全文获取类型
收费全文 | 30467篇 |
免费 | 4832篇 |
国内免费 | 4264篇 |
专业分类
化学 | 22862篇 |
晶体学 | 350篇 |
力学 | 2075篇 |
综合类 | 344篇 |
数学 | 3792篇 |
物理学 | 10140篇 |
出版年
2024年 | 63篇 |
2023年 | 521篇 |
2022年 | 711篇 |
2021年 | 893篇 |
2020年 | 1068篇 |
2019年 | 1154篇 |
2018年 | 891篇 |
2017年 | 924篇 |
2016年 | 1361篇 |
2015年 | 1306篇 |
2014年 | 1577篇 |
2013年 | 2084篇 |
2012年 | 2602篇 |
2011年 | 2596篇 |
2010年 | 2002篇 |
2009年 | 1716篇 |
2008年 | 2010篇 |
2007年 | 1786篇 |
2006年 | 1745篇 |
2005年 | 1543篇 |
2004年 | 1251篇 |
2003年 | 1187篇 |
2002年 | 1366篇 |
2001年 | 1220篇 |
2000年 | 838篇 |
1999年 | 770篇 |
1998年 | 563篇 |
1997年 | 484篇 |
1996年 | 524篇 |
1995年 | 401篇 |
1994年 | 355篇 |
1993年 | 263篇 |
1992年 | 260篇 |
1991年 | 268篇 |
1990年 | 188篇 |
1989年 | 157篇 |
1988年 | 126篇 |
1987年 | 94篇 |
1986年 | 114篇 |
1985年 | 87篇 |
1984年 | 69篇 |
1983年 | 62篇 |
1982年 | 43篇 |
1981年 | 41篇 |
1980年 | 33篇 |
1978年 | 24篇 |
1977年 | 25篇 |
1976年 | 23篇 |
1974年 | 21篇 |
1973年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
Chih-Wei HuangMuthian Shanmugasundaram Hao-Ming ChangChien-Hong Cheng 《Tetrahedron》2003,59(20):3635-3641
An efficient method for the preparation of various monosubstituted arylallenes, disubstituted allenes and alkenylallenes via palladium-catalyzed coupling of allenylstannanes with aryl iodides or alkenyl iodides is described. The coupling reaction was carried out in the presence of Pd(PPh3)4 and LiCl using DMF as solvent. The possible role of LiCl in this coupling process is discussed based on the 119Sn NMR studies. 相似文献
973.
Weiqiang Zhong Li Chen Renzhi Wang 《Journal of Radioanalytical and Nuclear Chemistry》1996,206(2):227-232
Human prostate specific antigen (hPSA) RIA was developed with hPSA and anti-PSA prepared in our laboratory. Its standard curve was linear with a sensitivity of 0.5 g/L. Serum PSA levels of 130 normal males ranged from 0 to 3.5 g/L (1.15±0.93 g/L), which are consistent with the results of other conventional RIA. The recovery, intra- and inter-assay coefficients of variation conform to the demands of RIA, and the results of 41 samples obtained by both the PSA RIA and PSA RIA of DPC were well correlated (=0.990). PSA level of 23 patients with prostatic carcinoma was 10400 g/L. 相似文献
974.
975.
The molar heat capacity of the azeotropic mixture composed of ethanol and toluene was measured by a high precision adiabatic calorimeter from 80 to 320 K. The glass transition and phase transitions of the azeotropic mixture were determined based on the heat capacity measurements. A glass transition at 103.350 K was found. A solid-solid phase transition at 127.282 K, two solid-liquid phase transitions at 153.612 and 160.584 K were observed, which correspond to the transition of metastable crystal to stable crystal of ethanol and the melting of ethanol and toluene, respectively. The thermodynamic functions and the excess ones of the mixture relative to the standard temperature 298.15 K were derived based on the relationships of the thermodynamic functions and the function of the measured heat capacity with respect to temperature. 相似文献
976.
The lower singlet excited states for dimethylaminobenzophenone have been investigated as a function of the twisting motion with inclusion of solvent effects. Theoretical calculations have been performed using time-dependent density functional theory. The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) basis set have been used to compute transition energies. The solvent effects have been described within the polarizable continuum model. Ground-state geometries are optimized using density functional theory with both B3LYP and MPW1PW91 functionals combined with 6-31G(d) basis sets. Vertical absorption energy calculations characterize the lower singlet excited states both in vacuum and in different kinds of solvents. A large redshift of the absorption maximum in the polar solvent suggests an intramolecular charge transfer character of the excited state. We have constructed the potential energy curves of two possible twisting motions of the excited states both in vacuum and in the polar solvent of acetonitrile: the twisting of only the dimethylamino group and the twisting of the dimethylaminophenyl group with respect to the benzoyl group. Both twisting processes predict the formation of the twisted intramolecular charge transfer state associated with the crossing of a low barrier. The presence of the polar solvent significantly changes the shape of the energy curves. Calculated emission energies for both the isolated and the solvated systems show a large Stokes shift between the absorption and fluorescence maxima. Two possible twisting motions produce similar fluorescence spectroscopic consequences. Our results including solvent effects explain the weak "dual-fluorescence" feature of dimethylaminobenzophenone, and imply that the two possible twisting motions may occur in the excited-state relaxation dynamics, but the twisting of the dimethylamino group seems to take place easier. 相似文献
977.
Weakly bound linear and bent dimers, FH—X (where X = CO, OC, CNH, NCH, N2O and ON2), are investigated using the DFT B3LYP and ab initio MP2 methods with the same basis sets (6–311++G(3df,2pd)). The strengths of the H—C or H—N H‐bonds in dimers FH—CO, FH—CNH, and FH—N2O are compared with those of the H—O or H—N H‐bonds in dimers FH—OC, FH—NCH, and FH—ON2. The results obtained for the H‐bond distances, the elongation effect of the HF bond, the red shift of the HF stretching frequency, and the energy difference between the dimer and the charge transfer reveal that the H‐bonds of the first group of dimers are stronger than those of the second. The Gibbs energies calculated for the six dimer formations indicate that the weakly bound dimers are unstable at room temperature (T = 298 K) (FH—X's → FH + X's, ΔG < 0). 相似文献
978.
Three novel Schiff base cadmium(II) complexes, derived from the end‐on (μ‐1,1‐N3) azide or end‐to‐end (μ‐1,3‐NCS) thio cyanate bridges and similar tridentate Schiff base ligands, have been synthesized under similar synthetic procedures and their crystal structures determined by X‐ray diffraction methods. They are the dinuclear double end‐on azide‐bridged [Cd2(L1)2(N3)2(μ‐1,1‐N3)2] ( 1 ), the dinuclear double end‐on azide‐bridged [Cd2(L2)2(N3)2(μ‐1,1‐N3)2] ( 2 ), and the dinuclear double end‐to‐end thiocyanate‐bridged [Cd2(L3)2(NCS)2(μ1,3‐NCS)2] ( 3 ), where L1, L2 and L3 are three similar tridentate Schiff bases obtained by condensation of 2‐pyridylaldehyde with N,N‐diethylethane‐1,2‐diamine, of 2‐pyridylaldehyde with N‐isopropylethane‐1,2‐diamine, and of 2‐pyridylaldehyde with N,N‐dimethylpropane‐1,3‐diamine, respectively. Each cadmium(II) centre in the complexes is in a distorted octahedral coordination. There is a crystallographic inversion centre in each of the complexes. The similar small ligands used as the secondary ligands in the preparation of the cadmium(II) complexes with similar Schiff bases can result in similar structures. 相似文献
979.
2-Halophenyl ketones 1a-e (1a, o-IC(6)H(4)COCH(3)) undergo carbocyclization with alkyl propiolates (2a, CH(3)(CH(2))(4)C[triple bond]CCO(2)CH(3); 2b, TMSC[triple bond]CCO(2)Et 2c, CH(3)C[triple bond]CCO(2)CH(3); 2d, CH(3)OCH(2)C[triple bond]CCO(2)CH(3); 2e, CH(3)(CH(2))(3)C[triple bond]CCO(2)CH(3); 2f, PhC[triple bond]CCO(2)CH(3); and 2g, (CH(3))(3)C[triple bond]CCO(2)CH(3)) in the presence of Ni(dppe)Br(2) and zinc powder in acetonitrile at 80 degrees C to afford the corresponding indenol derivatives 3a-m with remarkable regioselectivity in good to excellent yields. The nickel-catalyzed carbocyclization reaction was successfully extended to other simple disubstituted alkynes. Thus, the reaction of 2-halophenyl ketones 1a-e with disubstituted alkynes (2h, PhC[triple bond]CPh; 2i, CH(3)C(6)H(4)C[triple bond]CC(6)H(4)CH(3); 2j, CH(3)CH(2)C[triple bond]CCH(2)CH(3); 2k, PhC[triple bond]CCH(3); 2l, TMSC[triple bond]CCH(3); and 2m, PhC[triple bond]C(CH(2))(3)CH(3)) proceeded smoothly to afford the corresponding indenols 4a-t in good to excellent yields. For unsymmetrical alkynes 2k-m, the carbocyclization gave two regioisomers with regioselectivities ranging from 1:2 to 1:12 depending on the substituents on the alkyne and on the aromatic ring of halophenyl ketone. A possible mechanism for this nickel-catalyzed carbocyclization reaction is also proposed. 相似文献
980.
Both STO-3G ab initio and s-p separation-type-modified INDO semiempirical methods were applied to molecular-orbital calculation of the N20 molecule. From these two methods, the optimized bond distances between the nearest N atoms (dn-n) and the most calculated thermodynamic data are close to each other. The positive values of ΔHa° and ΔGa° for the atomization reaction in this work prove that N20 is stable. In contrast to conventional INDO and MINDO/3, but similar to former AMI and MNDO calculations, both ΔHr° and ΔGr° are positive in the formation reaction, which indicates that N20 belongs to the category of high-energy molecules. 相似文献