全文获取类型
收费全文 | 1480篇 |
免费 | 97篇 |
国内免费 | 172篇 |
专业分类
化学 | 399篇 |
晶体学 | 10篇 |
力学 | 39篇 |
综合类 | 15篇 |
数学 | 1051篇 |
物理学 | 235篇 |
出版年
2024年 | 3篇 |
2023年 | 11篇 |
2022年 | 27篇 |
2021年 | 30篇 |
2020年 | 46篇 |
2019年 | 60篇 |
2018年 | 48篇 |
2017年 | 59篇 |
2016年 | 39篇 |
2015年 | 30篇 |
2014年 | 49篇 |
2013年 | 350篇 |
2012年 | 39篇 |
2011年 | 52篇 |
2010年 | 43篇 |
2009年 | 63篇 |
2008年 | 62篇 |
2007年 | 85篇 |
2006年 | 75篇 |
2005年 | 59篇 |
2004年 | 49篇 |
2003年 | 55篇 |
2002年 | 103篇 |
2001年 | 51篇 |
2000年 | 36篇 |
1999年 | 44篇 |
1998年 | 35篇 |
1997年 | 28篇 |
1996年 | 30篇 |
1995年 | 22篇 |
1994年 | 14篇 |
1993年 | 12篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
1966年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1749条查询结果,搜索用时 0 毫秒
71.
P. Valtazanos S. T. Elbert S. Xantheas K. Ruedenberg 《Theoretical chemistry accounts》1991,78(5-6):287-326
Summary The global features of the groundstate ring opening of cyclopropylidene to allene are studied by means ofab-initio FORS MCSCF calculations based on a minimal AO basis set. The energy surface is completely mapped out in terms of three reaction coordinates, namely the CCC ring-opening angle and two angles describing the rotations of the CH2 groups. For each choice of these three variables, the twelve remaining internal coordinates are optimized by energy minimization. In the initial phase of the reaction, as the CCC angle opens, the CH2 groups rotate in a disrotatory manner, maintainingC
s symmetry. This uphill reaction path leads to a transition region which occurs early, for a CCC angle of about 84°. In this transition region the reaction path branches into two pathways which are each others' mirror images. The system exhibits thus abifurcating transition region. Passed this region, the two pathways are overall conrotatory in character. However, these downhill reaction paths to the products are poorly defined because, from a CCC opening angle of about 90° on,the CH
2
groups can rotate freely and isoenergetically in a synchronized, cogwheel-like manner and this disrotatory motion can mix unpredictably with the conrotatory downhill motion. There is no preference for any one of the two reaction pathways yielding the two stereoisomers of allene and the reaction is thereforenonstereospecific with respect to the numbered hydrogen atoms. The global surface is documented by means of contour maps representing slices corresponding to constant CCC angles. The bifurcating transition region is mapped in detail.Operated for the U.S. Department of Energy by Iowa State University under Contract No. 7405-ENG-82. This work was supported by the office of Basic Energy Sciences 相似文献
72.
Attempted reaction of indolenines (which represent rather sterically hindered cyclic imines) with a series of dicarboxylic acid anhydrides yielded no expected product, the Castagnoli-Cushman lactam. Instead, products presumably formed via N-acyliminium species trapping by a carboxylate anion. Among them, hydrolytically labile 2:2 adducts of an indolenine and a cyclic anhydride, containing a 16-membered cyclic core, are particularly intriguing. This result contradicts the recently reported successful Castagnoli-Cushman reaction of indolenines with homophthalic anhydride suggesting a mechanistic switch in the course of the reaction. 相似文献
73.
74.
关于Liesegang环形成机理的研究 总被引:1,自引:0,他引:1
本文研究了Liesegang环的形成,受到两扩散电解质溶液的浓度比、离子浓度积及光照等条件影响。侧重对成环的微观汇聚过程进行了探讨。 相似文献
75.
76.
Nobuhiro Kawatsuki Imin Sai Tohei Yamamoto 《Journal of polymer science. Part A, Polymer chemistry》1999,37(21):4000-4006
We present (1) the linearly polarized (LP) ultraviolet (UV) photoreaction of block copolyesterethers comprising a hard segment of polyester and soft segment of poly(tetramethylene ether) (PTMG) and (2) the alignment behavior of liquid crystals (LCs) on the resultant photoreacted polymer films. Two kinds of copolyesterethers with different polyester segments of poly(hexamethylene p-phenylenediacrylate) (1a) or poly(hexamethylene p-phenylenediacrylate)-co-poly(hexamethylene 1,4-dibenzoate) (1b) were used in this study. The LP–UV irradiation resulted in a negative optical anisotropy for both polymer films, and a homogeneous photoalignment of LCs was achieved on the photoreacted polymer film. The LC alignment was perpendicular to the electric vector of the incident polarized light for 1a, whereas a reversion of the alignment from parallel to perpendicular was observed for 1b, depending on the irradiation doses. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4000–4006, 1999 相似文献
77.
This communication describes a convenient synthesis of fused bicyclic pyrazolones in a one-pot reaction procedure. Modified protocols are also developed to access 5,5-bicyclic and bulky pyrazolones in good yields. The method reported herein represents a practical approach to structurally diversified bicyclic pyrazolones. 相似文献
78.
Estimation of diffusion coefficient of Ba++ ions in silica gel from periodic precipitation of BaMoO4
Formation of Liesegang rings on the growth of BaMoO4 crystals in silica gel by single diffusion is discussed. The rhythmic precipitation is found to be influenced profoundly by (a) the molarity of inner and outerelectrolytes (b) the age of gel and (c) the pH of the gel medium. The time law, spacing law and the law relating diffusion depth and width are verified. A method to estimate the diffusion coefficient of the outer electrolyte in the gel medium is developed and calculated for Ba++ions. 相似文献
79.
80.