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81.
A. Jayakrishnan D. O. Shah 《Journal of polymer science. Part A, Polymer chemistry》1983,21(11):3201-3208
The kinetics of phase-transfer-catalyzed free-radical polymerization of methyl methacrylate using the system ammonium peroxydisulfate/hexadecyl pyridinium chloride have been investigated in the temperature range of 50–60°C in ethyl acetate/water two-phase systems. The superior efficiency of the phase-transfer-catalyzed reaction in comparison with polymerization initiated by oil soluble initiators such as 2,2′-azobisisobutyronitrile and benzoylperoxide has been demonstrated. Rate of polymerization increased with increase in concentration of the quaternary salt but showed a tendency to level off at higher concentrations. Increase in the peroxydisulfate concentration also resulted in an increase in the rate at lower concentrations, but was found to decrease at higher concentrations. The order with respect to the monomer was found to be approximately unity. The title reaction has been found to be not truly phase-transfer catalyzed in character as claimed by previous workers. A suitable kinetic scheme has been proposed to account for the experimental data and its significance discussed. 相似文献
82.
W. Clark Still Lawrence J. Macpherson Toshiro Harada James F. Callahan Arnold L. Rheingold 《Tetrahedron》1984,40(12):2275-2281
The conformations of macrocyclic intermediates provide a useful medium through which distant chiral centers may control chemical reactions. In this paper, we show that macrocycles made by cyclization of simple acyclic starting materials with an auxiliary spacer may be used to prepare stereochemically complex acyclic products. 相似文献
83.
Hot water extracts of Ginkgo biloba seeds were analyzed for the presence of ginkgotoxin (4'-O-methylpyridoxine) by reversed-phase liquid chromatography (LC) using methanol-0.05M KH2PO4 (1 + 9, v/v) adjusted to pH 3 as mobile phase. Detection was by fluorescence (excitation 280 nm, emission 370 nm). A straight line calibration curve was obtained for the 10-100 ng injected. After addition of beta-glucosidase (37 degrees C/h), an earlier eluting peak disappeared and the ginkgotoxin peak increased. The identity of the ginkgotoxin was confirmed by LC/MS and LC/MS/MS. LC/MS/MS also confirmed the 5'-glucoside by comparison with the 3-glucoside. This is the first identification of a glucoside of ginkgotoxin in Ginkgo biloba. An unknown compound of MW 267 also observed in the Ginkgo biloba seed extract was shown not to be 3,5'-diacetylginkgotoxin by its different LC retention time. Extraction of ground Ginkgo biloba seeds with boiling water in a Soxhlet for 2 x 2 h yielded a total of 179 microg/g of free ginkgotoxin. The concentration in powder from Ginkgo biloba capsules was several times lower than this (17-64 microg/g) in 3 samples but higher in another (457 microg/g). Canned ginkgo seeds (white nuts) contained no detectable free ginkgotoxin but the glucoside was present. Different extraction times were studied: 0.5 h gave only 52 microg/g free ginkgotoxin in the ginkgo seeds. However, boiling an extract for 4 h showed about 15% loss of ginkgotoxin and its glucoside. 相似文献
84.
Mohammad A. Khadim Lawrence D. Colebrook Laurance D. Hall 《Journal of heterocyclic chemistry》1980,17(4):651-656
Proton spin-lattice relaxation rates (R1 values) have been measured, at 270 MHz, for a series of N-aryl isoindolinones. A normalization procedure has been used to enable comparison of R1 values in different compounds by minimizing the effects on relaxation rates of changes in motional correlation times accompanying changes in substitution patterns. A substantial (4.3-fold) dynamic range of R1 values has been observed, and individual values have been correlated with the molecular environments of the nuclei. There is evidence for an interring relaxation process. 相似文献
85.
Gas-phase activation energies were calculated for three lithium enolate reactions by using several different ab initio and density functional theory (DFT) methods to determine which levels of theory generate acceptable results. The reactions included an aldol-type addition of an enolate to an aldehyde, a proton transfer from an alcohol to a lithium enolate, and an S(N)2 reaction of an enolate with chloromethane. For each reaction, the calculations were performed for both the monomeric and dimeric forms of the lithium enolate. It was found that transition state geometry optimization with B3LYP followed by single point MP2 calculations generally provided acceptable results compared to higher level ab initio methods. 相似文献
86.
A series of gold(III) cations of the type cis-[CH3)2AuL2]+ X? where L Ph3, PMePh2, PMe2Ph, PMe3, AsPh3, AsPh3, SbPh3, H2NCH2CH2NH2, Ph2PCH2CH2-PPh2, Ph2AsCH2CH2AsPh2, and o-C6H4(AsMe2)2 and X BF4?, PF6?, ClO4?, and F3CSO3? has been prepared. In addition, the cis complexes [(CH3)(CD3)-Au(PPh3)2]F3CSO3, [(C2H5)2Au(PPh3)2]F3CSO and [(n-C4H9)2Au(PPh3)2]F3-CSO3 have been synthesized. All have been characterized by PMR, Raman and infrared spectroscopy. These [R2AuL2]X compounds yield only ethane, butane, or octane via reductive elimination, and no disproportionation is observed. The alkane eliminations have been studied in CHCl3, CH3Cl2, and CH3COCH3 solution as a function of temperature, concentration of the complex, and concentration of added ligand L. Elimination is fastest when L is bulky (PPh3 > PMePh2 > PMe2Ph > PMe3), decreases in the sequence SbPh3 > AsPh3 > PPh3, is slow with chelating ligands, is inhibited by excess ligand, and there is small anion effect as X is varied. As R is varied, the rate of elimination decreases Bu ? Et > Me. An intramolecular dissociative mechanism is proposed which involves rapid elimination of alkane from an electron deficient dialkylgold(III) complex with nonequivalent gold—carbon bonds and produces the corresponding [AuL2]X complex. 相似文献
87.
Abstract— The lowest-lying triplet states of a variety of aromatic molecules and complexes have been generated by the irradiation of these compounds in solvent glasses with plane-polarized light. Measurements of the allowed (Δ M=± 1) ESR transitions clearly demonstrate that the triplets so formed are oriented with respect to the external magnetic field. By this method the triplet zero-field splitting parameters, D and E , can be evaluated simply and reliably. Intramolecular energy transfer is postulated to explain the triplet spectra of Zn( o -phen)2 (NO3 )2 and Zn( o -phen)3 (NO3 )2 . It was observed that in triplet-triplet energy transfer from benzophenone to naphthalene there is no apparent orientation requirement between the donor and acceptor molecules. Further areas of significance and application of this technique are suggested. 相似文献
88.
Abdel-Magid AF Carson KG Harris BD Maryanoff CA Shah RD 《The Journal of organic chemistry》1996,61(11):3849-3862
Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4). 相似文献
89.
L. G. Chalmet R. L. Francis J. F. Lawrence 《Journal of Optimization Theory and Applications》1980,32(2):135-149
An example of design might be a warehouse floor (represented by a setS) of areaA, with unspecified shape. Givenm warehouse users, we suppose that useri has a known disutility functionf
isuch thatH
i(S), the integral off
iover the setS (for example, total travel distance), defines the disutility of the designS to useri. For the vectorH(S) with entriesH
i(S), we study the vector minimization problem over the set {H(S) :S a design} and call a design efficient if and only if it solves this problem. Assuming a mild regularity condition, we give necessary and sufficient conditions for a design to be efficient, as well as verifiable conditions for the regularity condition to hold. For the case wheref
iis thel
p-distance from warehouse docki, with 1<p<, a design is efficient if and only if it is essentially the same as a contour set of some Steiner-Weber functionf
=1
f
1++
m
f
m
,when the
i
are nonnegative constants, not all zero.This research was supported in part by the Interuniversity College for PhD Studies in Management Sciences (CIM), Brussels, Belgium; by the Army Research Office, Triangle Park, North Carolina; by a National Academy of Sciences-National Research Council Postdoctorate Associateship; and by the Operations Research Division, National Bureau of Standards, Washington, D.C. The authors would like to thank R. E. Wendell for calling Ref. 16 to their attention. 相似文献
90.
Lawrence E. Thomas 《Communications in Mathematical Physics》1980,77(3):211-218
Let 1 and 2 be thermodynamic Gibbs measures on
m
and
n
, respectively. Diffusions are constructed having 1, and 2 as invariant measures. These diffusions are then coupled; inequalities between expectations of certain random variables on the two spaces result.Partially supported by NSF-MCS 74-07313-A03 相似文献