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491.
A new tetracyclic triterpene 9,19-cyclolanost-22(22'), 24-diene-3beta-ol, named as nerifoliene (2) along with euphol (1) were isolated from the fresh latex of Euphorbia nerifolia. Their structures were elucidated on the basis of spectral (IR, 1H and 13C NMR, FAB and EI Mass) data. 相似文献
492.
We report the detailed results of magnetization and magnetoresistance measurements in the Ru doped layered manganite system La1.2Sr1.8Mn2−xRuxO7 (x=0, 0.1, 0.5, 1.0). High-resolution measurements of magnetization and magnetoresistance were carried out as functions of temperature, magnetic field and time. We find evidence for the existence of competing ferromagnetic and antiferromagnetic interactions resulting in the formation of a frustrated spin-glass-like state at low temperatures. The time dependent magnetization follows the relation very well. We find that Ru doping enhances the coercive field and drives the system towards a magnetically mixed phase at low temperatures. Large negative magnetoresistance values are observed in all samples and at low temperatures the magnetoresistance varies as the square root of the applied magnetic field. 相似文献
493.
494.
A series of FeIII and MnIII porphyrins with various tolyl and naphthyl substituents at the meso positions, and their perbromoderivatives with Br substituents at the -pyrrole positions, have been synthesised and investigated. As seen in the case of the free-base porphyrins, both FeIII and MnIII derivatives of the Br-substituted porphyrins also exhibit pronounced red-shifts in both B and Q bands compared to their nonbrominated analogues. This is attributed to the electron-withdrawing ability of eight Br substituents at -pyrrole positions and is also due to distortion brought about in the -framework by the bulky substituents including those at the meso positions. The naphthyl groups seem to be making mesomeric contributions for both nonbrominated and brominated porphyrins of these metal ions as is evident from the higher wavelength absorption of the B band as compared to the tolyl derivatives. While the meso-substituent do not exhibit any isomer dependent change on the electronic properties of FeIII porphyrins, they show a noticeable effect in the MnIII derivatives. During the metallation of meso-tetratolylporphyrins by FeIII ions -oxo dimeric compounds are formed, while the naphthyl porphyrins and the bromoderivatives do not form such dimeric species. The presence of bulky groups at the meso positions and heavy bromines on the -pyrrole positions can be considered to prevent the formation of catalytically inert -oxo dimers. 相似文献
495.
For any finitely based variety of orthomodular lattices, we determine the sizes
of all equational bases that are both irredundant and self-dual. 相似文献
496.
497.
In this paper we construct two maps between the polynomials of abelian groups and the polynomials of idempotent reducts of
abelian groups and show that these can be used to “lift” the finite basis property from abelian groups to their idenpotent
reducts. It follows that this equational class of all idempotent reducts of abelian groups has the finite basis property.
The research of this author was supported by a grant from the National Research Council of Canada.
Presented by J. Mycielski 相似文献
498.
499.
T Padmanabhan 《Pramana》1990,35(2):L199-L204
A definition for the intrinsic time co-ordinate is proposed, using the phase of the wave function of the universe. This definition
generalizes the notion of time co-ordinate which arises in the semiclassical cosmology. It also leads to acceptable results
for the evaluation of expectation values of physical variables. 相似文献
500.