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1.
2.
Xavier Fradera Michael De Rosa Modesto Orozco F. Javier Luque 《Theoretical chemistry accounts》2004,111(2-6):223-230
The tautomeric preferences of the conjugated acids of 2-aminopyrrole derivatives have been examined both in the gas phase and in aqueous solution by using a combination of quantum mechanical, self-consistent reaction field and Monte Carlo–free-energy perturbation methods. The results show that the nature of substituents, the solvent and the presence of cosolute are relevant factors in modulating the relative stability between the tautomeric conjugate acids protonated at the heterocyclic ring and at the exocyclic amino nitrogen. Thus, attachment of electron-withdrawing groups to the ring, solvation in polar solvents, and the presence of negatively charged cosolutes tend to favor protonation at the exocyclic amino nitrogen. Nevertheless, none of these factors alone suffice to change the tautomeric preference for the ring-protonated forms. The results point out that the concerted occurrence of the three factors is necessary to shift the tautomeric preference towards the conjugated species protonated at the exocyclic nitrogen.Contribution to the Jacopo Tomasi Honorary Issue 相似文献
3.
P.?Winget C. J.?CramerEmail author D. G.?TruhlarEmail author 《Theoretical chemistry accounts》2004,112(4):217-227
Equilibrium free-energy cycles relating oxidation and reduction potentials in solution to ionization potentials and electron affinities in the gas phase are constructed and the utilities of various levels of theory for computing particular free-energy changes within these cycles are discussed within the context of several examples. Emphasis is placed on the use of quantum-mechanical continuum solvation models to compute free energies of solvation. Key systems discussed include quinones, substituted anilines, substituted phenols, and reductive dechlorination reactions.Dedicated to Prof. Jean-Louis Rivail, whose pioneering efforts in developing and exploiting continuum solvent models were critical in making quantum chemistry more applicable to solution phenomenaProceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail 相似文献
4.
In this work, the complexes formed between formamide and water were studied by means of the SAPT and AIM methods. Complexation leads to significant alterations in the geometries and electronic structure of formamide. Intermolecular interactions in the complexes are intense, especially in the cases where the solvent interacts with the carbonyl and amide groups simultaneously. In the transition states, the interaction between the water molecule and the lone pair on the amide nitrogen is also important. In all the complexes studied herein, the electrostatic interactions between formamide and water are the main attractive force, and their contribution may be five times as large as the corresponding contribution from dispersion, and twice as large as the contribution from induction. However, an increase in the resonance of planar formamide with the successive addition of water molecules may suggest that the hydrogen bonds taking place between formamide and water have some covalent character. 相似文献
5.
A simple electrostatic analysis is given of the virtual charge (solvaton) model to represent the environment effect on the electronic wave function of a solute immersed in a polarizable surrounding. New features of this model are found. The classical aspects are discussed and secondly the quantal implications are considered. A correct Hartree-Fock-like operator is derived which represents an electron in a molecular orbital subjected to the average effect of the other electrons and to the reaction field produced by the virtual charges on the atomic centers.A general formalism based on the preceding model is presented in appendix. The final equations have a form similar to Newton's equation to represent a solvated electron. Unlike some other theories in this field, there is no cut-off involved in the evaluation of the molecular integrals. 相似文献
6.
Lluís Blancafort Annapaola Migani 《Journal of photochemistry and photobiology. A, Chemistry》2007,190(2-3):283-289
Two low-energy deactivation paths for singlet excited cytosine, one through a S1/S0 conical intersection of the ethylene type, and one through a conical intersection that involves the (nN, π*) state, are calculated in the presence of water. Water is included explicitly for several cytosine monohydrates, and as a bulk solvent, and the calculations are carried out at the complete active space self-consistent field (CASSCF) and complete active space second order perturbation (CASPT2) levels of theory. The effect of water on the lowest-energy path through the ethylenic conical intersection is a lowering of the energy barrier. This is explained by stabilization of the excited state, which has zwitterionic character in the vicinity of the conical intersection due to its similarity with the conical intersection of ethylene. In contrast to this, the path that involves the (nN, π*) state is destabilized by hydrogen bonding, although the bulk solvent effect reduces the destabilization. Overall, this path should remain energetically accessible. 相似文献
7.
The solvation parameter model is used to elucidate the retention mechanism on a perfluorohexylpropylsiloxane-bonded (Fluophase
RP) and octadecylsiloxane-bonded (Betasil C18) stationary phases based on the same silica substrate with acetonitrile–water
and methanol–water mobile phase compositions. Dewetting affects the retention properties of Fluophase RP at mobile phase compositions
containing less than 20% (v/v) acetonitrile or 40% (v/v) methanol. It results in a loss of retention due to an unfavorable change in the phase ratio as well as changes in specific
intermolecular interactions. Steric repulsion reduces retention of bulky solutes on fully solvated Betasil C18 with methanol–water
(but not acetonitrile–water) mobile phase compositions but is not important for Fluophase RP. The retention of weak bases
is affected by ion-exchange interactions on Fluophase RP with acetonitrile–water, and to a lesser extent, methanol-water mobile
phases but these are weak at best for Betasil C18. The system constants of the solvation parameter model and retention factor
scatter plots are used to compare selectivity differences for Fluophase RP, Betasil C18 and a perfluorophenylpropylsiloxane-bonded
silica stationary phase Discovery HS F5 for conditions where incomplete solvation, steric repulsion and ion-exchange do not
significantly contribute to the retention mechanism. Lower retention on Fluophase RP results from weaker dispersion and/or
higher cohesion moderated to different extents by polar interactions since solvated Fluophase RP is a stronger hydrogen-bond
acid and more dipolar/polarizable than Betasil C18. Retention factors for acetonitrile–water mobile phases are highly correlated
for Fluophase RP and Betasil C18 except for compounds with a large excess molar refraction and weak hydrogen-bonding capability.
Selectivity differences are more significant for methanol–water mobile phases. Retention factors on Fluophase RP are strongly
correlated with those on Discovery HSF5 for acetonitrile–water mobile phases while methanol–water mobile phases retention
on Fluophase RP is a poor predictor of the retention order on Discovery HS F5. 相似文献
8.
Cheng-Hua DengChuan-Jin Guan Man-Hua ShenCheng-Xue Zhao 《Journal of fluorine chemistry》2002,116(2):109-115
Some newly synthesized fluorinated nitroxides, such as t-butyl perfluoroalkyl nitroxides ButN(O) Rf (Rf=CF3, 5; C2F5, 6; n-C3F7, 7) and s-butyl perfluoroacyl nitroxides BusN(O) CORf (Rf=CF3, 9; n-C3F7, 10) have been employed as ESR probes of solvation in different common organic solvents. In aprotic solvents, the measured aN values for each of the nitroxyl probes show a linear correlation with the cybotactic polar solvent parameters ET (Dimroth) and Z (Kosowar), i.e. aN=bET+c, and aN=b′Z+c′. The physical significance for the slope (b or b′), the slope×ET or slope×Z, the extrapolated intercept on aN axis, c or c′, are linked, respectively, to the sensitivity of a specific nitroxide toward solvation, the magnitude of the overall solvation effect on the aN value, and the intrinsic aN value of each nitroxide in the ideal gaseous state. The intercept on the aN axis may also serve as a new measure of electronegativity for perfluoroalkyl groups, CF3, C2F5, n-C3F7, and perfluoroacyl groups, CF3CO, n-C3F7CO. In protic solvents, i.e. alcohols and carboxylic acids, however, aN values of all the probes, kept almost no change with the increase in ET and Z. Furthermore, the plots of aN versus non-cybotactic solvent constants, such as dipolar moment (μ) and dielectric constant (ε), all show random variations. 相似文献
9.
Solvation of the thallous ion in dilute solutions of six binary solvent systems (formamide/water,N-methylformamide/water,N-ethylformamide/water, formamide/N-methylformamide, formamide/N-ethylformamide, andN-methylformamide/N-ethylformamide) was studied with205Tl NMR spectroscopy. An attempt was made to separate solvation effects related to the electrondonating ability (Lewis basicity) of the solvents from effects resulting from structural changes in the solvation sphere. Structural effects were found to be greatest in theN-methylformamide/water system and least in theN-methylformamide/formamide system. 相似文献
10.