全文获取类型
收费全文 | 8009篇 |
免费 | 277篇 |
国内免费 | 26篇 |
专业分类
化学 | 5266篇 |
晶体学 | 43篇 |
力学 | 250篇 |
数学 | 1361篇 |
物理学 | 1392篇 |
出版年
2023年 | 86篇 |
2022年 | 198篇 |
2021年 | 228篇 |
2020年 | 192篇 |
2019年 | 178篇 |
2018年 | 151篇 |
2017年 | 150篇 |
2016年 | 334篇 |
2015年 | 256篇 |
2014年 | 278篇 |
2013年 | 436篇 |
2012年 | 442篇 |
2011年 | 525篇 |
2010年 | 323篇 |
2009年 | 295篇 |
2008年 | 429篇 |
2007年 | 420篇 |
2006年 | 407篇 |
2005年 | 358篇 |
2004年 | 322篇 |
2003年 | 267篇 |
2002年 | 236篇 |
2001年 | 103篇 |
2000年 | 89篇 |
1999年 | 101篇 |
1998年 | 74篇 |
1997年 | 71篇 |
1996年 | 103篇 |
1995年 | 76篇 |
1994年 | 62篇 |
1993年 | 56篇 |
1992年 | 61篇 |
1991年 | 63篇 |
1990年 | 45篇 |
1989年 | 36篇 |
1988年 | 26篇 |
1987年 | 34篇 |
1986年 | 39篇 |
1985年 | 46篇 |
1984年 | 34篇 |
1983年 | 38篇 |
1982年 | 32篇 |
1981年 | 33篇 |
1980年 | 37篇 |
1979年 | 36篇 |
1978年 | 20篇 |
1977年 | 36篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1934年 | 17篇 |
排序方式: 共有8312条查询结果,搜索用时 0 毫秒
171.
Gómez-Zavaglia A Kaczor A Cardoso AL Pinho e Melo TM Fausto R 《The journal of physical chemistry. A》2006,110(26):8081-8092
The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions. 相似文献
172.
Two strategies have been explored for the synthesis of steroidal 20,16-γ-carbolactones from the corresponding 17-ketoandrostane. A highly efficient, stereospecific protocol has been developed for the β-oriented cis-γ-lactone, based on a Michael addition of cyanide to a conjugated ketone. A different approach, involving prior attachment of a 3-carbon side chain on C-17 of a 17-oxo-16β-acetoxy-androstane led to the epimeric, α-oriented lactone. 相似文献
173.
Various techniques, already suggested for improving the computational efficiency of the force relaxation method, have been applied here in conjunction with the variable metric algorithms suggested by Murtagh and Sargent and by Fletcher. Various tests have been performed to assess the computational efficiency of the resulting minimization procedures. In addition, procedures where the steps are defined in terms of a parabolic fitting of the forces in approximated normal coordinates have been tested. In all cases the forces are computed analytically. 相似文献
174.
Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics 下载免费PDF全文
Kristin S. Grußmayer Florian Steiner Prof. Dr. John M. Lupton Prof. Dr. Dirk‐Peter Herten Dr. Jan Vogelsang 《Chemphyschem》2015,16(17):3578-3583
Blinking of the photoluminescence (PL) emitted from individual conjugated polymer chains is one of the central observations made by single‐molecule spectroscopy (SMS). Important information, for example regarding excitation energy transfer, can be extracted by evaluating dynamic quenching. However, the nature of trap states, which are responsible for PL quenching, often remains obscured. We present a detailed investigation of the photon statistics of single poly(3‐hexylthiophene) (P3HT) chains obtained by SMS. The photon statistics provide a measure of the number and brightness of independently emitting areas on a single chain. These observables can be followed during blinking. A decrease in PL intensity is shown to be correlated with either 1) a decrease in the average brightness of the emitting sites; or 2) a decrease in the number of emitting regions. We attribute these phenomena to the formation of 1) shallow charge traps, which can weakly affect all emitting areas of a single chain at once; and 2) deep traps, which have a strong effect on small regions within the single chains. 相似文献
175.
The rate constant of the comparably slow bimolecular NCN radical reaction NCN + O2 has been measured for the first time under combustion relevant conditions using the shock tube method. The thermal decomposition of cyanogen azide (NCN3) served as a clean high‐temperature source of NCN radicals. NCN concentration–time profiles have been detected by narrow‐bandwidth laser absorption at cm?1. The experiments behind incident shock waves have been performed with up to 17% O2 in the reaction gas mixture. At such high O2 mole fractions, it was necessary to take O2 relaxation into account that caused a gradual decrease of the temperature during the experiment. Moreover, following fast decomposition of NCN3 and collision‐induced intersystem crossing of the initially formed singlet NCN to its triplet ground state, an unexpected and slow additional formation of triplet NCN has been observed on a 100‐μs timescale. This delayed NCN formation was attributed to a fast recombination of 1NCN with O2 forming a 3NCNOO adduct acting as a reservoir species for NCN. Rate constant data for the reaction NCN + O2 have been measured at temperatures between 1674 and 2308 K. They are best represented by the Arrhenius expression . No pressure dependence has been observed at pressures between 216 and 706 mbar. 相似文献
176.
Emi Evangelio Dr. Marie‐Laure Bonnet Dr. Miquel Cabañas Dr. Motohiro Nakano Dr. Jean‐Pascal Sutter Dr. Andrea Dei Prof. Vincent Robert Dr. Daniel Ruiz‐Molina Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(22):6666-6677
The different thermally induced intermolecular electron transfer (IET) processes that can take place in the series of complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)2], for which M=Co ( 2 ), Fe ( 3 ) and Ni( 4 ), and Cat‐N‐BQ and Cat‐N‐SQ denote the mononegative (Cat‐N‐BQ?) or dinegative (Cat‐N‐SQ2?) radical forms of the tridentate Schiff‐base ligand 3,5‐di‐tert‐butyl‐1,2‐quinone‐1‐(2‐hydroxy‐3,5‐di‐tert‐butylphenyl)imine, have been studied by variable‐temperature UV/Vis and NMR spectroscopies. Depending on the metal ion, rather different behaviors are observed. Complex 2 has been found to be one of the few examples so far reported to exhibit the coexistence of two thermally induced electron transfer processes, ligand‐to‐metal (IETLM) and ligand‐to‐ligand (IETLL). IETLL was only found to take place in complex 3 , and no IET was observed for complex 4 . Such experimental studies have been combined with ab initio wavefunction‐based CASSCF/CASPT2 calculations. Such a strategy allows one to solicit selectively the speculated orbitals and to access the ground states and excited‐spin states, as well as charge‐transfer states giving additional information on the different IET processes. 相似文献
177.
Andrea Guerrini Greta Varchi Rizzo Daniele Cristian Samorì Arturo Battaglia 《Tetrahedron》2007,63(33):7949-7969
Reactions of chiral (2S)-enolates of dioxolan-4-ones, derived from lactic, mandelic, and phenyllactic acids, with aliphatic (SS)- and (SR)-tert-butylsulfinyl aldimines afforded conformationally restrained C2-disubstituted N,O-orthogonally protected 3-amino-2-hydroxyalkanoates in the form of N-sulfinyl protected 1′-aminodioxolan-4-ones. The product distribution showed that there is significant kinetic selectivity, due to the presence of ‘matched’ and ‘mismatched’ components, between the (S)- or (R)-tert-butylsulfinyl aldimines and the (2S)-enolates of the 1,3-dioxolan-4-ones. Selective methoxide-induced removal of the acetal group of the N-sulfinyl-1′-aminodioxolanones yielded the corresponding N-sulfinyl protected methyl alkanoates. In addition, the selective acid-induced removal of the sulfinyl group of the N-sulfinyl-1′-aminodioxolanones provided the corresponding N-unprotected 1′-aminodioxolanones, whose base-induced cyclization afforded the corresponding β-lactams. 相似文献
178.
Curcio A Marotta R Riedinger A Palumberi D Falqui A Pellegrino T 《Chemical communications (Cambridge, England)》2012,48(18):2400-2402
We here exploit pH-responsive nanogels as carriers to deliver functional anti-GFP siRNA and superparamagnetic IONPs to HeLa-GFP cells. The siRNA release via pH-mediated endosomal escape is shown. The IONPs act first as magnetofection agents to boost cellular uptake and then as probes to track the release mechanism by electron microscopy. 相似文献
179.
Time-resolved fluorescence spectra from the RuN719 dye exhibit very short lifetimes (<30 fs) in solutions, on non-injecting substrates and on injecting ones. This reveals <10 fs intramolecular energy redistribution competing with the injection. We conclude that injection proceeds on a sub-10 fs time scale from non-thermalized levels of the dye. 相似文献
180.
Golub IE Filippov OA Gutsul EI Belkova NV Epstein LM Rossin A Peruzzini M Shubina ES 《Inorganic chemistry》2012,51(12):6486-6497
The mechanism of transition-metal tetrahydroborate dimerization was established for the first time on the example of (Ph(3)P)(2)Cu(η(2)-BH(4)) interaction with different proton donors [MeOH, CH(2)FCH(2)OH, CF(3)CH(2)OH, (CF(3))(2)CHOH, (CF(3))(3)CHOH, p-NO(2)C(6)H(4)OH, p-NO(2)C(6)H(4)N═NC(6)H(4)OH, p-NO(2)C(6)H(4)NH(2)] using the combination of experimental (IR, 190-300 K) and quantum-chemical (DFT/M06) methods. The formation of dihydrogen-bonded complexes as the first reaction step was established experimentally. Their structural, electronic, energetic, and spectroscopic features were thoroughly analyzed by means of quantum-chemical calculations. Bifurcate complexes involving both bridging and terminal hydride hydrogen atoms become thermodynamically preferred for strong proton donors. Their formation was found to be a prerequisite for the subsequent proton transfer and dimerization to occur. Reaction kinetics was studied at variable temperature, showing that proton transfer is the rate-determining step. This result is in agreement with the computed potential energy profile of (Ph(3)P)(2)Cu(η(2)-BH(4)) dimerization, yielding [{(Ph(3)P)(2)Cu}(2)(μ,η(4)-BH(4))](+). 相似文献