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1.
Yagi H Ramesha AR Kalena G Sayer JM Kumar S Jerina DM 《The Journal of organic chemistry》2002,67(19):6678-6689
We describe a novel and efficient synthesis (62-84% yields) of the eight possible, diastereomerically pure, cis and trans, R and S O(6)-allyl-protected N(2)-dGuo phosphoramidite building blocks derived through cis and trans opening of (+/-)-3alpha,4beta-dihydroxy-1beta,2beta-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-1 (1)] and (+/-)-3alpha,4beta-dihydroxy-1alpha,2alpha-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-2 (2)] by hexafluoropropan-2-ol (HFP)-mediated addition of O(6)-allyl-3',5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine (3) at C-1 of the epoxides. Simply changing the relative amount of HFP used in the reaction mixture can achieve a wide ratio of cis/trans addition products. Thus, the observed cis/trans adduct ratio for the reaction of DE-1 (1) in the presence of 5 equiv of 3 varied from 17/83 to 91/9 over the range of 5-532 equiv of HFP. The corresponding ratios for DE-2 (2) varied from 2/98 to 61/39 under the same set of conditions. When 1 or 2 was fused with a 20-fold excess of 3 at 140 degrees C in the absence of solvent HFP, almost exclusive trans addition (>95%) was observed for the both DEs. Through the use of varying amounts of HFP in the reaction mixture as described above, each of the eight possible phosphoramidite oligonucleotide building blocks (DE-1/DE-2, cis/trans, R/S) of the BcPh DE N(2)-dGuo adducts can be prepared in an efficient fashion. To rationalize the varying cis-to-trans ratio, we propose that the addition of 3 to 1 or 2 in the absence of solvent or in the presence of small amounts of HFP proceeds primarily via an S(N)2 mechanism to produce mainly trans-opened adducts. In contrast, increasing amounts of HFP promote increased participation of an S(N)1 mechanism involving a relatively stable carbocation with two possible conformations. One of these conformations reacts with 3 to give mostly trans adduct, while the other conformation reacts with 3 to give mostly cis adduct. 相似文献
2.
Our earlier model of re-entrant liquid crystalline phases exhibited by highly polar compounds, in which the mutual orientation of near-neighbour molecules can change from an antiparallel to a parallel configuration, has been extended to include both nematic and smectic interactions. We show that, as the McMillan parameter alpha is decreased, the SmAd-SmA1 line goes over to the SmAd-N1Re line, finally becoming the NdRe-N1Re transition line, the latter ending in a critical point. This sequence is in agreement with the predictions of Prost's Landau model as well as with an experimental result. The phase sequence N-SmAd-NdRe-N1Re-SmA1 is obtained on cooling for a range of alphaA values. 相似文献
3.
Hiromu Kurosaki April A. Tucker Sarah E. Iden Shirley M. Sexton Benito D. Gonzalez Govind R. Rao 《Journal of Radioanalytical and Nuclear Chemistry》2013,298(2):1017-1022
Routine monitoring of urine is an effective way to detect occupational intake of radioactive material. Historically, determinations of uranium isotopic ratios have been performed by radiochemical separation followed by alpha spectrometry. With recent advancements in technology, inductively coupled plasma-mass spectrometry (ICP-MS) has become widely available for the determination of trace metals as well as radioactive nuclides with long half-lives, such as 238U in urine. Furthermore, ICP-MS measurements of 238U do not require radiochemical separation since the number of atoms in the sample is determined instead of the number of alpha particles emitted. However, this method does not provide good sensitivity for the determination of 235U due to its shorter half-life. An improved procedure using pre-concentration of uranium and determination by ICP-MS decreases the detection limit by a factor of ten or greater with only slight increase in total analysis time. The method also has the capability of accurately determining the isotopic ratio of the sample, which is very important in cases where enriched or depleted uranium is involved. 相似文献
4.
Awalekar Ramchandra Mohire Priyanka Patravale Ajinkya Salunkhe Shilpa Usmani Shams Jamale Dattatray Hangirgekar Shankar Kolekar Govind Anbhule Prashant 《Chemistry of Natural Compounds》2021,57(6):1000-1004
Chemistry of Natural Compounds - (3E,7Z)-Tetradecadienyl acetate, the major sex pheromone component of the potato pest Symmetrischema tangolias (Gyen), was stereoselectively synthesized from the... 相似文献
5.
Jaspreet Sandhu Amit Kumar Singh Chauhan Govind 《Journal of nanoparticle research》2011,13(8):3503-3509
The room temperature growth of gallium atoms on the highly oriented pyrolytic graphite (HOPG) surface has been performed.
The gallium atoms were deposited by thermal evaporation method in an ultra high vacuum system at a base pressure 5 × 10−10 torr. The X-ray photo electron spectroscopy (XPS) studies had been performed to confirm the presence of gallium atoms on
HOPG surface. Scanning tunneling spectroscopy (STM) technique was employed to study the surface morphology of the clean HOPG
surface and gallium covered HOPG surfaces which recognize the formation of gallium induced nanostructures. The deconvoluted
XPS core level spectra of C (1s) and Ga (3d) demonstrate the possible interaction between substrate and the adsorbate atoms.
The STM analysis revealed that the gallium deposition on HOPG led to significant change in the surface morphology. It was
observed that the Ga atoms adsorbed as layer structure on HOPG surface for low coverage while quasi one-dimensional chain
like nanostructure (1 ± 0.2 nm) has been formed for higher Ga coverage. The nanostructured surfaces induced by Ga deposition
are found to be stable and could be used as a template for the growth of metallic nanostructures. 相似文献
6.
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8.
Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski-Krastanov (S-K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S-K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages. 相似文献
9.
The EPR study of the Cu2+-doped tris-sarcosine calcium chloride (TSCC) at room temperature is reported. Two magnetically inequivalent sites for Cu2+ were observed. The rhombic spin Hamiltonian parameters are determined by fitting the EPR spectra for two centres: Cu2+(I) g1 = 2.0276, g2 = 2.0517, g3 = 2.4019, A1 = 82, A2 = 128, A3 = 152 [G] and Cu2+(II) g1 = 2.0231, g2 = 2.0368, g3 = 2.5294, A1 = 76, A2 = 92, A3 = 156 [G]. The ground state wave function is also determined. The g-anisotropy is evaluated and compared with the experimental value. Further, the optical study of the crystal at room temperature is carried out and the nature of bonding in the complex is discussed. 相似文献
10.
Jian Tong Duncan L. MacFarlane Issa Panahi L. Roberts Hunt Govind Kannan Gary A. Evans 《Fiber and Integrated Optics》2013,32(4):201-215
Abstract An integrated photonic architecture is introduced and used to realize an optical filter with direct form I realization. The architecture offers gain from semiconductor optical amplifiers, and this gain results in an active optical filter whose filter response depends on the individual gains. The presence of gain provides advantages in filter performance, and tunable and adaptive functionality. The optical filter is modeled as a discrete time system and the z-transform is used in its analysis and design. A low-pass filter design example is presented and the filter coefficients are derived in terms of gains and coupler splitting ratios. The region of stable operations is derived by applying the Schur-Cohn stability test. 相似文献