全文获取类型
收费全文 | 361篇 |
免费 | 6篇 |
国内免费 | 3篇 |
专业分类
化学 | 185篇 |
晶体学 | 29篇 |
力学 | 5篇 |
数学 | 76篇 |
物理学 | 75篇 |
出版年
2020年 | 5篇 |
2016年 | 5篇 |
2015年 | 4篇 |
2013年 | 12篇 |
2012年 | 10篇 |
2011年 | 10篇 |
2010年 | 5篇 |
2009年 | 6篇 |
2008年 | 11篇 |
2007年 | 7篇 |
2006年 | 20篇 |
2005年 | 10篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 9篇 |
2001年 | 5篇 |
1998年 | 3篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 11篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 11篇 |
1974年 | 6篇 |
1973年 | 8篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1968年 | 5篇 |
1967年 | 8篇 |
1966年 | 2篇 |
1960年 | 2篇 |
1954年 | 2篇 |
1935年 | 3篇 |
1902年 | 2篇 |
排序方式: 共有370条查询结果,搜索用时 15 毫秒
101.
The dynamic behavior of a simple elastic tensegrity structure is examined, in order to validate observations that the natural damping of the elastic elements in such a structure is poorly mobilized, due to the natural flexibility of the equilibrium position of the structure. It is confirmed, analytically and numerically, that the energy decay of such a system is slower than that of a linearly-damped system. 相似文献
102.
Irving J. Borowitz Whei-Oh Lin Tze-Chein Wun Robert Bittman Louis Weiss Vladimir Diakiw Grace B. Borowitz 《Tetrahedron》1977,33(14):1697-1705
The preparation of a series of neutral ligands featuring ether and N-methyl-N-carbethoxypentylamide groups is described. These ligands as well as related ones bearing other diamide groups are shown to selectively chelate Group IIA cations by picrate extraction from water to methylene chloride. The changes in UV absorption of aromatic rings and amide groups in the ligands upon titration with metal salts in methanol allow the estimation of the stoichiometry of complexation and the ordering of cation binding. The observed selectivity sequences of cation extraction and binding are briefly discussed. Preliminary proton and 13C NMR studies on the effect of addition of Group IIA cation salts to several of the ligands in methanol suggest that most of the complexation occurs at the central ether and amide groups. 13C NMR T1 changes by the Inversion Recovery Fourier Transform method are in agreement with the cation-induced shift data. 相似文献
103.
Irving M Krueger CA Wade JV Hodges JC Leopold K Collins N Chan C Shaqair S Shornikov A Yan B 《Journal of combinatorial chemistry》2004,6(4):478-486
A 4-amido-pyrrolidone library that was intentionally synthesized as pairs of diastereomers was produced by solution-phase parallel syntheses and purified by an automated high-throughput purification system. A total of 2592 4-amido-pyrrolidinones were ultimately isolated as single diastereomers from a matrix of 1920 syntheses. After the four-step synthesis and HPLC purification, the average yield of a single diastereomer was 36.6%. The average chemical purity was >90%, and the average diastereomeric purity was >87%. The choice of chiral amines used to make amides with heterocyclic acid chlorides had a dramatic effect on success. Analysis of the relationship between amines used for synthesis and the diastereomeric separation showed that amides made from chiral 1,2-amino alcohols gave superior separation to amides from chiral morpholines. The presence of a hydrogen bond donor on the amide side chain seems to be required for a better diastereomeric separation. 相似文献
104.
trans-2-Tribromomethyl-5-trichloromethyl-1,3-dioxolan-4-one is monoclinic,P21/c,a=6.381(6),b=17.116(6),c=10.598(5) Å,=94.07(6)°,Z=4. The structure was solved by direct methods from data collected at room temperature on an Enraf-Nonius CAD4 diffractometer, and refined by full-matrix least squares to a finalR value of 0.042 using 1274 reflections. The conformation of the molecule is very similar to that of chloralide with the 5-mem-bered ring being almost planar. Evidence is provided for a possible C-HO intermolecular interaction. The average C-Br distance is 1.931(11) Å and the average Br-C-Br angle is 109.7(9)°. A table of average C-Br bond lengths and Br-C-Br angles, with the ranges for these parameters, in compounds containing -CBr3 groups is given. Some1H NMR and mass spectra data are included for the title compound and for chloralide. 相似文献
105.
The host compound forms a coordinato-clathrate in a coordination assisted host lattice. The host lattice is provided by hydrogen bonding between the nitrogen atom of the cyano group and two hydrogen atoms from the phosphonium methyl group. Guest molecules ofp-xylene occupy constricted channels in the lattice. The twop-xylene molecules participate in C–H...(Ar)...H–C interactions. Results from searches of the Cambridge Structural Database for other C–H...(Ar)...H–C and C–H...(Ar) interactions are presented. 相似文献
106.
Anne Irving 《Journal of chemical crystallography》1992,22(5):533-540
8-Acetylamino-6-methyl-2,4-bis(trichloromethyl)-1,3-benzdioxin, (I), is monoclinic,P21/c,a=15.174(4),b=11.977(7),c=9.911(3)Å,=99.72(2)°. 6-Acetylamino-2,4-bis(trichloromethyl)-1,3-benzdioxin, (II), is monoclinic,P21/n,a=5.927(4),b=40.623(1),c =7.120(3)Å,=91.39(4)°. In compound (I) the imino hydrogen atom is locked in an intramolecular hydrogen bond to the proximate oxygen atom of the heterocyclic ring and is not available for intermolecular hydrogen bonding. Instead the weakly acidic hydrogen atom [Cl3C-C(2)]H takes part in a hydrogen bond to the carbonyl oxygen atom in another molecule. In compound (II) a normal intermolecular hydrogen bond of the type N-HO=C occurs. The 6-acetylam-ino group is twisted about the (CAr-N) bond such that the angles NHO=C, CArHArO=C, NHOHArCAr, at the carbonyl oxygen group total 360° (where NH is in the related molecule). The packing in both compounds takes the form of infinite chains and in compound (II) partial overlap of the aromatic ring and the acetylamino group, with very little offset, also occurs. 相似文献
107.
2,6-bis (trichloromethyl)-4-dichloromethylene-1-oxa-3,5-dithian is triclinic,P¯1,a=10.125(3),b=10.294(3),c=14.631(10) Å,=92.84(5),=96.51(5), =96.18(2)°,V=1503 Å3,Z=4. The structure was solved by direct methods, from data collected at room temperature on an Enraf-Nonius CAD4 diffractometer, and refined by least squares to a finalR value of 0.035 using 3961 reflections. The conformation of the molecules is twist-boat, one molecule being almost symmetrically twisted about the O-C(Cl2) direction. The range of endocyclic O-C-S angles is 111.8(3) to 115.1(3)° and data are given which support the possibility of there being a correlation between the endocyclic O-C(X)-S angle, the C-O-C(X)-C(Cl3) torsion angle, and the C-S-C(X)-C(Cl3) torsion angle. The -CCl3 groups are in pseudoequatorial positions. Average bond lengths (Å) for the two molecules are C-O 1.405, C(sp
3)-S 1.822, C(sp
2)-S 1.759, C-Ceq 1.529 and average endocyclic angles (°) are O 117.3, C (sp
3) 113.5, S 97.4, C(sp
2) 119.3, the esds on the individual bond lengths and angles being in the ranges of 0.003–0.004 Å and 0.2–0.4° respectively. 相似文献
108.
6-nitro-2-trichloromethyl-1,3-benzdioxin-4-one is monoclinic,P21/n;a=5.969(2),b=11.646(1),c=16.953(1) Å;=98.03(2)°;Z=4. The structure was solved by direct methods, from data collected at room temperature on an Enraf-Nonius CAD4 diffractometer, and refined by least squares to a finalR value of 0.030 using 1530 reflections.The heterocyclic ring is a distorted half-boat and is substituted equatorially by a trichloromethyl group. Dihedral angles are (a) 4.87(7)° between the mean plane of the aromatic ring and the plane containing the two carbon atoms common to both rings plus the two atoms bonded to these, and (b) 16.8(2)° between the carbonyl group and the aromatic ring. The conformation is discussed with reference to the two previously reported X-ray structures of 1,3-benzdioxin-4-ones and to two isocoumarins. 相似文献
109.
6-Nitro-2,4-bis(dichloromethyl)-1,3-benzdioxin is monoclinic,P21/c, witha=8.514(1),b=22.187(3),c=7.931(1) Å,=117.96(1)°,Z=4. The structure was solved, from data collected with MoK radiation on an Enraf-Nonius CAD4 diffractometer, by direct methods and refined by full-matrix least squares using 2170 unique reflections to give a finalR factor of 0.032. The heterocyclic ring in the title compound has an envelope conformation with thecis-dichloromethyl groups in pseudoequatorial positions. The average C-C(HCl2) distance is 1.516(4) Å. A comparison is given with the structure of the analogous compound 6-nitro-2,4-bis(trichloromethyl)-1,3-benzdioxin (Irving, A., and Irving, H. M. N. H. (1986)J. Crystallogr. Spectrosc. Res.
16, 703) which also containscis-halogenoalkyl groups but in which the –CCl3 group at the 4-position occupies a pseudoaxial position. 相似文献
110.