全文获取类型
收费全文 | 350086篇 |
免费 | 3837篇 |
国内免费 | 1177篇 |
专业分类
化学 | 191388篇 |
晶体学 | 5687篇 |
力学 | 14738篇 |
综合类 | 18篇 |
数学 | 35741篇 |
物理学 | 107528篇 |
出版年
2021年 | 2412篇 |
2020年 | 2721篇 |
2019年 | 2983篇 |
2018年 | 3289篇 |
2017年 | 3383篇 |
2016年 | 5629篇 |
2015年 | 3934篇 |
2014年 | 5780篇 |
2013年 | 15848篇 |
2012年 | 12021篇 |
2011年 | 14899篇 |
2010年 | 9879篇 |
2009年 | 10072篇 |
2008年 | 13464篇 |
2007年 | 13348篇 |
2006年 | 12696篇 |
2005年 | 11607篇 |
2004年 | 10537篇 |
2003年 | 9375篇 |
2002年 | 9067篇 |
2001年 | 11036篇 |
2000年 | 8411篇 |
1999年 | 6417篇 |
1998年 | 4976篇 |
1997年 | 4949篇 |
1996年 | 4793篇 |
1995年 | 4385篇 |
1994年 | 4184篇 |
1993年 | 3923篇 |
1992年 | 4807篇 |
1991年 | 4701篇 |
1990年 | 4407篇 |
1989年 | 4402篇 |
1988年 | 4430篇 |
1987年 | 4388篇 |
1986年 | 4171篇 |
1985年 | 5713篇 |
1984年 | 5709篇 |
1983年 | 4527篇 |
1982年 | 4768篇 |
1981年 | 4732篇 |
1980年 | 4485篇 |
1979年 | 4732篇 |
1978年 | 4827篇 |
1977年 | 4883篇 |
1976年 | 4691篇 |
1975年 | 4412篇 |
1974年 | 4304篇 |
1973年 | 4317篇 |
1972年 | 2777篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Electric fields of the anions, cations and neutral forms of 2-aminopurine and 6-thioguanine have been mapped. Certain important
features of the maps are similar to those found earlier in the neutral and ionic forms of adenine and guanine. The computed
electric field patterns satisfactorily explain reactive sites and biological activity of the molecules. 相似文献
122.
P. C. Chen 《中国化学会会志》1995,42(5):755-760
Three ab initio calculations (HF/6-3IG, HF/6-3IG*, and HF/6-3IG**) on 2,4,6-trinitrotoluene were made, The results compare well with xray data, except dihedral angles of NO2 relative to the plane of the benzene ring. The deviations are attributed to packing forces and steric effects in the crystal. The most stable structure was a torsional angle 10° of the methyl top with the benzene ring, unlike toluene. The rotational barriers of the methyl top and the 4-nitro group are small. Hydrogen bonding, dipole moments and total atomic charges arc calculated. 相似文献
123.
The determination of the enantiomeric purity of 2 or 3-substituted cyclanones, particularly cyclohexanones, is conveniently achieved by derivatisation into cyclic aminals with commercially available (R, R)-1,2-diphenylethylenediamine. The derivatisation procedure is directly done into the NMR tube, instantaneously, and 13C NMR allows an accurate measure of the ee. 相似文献
124.
A novel method for measuring AMP-deaminase activity in human erythrocytes is presented, based on the determination of the reaction product, IMP, using high performance liquid chromatography. IMP formation was found to be proportional both to the incubation time and the amount of haemolysate over a wide range. The minimal detectable AMP-deaminase activity was more than 1000 times lower than the mean activity found in healthy controls (1083 nmol/h/mg Hb). No marked difference of activity was found in the patients with the following inherited purine disorders: familial juvenile gouty nephropathy and deficiencies of adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase or adenine phosphoribosyltransferase. The activity in the erythrocytes of patients with chronic renal failure was also similar to controls. The existence of subjects with low erythrocyte AMP-deaminase activity in the population has been confirmed. 相似文献
125.
C. R. Lavers 《Liquid crystals》1992,11(6):819-832
Prism-coupling work in polymer aligned liquid crystal layers is presented with special emphasis being placed on the ferroelectric chiral smectic C phase, of interest to electro-optic device fabrication. Experimental results as a function of temperature, wavelength and DC applied voltage are presented, together with a study of sp mixing which may have potential device applications as well as being an elegant technique with which to establish the optical dielectric tensor configuration in thin, aligned liquid crystal layers. 相似文献
126.
Novel 3-phenyloxy substituted propane-1,2-diol derivatives, 4-(4-n-hexyloxyphenyl)-butane-1,2-diol and 4-(trans-4-n-pyropylcyclohexyl)-butane-1,2-diol have been synthesized and their thermal behaviour has been studied. These compounds exhibit thermotropic and, after addition of water, also lyotropic liquid-crystalline behaviour. The clearing temperatures of the smectic mesophases were found to be strongly dependent on the alkyl chain length and on the structural units that link the aromatic ring to the alkyl chain and to the diol unit. The behaviour of the aromatic compounds is compared with that of the cyclohexane derivative. Thereby it has been realized that the mesophase stability of the amphiphilic diols incorporating a rigid unit is largely determined by both, the molecular geometry (molecular shape and intramolecular flexibility) and the amphiphilic structural pattern. 相似文献
127.
Fifteen patients with sarcoidosis underwent thoracic MRI examinations. The T2 signal intensity of lymphadenopathy varied, with no characteristic pattern noted. Three of four patients with bright lymph nodes on T2 images had stage I sarcoidosis, but low intensity nodes were also seen in stage I patients. Coronal images were complementary to axial images and better depicted subcarinal adenopathy. MRI does not distinguish the lymphadenopathy of sarcoidosis from other entities, but is useful for defining the anatomic extent of disease and differentiating pulmonary artery enlargement from adenopathy. 相似文献
128.
129.
This contribution discusses the phenomena of retrograde condensation of one or two liquids. It w1 be shown that both phenomena can be well understood. Also the relation of retrograde condensation of one liquid phase with the condensation behavior of natural gas will be discussed. Similarly that of two liquid phases with multiple phase behavior occurring in low temperature reservoir fluids will be pointed out. 相似文献
130.