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91.
D. D. K. Chingakule P. Gans J. B. Gill P. J. Longdon 《Monatshefte für Chemie / Chemical Monthly》1992,123(6-7):521-535
Summary The vibrational spectra of solutions have been analyzed to assess both qualitatively and quantitatively the changes in enthalpy and entropy for ion pair formation in solutions of LiNCS, Mg(NCS)2, and LiN3 in liquid ammonia, dimethylformamide, dimethylsulphoxide and acetonitrile. Contrary to predictions both the H
ass
and S
ass
terms are all positive in the cases examined, indicating that the driving force in the ion association process derives from solvent-solute restructuring, and not the energy of the interaction between the cation and anion. This characteristic of contact ion pair formation is likely to be found to be applicable over a wide range of solvents. The following specific values of the thermodynamic parameters at 298 K have been obtained: LiNCS/DMF, G=–1.3 (1) kJ mol–1, H
ass
=+1.8 (5) kJ mol–, S
ass
=+10 (2) J mol–1 K–1; LiNCS/DMSO, G=+0.9 (2) kJ mol–1, H
ass
=+0.3 (3) kJ mol–1; Mg(NCS)2/DMF, G
ass
=–4.0 (3) kJ mol–1, H
ass
=+15 (4) kJ mol–1, S=+64 (17) kJ mol–1; LiN3/DMSO, G
ass
=–2.5 (3) kJ mol–1, H
ass
=+4.9 (9) kJ mol–1, S
ass
=+25 (10) J K–1 mol–1.Submitted to celebrate the 70th Birthday of Professor Viktor Gutmann, and in recognition of his considerable contributions towards the better understanding of Chemistry in the Solution Phase 相似文献
92.
Miaojun Wang 《Journal of solid state chemistry》2005,178(4):1182-1189
Lithium substituted Li1+xMn2−xO4 spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na2O·4MoO3 at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn3+ to Mn4+ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn3+ to Mn4+ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction. 相似文献
93.
Rimma V. Golovnya Diana N. Grigoryeva Alexei V. Vasilyev 《Journal of separation science》1991,14(11):741-744
The enthalpy and entropy of sorption of methylene units in the homologous series of n-alkyl acetates; methyl, n-butyl, and phenyl n-alkyl ketones; and the methyl esters and chloroanhydrides of n-aliphatic carboxylic acids have been determined on an SE-54 capillary column. The enthalpy of sorption of the first methylene unit is anomalously high when the growing n-alkyl chain is connected directly to a carbonyl group. This effect is a result of intramolecular interaction between neighboring methyl and carbonyl groups. It has been shown by measurement of the enthalpy and entropy of sorption of the difluoromethylene unit in the series of n-hexyl esters of perfluorinated carboxylic acids that for these compounds the effect is absent. The intramolecular interaction was found to increase in the order methyl butyl ketone < methyl phenyl ketone < methyl acetate < acetyl chloride < acetone. 相似文献
94.
Aliaksei A. Strechan Gennady J. Kabo Yauheni U. Paulechka 《Fluid Phase Equilibria》2006,250(1-2):125-130
Correlation relations based on Stefan's rule, which defined dependence between the enthalpy of vaporization, the surface tension, the molar volume and the molar mass of a substance, were obtained. For development of the correlation equations two computational procedures were used: a method of the least squares and a method of artificial neural networks. The method of artificial neural networks was shown to give somewhat better results than the linear least-squares procedure. The average deviation of the calculated values from the experimental ones did not exceed 6% for training set of substances and 10% for control set (the method of the least squares). For the method of artificial neural networks it is 3% and 8%, respectively. 相似文献
95.
等电子-等自旋与非等旋反应的G2(MP2)和G2研究 总被引:1,自引:1,他引:1
我们在前文[1-2]中分别用MP2-4/6-31G**//MP2/6-31G**及MP4/6-311G(2df,Zpd)//MP2/6-31G**研究了一些双原子氢化物、卤化物、硫化物和氧化物的化学反应的烂变·这些化学反应按如下类型分为四组,即(1)反应物与生成物之间为等电子一等自旋关系,(2)价层等电一等旅,(3)等施和(4)非等旅·结果表明,MP4/6-3fiG(2才,ZPd)对于(1),(2)和(3)类反应,基本上与实验误差小于士15kJ·mo-‘而对非等旋反应仍有较大误差;MPZ-4/6-31G””只对(1)类反应较好.由于PoPle等人近几年来创立的Gaussi… 相似文献
96.
Rafael Notario Maria Victoria Roux Daniel A. Bors Joel F. Liebman 《Structural chemistry》2007,18(3):395-398
Acrylic acid is a key industrial compound with numerous uses. Despite its importance, its enthalpy of formation is still contentious—even
ignoring “ancient” determinations, there is a 12 kJ mol−1 range of values reported for the gas phase quantity, −320 to −332 kJ mol−1. Our quantum chemical calculations using current methodology suggest the value of −321 ± 3 kJ mol−1. 相似文献
97.
So-Nhu Le 《Journal of solid state chemistry》2006,179(12):3731-3738
Thermal behavior, relative stability, and enthalpy of formation of α (pink phase), β (blue phase), and red NaCoPO4 are studied by differential scanning calorimetry, X-ray diffraction, and high-temperature oxide melt drop solution calorimetry. Red NaCoPO4 with cobalt in trigonal bipyramidal coordination is metastable, irreversibly changing to α NaCoPO4 at 827 K with an enthalpy of phase transition of −17.4±6.9 kJ mol−1. α NaCoPO4 with cobalt in octahedral coordination is the most stable phase at room temperature. It undergoes a reversible phase transition to the β phase (cobalt in tetrahedra) at 1006 K with an enthalpy of phase transition of 17.6±1.3 kJ mol−1. Enthalpy of formation from oxides of α, β, and red NaCoPO4 are −349.7±2.3, −332.1±2.5, and −332.3±7.2 kJ mol−1; standard enthalpy of formation of α, β, and red NaCoPO4 are −1547.5±2.7, −1529.9±2.8, and −1530.0±7.3 kJ mol−1, respectively. The more exothermic enthalpy of formation from oxides of β NaCoPO4 compared to a structurally related aluminosilicate, NaAlSiO4 nepheline, results from the stronger acid-base interaction of oxides in β NaCoPO4 (Na2O, CoO, P2O5) than in NaAlSiO4 nepheline (Na2O, Al2O3, SiO2). 相似文献
98.
Maria D.M.C. Ribeiro da Silva Mnica A.A. Vieira Chelsea Givens Stephanie Keown William E. Acree Jr. 《Thermochimica Acta》2006,450(1-2):67-70
The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline 2,3,5-trimethylpyrazine-1,4-dioxide and 2,3,5,6-tetramethylpyrazine-1,4-dioxide were measured, at T = 298.15 K, by static bomb calorimetry and the standard molar enthalpies of sublimation, at T = 298.15 K, were obtained from Calvet microcalorimetric measurements. These values were used to derive the respective standard molar enthalpies of formation in gaseous phase. The mean N–O bond dissociation enthalpy has been calculated for both compounds. 相似文献
99.
Tuhina Banerjee 《Thermochimica Acta》2004,411(2):195-201
A differential scanning calorimetry study on the thermal denaturation of concanavalin A at pH 5.2 where it exists in the dimeric form was carried out. The calorimetric transitions were observed to be irreversible and the transition temperature of the protein increased with increasing scan rate, indicating that the thermal denaturation process is under kinetic control. The thermal unfolding, and its scan rate dependence could be explained according to the kinetic scheme with k as first-order kinetic constant whose change with temperature is given by the Arrhenius equation. Using this model, rate constant as a function of temperature and activation energy of the process have been calculated. The average activation energy of the kinetic process using different approaches is 129±10 kJ mol−1. The differential scanning calorimetric results on transition temperatures and calorimetric enthalpies supported by intrinsic fluorescence indicate that the irreversibility in the calorimetric transitions of concanavalin A includes a combination of post-transition aggregation, chain separation and loss of cofactor. 相似文献
100.