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301.
302.
In this communication we have reported the steric effect on the fluorescence quenching rate constants of the electron transfer (ET) process. We have done a comparative study using donor (D)-acceptor (A) systems with different exergonicity (-deltaG(f)). Different carbazole derivatives (CZ): 1,4-dicyanobenzene (DCB) systems (-deltaG(f) = 0.7-0.8 eV) were found to be among those limiting systems that show a clear-cut steric dominance in the process of fluorescence quenching. It is known that with increasing exergonicity the ET distance increases and hence steric dependence becomes insignificant. On the other hand, with decreasing exergonicity the ET distance decreases and a pronounced steric dominance should be observed. However, in the D-A systems having lower exergonicity compared to CZ-DCB systems, this steric dominance is observed only in polar medium. In non-polar medium due to exciplex formation the D-A distance effectively becomes much longer and therefore no steric dominance is observed.  相似文献   
303.
Complexes of formulation [Cu(Tp(Ph))(L)](ClO(4)) (1-4), where Tp(Ph) is anionic tris(3-phenylpyrazolyl)borate and L is N,N-donor heterocyclic base, viz. 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyridoquinoxaline (dpq, 3), and dipyridophenazine (dppz, 4), are prepared from a reaction of copper(II) acetate.hydrate with KTp(Ph) and L in CH(2)Cl(2) and isolated as perchlorate salts. The complexes are characterized by analytical, structural, and spectral methods. The crystal structures of complexes 1-4 show the presence of discrete cationic complexes having the metal, Tp(Ph), and L in a 1:1:1 ratio and a noncoordinating perchlorate anion. The complexes have a square-pyramidal 4 + 1 coordination geometry in which two nitrogens of L and two nitrogens of the Tp(Ph) ligand occupy the basal plane and one nitrogen of Tp(Ph) binds at the axial site. Complexes 3 and 4 display distortion from the square-pyramidal geometry. The Cu-N distances for the equatorial and axial positions are approximately 2.0 and 2.2 A, respectively. The phenyl groups of Tp(Ph) form a bowl-shaped structure that encloses the [CuL] moiety. The steric encumbrance is greater for the bpy and phen ligands compared to that for dpq and dppz. The one-electron paramagnetic complexes (mu approximately equal to 1.8 mu(B)) exhibit axial EPR spectra in CH(2)Cl(2) glass at 77 K giving g(parallel) and g(perpendicular) values of approximately 2.18 (A(parallel) = 128 G) and approximately 2.07. The data suggest a [d(x(2)-y(2))](1) ground state. The complexes are redox-active and display a quasireversible cyclic voltammetric response for the Cu(II)/Cu(I) couple near 0.0 V versus SCE with an i(pc)/i(pa) ratio of unity in CH(2)Cl(2) or DMF-0.1 M TBAP. The E(1/2) values of the couple vary in the order 4 > 3 > 2 > 1. A profound effect of steric encumbrance caused by the Tp(Ph) ligand is observed in the reactivity of 1-4 with the calf thymus (CT) and supercoiled (SC) DNA. Complexes 2-4 show similar binding to CT DNA. The propensity for the SC DNA cleavage varies as 4 > 3 > 2. The bpy complex does not show any significant binding or cleavage of DNA. Mechanistic investigations using distamycin reveal minor groove binding for 2 and 3 and a major groove binding for 4. The scission reactions that are found to be inhibited by hydroxyl radical scavenger DMSO are likely to proceed through sugar hydrogen abstraction pathways.  相似文献   
304.
It is theoretically shown that under the influence of an appropriately directed external electric field (ε) in H2CO or a similar molecule (of the XYC = O type), the barrier to planar inversion in the 3n π* state decreases as the field strength increases. The molecule assumes a forced planar geometry in the 3n π* state rather abruptly when ε(applied) > ε(critical). This phenomenon of field-induced change of structure of an isolated molecule has the attributes of a first order phase transition. The effects should be spectroscopically detectable in the inversion spectrum. The existence of another critical field strength at which the molecule makes a more or less continuous transition from the normal convalent to a purely ionic structure is demonstrated and rationalized through a simple model.  相似文献   
305.
Electron donor–acceptor (EDA) complex formation of o-chloranil with six different phosphine oxides and tri-n-butyl phosphate (TBP) has been studied in CCl 4 solution by the UV-VIS absorption spectrophotometric technique. An absorption band due to a charge–transfer (CT) transition is observed in the visible region. Utilizing the CT transition energy, the electron affinity of o-chloranil in solution has been calculated. Degrees of charge transfer, and oscillator and transition dipole strengths have also been calculated for all of the investigated EDA complexes. Except for TBP, other phosphine oxides, viz., tri-n-octyl phosphine oxide, tri-n-butyl phosphine oxide, triphenyl phosphine, octyl(phenyl)-N,N-diisobutylcarbamoylphosphine oxide, octyl(phenyl)-N,N-dicyclohexylcarbamoylmethylphosphine oxide and octyl(phenyl)-N,N-diisopropylcarbamoylmethylphosphine oxide have been shown to form stable 1:1 EDA complexes with o-chloranil. The complex of TBP with o-chloranil decays slowly into a secondary product. Formation constants of the EDA complexes have been determined.  相似文献   
306.
Semiempirical molecular orbital calculations at the AM1 level have been carried out to obtain specific solvation interaction energies involving hydrogen bonding for various solute-protic solvent systems. A scale for hydrogen bond donating ability of the protic solvents is proposed. A comparison of the calculated parameters with solvatochromic parameters is presented.  相似文献   
307.
The INDO/2 version of the average hole potential (AHP) model is analyzed. The model is applied to study the geometric features, molecular inversion barriers, singlet-triplet splittings, etc., of a few small carbonyl molecules (H2CO, HFCO, F2CO) in the 1,3nπ* states with partial as well as complete optimization of all geometric parameters in the excited states. The results are compared with those obtained by a simple hole-potential (HP) model.  相似文献   
308.
To improve the understanding of the charge transfer (CT) interaction of [70]fullerene with electron donors, interaction of [70]fullerene with a series of phenols, e.g., phenol, resorcinol and p-quinol were studied in 1,4-dioxan medium using absorption spectroscopy. An absorption band due to CT transition was observed in the visible region. The experimental CT transition energies (h nuCT) are well correlated (through Mulliken's equation) with the vertical ionisation potentials (I(D)v) of the series of phenols studied. From an analysis of this correlation degrees of charge transfer for the [70]fullerene-phenol complexes were estimated. The degrees of charge transfer in the ground state of the complexes have been found to be very low (<2%). The h nuCT values change systematically as the number and position of the -OH groups change on the aromatic ring of the phenol moiety. From the trends in the h nuCT values, the Hückel parameters (h(O) and k(C-O)) for the -OH group were obtained in a straightforward way and the values so obtained, viz., 1.91 and 1.0, respectively, are close to the ones (1.8 and 0.8) recommended by Streitwieser on the basis of other evidence. Oscillator strengths, transition dipole strengths and resonance energies of the [70]fullerene-phenol complexes were determined. Formation constants of the CT complexes were determined at four different temperatures from which enthalpies and entropies of formation of the complexes were estimated.  相似文献   
309.
Detailed (1)H and (13)C NMR spectrometric studies have been carried out to gain insight into the nature of molecular interactions of the electron donor-acceptor (EDA) complexes of [60]fullerene with a series of anisoles, namely, anisole, m-bromoanisole, and p-bromoanisole. [60]Fullerene has been shown to form 1:1 adducts with the above series of anisoles. Formation constants (K) for all the complexes have been determined from the systematic variation of the NMR chemical shifts of specific protons of the anisoles in the presence of [60]fullerene. The K values of [60]fullerene/anisole, [60]fullerene/m-bromoanisole, and [60]fullerene/ p-bromoanisole complexes yield good estimates of the Hammett rho constant for the complexation reaction. To the best of our knowledge, this paper reports for the first time a very fruitful technique by which the concentrations of EDA complexes can be estimated from systematic variations of the (13)C NMR signal.  相似文献   
310.
3-Formylindole and its 1-substituted and 1,5-disubstituted derivatives react with TOSMIC in presence of potassium carbonate in methanol under reflux to furnish 5-(3′-indolyl)oxazoles, new stable E-2-(3′-indolyl)-2-tosylethenamines and two diastereomers of N-[2-(3′-indolyl)-1,2-dimethoxy]ethylformamides. In contrast, 2-formylskatole furnishes N-(1-tosyl-2-skatolyl)ethenylformamide.  相似文献   
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