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31.
Yu.S. Nekrasov D.V. Zagorevskii V.F. Sizoi F.S. Denisov 《Journal of organometallic chemistry》1975,97(2):253-256
The mass spectra of C5H5(CO)2FeGECl2X (X = Cl, Me, Et, n-Pr or CH2Ph) show very intensive rearrangement ions which are formed due to cyclopentadienyl ring migration from iron to the germanium atom or chlorine (subsituent) migration from germanium to atom. 相似文献
32.
The kinetics of hydrogen exchange in molecular systems with H-bonds has been studied by means of kinetic IR spectroscopy and low-temperature NMR spectroscopy. The experimental values of the rate constants and activation energies for molecules capable of forming H-bonds as both proton donors and proton acceptor are collected and analyzed from the point of view of the influence of H-bond formation ability of the molecules-partners. The evidence available testifies to a molecular mechanism of the H-exchange reactions in inert solvents and in the gas phase via the formation of cyclic bimolecular intermediates. The different mechanisms and the structure of intermediate complex of molecular H-exchange process in inert media are discussed and the possible paths of experimental elucidation of reaction mechanism are offered. 相似文献
33.
The O-H bond dissociation energies (D O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R1O˙+RH→R1OH+R˙ and R i O˙+RH→R i OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions. 相似文献
34.
Denisov AY Hannoush RN Gehring K Damha MJ 《Journal of the American Chemical Society》2003,125(38):11525-11531
We have recently shown that hairpins containing 2',5'-linked RNA loops exhibit superior thermodynamic stability compared to native hairpins comprised of 3',5'-RNA loops [Hannoush, R. N.; Damha, M. J. J. Am. Chem. Soc. 2001, 123, 12368-12374]. A remarkable feature of the 2',5'-r(UUCG) tetraloop is that, unlike the corresponding 3',5'-linked tetraloop, its stability is virtually independent of the hairpin stem composition. Here, we determine the solution structure of unusually stable hairpins of the sequence 5'-G(1)G(2)A(3)C(4)-(U(5)U(6)C(7)G(8))-G(9)(U/T(10))C(11)C(12)-3' containing a 2',5'-linked RNA (UUCG) loop and either an RNA or a DNA stem. The 2',5'-linked RNA loop adopts a new fold that is completely different from that previously observed for the native 3',5'-linked RNA loop. The 2',5'-RNA loop is stabilized by (a). U5.G8 wobble base pairing, with both nucleotide residues in the anti-conformation, (b). extensive base stacking, and (c). sugar-base and sugar-sugar contacts, all of which contribute to the extra stability of this hairpin structure. The U5:G8 base pair stacks on top of the C4:G9 loop-closing base pair and thus appears as a continuation of the stem. The loop uracil U6 base stacks above U5 base, while the cytosine C7 base protrudes out into the solvent and does not participate in any of the stabilizing interactions. The different sugar pucker and intrinsic bonding interactions within the 2',5'-linked ribonucleotides help explain the unusual stability and conformational properties displayed by 2',5'-RNA tetraloops. These findings are relevant for the design of more effective RNA-based aptamers, ribozymes, and antisense agents and identify the 2',5'-RNA loop as a novel structural motif. 相似文献
35.
E. T. Denisov 《Russian Chemical Bulletin》2005,54(4):914-923
Interatomic distances in the reaction centers of the addition reactions of (i) H· to the C=C, C=O, N≡C, and C≡C bonds, (ii) ·CH3 radical to the C=C, C=O, and C≡C bonds, and (iii) alkyl, aminyl, and alkoxyl radicals to olefin C=C bonds were determined
using a new semiempirical method for calculating transition-state geometries of radical reactions. For all reactions of the
type X· + Y=Z → X— Y—Z· the r
#
X...Y distance in the transition state is a linear function of the enthalpy of reaction. Parameters of this dependence were determined
for seventeen classes of radical addition reactions. The bond elongation, Δr
#
X...Y, in the transition state decreases as the triplet repulsion, electronegativity difference between the atoms X and Y in the
reaction center, and the force constant of the attacked multiple bond increase.
__________
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 894–902, April, 2005. 相似文献
36.
Conclusions The parallel formation of hydroperoxide, lactone, and acids in the initial period of the initiated oxidation of cyclohexanone was established. The reaction RO2 + cyclohexanone RO+lactone, RO acid, explaining this fact, was proposed.Translated from Izvestiya Akademii Nauk SSSR, Setiya Khimicheskaya, No. 6, pp. 1108–1110, June, 1964 相似文献
37.
The parabolic model of radical abstraction reactions is used to analyze experimental data on monomolecular hydrogen-atom transfer in the reactionsRC.H(CH2)
n
CH2R1 RCH2(CH2)
n
C.HR1(n= 2, 3, 4)RCH(O.)(CH2)2CH2R1 RCH(OH)(CH2)2C.HR1
RCH(OO.)(CH2)
n
CH2R1 RCH(OOH)(CH2)
n
C.HR1(n= 1, 2).The activation energies and rate constants that specify each class of these reactions are calculated. Alkyl radical isomerization is characterized by the following activation energies of a thermally neutral reaction depending on the cycle size in the transition state (nis the number of atoms in a cycle): E
e
, 0(kJ/mol) = 46.6 (n= 6), 59.4 (n= 5), and 57.1 (n= 7). Alkoxy radicals isomerize with E
e
, 0(kJ/mol) = 53.4 (n= 6), whereas peroxy radicals isomerize with E
e
, 0(kJ/mol) = 53.2 (n= 6) and E
e
, 0(kJ/mol) = 54.8 (n= 7). The E
e
, 0value varies with changes in the cycle size and the strain energy in cycloparaffin C
n
H2n
in the same manner. The activation energies E
e
, 0for the intra- and intermolecular H-atom abstractions are compared. It is found that E
e
, 0(isomerization) < E
e
, 0(R.+ R1H) for alkyl radicals and that E
e
, 0(isomerization) E
e
, 0(RO.(RO.) + R1H) for alkoxy and peroxy radicals. 相似文献
38.
E. F. Panarin V. V. Kopeikin V. M. Denisov A. I. Kol'tsov 《Russian Chemical Bulletin》1976,25(3):532-535
1. | Carbon-chain polymers with -diketone and -keto ester functional groups in the side chains have been synthesized. |
2. | The tautomeric equilibrium of the polymeric -dicarbonyl compounds polymethacryloylacetone and poly(vinyl acetoacetate) and low-molecular models of these polymers has been studied. The effect of the macromolecular nature of the polymeric -diketones on the tautomeric equilibrium is expressed as a significant difference between their thermodynamic equilibrium parameters and those of the low-molecular analog. |
39.
Eremets MI Popov MY Trojan IA Denisov VN Boehler R Hemley RJ 《The Journal of chemical physics》2004,120(22):10618-10623
The high-pressure behavior of nitrogen in NaN(3) was studied to 160 GPa at 120-3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N(3-) are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions. 相似文献
40.
The infrared spectra of phosphinic acid R2POOH dimers (R=CH3, CH2Cl, C6H5) have been studied in CCl4 and CH2Cl2 solutions (T=300 K). The infrared spectra of deuterated R2POOD dimers (R=CH3, CH2Cl) were also studied in the gas phase (T=400–550 K) and solid state (T=100–300 K). They are compared with previously studied spectra of the light (non-deuterated) dimers in the gas phase, in the solid state and in low-temperature argon matrices (T=12–30 K) in the 4000–400 cm−1 spectral region. It is found that the strong and broad ν(OH) dimer bands have similar shapes, nearly equal values of bandwidth and low-frequency shift, and possess the Hadzi ABC structure irrespective of the type of acid, significant differences of dimerization enthalpies, influence of solvent, the type of H-bonded complexes (cyclic dimers in the gas phase, in solutions, and in inert matrices, and infinite chains in the solid state), and temperature in the range 12–600 K. Isotopic ratio of the first moments of light and deuterated acid bands has been measured. Analysis of the ν(OH/OD) band of hydrogen bonded dimers of phosphinic acids shows that the interaction between the two intermolecular bonds O–HOP in a cyclic complex plays virtually no role in the mechanism of the ν(OH/OD) band formation; the shape of ν(OH/OD) band is controlled mainly by the POOH(D)O fragment; and the band shape of strong hydrogen bonded complexes is formed by a number of vibrational transitions from the ground state to different combination levels in the region 3500–1500 cm−1. 相似文献