全文获取类型
收费全文 | 3852篇 |
免费 | 88篇 |
国内免费 | 8篇 |
专业分类
化学 | 2374篇 |
晶体学 | 9篇 |
力学 | 126篇 |
数学 | 655篇 |
物理学 | 784篇 |
出版年
2020年 | 53篇 |
2019年 | 39篇 |
2018年 | 25篇 |
2017年 | 32篇 |
2016年 | 67篇 |
2015年 | 60篇 |
2014年 | 51篇 |
2013年 | 112篇 |
2012年 | 104篇 |
2011年 | 135篇 |
2010年 | 76篇 |
2009年 | 87篇 |
2008年 | 128篇 |
2007年 | 119篇 |
2006年 | 111篇 |
2005年 | 98篇 |
2004年 | 86篇 |
2003年 | 81篇 |
2002年 | 60篇 |
2001年 | 52篇 |
2000年 | 63篇 |
1999年 | 48篇 |
1998年 | 42篇 |
1997年 | 44篇 |
1996年 | 60篇 |
1995年 | 44篇 |
1994年 | 42篇 |
1993年 | 49篇 |
1992年 | 47篇 |
1991年 | 76篇 |
1990年 | 57篇 |
1989年 | 51篇 |
1988年 | 53篇 |
1987年 | 42篇 |
1986年 | 36篇 |
1985年 | 55篇 |
1984年 | 60篇 |
1983年 | 50篇 |
1982年 | 60篇 |
1981年 | 57篇 |
1980年 | 63篇 |
1979年 | 54篇 |
1978年 | 65篇 |
1977年 | 40篇 |
1976年 | 41篇 |
1975年 | 45篇 |
1974年 | 25篇 |
1973年 | 30篇 |
1972年 | 26篇 |
1970年 | 27篇 |
排序方式: 共有3948条查询结果,搜索用时 15 毫秒
101.
The brown crystals of [NEt4]2[Se3Br8(Se2Br2)] ( 1 ) were obtained when selenium and bromine reacted in the solution of acetonitrile in the presence of tetraethylammonium bromide. The crystal structure of 1 has been determined by the X‐ray methods and refined to R = 0.0308 for 10433 reflections. The crystals are monoclinic, space group P21 with Z = 2 and a = 12.0393(3) Å, b = 11.8746(3) Å, c = 13.1946(3) Å, β = 96.561(1)° (123 K). In the solid state structure the anion of 1 is built up of Se3Br8 unit which consists of a triangular arrangement of three planar SeBr4 units sharing a common edge through two μ3‐bridging Br atoms, and one Se2Br2 molecule which is linked to one of μ3‐bridging Br atoms. The three SeII atoms form a triangle which is almost perpendicular to the planes given by three SeBr4 moieties. The contact between the μ3Br and the SeI atom of the Se2Br2 molecule is 3.1711(8) Å and can be interpreted as a bond of the donor‐acceptor type with the μ3Br as donor and the Se2Br2 molecule as acceptor. The terminal SeII‐Br and μ3Br‐SeII bond lengths are in the ranges 2.3537(7)–2.4737(7) Å and 2.7628(7)–3.1701(7) Å, respectively. The bond lengths in coordinated Se2Br2 molecule are: SeI‐SeI = 2.2636(9) Å, SeI‐Br = 2.3387(11) and 2.3936(8) Å. 相似文献
102.
A Lainez Emmerich Wilhelm G Roux-Desgranges J.-P.E Grolier 《The Journal of chemical thermodynamics》1985,17(12):1153-1161
Excess molar volumes VmE at 298.15 K were obtained, as a function of mole fraction x, for series I: {x1-C4H9Cl + (1 ? x)n-ClH2l + 2}, and II: {x1,4-C4H8Cl2 + (1 ? x)n-ClH2l + 2}, for l = 7, 10, and 14. 10, and 14. The instrument used was a vibrating-tube densimeter. For the same mixtures at the same temperature, a Picker flow calorimeter was used to measure excess molar heat capacities Cp, mE at constant pressure. VmE is positive for all mixtures in series I: at x = 0.5, VmE/(cm3 · mol?1) is 0.277 for l = 7, 0.388 for l = 10, and 0.411 for l = 14. For series II, VmE of {x1,4-C4H8Cl2 + (1 ? x)n-C7H16} is small and S-shaped, the maximum being situated at xmax = 0.178 with VmE(xmax)/(cm3 · mvl?1) = 0.095, and the minimum is at xmin = 0.772 with VmE(xmin)/(cm3 · mol?1) = ?0.087. The excess volumes of the other mixtures are all positive and fairly large: at x = 0.5, VmE/(cm3 · mol?1) is 0.458 for l = 10, and 0.771 for l = 14. The Cp, mEs of series I are all negative and |Cp, mE| increases with increasing l: at x = 0.5, Cp, mE/(J · K?1 · mol?1) is ?0.56 for l = 7, ?1.39 for l = 10, and ?3.12 for l = 14. Two minima are observed for Cp, mE of {x1,4-C4H8Cl2 + (1 ? x)n-C7H16}. The more prominent minimum is situated at x′min = 0.184 with Cp, mE(x′min)/(J · K?1 · mol?1) = ?0.62, and the less prominent at x″min = 0.703 with Cp, mE(x″min)/(J · K?1 · mol?1) = ?0.29. Each of the remaining two mixtures (l = 10 and 14) has a pronounced minimum at low mole fraction (xmin = 0.222 and 0.312, respectively) and a broad shoulder around x = 0.7. 相似文献
103.
Lipophilic trifluoroacetophenone derivatives incorporated in plasticized PVC membranes are able to selectively extract water and alcohols from the sample solution into the organic membrane phase, reversibly forming hydrates and hemi-acetals, respectively. Since this is accompanied by a change in the absorption spectrum of the acetophenone isologue, the chemical recognition process can directly be translated into an optical signal. With N-acetyl-N-dodecyl-4-trifluoroacetylaniline (ETH 6022) as the electrically neutral, lipophilized carrier ethanol can be determined from 0.5 to 35% (v/v) in aqueous solutions. The calibration curve for different ethanol-water mixtures shows a good correlation with the mathematically derived formalism and thus confirms the theoretically expected behavior. Besides high reproducibility of the optical signals, very short response times of less than 30 s were realized. The optode membrane presented exhibits a preference for ethanol compared to water by a factor greater than 11. The selectivities for several primary alcohols, such as methanol, ethanol, 1-propanol and 1-n-butanol, are comparable, but isopropanol and tert.-butanol are rejected by a factor of about 10. The alcohol concentration in different beverages was determined to evaluate the reliability of the system. The values obtained for wine, beer and different spirits show an excellent correlation with those obtained by a conventional approach involving distillation and density measurements. A residual standard deviation of ± 0.27% (v/v) over the 0.7–40% (v/v) range was found. 相似文献
104.
The behaviour under electron impact (70 eV) which includes some rearrangement processes of some tetraorganodiphosphanedisulfides R2P(S)-P(S)R2 (R ? CH3, C2H5, n-C3H7, n-C4H9, C3H5, C6H5) and CH3RP(S)–P(S)CH3R (R ? C2H5, n-C3H7, n-C4H9, C6H5, C6H5, C6H5,CH2) is reported and discussed. Fragmentation patterns which are consistent with direct analysis of daughter ions and defocusing metastable spectra are given. The atomic composition of many of the fragment ions was determined by precise mass measurements. In contrast to compounds R3P(S) loss of sulphur is not a common process here. The first step in the fragmentation of these compounds is cleavage of one P–C bond and loss of a substituent R?. The second step is elimination of RPS leading to [R2PS]+ from which the base peaks in nearly all the spectra arise. The phenyl substituted compounds give spectra with very abundant [(C6H5)3P]+. and [(C6H5)2CH3P]+. ions respectively, resulting from [M]+. by migration of C6H5. Rearrangement of [M]+. to a 4-membered P-S ring system prior to fragmentation is suggested. 相似文献
105.
A recently characterized oviposition-deterring pheromone (ODP, structure 1) of the European cherry fruit fly was used as a test case for probing the potential of tandem mass spectrometry (MS/MS) in structure elucidation as a stand-alone technique. The glycolipid-taurinate 1 was subjected to MS/MS analyses under a variety of conditions with and without preceding chemical degradation. Acidic methanolysis of 1 and subsequent in-batch derivatization (trideuterioacetylation) yielded methyl 2,3,4,6-tetrakis-O-trideuterioacetyl-glucopyranoside (2), methyl 8,15-bis-trideuterioacetoxy-palmitate (3), and taurine (4) as suitable target compounds for direct mixture analysis.Low energy collision induced dissociation (CID) on selected precursor ions (MS/MS on [M + H – CH3OH]+ and [M + H]+ produced by fast atom bombardment (FAB)) allowed direct identification of 2 and 4, respectively, by comparison with appropriate reference ions. In the case of 3, low energy CID (desorption chemical ionization (DCI) instead of FAB, MS/MS on [M + H]+) permitted deduction of gross molecular structure, but failed to provide positional detail. In sharp contrast,high energy CID of trideuterioacetylated intact 1 (FAB-MS/MS on [M – H]– ions of la) clearly revealed a linear 8,15-hydroxylated palmitic acid backbone. Less certain was assignment of 15-O-glucosylation by this approach. 相似文献
106.
A. Lainez M. M. Rodrigo Emmerich Wilhelm J. -P. E. Grolier 《Journal of solution chemistry》1992,21(1):49-65
Excess molar volumes V
E
and excess molar heat capacities C
P
/E
at constant pressure have been obtained, as a function of mole fraction x1, for several binary liquid mixtures belonging either to series I: pyridine+n-alkane (ClH2l+2), with l=7, 10, 14, 16, or series II: piperidine+n-alkane, with l=7, 8, 10, 12, 14. The instruments used were a vibrating-tube densimeter and a Picker flow microcalorimeter, respectively. V
E
of pyridine+n-heptane shows a S-shaped composition dependence with a small negative part in the region rich in pyridine (x1>0.90). All the other systems show positive V
E
only. The excess volumes increase with increasing chain length l of the n-alkane. The excess molar heat capacities of the mixtures belonging to series II are all negative, except for a small positive part for piperidine+n-heptane in the region rich in piperidine (x1>0.87). The C
P
/E
at the respective minima, C
P
/E
(x1,min
), become more negative with increasing l, and the x1,min
values range from about 0.26 (l=7) to 0.39 (l=14). Most interestingly, mixtures of series I exhibit curves of C
P
/E
against x1 with two minima and one maximum, the so-called W-shape curves.Dedicated to Professor A. Néckel on the occasion of his 65th birthday. Communicated in part at the XVIIèmes Journées de Calorimétrie, d'Analyse Thermique et de Thermodynamique Chimique, Ferrara, Italy, 27–30 October, 1986. 相似文献
107.
Results of voltammetric, potentiometric, chronoamperometric, ion transport, and extraction studies on neutral carrier based, plasticized poly(vinyl chloride) membranes are summarized. They unambiguously confirm that such bulk membranes dispose of immobile anionic sites. These fixed sites lead to a Donnan exclusion of other anions from the membrane and thus to a permselectivity for cations. The results are in perfect agreement with the predictions of earlier membrane models. A rigorous Poisson-Boltzmann analysis of macroscopic liquid membranes clearly indicates that space-charge at the membrane/solution interfaces does not influence the electrochemical properties and the ion-selectivity behavior at steady state. 相似文献
108.
Wurche F Sicking W Sustmann R Klärner FG Rüchardt C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(11):2707-2721
The effect of pressure on the oxidation of hydroarenes 3-9 with 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ; 1 a) or o-chloranil (10), leading to the corresponding arenes, has been investigated. The activation volumes were determined from the pressure dependence of the rate constants of these reactions monitored by on-line UV/Vis spectroscopic measurements in an optical high-pressure cell (up to 3500 bar). The finding that they are highly negative and only moderately dependent on the solvent polarity (DeltaV( not equal ) = -13 to -25 in MTBE and -15 to -29 cm(3) mol(-1) in MeCN/AcOEt, 1:1) rules out the formation of ionic species in the rate-determining step and is good evidence for a hydrogen atom transfer mechanism leading to a pair of radicals in the rate-determining step, as was also suggested by kinetic measurements, studies of kinetic isotope effects, and spin-trapping experiments. The strong pressure dependence of the kinetic deuterium isotope effect for the reaction of 9,10-dihydroanthracene 5/5-9,9,10,10-D(4) with DDQ (1 a) can be attributed to a tunneling component in the hydrogen transfer. In the case of formal 1,3-dienes and enes possessing two vicinal C--H bonds, which have to be cleaved during the dehydrogenation, a pericyclic hydrogen transfer has to considered as one mechanistic alternative. The comparison of the kinetic deuterium isotope effects determined for the oxidation of tetralin 9/9-1,1,4,4-D(4)/9-2,2,3,3-D(4)/9-D(12) either with DDQ (1 a) or with thymoquinone 1 c indicates that the reaction with DDQ (1 a) proceeds in a stepwise manner through hydrogen atom transfer, analogously to the oxidations of 1,4-dihydroarenes, whereas the reaction with thymoquinone 1 c is concerted, following the course of a pericyclic hydrogen transfer. The difference in the mechanistic courses of these two reactions may be explained by the effect of the CN and Cl substituents in 1 a, which stabilize a radical intermediate better than the alkyl groups in 1 c. The mechanistic conclusions are substantiated by DFT calculations. 相似文献
109.
The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms. 相似文献
110.
Max Winter Karl H. Schulte-Elte Alain Velluz Josef Limacher Wilhelm Pickenhagen Günther Ohloff 《Helvetica chimica acta》1979,62(1):131-134
Aroma Constituents of the Purple Passion Fruit. Two New Edulan Derivatives The isolation of (2R*, 4S*, 4aS*, 8aS*)-4,4a-epoxy-4,4a-dihydroedulan ( 1 ), and (2R*, 3S*, 8aS*)-3-hydroxyedulan ( 2 ), two new constituents of the purple passion-fruit (Passiflora edulis SIMS ), is reported. Racemic epoxide 1 was synthesized by oxidation of edulan 6 with peracidic acid, and racemate of alcohol 2 was obtained by reduction of ketone 7 , one of the chromic acid oxidation-products of edulan 6 . 相似文献