全文获取类型
收费全文 | 897篇 |
免费 | 8篇 |
专业分类
化学 | 598篇 |
晶体学 | 4篇 |
力学 | 4篇 |
数学 | 66篇 |
物理学 | 233篇 |
出版年
2016年 | 9篇 |
2015年 | 10篇 |
2014年 | 10篇 |
2013年 | 24篇 |
2012年 | 13篇 |
2011年 | 33篇 |
2010年 | 17篇 |
2009年 | 17篇 |
2008年 | 27篇 |
2007年 | 19篇 |
2006年 | 21篇 |
2005年 | 21篇 |
2004年 | 10篇 |
2003年 | 15篇 |
2002年 | 16篇 |
2001年 | 23篇 |
2000年 | 17篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 16篇 |
1996年 | 25篇 |
1995年 | 28篇 |
1994年 | 22篇 |
1993年 | 27篇 |
1992年 | 31篇 |
1991年 | 23篇 |
1990年 | 20篇 |
1989年 | 20篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 21篇 |
1984年 | 14篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 19篇 |
1979年 | 12篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 14篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1973年 | 14篇 |
1972年 | 11篇 |
1971年 | 14篇 |
1970年 | 10篇 |
1969年 | 14篇 |
1968年 | 15篇 |
1967年 | 14篇 |
1966年 | 9篇 |
排序方式: 共有905条查询结果,搜索用时 0 毫秒
31.
32.
Perphenylcyclopolysilanes [Si(C6H5)2]n (n = 4, 5) are reduced by potassium to radical anions. Their simple ESR spectra demonstrate, that the extra electron is confined to the inner SinC2n skeleton of the uncleaved and presumably planarized cyclopolysilanes. 相似文献
33.
Transport properties of pure carbon dioxide have been calculated from the intermolecular potential using the classical trajectory method. Results are reported in the dilute-gas limit for volume viscosity, depolarized Rayleigh scattering, and nuclear spin relaxation for temperatures ranging from 200 to 1000 K. Three recent carbon dioxide potential energy hypersurfaces have been investigated. Calculated values for the rotational collision number for all three intermolecular surfaces are consistent with the measurements and indicate that the temperature dependence of the Brau-Jonkman correlation is not applicable for carbon dioxide. The results for the depolarized Rayleigh scattering cross section and the nuclear spin relaxation cross section show that calculated values for the generally more successful potentials differ from the observations by 9% at about 290 K, although agreement is obtained for nuclear spin relaxation at about 400 K. 相似文献
34.
Calculations on performic acid at the 4-31G level, with and without bond functions with complete geometry optimization, and at the (9, 5) level, with and without polarization functions and rigid rotation, all give no sign of a well in the potential energy curve for rotation about the O/O bond axis in the region of 50° – 90° ; and all but the unaugmented 4-31G basis set find the cis-cis planar conformer to be the most stable form. Calculations at the (9,5) level with rigid rotation find the energies of the other planar conformers, relative to the cis-cis conformer, to be 0.94, 1.50 and 14.80 kcal mol?1 for the trans-trans, cis-trans and trans-cis structures respectively. These energies and also that for the barrier separating the cis-cis and cis-trans conformers, 1–2 kcal mol?1, are discussed in relation to corresponding data for formic acid, hydrogen peroxide and several planar four heavy-atom molecules. Dipole moment calculations using the same basis sets would seem to favor a skew conformation as the most stable for performic acid, but comparisons between calculated and experimental values for formic acid and for hydrogen peroxide cast doubt on the validity of such results. 相似文献
35.
Single crystal structure determinations prove the two pyridine substituents in di(2-pyridyl)ketone (H4C4NC)2C=O to be twisted out of the carbonyl skeleton plane by torsion angles (OCCN) of 41° and –163°, in contrast to their planar arrangement in azo-di(2-pyridine) H4C4NC)-N=N-(CNC4H4). In order to rationalize the surprising difference between the two isoelectronic molecules, approximate PM3 enthalpy of formation hypersurfaces have been calculated for each of the two ring torsions, which are assumed to be the dominant ones among the 3N – 6 = 60 degrees of freedom. For both the ketone and the azo derivative, global minima are calculated, the torsion angles of which deviate from the crystal structure results, and, therefore, support the assumption that both the experimentally determined twisting of di(2-pyridyl)ketone as well as the flattening of azo-di(2-pyridine) might be affected by the crystal packing. 相似文献
36.
37.
Philip George Charles W. Bock Mendel Trachtman 《Journal of computational chemistry》1982,3(3):283-296
Ab initio calculations using the unscaled 4-31G basis set have been carried out on the cc, tc, and tt conformers of carbonic acid and the bicarbonate ion, with full geometry optimization assuming the structures to be planar. The complete harmonic force field is reported for the (most stable) tt conformer and for the bicarbonate ion, also selected quadratic force constants for the cc and tc conformers. The changes in certain bond lengths and stretching force constants in the cc → tc, tc → tt, and cc → tt conformer conversion reactions are indicative of intramolecular hydrogen bonding, C?O…H? O and H? O…H? O, which is examined in greater detail by partitioning the overall conformer conversion energy into distortion and bonding energy components. The fundamental vibration frequencies for the tt conformer and the bicarbonate ion are calculated from the force constant matrices, and hence, using a scaling factor based on a comparison of calculated and experimental values for the bicarbonate ion and trans-formic acid, a value is predicted for the zero-point energy of the tt conformer. A new estimate of ΔH? for the hydration reaction, H2O + CO2 → H2CO3, at 298 K in the gas phase; is made from thermochemical data, +20.2 ± 3.4 kJ mol?1, which, together with estimates of (H298? – H0?) and the zero-point energy for H2CO3, gives +8.1 ± 7.0 kJ mol?1 for ΔET(expt). ΔET calculated from the 4-31G basis set data is -29.1 kJ mol?1. Comparison of the experimental value, the Hartree–Fock limit value, and values calculated with a variety of basis sets for the bond separation reaction, CO2 + CH4 → 2H2CO, suggests that the differences, ΔET(expt) minus ΔET(SCF ), are due mainly to basis set limitations and not substantial correlation energy contributions. 相似文献
38.
39.
The geometry and energy of 1,3-butadiene have been calculated using the 6-311G** basis set as a function of the CCCC dihedral angle-0 ° (trans), 30 °, 60 °, 75 °, 90 °, 120 °, 135 °, 150 °, 165 ° and 180 ° (cis)-assuming that the vinyl groups remain planar. Potential minima are located at 0 ° and 141.4 °, with the trans structure more stable than the gauche by 13.2 kJ mol–1. Potential maxima are located at 76.7 °, giving a barrier height of 25.4 kJ mol–1 relative to the trans structure, and at 180 ° giving a barrier height of 3.0 kJ mol–1 relative to the 141.4 °-gauche structure. Using the 6-31G* basis set the inclusion of electron correlation, accounting for about 52% of the correlation energy, was found to produce no significant change in the shape of the potential energy curve. The magnitude of the expectation energy differences is such that both barriers with respect to the 14l.4 °-gauche maximum structure can be categorized unequivocally as attractive-dominant, whereas the values for the energy barrier with respect to the trans structure, although characteristic of a repulsive-dominant barrier at the 6–311G** level, are sufficiently small that higher level calculations might give the opposite result. Analysis of V
nn
for the conversion reactions cis 150 °-gauche, trans 60 °-gauche, and trans 90 °-gauche in terms of the individual contributions from the various internuclear interactions shows that nonbonded interactions are important, not only in initiating the destabilization of the crowded cis structure, but also through-out the entire range of CCCC dihedral angles, 0 ° to 180 °. 相似文献
40.
The ESR. spectra of the radical anions of trimethylsilyl-substituted derivatives of p-benzoquinone, phenylketone, diimine, p-benzoquinone-diimide, aniline and p-phenylenediamine are reported. In most cases the coupling constants of 29Si isotopes in natural abundance have been measured. The comparison of the ESR. data for the radical anions of the trimethylsilyl derivatives with those for the radical anions of the corresponding parent hydrocarbons and/or alkyl-substituted compounds provide additional evidence for the overall electron withdrawing effect of the trimethylsilyl substituent (Si←Cπ and Si←nπ delocalization). 相似文献