全文获取类型
收费全文 | 3130篇 |
免费 | 42篇 |
国内免费 | 3篇 |
专业分类
化学 | 2232篇 |
晶体学 | 11篇 |
力学 | 42篇 |
数学 | 480篇 |
物理学 | 410篇 |
出版年
2018年 | 26篇 |
2017年 | 26篇 |
2016年 | 33篇 |
2015年 | 33篇 |
2014年 | 31篇 |
2013年 | 84篇 |
2012年 | 80篇 |
2011年 | 104篇 |
2010年 | 77篇 |
2009年 | 77篇 |
2008年 | 104篇 |
2007年 | 119篇 |
2006年 | 99篇 |
2005年 | 111篇 |
2004年 | 110篇 |
2003年 | 88篇 |
2002年 | 94篇 |
2001年 | 53篇 |
2000年 | 76篇 |
1999年 | 66篇 |
1998年 | 50篇 |
1997年 | 58篇 |
1996年 | 49篇 |
1995年 | 47篇 |
1994年 | 39篇 |
1993年 | 65篇 |
1992年 | 54篇 |
1991年 | 41篇 |
1990年 | 29篇 |
1989年 | 35篇 |
1988年 | 42篇 |
1987年 | 35篇 |
1986年 | 50篇 |
1985年 | 61篇 |
1984年 | 41篇 |
1983年 | 31篇 |
1982年 | 38篇 |
1981年 | 50篇 |
1980年 | 40篇 |
1979年 | 54篇 |
1978年 | 41篇 |
1977年 | 42篇 |
1976年 | 37篇 |
1975年 | 31篇 |
1974年 | 36篇 |
1973年 | 23篇 |
1972年 | 25篇 |
1971年 | 23篇 |
1970年 | 24篇 |
1934年 | 21篇 |
排序方式: 共有3175条查询结果,搜索用时 343 毫秒
111.
[Cl3PNPCl3][MoNCl4], a Compound having Columns of Stacked Anions The title compound is formed by the reaction of [Cl3PNPCl3]Cl with MoNCl3 in CH2Cl2 and subsequent precipitation with CCl4 in from of orange-red crystals. According to the 31P-NMR spectrum, the compound exists as its isomer phosphaneiminate [Cl5Mo(NPCl2NPCl3)] in CD2Cl2/CH3CN solution. The crystal structure of [Cl3PNPCl3][MoNCl4] is isotypic with that of [Cl3PNPCl3][MoOCl4] and shows the same kind of two-dimensional disorder. X-ray diffraction patterns show planes of diffuse scattering as well as Bragg reflexions. The latter correspond to an averaged structure with a = 1590.0, b = 1141.6, c = 418.0 pm, space group Pba2, Z = 2. In the averaged structure (606 reflexions, R = 0.071) the atom sites have fractional occupation. The real structure consists of square-pyramidal [MoNCl4]? ions stacked to form columns with alternating MoN distances of 175 and 243 pm. The packing of the columns is disordered in that the [MoNCl4]? pyramids point either in the +c or ?c direction. The [Cl3PNPCl3]+ ions are stacked in the c direction and show two types of disorder, namely a displacement parallel to c and a rotation by 120° about the P? P axis. 相似文献
112.
Kurt Burdeska Hermann Fuhrer Guglielmo Kabas Adolf Emil Siegrist 《Helvetica chimica acta》1981,64(1):113-152
Preparation of Styryl and Stilbenyl Derivatives of Pyrimidines 2- and 4-(p-Tolyl)-substituted pyrimidines react with anils of hetero-aromatic aldehydes in the presence of dimethylformamide and potassium hydroxide or potassium t-butoxide to yield the corresponding 2- and 4-[4″-(heteroaryl)stilben-4′-yl]pyrimidines or the 2- and 4-[a-(heteroaryl)-4′-styryl]pyrimidines respectively (‘Anil synthesis’). Furthermore, the Schiff′s bases derived from p-chloroaniline and 4-(pyrimidine-2-yl and 4-yl)benzaldehydes give, with methyl- and with p-tolyl-substituted heterocycles, the corresponding heterocyclic substituted styryl and stilbenyl derivatives. Alkyl-, alkoxy- or phenyl-substituted pyrimidines undergo also the ‘Anil synthesis’. 相似文献
113.
Kurt Rüfenacht 《Helvetica chimica acta》1973,56(7):2186-2204
To complete results presented in this and in previous papers of this series as well as published in patents of other authors a review is given on known and new variations of the heterocyclic moiety in GS 13005 type thio- and dithiophosphoric acid esters ( 1, 2 ) by modification of the 1,3,4-thiadiazol-2(3H)-one ring 5 and by its replacement by analogue five- and homologue six-membered rings. Among new esters of this type some containing the pyrazolinone ring 3 or a 2-alkoxy-4H, 6H-1,3,4-thiadiazin-5-one ring 10 (homologue of the original 5-methoxy-1,3,4-thiadiazol-2(3H)-one ring in GS 13005) show no remarkable pesticidal activity, some others containing a pyrazolering 7 or a 3(2H)-pyridazinonc ring 8 are moderately to highly active but toxic to inauinials in the same proportion. Attempts to prepare seven-membered 2-alkoxy- and 2-alkylthio-6,7-dihydro-l, 3,4-thiadiazepin-S(4H)-ones 11 , Z-rnethoxy-l,3,4-thiadiazepin-S(4H)-one 12 (ring vinylogue of the original 5-methoxy-l,3,4-thiadIazol-2(3H)-one ring in GS 13005) and its 7-methyl-derivative have been unsuccessful due to unexpected side reactions, such as: five-ring closure of 3-(3-chloropropionyl)-thio- and -dithiocarbazic acid esters 22 to pyrazolidinone derivatives 23 , pyrazolinone ring closure of a 3-(acetoacety1)-thiocarbazic acid O-methyl ester derivative 26 , bromine attack on sulfur in 3-(2-alkenoyl)-thiocarbazic O-methyl esters 29 instead of bromine addition at the double bond, and halogen splitting off without ring closure in 3-(2,3-dihalogeno-alkanoyl) -thiocarbazic O-methyl esters 30 prepared by acylation of thiocarbazic acid O-methyl ester with dihalogeno-alkanoyl-chlorides. 相似文献
114.
Zusammenfassung Flüssige und feste Legierungen im System Au–Cd wurden von 60 bis 90 At% Cd zwischen 653° und 873°K mit derEMK-Methode untersucht und thermodynamische Eigenschaften berechnet. Die Aktivitäten von Cd zeigen stark negative Abweichungen vom idealen Verhalten. Die partiellen molaren Enthalpien sind negativ und verschieben sich mit zunehmender Temperatur zu weniger negativen Werten, die partiellen molaren Entropien sind positiv und nehmen mit zunehmender Temperatur zu. Aus eigenen Werten und Literaturdaten wurden integrale thermodynamische Größen für die Schmelzen im gesamten System Au–Cd bei 1000°K berechnet. Für einige feste Cd-reiche Phasen wurden ebenfalls Aktivitäten erhalten und aus den Knickpunkten derEMK-Temperatur-Kurve Temperaturen von Phasenübergängen bestimmt.
Mit 5 Abbildungen
Herrn Prof. Dr.H. Nowotny gewidmet. 相似文献
Thermodynamic investigations in theAu–Cd-system
Liquid and solid alloys in the Au–Cd system were investigated between 60 and 90 at % Cd and 653°–873°K by theemf method, and thermodynamic properties were calculated. The activities of Cd exhibit strong negative deviations from ideal behavior. Partial molar enthalpies are negative and become less so with increasing temperature; partial molar entropies are positive and increase with increasing temperature. A consistent set of integral thermodynamic data for liquid solutions of the entire Au–Cd system at 1000°K were calculated using values taken from the literature. Activities for some solid Cd-rich phases were also obtained. From discontinuities in theemf vs. temperature curve temperatures of phase transitions were determined.
Mit 5 Abbildungen
Herrn Prof. Dr.H. Nowotny gewidmet. 相似文献
115.
Uyeda HT Zhao Y Wostyn K Asselberghs I Clays K Persoons A Therien MJ 《Journal of the American Chemical Society》2002,124(46):13806-13813
The syntheses and electrooptic properties of a new family of nonlinear optical chromophores are reported. These species feature an ethyne-elaborated, highly polarizable porphyrinic component and metal polypyridyl complexes that serve as integral donor and acceptor elements. Examples of this structural motif include ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)(2+) bis-hexafluorophosphate (Ru-PZn); osmium(II) [5-(4'-ethynyl-(2,2';6',2'-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2'-terpyridine)(2+) bis-hexafluorophosphate (Os-PZn); ruthenium(II) [5-(4'-ethynyl-(2,2';6',2'-terpyridinyl))-15-(4'-nitrophenyl)ethynyl-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phen-yl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)(2+) bis-hexafluorophosphate (Ru-PZn-A); osmium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-15-(4'-nitrophenyl)ethynyl-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)(2+) bis-hexafluorophosphate (Os-PZn-A); and ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))osmium(II)-15-(4'-ethynyl-(2,2';6',2'-terpyridinyl))-10,20-bis (2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2';6',2'-terpyridine)(4+) tetrakis-hexafluorophosphate (Ru-PZn-Os). The frequency dependence of the dynamic hyperpolarizability of these compounds was determined from hyperRayleigh light scattering (HRS) measurements carried out at fundamental incident irradiation wavelengths (lambda(inc)) of 800, 1064, and 1300 nm. These data show that (i) coupled oscillator photophysics and metal-mediated cross-coupling can be exploited to elaborate high beta(0) supermolecules that exhibit significant excited-state electronic communication between their respective pigment building blocks; (ii) high-stability metal polypyridyl compounds constitute an attractive alternative to electron releasing dialkyl- and diarylamino groups, the most commonly used donor moieties in a wide range of established nonlinear optical dyes; (iii) this design strategy enables ready elaboration of chromophores having extraordinarily large dynamic hyperpolarizabilities (beta(lambda) values) at telecommunication relevant wavelengths; and (iv) porphyrin B- and Q-state-derived static hyperpolarizabilities (beta(0) values) can be designed to have the same or opposite sign in these species, thus providing a new means to regulate the magnitude of lambda(inc)-specific dynamic hyperpolarizabilities. 相似文献
116.
Paulson EK Morcombe CR Gaponenko V Dancheck B Byrd RA Zilm KW 《Journal of the American Chemical Society》2003,125(51):15831-15836
A new indirect detection scheme for obtaining (15)N/(1)H shift correlation spectra in crystalline proteins is described. Excellent water suppression is achieved without the need for pulsed field gradients, and using only a 2-step phase cycle. Careful attention to overall NMR instrument stability was found critical for obtaining the best resolution and sensitivity. Magnetic dilution by deuteration of the protein in combination with high-speed magic angle spinning produces (1)H resonances averaging only 0.22 ppm in width, and in some cases lines as narrow as 0.17 ppm are obtained. In application to two different polymorphs of ubiquitin, structure dependent differences in both (15)N and (1)H amide chemical shifts are observed. In one case, distinct shifts for different molecules in the asymmetric unit are seen, and all differ substantially from solution NMR shifts. A gain of 7 in sensitivity makes the method competitive with solution NMR as long as nanocrystalline samples are available. 相似文献
117.
Phosphoraneiminato Complexes of Iodine. Syntheses and Crystal Structures of Ph3PNIO2 and Ph3PNSiMe3 · I2 Ph3PNIO2 has been prepared as yellow crystals by the reaction of Ph3PNSiMe3 with I2O5 in boiling acetonitrile, whereas the molecular complex Ph3PNSiMe3 · I2 is formed as brown crystals by the reaction of Ph3PNSiMe3 with iodine in acetonitrile solution. Both complexes were characterized by crystal structure determinations. Ph3PNIO2: Space group P21/n, Z = 4, 2 858 observed unique reflections, R = 0.039. Lattice dimensions at 19°C: a = 972.8(2), b = 1 743.4(3), c = 1 073.7(2) pm, β = 115.46(3)°. The compound forms monomeric molecules with pyramidal geometry at the iodine atom. The bond angle PNI (126.9°) is unusually small; the PN bond length of 159.2 pm corresponds with a double bond. Ph3PNSiMe3 · I2: Space group P1 , Z = 2, 3 560 observed unique reflections, R = 0.033. Lattice dimensions at 19°C: a = 941.2(2), b = 1 041.7(2), c = 1 287.4(3) pm, α = 78.34(1)°, β = 72.00(2)°, γ = 86.08(2)°. The compound forms monomeric molecules, in which the I2 molecule and the nitrogen atom of the phosphoraneimine molecule realize a linear N? I? I axis with a bond length N? I of 243.2 pm. 相似文献
118.
Serge Schubert Philippe Renaud Pierre-Alain Carrupt Kurt Schenk 《Helvetica chimica acta》1993,76(7):2473-2489
Radical addition to enamines using Bu3SnH as reducing agent are reported (Schemes 2 and 4). The diastereoselectivity of these reactions was examined in different systems (Tables 1 and 2). Enamines derived from cyclic ketones such as cyclohexanone were alkylated with high diastereoselectivity with preferential formation of the cis-disubstituted cycloalkanes. In acyclic systems such as enamines derived from propiophenone and diethyl ketone, moderate to high stereoselectivities were observed in the H-abstraction step. A model based principally on minimization of allylic 1,3-strain (A1,3 strain) was deduced from the experimental results and semi-empirical (AM1) calculations. 相似文献
119.
Condensation of chlorocarbonylsulfenyl chloride 1 with 1-(5-(1,1-dimethylethyl)-1,2-oxazol-3-yl)-3-methylurea 4a has been found to give isomeric 2,4-disubstituted-1,2,4-thiazolidine-3,5-diones 5 and 6 . Assignments are confirmed by the X-ray structure data of 6 . Sulfenylation with alkoxycarbonylsulfenyl chlorides 7 and trichloromethylsulfenyl chloride 10 of 4 occurs exclusively on N-1 rather than N-3 of the urea moiety. With the isomeric 1-[3-(1,1-dimethylethyl)-1,2-oxazol-5-yl]-3-methylurea 14 and ethoxycarbonylsulfenyl chloride 7b , C-sulfenylated derivatives 15 and 16 are formed in low yield. 相似文献
120.
Hans-Ulrich Gonzenbach Inga-Mai Tegmo-Larsson Jean-Pierre Grosclaude Kurt Schaffner 《Helvetica chimica acta》1977,60(3):1091-1123
2-Cyclopentenyl and 3-phenyl-2-cyclopentenyl methyl ketones (15–18, 30, 31) undergo a 1,3-acetyl shift on direct irradiation, and the oxa-di-π-methane rearrangement to photochemically non-interconverting endo and exo bicyclo-[2.1.0]pentyl methyl ketones on triplet sensitization. Exceptions include the 2-methyl-3-phenyl-2-cyclopentenyl methyl ketone 32 and the 1-phenyl-2-cyclo-pentenyl methyl ketone 44 which are unreactive on direct irradiation and on triplet sensitization, respectively, and the 2-phenyl-2-cyclopentenyl methyl ketones 42 and 43 which do not react under either condition. The reactive triplet of the 3-phenyl-2-cyclopentenyl methyl ketone 30 has been identified as the localized styrene π,π*-state of ET=59 kcal/mol by comparison of its phosphorescence at 77K in rigid glasses with that of 1-phenyl-cyclopentene, and by the independence of the quantum yield on sensitizer energy in the range of 61–74 kcal/mol. 相似文献