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1.
A preparative route to cis- and trans-1,2-dibromocyclopropane () was developed via the Hunsdiecker reaction of silver cyclopropane-1,2-dicarboxylate (). Cis- and trans- gave the same ratio of cis- and trans- (1:3.2). The mechanism of this reaction is briefly discussed. 相似文献
2.
Schultz György Hargittai István Rot Nicolette Bickelhaupt Friedrich 《Structural chemistry》1998,9(3):209-214
The molecular structure of 1,3,5-tris (trimethylstannyl) benzene has been determined by gas-phase electron diffraction. The C — C bond length is in good agreement with that in benzene. In agreement with the somewhat electron-releasing character of the substituents, the endocyclic bond angles at the substituents are somewhat smaller than 120°. The mean value of Sn — C bond lengths is greater than that in tetraphenyltin and tetramethyltin. The SnMe3 groups appear freely rotating around the Caryl — Sn bonds. The following bond lengths (r
g) and bond angles were determined: (Sn — C)mean 2.150 ± 0.007 Å, C — C 1.399 ± 0.005 Å, (C — H)mean 1.105 ± 0.006 Å, < C — C(Sn) — C 117.7 ± 1.7º, < Caryl — Sn — Cmethyl 106.7 ± 0.7º < Sn — C — H 111.5 ± 0.9º. 相似文献
3.
Swart M Fonseca Guerra C Bickelhaupt FM 《Journal of the American Chemical Society》2004,126(51):16718-16719
Recently, Vakonakis and LiWang (J. Am. Chem. Soc. 2004, 126, 5688) reported experimental evidence for stronger hydrogen bonds in RNA A:U than in DNA A:T base pairs, which was based on differences in NMR shielding for adenine C2. We have analyzed the proposed correlation between NMR shielding and hydrogen-bond strength using density functional theory. Although we agree with the conclusion that A:U is more strongly bound, we find no correlation between the hydrogen-bond strength and the NMR shielding of C2. Our study shows that NMR merely probes the presence/absence of the methyl group in thymine/uracil, without any relation to the strength of the hydrogen bonds involved. In other words, one cannot infer the Watson-Crick hydrogen-bond strength from the NMR shielding constant of adenine C2. 相似文献
4.
Quantum chemical DFT calculations using the B3LYP functionals have been carried out for the electronically unsaturated 16 VE five-coordinate osmium boryl-complexes [(PH3)2(CO)ClOs-BR2] and the 18 VE six-coordinate complexes [(PH3)2(CO)2ClOs-BR2] with BR2 = BH2, BF2, B(OH)2, B(OHC=CHO), and Bcat (cat = catecholate O2C6H4). The bonding situation of the Os-BR2 bond was analyzed with the help of the NBO partitioning scheme. The Os-B bond dissociation energies of the 16 VE complexes are very high, and they do not change very much for the different boryl ligands. The 18 VE complexes have only slightly lower bond energies than the 16 VE species. The Os-B bond in both classes of compounds is provided by a covalent sigma-bond which is polarized toward osmium and by strong charge attraction. Os-->B pi-donation is not important for the Os-B binding interactions, except for the Os-BH2 complexes. The stability of the boryl complexes [Os]-BR2 comes mainly from B<--R pi-donation, which is clearly higher than the Os-->B pi-donation. The intraligand charge distribution of the BR2 group changes little when the Os-B bond is formed, except for BH2. The CO ligand in [(PH3)2(CO)2ClOs-BR2] which is trans to BR2 has a relatively weak bond to the osmium atom. 相似文献
5.
Senthilkumar K Grozema FC Guerra CF Bickelhaupt FM Siebbeles LD 《Journal of the American Chemical Society》2003,125(45):13658-13659
The effective energy of a positive charge when it is localized at a specific guanine nucleobase in DNA was calculated using density functional theory. The results demonstrate that the efficiency of a guanine to act as a hole-trap in DNA strongly depends on the nature of the flanking nucleobases. The presence of a pyrimidine base at the 3' position adjacent to a guanine significantly increases the localization energy of the positive charge. The calculated distributions of a positive charge in sequences of two or three adjacent guanines, flanked by other nucleobases, provide an explanation for experimental literature data on the site-selective oxidation of DNA. 相似文献
6.
Bickelhaupt FM DeKock RL Baerends EJ 《Journal of the American Chemical Society》2002,124(7):1500-1505
Exceptionally short N [bond] F bond distances of only 1.217 A (crystal) and 1.246 A (gas phase) have been reported for N(2)F(+), making it the shortest N [bond] F bond ever observed. To trace the origin of this structural phenomenon, we have analyzed the model systems N(2)X(+), NF(3)X(+), and NH(3)X(+) (X [double bond] F, H) using generalized gradient approximation density functional theory at BP86/TZ2P. In good agreement with experiment, the computations yield an extremely short N [bond] F bond for N(2)F(+): we find N [bond] F bond distances in N(2)F(+), NF(4)(+), and NH(3)F(+) of 1.245, 1.339, and 1.375 A, respectively. The N [bond] X bonding mechanisms are quantitatively analyzed in the framework of Kohn-Sham MO theory. At variance with the current hypothesis, reduced steric and other Pauli repulsion (of substituents or lone pairs at N with F) rather than the extent of s [bond] p hybridization of N (i.e., sp versus sp(3)) are responsible for the much shorter N [bond] F distance in N(2)F(+) compared to NF(4)(+). The results for our nitrogen compounds are furthermore discussed in the more general context of how bond lengths are determined by both bonding and repulsive orbital interactions. 相似文献
7.
The reaction of 6-bromo-1-hexene with Mg was studied in order to obtain information on the role of the solvent during the formation of Grignard reagents. The 5-hexenyl radical (Rnc.) is known to cyclize rapidly and irreversibly to the cyclopentylmethyl radical (Rc.). Changes in yields of the cyclized and non-cyclized Grignard compounds have been found on varying the solvent. Information on the radical pairs involved is obtained from the yields of the three possible coupling products (RncRnc, RncRc and RcRc). Results are correlated to the intensity of the CIDNP spectra of the Grignard compounds. It is found that basicity and viscosity of the solvents influence the reactions at the site of single electron transfer. Formation of Grignard compounds via radical pairs increases: (a) with decreasing basicity of the solvent, (b) with decreasing viscosity of the solvent, and (c) on dilution of THF with benzene. It is proposed that interaction between the radical and the π-electron rich solvent benzene plays a role in reactions run in Bz/THF mixtures. 相似文献
8.
Dr. Laura Orian Lando P. Wolters Prof. Dr. F. Matthias Bickelhaupt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(40):13337-13347
A mechanistic density functional theory study of acetylene [2+2+2] cyclotrimerization to benzene catalyzed by RhI half metallocenes is presented. The catalyst fragment contains a heteroaromatic ligand, that is, the 1,2‐azaborolyl (Ab) or the 3a,7a‐azaborindenyl (Abi) anions, which are isostructural and isoelectronic to the hydrocarbon cyclopentadienyl (Cp) and indenyl (Ind) anions, respectively, but differ from the last ones on having two adjacent carbon atoms replaced with a boron and a nitrogen atom. The better performance of either the classic hydrocarbon or the heteroaromatic catalysts is found to depend on the different mechanistic paths that can be envisioned for the process. The present analyses uncover and explain general structure–reactivity relationships that may serve as rational design principles. In particular, we provide evidence of a reverse indenyl effect. 相似文献
9.
Dr. Israel Fernández Prof. Dr. Miquel Solà Prof. Dr. F. Matthias Bickelhaupt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(23):7416-7422
The origin of the experimentally known preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C60 has been computationally explored. To this end, the Diels–Alder reaction between cyclopentadiene and C60 has been analysed by means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method. Other issues, such as the aromaticity of the corresponding transition states, have also been considered. These results indicate that the major factor controlling the observed regioselectivity is the more stabilising interaction between the deformed reactants in the [6,6] reaction pathway along the entire reaction coordinate. 相似文献
10.
1,3-Di-tert-butylbenzene (4) can conveniently be prepared from 1-bromo-3,5-di-tert-butylbenzene (2) which, in turn, is obtained from the easily available 1,3,5-tri-tert-butylbenzene (1). The use of 4 is illustrated by the preparation of the arylphosphines 6 and 7. 相似文献