全文获取类型
收费全文 | 2038篇 |
免费 | 58篇 |
国内免费 | 2篇 |
专业分类
化学 | 1443篇 |
晶体学 | 37篇 |
力学 | 63篇 |
数学 | 166篇 |
物理学 | 389篇 |
出版年
2023年 | 20篇 |
2022年 | 39篇 |
2021年 | 49篇 |
2020年 | 44篇 |
2019年 | 74篇 |
2018年 | 50篇 |
2017年 | 48篇 |
2016年 | 63篇 |
2015年 | 42篇 |
2014年 | 63篇 |
2013年 | 156篇 |
2012年 | 129篇 |
2011年 | 144篇 |
2010年 | 77篇 |
2009年 | 56篇 |
2008年 | 102篇 |
2007年 | 99篇 |
2006年 | 91篇 |
2005年 | 85篇 |
2004年 | 100篇 |
2003年 | 49篇 |
2002年 | 36篇 |
2001年 | 21篇 |
2000年 | 18篇 |
1999年 | 24篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 13篇 |
1995年 | 16篇 |
1993年 | 14篇 |
1992年 | 14篇 |
1990年 | 10篇 |
1989年 | 17篇 |
1988年 | 10篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 26篇 |
1979年 | 12篇 |
1978年 | 14篇 |
1977年 | 23篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1971年 | 8篇 |
排序方式: 共有2098条查询结果,搜索用时 46 毫秒
21.
22.
Bonner BE Buchanan JA Clement JM Corcoran MD Krishna NM Kruk JW Miettinen HE Moss RM Mutchler GS Nessi-Tedaldi F Nessi M Phillips GC Roberts JB Stevenson PM Tonse SR Birman A Chung SU Etkin A Fernow RC Kirk H Protopopescu SD Willutzki H Hallman T Madansky L Bar-Yam Z Dowd J Kern W King E Mayes BW Pinsky LS 《Physical review D: Particles and fields》1990,41(1):13-16
23.
Bonner BE Buchanan JA Clement JM Corcoran MD Krishna NM Kruk JW Lincoln DW Miettinen HE Mutchler GS Nessi-Tedaldi F Nessi M Phillips GC Roberts JB Stevenson PM Tonse SR White JL Chung SU Etkin A Fernow RC Protopopescu SD Willutzki H Hallman T Madansky L Pinsky LS 《Physical review letters》1989,62(14):1591-1594
24.
Krishna B. Pandeya Trilochan S. Waraich Ramesh C. Gaur Rajendra P. Singh 《Transition Metal Chemistry》1982,7(3):146-148
Summary Tris-chelates of chromium(III) have been synthesised with five new dithiocarbamates, [RR'NCS2]–, where R=PhCH2 and R/t'=H, PhCH2, Me, Et and i-Pr. Magnetic moments together with electronic, i.r. and e.s.r spectra of the complexes have been described. Various ligand-field parameters have been evaluated and discussed. 相似文献
25.
Reddy PN Srikanth R Swamy NS Srinivas R Sharma GV Nagendar P Krishna PR 《Journal of mass spectrometry : JMS》2005,40(11):1429-1438
Positive and negative ion electrospray ionization (ESI) tandem mass spectral study of a new series of hybrid peptides, viz, BocN-alpha,beta-peptides and BocN-beta,alpha-peptides, synthesized from C-linked carbo-beta3-amino acids [Caa (S)] and L-Ala has been carried out. The alpha,beta-peptides have been differentiated from beta,alpha-peptides by the collision-induced dissociation (CID) of [M + H]+ and [M - H]- ions in positive and negative ion ESI-MS respectively. The fragment ion [M + H - C(CH3)3 + H]+ formed from [M + H]+ ions by the loss of 2-methyl-prop-2-ene in alpha,beta-peptides with L-Ala at the N-terminus is insignificant or totally absent for beta,alpha-peptides which have the Caa (S) at N-terminus. The fragment ion [M - H-C(CH3)3OH - HNCO]- formed from [M - H]- of beta,alpha-peptide acids is totally absent for alpha,beta-peptide acids. This has been attributed to the absence of the beta-methylene group in alpha,beta-peptides, and the participation of the beta-methylene group in the loss of HNCO in beta,alpha-peptide acids is confirmed by the deuteration experiments. The CID of [M + H-Boc + H]+ ions of these peptides also produce characteristic fragmentation. In the CID spectra of alpha,beta-peptides, the b(n)+ ions and the resulting y(n)+ ions occur at a mass difference of 243 and 71 Da corresponding to the successive losses of Caa and L-Ala, whereas a mass difference of 71 and 243 Da is observed for beta,alpha-peptides. In contrast to the CID of protonated peptides, the CID of [M - H]- ions of the alpha,beta- and beta,alpha-peptide acids do not give b(n)- ions and show abundant z(n) (-) ions. Further, a pair of diastereomeric dipeptide esters and acids have been distinguished by the CID of [M + H]+ ions. The loss of 2-methyl-prop-2-ene is more pronounced for Boc-NH-Caa(R)-D-Ala-OCH3 (21) and Boc-NH-Caa(R)-D-Ala-OH (23) with Caa (R) at the N-terminus, whereas it is totally absent for Boc-NH-Caa (S)-D-Ala-OCH3 (22) and Boc-NH-Caa(S)-D-Ala-OH (24) peptides, which have Caa (S) at the N-terminus. Thus, on the basis of our previous and present studies, we propose that the CID of [M + H]+ ions provides a simple and useful method for distinguishing the configuration of Caa (S or R) at the N-terminus of BocN-carbo beta,alpha- and beta,beta-dipeptides. 相似文献
26.
Krishna Kumar Gollapelli Vaibhav B. Patil Allam Vinaykumar Rambabu Chegondi 《Chemical science》2021,12(4):1544
A Rh(i)-catalyzed highly stereoselective desymmetrization of 2-alkynylbenzaldehyde-tethered cyclohexadienones triggered by intramolecular Huisgen-type [3 + 2] cycloaddition has been developed. This method enables convergent construction of complex epoxy-bridged polycyclic ring systems with five contiguous stereocenters with excellent exo-selectivity and broad substrate scope. The highly atom-economical process involves 6-endo-dig cyclization of carbonyl oxygen onto an activated alkyne resulting in a highly reactive metal–benzopyrylium intermediate, which readily undergoes intramolecular [3 + 2] annulation/hydration. Asymmetric induction is also achieved for the first time in Rh(i)-catalyzed 1,3-dipolar cycloaddition using an easily accessible chiral diene as the ligand.A Rh(i)-catalyzed highly stereoselective desymmetrization of 2-alkynylbenzaldehyde-tethered cyclohexadienones triggered by intramolecular Huisgen-type [3 + 2] cycloaddition has been developed. 相似文献
27.
We report here a methodology for the construction of a conjugated cyanodiene synthon and the propensity of such synthons to participate in the olefin metathesis reaction. To this end, we have developed a strategy for the construction of the C11-C15 fragment of borrelidin and demonstrated the utility of the RCM reaction in the preparation of the final macrolide. To our knowledge, this is the first example of a RCM with a nitrile functionality on a diene. 相似文献
28.
G.V.M. Sharma J. Janardhan Reddy P. Sree Lakshmi Palakodety Radha Krishna 《Tetrahedron letters》2004,45(41):7729-7732
A practical and efficient electrophilic substitution reaction of indoles with a variety of aldehydes was carried out using catalytic trichloro-1,3,5-triazine (10 mol %) in acetonitrile to furnish the corresponding bis(indolyl)methanes in excellent yields. Similarly, sugar derived aldehydes gave hitherto unknown bis(indolyl)glycoconjugates in very good yields. 相似文献
29.
Krishna Gopal Dongol M. Cristina Melo e Silva Kouki Matsubara Taisuke Matsumoto Shuntaro Mataka Thies Thiemann 《无机化学与普通化学杂志》2003,629(6):945-947
The preparation of two η6‐estra‐1, 3, 5(10), 6‐tetraene tricarbonylchromium complexes 4 and 6 are described. In both cases only one stereoisomer can be isolated, in contrast to other estrane‐tricarbonylchromium complexes, where complexations are non‐stereoselective. X‐ray crystal structural analysis of 4 discloses that only the more sterically hindered β‐facial isomer is formed. It is assumed that the 6, 7‐olefinic moiety exerts a directive influence on the complexation. 相似文献
30.
The condensation products of 5-amino-1,3-disubstituted-pyrazoles with aromatic aldehydes were identified as 2,4-dihydro-2,5-diphenyl-4-(phenylmethylene)-3H-pyrazol-5-imine derivatives Treatment of these products with mercaptoacetic acid gave new fluorine containing 1H-pyrazolo[3,4-e][1,4]thiazepin-7-ones. 相似文献