is the set of all continuous functions which vanish on a neighborhood of p. In this paper, we reverse some inclusions and study the union of all of the minimal prime ideals contained in M p ; particularly in the case when this set-theoretic union is all of M p . When this occurs, we call X a UMP-space. By making use of the well-known theorem of Gelfand and Kolmogoroff, we obtain new results without assuming that X is compact. It turns out that all UMP-spaces have the property that each of its nonempty zero-sets has nonempty interior. That is, X is an almost P-space. But this condition is far from sufficient. 相似文献
70.
Hydroxyapatite coating on stainless steel pre-coated with bovine serum albumin at ambient conditions
Nayar S Pramanick AK Sharma BK Mishra RK Bansal SK Prajapati A Sahu KK Das SK Pathak L Sinha A 《Colloids and surfaces. B, Biointerfaces》2006,48(2):183-187
A biomimetic process for coating of nanosized hydroxyapatite on stainless steel, which capitalises the dual nature of the protein bovine serum albumin in both metal binding and a strong affinity for calcium ions, has been developed. The novelty of the process lies in pre-conditioning the metallic surface using the above protein prior to its mineralization with hydroxyapatite at ambient conditions. The microporous morphology of these coatings may provide favourable solubility and resorbability as desired by many orthopaedic and orthodontic applications. 相似文献