全文获取类型
收费全文 | 109篇 |
免费 | 2篇 |
国内免费 | 1篇 |
专业分类
化学 | 95篇 |
力学 | 5篇 |
数学 | 4篇 |
物理学 | 8篇 |
出版年
2018年 | 1篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 3篇 |
2013年 | 5篇 |
2012年 | 6篇 |
2011年 | 9篇 |
2010年 | 2篇 |
2009年 | 2篇 |
2008年 | 3篇 |
2007年 | 6篇 |
2006年 | 11篇 |
2005年 | 8篇 |
2004年 | 4篇 |
2003年 | 8篇 |
2002年 | 6篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1995年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有112条查询结果,搜索用时 46 毫秒
1.
2.
The molecular structures of phenylphosphine and its analogs, aniline and phenylarsine, were studied by gas-phase electron diffraction and quantum-chemical methods. The geometry and force constants were calculated at the HF/6-31G and B3LYP/6-31G* levels of theory. Phenylphosphine and phenylarsine possess a bisector conformation with asymmetric phenyl rings. The main geometric parameters are as follows (r
a): P-C 1.833(6), C-Cav 1.397(1)Å. The structures of molecules like X2YPh (X = H, F, Cl; Y = N, P, As) were discussed. 相似文献
3.
Svein G. Dahl Peter A. Kollman Shashidhar N. Rao U. Chandra Singh 《Journal of computer-aided molecular design》1992,6(3):207-222
Summary The side-chain conformations of psychoactive phenothiazine drugs in crystals are different from those of biologically inactive ring sulfoxide metabolites. This study examines the potential energies, molecular conformations and electrostatic potentials in chlorpromazine, levomepromazine (methotrimeprazine), their sulfoxide metabolites and methoxypromazine. The purpose of the study was to examine the significance of the different crystal conformations of active and inactive phenothiazine derivatives, and to determine why phenothiazine drugs lose most of their biological activity by sulfoxidation. Quantum mechanics and molecular mechanics calculations demonstrated that conformations with the side chain folded over the ring structure had lowest potential energy in vacuo, both in the drugs and in the sulfoxide metabolites. In the sulfoxides, side chain conformations corresponding to the crystal structure of chlorpromazine sulfoxide were characterized by stronger negative electrostatic potentials around the ring system than in the parent drugs. This may weaken the electrostatic interaction of sulfoxide metabolites with negatively charged domains in dopamine receptors, and cause the sulfoxides to be virtually inactive in dopamine receptor binding and related pharmacological tests. 相似文献
4.
Stølen S Mohn CE Ravindran P Allan NL 《The journal of physical chemistry. B》2005,109(25):12362-12365
We discuss the importance of the topography of the potential energy hypersurface for the ionic conductivity of perovskite-related A(2)B(2)O(5) oxides. A correlation between the energetic preference of the cations for different coordination geometries and the ionic conductivity is proposed based on a first principles periodic density functional theory study of selected possible structures for Ba(2)In(2)O(5), Sr(2)Fe(2)O(5), Sr(2)Mn(2)O(5), and La(2)Ni(2)O(5). There are a large number of low-energy local minima on the potential energy hypersurfaces of the two first compounds due to an energetic preference for BO(4) tetrahedra. Tetrahedral environments are energetically unfavorable for Mn(III) in Sr(2)Mn(2)O(5) and for Ni(II) in La(2)Ni(2)O(5), and the number of low-energy configurations is relatively low in these two cases. Consistent with our findings, in contrast to Sr(2)Fe(2)O(5) and Ba(2)In(2)O(5), Sr(2)Mn(2)O(5) and La(2)Ni(2)O(5) do not exhibit transitions to disordered phases on heating, and there appear to be no reports of enhanced ionic conductivity for these compounds. Thus we suggest that the possibility of many different oxygen orderings associated with a variety of low-energy connectivity schemes within tetrahedral layers such as in the brownmillerite-based structures of Sr(2)Fe(2)O(5) and Ba(2)In(2)O(5) is a prerequisite for high ionic conductivity in perovskite-related A(2)B(2)O(5) oxides. 相似文献
5.
The molecular structure of gaseous OVF3 has been determined by electron diffraction to be: rg(V-O) = 1.570(5) Å, rg(V-F) = 1.729(2) Å and ∠α(OVF) = 107.5(4)°. A modified force field has been fitted to results from spectroscopic as well as diffractional studies. A similar attempt to determine the force field for OVCl3 was not as successful as for OVF3, probably because the Coriolis constants are less accurately determined for that molecule. 相似文献
6.
The enthalpy of oxidation of SrFe1-xCoxO3-d with x=0.33 and 0.67 has been determined by adiabatic calorimetry; average values for x=0.33 and 0.67 are -67±11 and -49.5±9 kJ (mol O2)-1. These data and the previously reported value for SrFeO3-d suggest that the enthalpy of oxidation for pure (perovskite-type) SrCoO3-d is close to zero. Earlier reported composition - partial pressure data for SrFe0.67Co0.33O3-d are reproduced when preferential oxidation of iron is assumed for low partial pressures of oxygen.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
7.
V. P. Novikov M. V. Popik S. Samdal L. V. Vilkov H. V. Volden 《Journal of Structural Chemistry》1995,36(5):772-783
A numerical algorithm for the nonparametric determination of the internal rotation potential from electron diffraction data using Tikhonov's regularization method is described. The range of admissible values of the regularization parameter is estimated using Hamilton's statistical criterion. m-Bromonitrobenzene was reinvestigated using this approach. It was found that the form of the potential may be reliably established only in the region of the minimum corresponding to the planar conformation of the molecule. Fourier series approximation of the experimental values of the potential in the region of the minimum gives the rotation barrier of 4.6–5.4 kcal/mole. The following basic geometrical parameters have been obtained (ra in Å, ∠α in deg, the error equals triple standard deviation): r(C?C)ave=1.399(3), r(C?N)=1.459(16), 1.459(16), r(N=O)=1.244(3), r(C?Br)=1.884(6), r(C?H)=1.099(20), ∠CCNC=123.9(1.4), ∠CNCCBr=116.8(1.5), ∠CNCC=116.6(1.9), ∠CNO=118.8(0.8). The results are compared with the data for the related compounds. 相似文献
8.
V. A. Naumov S. Samdal A. V. Naumov S. Gundersen H. V. Volden 《Russian Journal of General Chemistry》2005,75(12):1956-1961
The molecular structure of triphenylamine was studied by gas-phase electron diffraction in combination with ab initio calculations. It is found that in the gas phase at 160°C the molecule possesses C3 symmetry. The principal geometric parameters are as follows (r a structure): N-C 1.421(4), C-Cmean 1.399(1), C-H 1.123(2) Å, bond angles NCC 123.6(10)° and 117.2(7)°, and CNC 119.9(2)°. Torsion angles around C-N bonds are ?39° and ?45°. Phenyl groups are rotated by 48° from the position in which the C3 axis lies in the phenyl ring plane. 相似文献
9.
The potential energy surfaces of both singlet and triplet B2N2 have been investigated computationally at the coupled-cluster level with a polarized triple zeta basis set augmented with diffuse functions. Calculated vibrational frequencies and intensities are also reported. The triplet species are consistently more stable than their singlet analogs and the stabilities of the linear B2N2 isomers increase with increasing number of B–N bonds. The most stable isomer is the linear triplet BNBN isomer with a rhombic form with a short diagonal BB distance close in energy. Our results are consistent with the results of the matrix IR studies of Andrews et al. nucleus-independent chemical shift (NICS) values were calculated for the singlet D2h rhombic form and its C2v dication, and these were compared to those of the D2h cyclobutadiene and its D2d dication, respectively. Electron density plots for the linear and rhombic B2N2 minima showed similar distributions for the singlet and triplet states. These plots confirmed weak BB bonding interactions in both rhombic forms but larger BN bond orders. 相似文献
10.
Coupled-cluster investigations (CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ) of singlet cyclobutadiene and fifteen-substituted analogs were conducted. A local minimum with a square frame does not exist on their potential surfaces. The well-known rectangular D2h minimum, the square D4h transition state, and two additional stationary points were found on cyclobutadiene’s potential surface. This included a transition state with a rhombic carbon ring and C2h symmetry, separating two equivalent puckered C2v local minima. The predicted barriers were 19.7 and 19.8 kcal/mol at the CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ levels, respectively. The relative strain energies of rectangular D2h cyclobutadiene and all fifteen-substituted analogs were obtained from isodesmic reactions. Progressive substitution with methyl or BH2 groups continuously lowers ring strain while increasing substitution with fluorines or trifluoromethyl groups steadily increases ring strain. C4(BH2)4 is 16.6 and 13.3 kcal/mol less strained than cyclobutadiene while C4F4 is 17.7 and 21.5 kcal/mol more strained at the levels above. Cyclobutadiene is more strained than both cyclopropene and cyclobutene by 12.2 and 37.0 kcal/mol, respectively. Electron density contours indicate that fluorine substitution raised the electron density especially in the short C=C ring bonds above/below the ring plane (π-electrons) but not in the ring plane (σ-electrons). BH2-substitutions lower the ring π-electron density with little effect in the ring plane. Methyl substituents have little effect on electron densities. All rings retain a strong bond alternation tendency (rectangular) whether substituted with electron-donating or -attracting groups. One-bond coupling constants and the percent p-character in ring C-to-C and C-to-substituent bonds are described. 相似文献