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41.
Winfried Wendelin Edith Gößnitzer Dalal Abou El Ella 《Monatshefte für Chemie / Chemical Monthly》2000,131(4):353-374
Summary. The cyclization reactions of trans-3a-hexahydro-2-benzimidazolamine with 2-alkylidene- and 2-benzylidenecyclohexanones and α-tetralones, respectively, yield
mixtures of two isomeric condensates each. Thorough high resolution NMR analyses (1H and 13C NMR, HH-COSY, gs-HSQC, gs-HMBC, 1D TOCSY, and 1D NOE difference spectra) revealed that the corresponding isomers are in all cases linearly fused diastereomeric
12α- and 12β-substituted trans-6aα-dodecahydrobenzimidazo[2,1-b]quinazolines and 7α-substituted trans-8aβ- and trans-8aα-decahydrobenzimidazo[2,1-b]benzo[h]quinazolines, respectively. The formation of corresponding angularly fused regioisomers was not observed so far. The stereochemistry
and the tautomerism of some bases and their hydrochlorides as well as the regiospecific course of the cyclization reactions
are discussed. Biological tests showed that the novel compounds don’t exert remarkable antibacterial and antimycotic effects.
Received November 3, 1999. Accepted November 30, 1999 相似文献
42.
The temperature dependence of the reorientation rates of the AsO44? centers in KH2AsO4 and NH4H2AsO4 as well as of the CrO43? centers in KH2PO4 shows a strong deviation from an Arrhenius behavior which can be quantitatively described as a result of the local “pseudo freeze-out” of the dynamics of these centers and the associated polarization cloud far above the transition temperature TC of the host crystal. 相似文献
43.
This work addresses the analytical and numerical investigations of the transmission performance of an optical Single Sideband (SSB) modulation technique generated by a Mach Zehnder Modulator (MZM) with a 90° and 120° hybrid coupler. It takes into account the problem of chromatic dispersion in single mode fibers in Passive Optical Networks (PON), which severely degrades the performance of the system. Considering the transmission length of the fiber, the SSB modulation generated by maintaining a phase shift of π/2 between the two electrodes of the MZM provides better receiver sensitivity. However, the power of higher-order harmonics generated due to the nonlinearity of the MZM is directly proportional to the modulation index, making the SSB look like a quasi-double sideband (DSB) and causing power fading due to chromatic dispersion. To eliminate one of the second-order harmonics, the SSB signal based on an MZM with a 120° hybrid coupler is simulated. An analytical model of conventional SSB using 90° and 120° hybrid couplers is established. The latter suppresses unwanted (upper/lower) first-order and second-order (lower/upper) sidebands. For the analysis, a varying quadrature amplitude modulation (QAM) Orthogonal Frequency Division Multiplexing (OFDM) signal with a data rate of 5 Gb/s is upconverted using both of the SSB techniques and is transmitted over a distance of 75 km in Single Mode Fiber (SMF). The simulation results show that the SSB with 120° hybrid coupler proves to be more immune to chromatic dispersion as compared to the conventional SSB technique. This is in tandem with the theoretical analysis presented in the article. 相似文献
44.
Dalal M. Alshamsi Ahmed A. Murad Ala Aldahan Xiaolin Hou 《Journal of Radioanalytical and Nuclear Chemistry》2013,298(3):1899-1905
Groundwater in arid and semiarid regions is vital resource for many uses and therefore information about concentrations of uranium isotopes among other chemical parameters are necessary. In the study presented here, distribution of 238U and 235U in groundwater of four selected locations in the southern Arabian peninsula, namely at two locations within the United Arab Emirates (UAE) and two locations in Oman are discussed. The analyses of the uranium isotopes were performed using ICP-MS and the results indicated a range of concentrations for 235U and 238 U at 3–39 ng L?1 (average: 18 ng L?1) and 429–5,293 ng L?1 (average: 2,508 ng L?1) respectively. These uranium concentrations are below the higher permissible WHO limit for drinking water and also comparable to averages found in groundwater from similar aquifers in Florida and Tunisia. Negative correlation between rainfall and uranium concentrations suggests that in lithologically comparable aquifers, climate may influence the concentration of uranium in subtropical to arid regions. 相似文献
45.
O'Reilly ME Del Castillo TJ Falkowski JM Ramachandran V Pati M Correia MC Abboud KA Dalal NS Richardson DE Veige AS 《Journal of the American Chemical Society》2011,133(34):13661-13673
Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO = [2,6-((t)BuC(6)H(3)O)(2)C(6)H(3)](3-), THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O(2) activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O(2) activation when dimerized. The product from O(2) activation, [(t)BuOCO]Cr(V)(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr(IV)-O-Cr(IV) dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are g(iso) = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O(2) was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O(2). Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh(3) accelerates O(2) activation by forming the five-coordinate complex trans-[(t)BuOCO]Cr(III)(OPPh(3))(2) (4). 相似文献
46.
47.
Hydrous ferric oxide was synthesised under alkaline conditions, aged in distilled water and calcined. Characterisation was
attempted through x-ray, differential thermal analysis, thermogravimetric and infrared analyses. Electrical conductivity was
measured and chemical reactivity towards hydrazine investigated. It was found that ageing as well as calcination facilitated
crystallisation toa-Fe2O3 and thereby decrease the reactivity, which in turn appears to be strongly influenced by the three types of water,viz. absorbed, bound and crystallisation. 相似文献
48.
Li-Hua Bi Sib Sankar Mal Nadeen H. Nsouli Michael H. Dickman Ulrich Kortz Saritha Nellutla Naresh S. Dalal Manuel Prinz Georg Hofmann Manfred Neumann 《Journal of Cluster Science》2008,19(1):259-273
The mixed-valence 24-vanadophosphate
(1) has been synthesized and characterized in the solid state by IR, magnetism, EPR, XPS, and elemental analysis. Single-crystal
X-ray analysis was carried out on (Na-1), which crystallizes in the triclinic system, space group , with a = 17.168(3) ?, b = 18.1971(14) ?, c = 20.1422(13) ?, α = 114.753(3)°, β = 99.390(4)°, γ = 95.124(4)°, and Z = 2. Polyanion 1 has an unusual, open structure composed of 2 RuIIIO6 octahedra, 2 VIVO6 octahedra, 14 VVO5 square-pyramids, 8 VVO4 tetrahedra, and 2 PO4 tetrahedra which are all directly linked via edges and corners. The outer surface of 1 is decorated with six RuII(dmso)3 groups. XPS studies on Na-1 confirm the presence of 2 RuIII and 6 RuII as well as 22 VV and 2 VIV centers. Magnetic susceptibility data on Na-1 show that the VIV–RuIII pairs are coupled antiferromagnetically, with J
1 = −13 K and J
2 ∼ −3 K. We did not detect any peak in our EPR measurements on Na-1, thus supporting the conclusion that Na-1 is diamagnetic in its ground state.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
In Memoriam Prof. F. A. Cotton 相似文献
49.
Gunaydin-Sen O Achey R Dalal NS Stowe A Autrey T 《The journal of physical chemistry. B》2007,111(4):677-681
We report high resolution 15N NMR probing of the solid-solid phase transition of 15N-labeled ammonia borane (NH3BH3) at 225 K. Both the 15N isotropic chemical shift (delta iso) and the spin-lattice relaxation rate (T1-1) exhibited strong anomalies around 225 K. The analysis of T1-1 using the Bloembergen, Purcell, and Pound model showed that the motional correlation time, tau, increased from about 1 to 100 ps and the corresponding Arrhenius activation energy increased from 6 to 14.5 kJ/mol on going through the transition toward lower temperatures. The temperature dependence of delta iso was interpreted by an extension of the Bayer model. The time scale of the underlying motion was found to be in a reasonable agreement with the T1-1 data. These results imply that the NH3 rotor motion plays a pivotal role in the transition mechanism and that the transition is of both order-disorder and displacive type. 相似文献
50.
High level ab initio electronic structure calculations at different levels of theory have been performed on HNP and HPN neutrals, anions, and cations. This includes standard coupled cluster CCSD(T) level with augmented correlation-consistent basis sets, internally contacted multi-reference configuration interaction, and the newly developed CCSD(T)-F12 methods in connection with the explicitly correlated basis sets. Core-valence correction and scalar relativistic effects were examined. We present optimized equilibrium geometries, harmonic vibrational frequencies, rotational constants, adiabatic ionization energies, electron affinities, vertical detachment energies, and relative energies. In addition, the three-dimensional potential energy surfaces of HNP(-1,0,+1) and of HPN(-1,0,+1) were generated at the (R)CCSD(T)-F12b∕cc-pVTZ-F12 level. The anharmonic terms and fundamentals were derived using second order perturbation theory. For HNP, our best estimate for the adiabatic ionization energy is 7.31 eV, for the adiabatic electron affinity is 0.47 eV. The higher energy isomer, HPN, is 23.23 kcal∕mol above HNP. HPN possesses a rather large adiabatic electron affinity of 1.62 eV. The intramolecular isomerization pathways were computed. Our calculations show that HNP(-) to HPN(-) reaction is subject to electron detachment. 相似文献