全文获取类型
收费全文 | 26537篇 |
免费 | 5123篇 |
国内免费 | 1803篇 |
专业分类
化学 | 26382篇 |
晶体学 | 234篇 |
力学 | 983篇 |
综合类 | 126篇 |
数学 | 520篇 |
物理学 | 5218篇 |
出版年
2024年 | 43篇 |
2023年 | 310篇 |
2022年 | 796篇 |
2021年 | 936篇 |
2020年 | 1648篇 |
2019年 | 1337篇 |
2018年 | 1163篇 |
2017年 | 1092篇 |
2016年 | 1734篇 |
2015年 | 1534篇 |
2014年 | 1662篇 |
2013年 | 2309篇 |
2012年 | 1669篇 |
2011年 | 1678篇 |
2010年 | 1418篇 |
2009年 | 1426篇 |
2008年 | 1359篇 |
2007年 | 1297篇 |
2006年 | 1227篇 |
2005年 | 1121篇 |
2004年 | 1085篇 |
2003年 | 878篇 |
2002年 | 1593篇 |
2001年 | 591篇 |
2000年 | 477篇 |
1999年 | 466篇 |
1998年 | 451篇 |
1997年 | 377篇 |
1996年 | 307篇 |
1995年 | 275篇 |
1994年 | 215篇 |
1993年 | 138篇 |
1992年 | 132篇 |
1991年 | 97篇 |
1990年 | 79篇 |
1989年 | 70篇 |
1988年 | 82篇 |
1987年 | 51篇 |
1986年 | 99篇 |
1985年 | 28篇 |
1984年 | 27篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 24篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1972年 | 13篇 |
1966年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
Aromatic nitroderivatives are compounds of considerable environmental concern, because some of them are phytotoxic (especially the nitrophenols, and particularly 2,4-dinitrophenol), others are mutagenic and potentially carcinogenic (e.g., the nitroderivatives of polycyclic aromatic hydrocarbons, such as 1-nitropyrene), and all of them absorb sunlight as components of the brown carbon. The latter has the potential to affect the climatic feedback of atmospheric aerosols. Most nitroderivatives are secondarily formed in the environment and, among their possible formation processes, photonitration upon irradiation of nitrate or nitrite is an important pathway that has periodically gained considerable attention. However, photonitration triggered by nitrate and nitrite is a very complex process, because the two ionic species under irradiation produce a wide range of nitrating agents (such as •NO2, HNO2, HOONO, and H2OONO+), which are affected by pH and the presence of organic compounds and, in turn, deeply affect the nitration of aromatic precursors. Moreover, aromatic substrates can highly differ in their reactivity towards the various photogenerated species, thereby providing different behaviours towards photonitration. Despite the high complexity, it is possible to rationalise the different photonitration pathways in a coherent framework. In this context, this review paper has the goal of providing the reader with a guide on what to expect from the photonitration process under different conditions, how to study it, and how to determine which pathway(s) are prevailing in the formation of the observed nitroderivatives. 相似文献
972.
Electrochemical blocking is a type of single-entity electrochemical measurement particularly well adapted to the detection of insulating particles. The digital detection of ultralow concentrations of artificial entities such as polymer particles or biotargets such as proteins and bacteria represents an exceptional opportunity for sensing applications. In this review, we explore the latest development in the field of electrochemical blocking and propose some perspectives. 相似文献
973.
974.
Emilie Boulard Vivien Zibulski Luisa Oertel Dr. Philip Lienau Dr. Martina Schäfer Dr. Ursula Ganzer Dr. Ulrich Lücking 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(19):4378-4388
A short synthetic approach with broad scope to access five- to seven-membered cyclic sulfoximines in only two to three steps from readily available thiophenols is reported. Thus, simple building blocks were converted to complex molecular structures by a sequence of S-alkylation and one-pot sulfoximine formation, followed by intramolecular cyclization. Seventeen structurally diverse cyclic sulfoximines were prepared in high overall yields. In vitro evaluation of these underrepresented, three-dimensional, cyclic sulfoximines with respect to properties relevant to medicinal chemistry did not reveal any intrinsic flaw for application in drug discovery. 相似文献
975.
Nehal F. Farid Ibrahim A. Naguib Nessreen S. Abdelhamid Basma H. Anwar Maimana A. Magdy 《Journal of separation science》2020,43(12):2330-2337
A novel ecofriendly, cost and time saving high‐performance thin‐layer chromatographic method was developed and validated for simultaneous determination of metoclopramide, ergotamine, caffeine, and paracetamol in bulk and pharmaceutical formulation. The separation was carried out on silica gel plates, using ethyl acetate:ethanol:ammonia (9:1:0.1, v/v/v) as a developing system. Ultraviolet detection was carried out at 272 nm. The resulting retention times were 0.15, 0.36, 0.49, and 0.74 min for metoclopramide, ergotamine, caffeine, and paracetamol, respectively. The greenness profile assessment was achieved to the proposed method to evaluate its greenness characters to the environment with acceptable results. Validation parameters were checked according to International Conference of Harmonization guidelines to achieve the international requirements for quality control analysis of the proposed drugs. 相似文献
976.
Raoul Plessius Vera Deij Prof. Dr. Joost N. H. Reek Prof. Dr. Ir. Jarl Ivar van der Vlugt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(58):13241-13248
Three Pt4L2L′2 heteroleptic rectangles ( 1 – 3 ), containing ditopic redox-active bis-pyridine functionalized perylene bisimide (PBI) ligands PBI-pyr2 ( L ) are reported. Co-ligand L′ is a dicarboxylate spacer of varying length, leading to modified overall size of the assemblies. 1H NMR spectroscopy reveals a trend in the splitting and upfield chemical shift of the PBI-hydrogens in the rectangles with respect to free PBI, most pronounced with the largest strut length ( 3 ) and least with the smallest strut length ( 1 ). This is attributed to increased rotational freedom of the PBI-pyr 2 ligand over its longitudinal axis (Npy-Npy), due to increased distance between the PBI-surfaces, which is corroborated by VT-NMR measurements and DFT calculations. The intramolecular motion entails desymmetrization of the two PBI-ligands, in line with cyclic voltammetry (CV) data. The first (overall two-electron) reduction event and re-oxidation for 1 display a subtle peak-to-peak splitting of 60 mV, whilst increased splitting of this event is observed for 2 and 3 . The binding of pyrene in 1 is probed to establish proof of concept of host-guest chemistry enabled by the two PBI-motifs. Fitting the binding curve obtained by 1H NMR titration with a 1:1 complex formation model led to a binding constant of 964±55 m −1. Pyrene binding is shown to directly influence the redox-chemistry of 1 , resulting in a cathodic and anodic shift of approximately 46 mV on the first and second reduction event, respectively. 相似文献
977.
Dr. Miguel Cortijo Dr. Ángela Valentín-Pérez Dr. Andrei Rogalev Dr. Fabrice Wilhelm Dr. Philippe Sainctavit Dr. Patrick Rosa Dr. Elizabeth A. Hillard 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(59):13363-13366
An original method for determining the handedness of individual non-centrosymmetric crystals in a mixture using a tightly-focused, circularly polarized X-ray beam is presented. The X-ray natural circular dichroism (XNCD) spectra recorded at the metal K-edge on selected crystals of [Δ-M(en)3](NO3)2 and [Λ-M(en)3](NO3)2 (M=CoII, NiII) show extrema at the metal pre-edge (7712 eV for Co, 8335 eV for Ni). A mapping of a collection of some 220 crystals was performed at the respective energies by using left and right circular polarizations. The difference in absorption for the two polarizations, being either negative or positive, directly yielded the handedness of the crystal volume probed by the beam. By using this technique, it was found that the addition of l -ascorbic acid during the synthesis of [Co(en)3](NO3)2 resulted in an enantiomeric enrichment of the Λ-isomer of 67±13 %, whereas the Ni analogue was similarly, but conversely, enriched in the Δ-isomer (65±22 %). 相似文献
978.
Dr. Anton A. Anisimov Dr. Fedor V. Drozdov Yulia S. Vysochinskaya Ekaterina O. Minyaylo Prof. Alexander S. Peregudov Prof. Fedor M. Dolgushin Dr. Olga I. Shchegolikhina Prof. Aziz M. Muzafarov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(50):11404-11407
This study presents the synthesis of organoboron derivatives of stereoregular 4-, 6-, and 12-unit phenylcyclosilsesquioxanes. All compounds obtained were isolated in good yields (70–80 %) and were fully characterized by 1H, 13C, 29Si, 11B NMR, IR spectroscopy, HRMS ESI, and elemental microanalysis. The structure of the key modifier, obtained for the first time, 4-(tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) dimethylvinylsilane, was also confirmed by single-crystal XRD. 相似文献
979.
利用高温固相法成功制备了Er~(3+)单掺、Er~(3+)/Yb~(3+)共掺杂Ca_(12)Al_(14)O_(32)F_2上转换发光样品。在980 nm激光激发下,Er~(3+)单掺和Er~(3+)/Yb~(3+)共掺杂样品均呈现出较强的绿光(528,549 nm)和较弱的红光(655 nm)发射,分别归因于Er~(3+)离子的~2H_(11/2),~4S_(3/2)→~4I_(15/2)和~4F_(9/2)→~4I_(15/2)能级跃迁。随着Er离子浓度的增加,单掺杂样品上转换发光强度先增大后减小,最佳掺杂浓度为0.8%。共掺杂Yb~(3+)后,Er~(3+)的发光强度明显增大。还原气氛下合成的样品上转换发光强度增大约两倍,可能和笼中阴离子基团变化有关。发光强度和激发光功率的关系表明所得上转换发射为双光子吸收过程,借助Er~(3+)-Yb~(3+)体系能级结构详细讨论了上转换发射的跃迁机制。 相似文献
980.
Pengcheng Li Jie Dai Yi Xu Qichao Ran Yi Gu 《Journal of polymer science. Part A, Polymer chemistry》2019,57(14):1587-1592
A difunctional benzoxazine (coPh‐apa) with a conjugated alkyne group is synthesized by the oxidative coupling reaction from a monocycle‐benzoxazine (Ph‐apa) containing an alkyne group. A model compound, 1,4‐diphenylbutadiyne (coPa), is used to study the curing reaction process of coPh‐apa by DSC, Fourier transform infrared spectroscopy, and 13C NMR, and the results suggest that the conjugated alkyne groups are involved in the crosslinking reaction via the trimerization reaction of the conjugated alkynyl groups and the Diels–Alder reaction. Furthermore, thermal properties of the polybenzoxazine are studied by dynamic thermomechanical analysis and thermogravimetric analysis. A glass‐transition temperature (Tgs) of as high as 412 °C and a char yield of 75.6% at 800 °C under nitrogen are obtained with the aid of the conjugated alkyne groups. Its excellent heat resistance dominates most thermosetting resins and will serve for heat shields. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1587–1592 相似文献