全文获取类型
收费全文 | 1407篇 |
免费 | 20篇 |
国内免费 | 7篇 |
专业分类
化学 | 717篇 |
晶体学 | 12篇 |
力学 | 46篇 |
数学 | 296篇 |
物理学 | 363篇 |
出版年
2021年 | 10篇 |
2020年 | 11篇 |
2019年 | 9篇 |
2017年 | 11篇 |
2016年 | 26篇 |
2015年 | 17篇 |
2014年 | 17篇 |
2013年 | 72篇 |
2012年 | 58篇 |
2011年 | 73篇 |
2010年 | 36篇 |
2009年 | 36篇 |
2008年 | 53篇 |
2007年 | 64篇 |
2006年 | 65篇 |
2005年 | 55篇 |
2004年 | 62篇 |
2003年 | 31篇 |
2002年 | 34篇 |
2001年 | 30篇 |
2000年 | 25篇 |
1999年 | 22篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 29篇 |
1995年 | 19篇 |
1994年 | 35篇 |
1993年 | 31篇 |
1992年 | 19篇 |
1991年 | 25篇 |
1990年 | 33篇 |
1989年 | 18篇 |
1988年 | 28篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 31篇 |
1984年 | 25篇 |
1983年 | 14篇 |
1982年 | 20篇 |
1981年 | 25篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 19篇 |
1975年 | 9篇 |
1974年 | 17篇 |
1973年 | 14篇 |
1972年 | 9篇 |
1969年 | 8篇 |
排序方式: 共有1434条查询结果,搜索用时 890 毫秒
61.
Aromatic primary amines are determined by injection into dilute hydrochloric acid carrier which merges sequentially with 4-N-methylaminophenol and dichromate. The purple-red color formed by oxidative coupling of amines with 4-N-methylaminophenol is measured at 530 nm. In contrast to the manual procedure, the flow-injection procedure avoided errors arising from the instability of the coupling intermediate, oxidation of the amine, and too great an excess of the oxidant. The method improves the selectivity for certain amines in the presence of those which are sterically hindered or have an electron-deficient aromatic nucleus. Nitrite is determined by diazotization of sulfanilamide and quantifying the residual sulfanilamide by oxidative coupling. The sample thourghout for the assay of amines (0.05–20 μg ml?1 NH2-N) and nitrite (1–10 μg ml?1 NO2--N) was 120 h?1. A system for the consecutive determination of aromatic primary amines and nitrite is decribed. 相似文献
62.
We present an accurate and fast wave tracking method that uses parametric representations of tracked fronts, combined with modifications of level set methods that use narrow bands. Our strategy generates accurate computations of the front curvature and other geometric properties of the front. We introduce data structures that can store discrete representations of the location of the moving fronts and boundaries, as well as the corresponding level set fields, that are designed to reduce computational overhead and memory storage. We present an algorithm we call stack sweeping to efficiently sort and store data that is used to represent orientable fronts. Our implementation features two reciprocal procedures, a forward ‘front parameterization’ that constructs a parameterization of a front given a level set field and a backward ‘field construction’ that constructs an approximation of the signed normal distance to the front, given a parameterized representation of the front. These reciprocal procedures are used to achieve and maintain high spatial accuracy. Close to the front, precise computation of the normal distance is carried out by requiring that displacement vectors from grid points to the front be along a normal direction. For front curves in two dimensions, a cubic interpolation scheme is used, and G 1 surface parameterization based on triangular patches is used for the three-dimensional implementation to compute the distances from grid points near the front. To demonstrate this new, high accuracy method we present validations and show examples of combustion-like applications that include detonation shock dynamics, material interface motions in a compressible multi-material simulation and the Stephan problem associated with dendrite solidification. 相似文献
63.
Ronald F. Stewart 《Molecular physics》2013,111(4):879-883
An analytical expression of dipolar absorption, obtained by truncating the memory function hierarchy to the first order is compared to experimental results on Furan and Chloroform at different temperatures. The proposed method permits the prediction of the form of the correlation functions if the high frequencies absorption spectrum only, is known. 相似文献
64.
65.
Aminomercuration and aminoselenation of electron rich alkenes provides a useful route to adenosine nucleoside analogs. 相似文献
66.
ABSTRACT In this paper, we present a systematic roadmap for developing a robust and parallel multi-material reactive hydrodynamic solver that integrates historically stable algorithms with new and current modern methods to solve explosive system design problems. The Ghost Fluid Method and Riemann solvers were used to enforce appropriate interface boundary conditions. Improved performance in terms of computational work and convergence properties was achieved by modifying a local node sorting strategy that decouples ghost nodes, allowing us to set material boundary conditions via an explicit procedure, removing the need to solve a coupled system of equations numerically. The locality and explicit nature of the node sorting concept allows for greater levels of parallelism and lower computational cost when populating ghost nodes. Non-linear numerical issues endemic to the use of real Equations of State in hydro-codes were resolved by using more thermodynamically consistent forms allowing us to accurately resolve large density gradients associated with high energy detonation problems at material interfaces. Pre-computed volume tables were implemented adding to the robustness of the solver base. 相似文献
67.
68.
Etienne V. Brouillet Alan R. Kennedy Tobias Krämer Robert E. Mulvey Stuart D. Robertson Alexander Stewart Stephen Towie 《无机化学与普通化学杂志》2020,646(13):726-733
Previous studies of different solvates of 2-methylpyridyllithium (2-picolyllithium) have uncovered electronic structures corresponding to aza-allyl and enamido resonance forms of the metallated pyridine-based compounds. Here, we report the synthesis and characterization of [2-CH2Li(THF)2C5H4N], a new THF solvate. X-ray crystallographic studies reveal a dimeric arrangement featuring a non-planar eight-membered [NCCLi]2 ring, in which the primary cation-anion interaction is between the central Li atom and the C atom of the deprotonated methyl group [length, 2.285(2) Å], suggesting a new carbanionic resonance structure for this 2-picolyllithium series. The significant carbanionic character of [2-CH2Li(THF)2C5H4N] was confirmed by gas-phase DFT calculations [B3LYP/6-311+G(d)] with the calculated electron density interrogated by means of quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. For comparison these computational analyses were also performed on the literature structures of [2-CH2Li(2-Picoline)C5H4N] and [2-CH2Li(PMDETA)C5H4N]. In a reactivity study, [2-CH2Li(THF)2C5H4N] was found to undergo nucleophilic addition to pyridine to generate dipyridylmethane in a good yield. 相似文献
69.
70.
The Chemistry of 10-Pn-3 Systems1: Tricoordinate Hypervalent Pnictogen Compounds and Related Systems
Anthony J. Arduengo III David A. Dixon Constantine A. Stewart 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-2):341-344
Abstract The unusual hypervalent tricoordinate pnictogen systems, 10-Pn-3 ADPnO, provide a convenient starting point for the study of a wide range of main group chemistry. Differences in reactivity patterns among the pnictogens are readily apparent from the variety of chemistry exhibited. The ADPnO systems also provide a model for the recently recognized edge inversion mechanism for 3 and 4 coordinate 8-electron main group species. 相似文献