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101.
Roger L. Dekock Evert Jan Baerends Paul M. Boerrigter Jaap G. Snijders 《Chemical physics letters》1984,105(3):308-316
Results of calculations on the uranylion using the LCAO MO Hartree—Fock—Slater method including relativistic effects are reported. The highest occupied molecular orbital is calculated to be σu, consisting predominantly of U 5f character. The σu orbital is the HOMO partly because of “pushing-from-below” by the U 6p orbital, but also as a result of the change in potential of the U 5f electrons with the uranium core elections brought about by relativistic contraction of the core electrons. This effect also determines the character of the first virtual levels (δu and Φu, respectively) in equatorial ligand fields. 相似文献
102.
103.
Célia Fonseca Guerra Evert Jan Baerends F. Matthias Bickelhaupt 《Central European Journal of Chemistry》2008,6(1):15-21
We have theoretically analyzed mimics of Watson-Crick AT and GC base pairs in which N-H···O hydrogen bonds are replaced by
N-H···S, using the generalized gradient approximation (GGA) of density functional theory at BP86/TZ2P level. The general effect
of the above substitutions is an elongation and a slight weakening of the hydrogen bonds that hold together the base pairs.
However, the precise effects depend on how many, and in particular, on which hydrogen bonds AT and GC are substituted.. Another
purpose of this work is to clarify the relative importance of electrostatic attraction versus orbital interaction in the hydrogen bonds involved in the mimics, using a quantitative bond energy decomposition scheme.
At variance with widespread believe, the orbital interaction component in these hydrogen bonds is found to contribute more
than 40% of the attractive interactions and is thus of the same order of magnitude as the electrostatic component, which provides
the remaining attraction.
相似文献
104.
Valentin Paul Nicu Johannes Neugebauer Stephen K. Wolff Evert Jan Baerends 《Theoretical chemistry accounts》2008,119(1-3):245-263
We describe the implementation of the rotational strengths for vibrational circular dichroism (VCD) in the Slater-type orbital
based Amsterdam Density Functional (ADF) package. We show that our implementation, which makes use of analytical derivative
techniques and London atomic orbitals, yields origin independent rotational strengths. The basis set dependence in the particular
case of Slater-type basis functions is also discussed. It turns out that the triple zeta STO basis sets with one set of polarization
functions (TZP) are adequate for VCD calculations. The origin- dependence of the atomic axial tensors is checked by a distributed
origin gauge implementation. The distributed and common origin gauge implementations yield virtually identical atomic axial
tensors with the Slater-type basis sets employed here, proving that our implementation yields origin independent rotational
strengths. We verify the implementation for a set of benchmark molecules, for which the dependence of the VCD spectra on the
particular choice of the exchange–correlation functional is studied. The pure functionals BP86 and OLYP show a particularly
good performance. Then, we apply this approach to study the VCD spectra of hexa- and hepta- helicenes. In particular we focus
on relationships between the sign of the rotational strengths of the two helicenes.
Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. 相似文献
105.
Rate coefficients for reactions between Cl radicals and four ketones were determined at 294 ± 1 K with a relative rate method using a laser photolysis technique. The experiments were conducted in synthetic air in a flow system at atmospheric pressure. A mixture of Cl2/ClONO2 was photolyzed and the formation of NO3 through the reaction Cl + ClONO2 → Cl2 + NO3 was measured with and without ketones in the reaction mixture. The NO3 radical concentration was measured by optical absorption using a diode laser as the light source. The rate coefficients for the Cl-ketone reactions could then be evaluated. The following rate coefficients were obtained (in units of cm3 molecule−1 s−1): cyclohexanone (7.00 ± 1.15) × 10−11; cyclopentanone (4.76 ± 0.33) × 10−11; acetone (1.69 ± 0.32) × 10−12; and 2,3-butanedione (7.62 ± 1.66) × 10−13. The accuracy of the method employed was tested by using the well-studied reaction between Cl and methane and a rate coefficient of (9.37 ± 1.04) × 10−14 cm3 molecule−1 s−1 was obtained, which is in good agreement with previous work. The errors are at the 95% confidence level. The results in this work indicate that a carbonyl group in a ketone lowers the reactivity towards α-hydrogen abstraction by Cl radicals, compared to the corresponding Cl-alkane reactions. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 195–201, 1997. 相似文献
106.
Oleg V. Gritsenko Robert Van Leeuwen Evert Jan Baerends 《International journal of quantum chemistry》1996,60(7):1375-1384
The optimal mixing coefficient C of the exchange energy Ex and the electron-electron interaction part of the exchange-correlation energy W1xc in the formula for the total exchange-correlation energy Exc was expressed through the ratio of the kinetic Tc and potential Wc contributions to the correlation energy Ec. This expression can be derived from a Heavyside step function model of the dependence of Wλxc on the coupling parameter of the electron interaction λ. Values of Tc and Wc obtained from ab initio wave functions were used to estimate C for a number of atoms and molecules. A strong dependence of Tc, Wc, and C on the bond distance was demonstrated for the case of the H2 molecule. Tc and C approach zero in the bond-dissociation limit; so for an electron-pair bond, the admixing of exact exchange to obtain an accurate Exc is strongly dependent on the bond length and has to disappear for weak interaction/large bond distances. The potential of the exchange-correlation hole constructed for H2 from an ab initio second-order density matrix was compared with its generalized gradient approximation (GGA). © 1996 John Wiley & Sons, Inc. 相似文献
107.
Sarka Langer Evert Ljungstr m Thomas Ellermann Ole J. Nielsen Jens Sehested 《Chemical physics letters》1995,240(5-6):499-505
UV spectra and kinetics for the reactions of alkyl and alkylperoxy radicals from methyl tert-butyl ether (MTBE) were studied in 1 atm of SF6 by the pulse radiolysis-UV absorption technique. UV spectra for the radical mixtures were quantified from 215 to 340 nm. At 240 nm. σR = (2.6 ± 0.4) × 10−18 cm2 molecule−1 and σRO2 = (4.1 ± 0.6) × 10−18 cm2 molecule−1 (base e). The rate constant for the self-reaction of the alkyl radicals is (2.5 ± 1.1) × 10−11 cm3 molecule−1 s−1. The rate constants for reaction of the alkyl radicals with molecular oxygen and the alkylperoxy radicals with NO and NO2 are (9.1 ± 1.5) × 10−13, (4.3 ± 1.6) × 10−12 and (1.2 ± 0.3) × 10−11 cm3 molecule−1 s−1, respectively. The rate constants given above refer to reaction at the tert-butyl side of the molecule. 相似文献
108.
Finazzo C Harmer J Bauer C Jaun B Duin EC Mahlert F Goenrich M Thauer RK Van Doorslaer S Schweiger A 《Journal of the American Chemical Society》2003,125(17):4988-4989
Methyl-coenzyme M reductase (MCR) catalyzes the reaction of methyl-coenzyme M (CH3-S-CoM) with coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. At the active site, it contains the nickel porphinoid F430, which has to be in the Ni(I) oxidation state for the enzyme to be active. How the substrates interact with the active site Ni(I) has remained elusive. We report here that coenzyme M (HS-CoM), which is a reversible competitive inhibitor to methyl-coenzyme M, interacts with its thiol group with the Ni(I) and that for interaction the simultaneous presence of coenzyme B is required. The evidence is based on X-band continuous wave EPR and Q-band hyperfine sublevel correlation spectroscopy of MCR in the red2 state induced with 33S-labeled coenzyme M and unlabeled coenzyme B. 相似文献
109.
Rosa A Ricciardi G Baerends EJ Zimin M Rodgers MA Matsumoto S Ono N 《Inorganic chemistry》2005,44(19):6609-6622
The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio group, NiOMTP (OMTP = 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D(2d) and hybrid (ruffled with a modest degree of saddling) D(2) conformations, both characterized by an up-down (ud) orientation of the vicinal methylthio groups are by far the preferred conformations in the "gas phase". In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and beta-alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthio groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway 1(pi,pi) --> 3(d(z2),d(x2-y2))--> ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands. 相似文献
110.
Dew A. McCormack Evert Jan Baerends Erik van Lenthe Nicholas C. Handy 《Theoretical chemistry accounts》2004,112(5-6):410-418
Calculations are presented to assess a theorem presented by S.F. Boys [(1969) Proc. R. Soc. A. 309:195], regarding the accuracy of numerical integration in quantum chemical calculations. The theorem states that the error due to numerical integration can be made proportional to the error due to basis set truncation, and thus goes to zero in the limit of a complete basis. We test this theorem on the hydrogen atom, showing that with a solution-spanning basis, the numerically exact orbital energy can indeed be calculated with a small number of integration points. Moreover, tests for H and H2+ demonstrate that even when only a near-complete basis is employed, Boys Theorem can significantly reduce integration error. However, for other systems, like the oxygen atom and the CO2 molecule, the theorem yields no advantage for some occupied orbitals. It is concluded that the theorem would be most useful for calculations that demand large basis sets. 相似文献