全文获取类型
收费全文 | 2223篇 |
免费 | 121篇 |
国内免费 | 14篇 |
专业分类
化学 | 1789篇 |
晶体学 | 11篇 |
力学 | 28篇 |
数学 | 191篇 |
物理学 | 339篇 |
出版年
2024年 | 2篇 |
2023年 | 36篇 |
2022年 | 110篇 |
2021年 | 100篇 |
2020年 | 76篇 |
2019年 | 79篇 |
2018年 | 52篇 |
2017年 | 44篇 |
2016年 | 119篇 |
2015年 | 104篇 |
2014年 | 79篇 |
2013年 | 135篇 |
2012年 | 171篇 |
2011年 | 202篇 |
2010年 | 115篇 |
2009年 | 88篇 |
2008年 | 110篇 |
2007年 | 111篇 |
2006年 | 100篇 |
2005年 | 87篇 |
2004年 | 66篇 |
2003年 | 59篇 |
2002年 | 44篇 |
2001年 | 14篇 |
2000年 | 15篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 29篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 19篇 |
1992年 | 22篇 |
1991年 | 12篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1925年 | 1篇 |
1924年 | 1篇 |
排序方式: 共有2358条查询结果,搜索用时 15 毫秒
61.
Frank J. Millero Sara Sotolongo David J. Stade Carmen A. Vega 《Journal of solution chemistry》1991,20(11):1079-1092
The oxidation of Fe(II) with H2O2 has been measured in NaCl and NaClO4 solutions as a function of pH, temperature T (K) and ionic strength (M, mol-L–1). The rate constants, k (M–1-sec–1), d[Fe(II)]/DT=-k[Fe(II)][2O2]at pH=6.5 have been fitted to equations of the formlog k = log k0+ AI
1/2+BI+CI
1/2/T
Where log k0=15.53-3425/T in water; A=–2.3, –1.35; B=0.334, 0.180; and C=391, 235, respectively, for NaCl (=0.09) and NaClO4 ( =0.08). Measurements made in NaCl solutions with added anions yield rates in the order B(OH)
4
–
>HCO
3
–
>ClO
4
–
>Cl–>NO
3
–
>SO
4
2–
and are attributed to the relative strength of the interactions of Fe2+ or FeOH+ with these anions. The FeB(OH)
4
+
species is more reactive while the FeCO
3
0
, FeCl+, FeNO
3
+
and FeSO
4
0
species are less reactive than the FeOH+ ion pair. The general trend is similar to our earlier studies of the oxidation of Fe(II) with O2 except for B(OH)
4
–
. The effect of pH on the logk was found to be a quadratic function of the concentration of H+ or OH– from pH=4 to 8. These results have been attributed to the different rate constants for Fe2+ (k0) and FeOH+ (k1) which are related to the measured k by, k=k0Fe + k1FeOH, where i is the molar fraction of species i. The rates increase due to the greater reactivity of FeOH+ compared to Fe2+. k0 is independent of composition and ionic strength but k1 is a function of ionic strength and composition due to the interactions of FeOH+ with various anions. 相似文献
62.
Kyle Clagg Sara Hold Archana Kumar Stefan G. Koenig Remy Angelaud 《Tetrahedron letters》2019,60(1):5-7
The direct application of Knochel-Hauser derivative arylmagnesium halides in Kumada-Corriu couplings is described. By utilizing the in situ-generated Grignard reagents, the transmetalation with stoichiometric zinc salts to known Negishi coupling conditions is avoided, thereby streamlining the transformation. Several aromatic hetereocycles participate in the deprotonation and couple with a variety of phenyl iodides and bromides in isolated yields up to 81%. The parent system is demonstrated on 6?g scale with 79% yield, and monitored via ReactIR to show the stability of the Grignard and progression of the deprotonation/CC coupling reaction. 相似文献
63.
Twenty-one R2R′2N+X− -type (R=methyl or ethyl, R′=alkyl, X=Br or I) quaternary ammonium (QA) halides have been prepared by using a novel one-pot synthetic route in which a formamide (dimethyl-, diethylformamide, etc.) is treated with alkyl halide in the presence of sodium or potassium carbonate. The formation of QA halides was verified with 1H-NMR, 13C-NMR, MS and elemental analysis. The crystal structures of four QA halides (two bromide and two iodide) were determined using X-ray single crystal diffraction, and the powder diffraction method was used to study the structural similarities between the single crystal and microcrystalline bulk material. The thermal properties of all compounds were studied using TG/DTA and DSC methods. The smallest compounds decomposed during or before melting. The decreasing trend of melting points was observed when the alkyl chain length was increased. The liquid ranges of 120-180 °C were observed for compounds with 5-6 carbon atoms in the alkyl chain. The low melting points and wide liquid ranges suggest potential applicability of these compounds for example as ionic liquids precursors. 相似文献
64.
A novel procedure for immobilization of liposomes inside fused-silica capillaries is demonstrated. First, the inner wall of the capillaries was coated with a positively charged polymer, composed of derivatized agarose. Subsequently, negatively charged liposomes were immobilized by electrostatic interaction on the polymer coating. The developed liposome coated capillaries were used as a nanoseparation tool for studying interactions between small drug compounds and liposomes. Part of this work was presented at the 15th International Symposium on Microscale Separations and Analysis, HPCE 2002, Stockholm, Sweden, April 2002. 相似文献
65.
66.
A 28-residue beta-hairpin dimer (WKWK)2 with two Trp and two Lys residues on one face of each beta-sheet was shown to form a complex with single-stranded oligonucleotides at low micromolar concentrations. Each beta-hairpin of the dimer contains a cross-strand Trp-Trp pair in a diagonal orientation which has previously been shown to create a cleft for the intercalation of aromatic guests such as adenine (J. Am. Chem. Soc. 2003, 125, 9580). The beta-hairpin dimer binds 5-residue ssDNA sequences 5'-AAAAA-3', 5'-TTTTT-3', and 5'-CCCCC-3' in water with dissociation constants in the range of 12-30 muM. A weak energetic preference for binding to sequence 5'-AAAAA-3' was observed, which is believed to result from stronger stacking interactions between Trp and the adenine base. The interaction of 5'-AAAAA-3' with the Lys and Trp residues of the peptide was evident by NMR, and a 1:1 association was demonstrated. The recognition of an 11-residue ssDNA sequence occurred with a dissociation constant of 3 muM under near-physiological ionic strength and pH, demonstrating that the beta-hairpin dimer binds ssDNA as strongly as many naturally occurring proteins. The salt dependence of the interaction of the 11-residue oligonucleotide with the peptide dimer indicates that Trp-nucleobase stacking interactions contribute about -4 kcal/mol to recognition, which is much greater than the contribution of nonionic interactions in unstructured peptides containing Trp. Moreover, recognition of the ssDNA demonstrated reduced salt dependence relative to the corresponding duplex, resulting in selectivity for ssDNA under high salt conditions. Peptide (WKWK)2 is a relevant mimic of OB-fold (oligonucleotide/oligosaccharide-binding) proteins which bind ssDNA on the surface of a beta-sheet. 相似文献
67.
Rate constants for the reactions of OH radicals and NO3 radicals with dimethyl phosphonate [DMHP, (CH3O)2P(O)H], dimethyl methylphosphonate [DMMP, (CH3O)2P(O)CH3], and dimethyl ethylphosphonate [DMEP, (CH3O)2P(O)C2H5] have been measured at 296 +/- 2 K and atmospheric pressure using relative rate methods. The rate constants obtained for the OH radical reactions (in units of 10(-12) cm3 molecule(-1) s(-1)) were as follows: DMHP, 4.83 +/- 0.25; DMMP, 10.4 +/- 0.6; and DMEP, 17.0 +/- 1.0, with a deuterium isotope effect of k(OH + DMMP)/k(OH + DMMP-d9) = 4.8 +/- 1.2. The rate constants obtained for the NO3 radical reactions (in units of 10(-16) cm3 molecule(-1) s(-1)) were as follows: DMHP, < 1.4; DMMP, 2.0 +/- 1.0; and DMEP, 3.4 +/- 1.4. Upper limits to the rate constants for the O3 reactions of < 8 x 10(-20) cm3 molecule(-1) s(-1) for DMHP and < 6 x 10(-20) cm3 molecule(-1) s(-1) for DMMP and DMEP were determined. Products of the reactions of OH radicals with DMHP, DMMP, and DMEP were investigated in situ using atmospheric pressure ionization mass spectrometry (API-MS) and, for the DMMP and DMEP reactions, Fourier transform infrared (FT-IR) spectroscopy. API-MS analyses showed the formation of products of molecular weight 96 and 126, attributed to CH3OP(O)(H)OH and (CH3O)2P(O)OH, respectively, from DMHP; of molecular weight 110, attributed to CH3OP(O)(CH3)OH, from DMMP; and of molecular weight 124 and 126, attributed to CH3OP(O)(C2H5)OH and (CH3O)2P(O)OH, respectively, from DMEP. FT-IR analyses showed formation (values given are % molar yields) of the following: from DMMP, CO, 54 +/- 6; CO2, 5 +/- 1 in dry air; HCHO, 3.9 +/- 0.7; HC(O)OH, < 1.4 in dry air; RONO2, approximately 4; and formate ester, approximately 8; and from DMEP, CO, 50 +/- 7; CO2, 11 +/- 4; CH3CHO, 18 +/- 8; HCHO, < 7; HC(O)OH, < 6; RONO2, < or = 5; and formate ester, 5.0 +/- 1.5. Possible reaction mechanisms are discussed. 相似文献
68.
69.