A. Abu El-Fadl

Bio: A. Abu El-Fadl is an academic researcher from Assiut University. The author has contributed to research in topics: Band gap & Dielectric. The author has an hindex of 16, co-authored 89 publications receiving 1285 citations. Previous affiliations of A. Abu El-Fadl include University of Mainz.

Complexation of alkali metal cations by conformationally rigid, stereoisomeric calix[4]arene crown ethers: a quantitative evaluation of preorganization

Abstract: Selective bridging of p-tert-butylcalix[4]arene (1) with tetra- and pentaethylene glycol ditosylate gives the 1,3- dihydroxy-p-tert-butylcalix[4]arene crown ethers 7 and 9 in good yields. The subsequent alkylation of the two phenolic groups of 7 and 9 with substituents (R) bulkier than CH 3 gives a series of conformationally rigid 1,3-dialkoxy-p-tert-butylcalix[4]arene crown ethers.

Stereochemical Aspects of The 'tert-Amino Effect”: 2. Enantio- and Diastereoselectivity in the Synthesis of Quinolines, Pyrrolo[l,2-a]quinolines, and [l,4]oxazino[4,3-a]quinolines

Abstract: Thermal isomerization of the optically pure 2-vinyl-N,N-dialkylanilines, with a methyl or an ethyl substituent (R2) at the α-position of the N,N-dialkyl moiety, 4a (R1= H), 4b (R1= CH3), 5a (R1= H), and 6a (Rl= H), affords enantioselectively the optically pure pyrrolo[l,2-α] quinolines 7a and 7b and the [1,4]oxazino[4,3-a]quinoline 11, with the methyl or ethyl substituent (R2) at the bridgehead carbon atom, and the quinoline 13, respectively. The optical purity of these quinoline derivatives was determined by 1H NMR spectroscopy in the presence of chiral shift reagents. Heating of the optically pure analogues in which R2is a methoxymethyl group (4c and 4d) in refluxing 1-butanol yields, besides the compounds 7c,d with the methoxymethyl group at the bridgehead carbon atom, also the regioisomers 8c,d and 9c,d that are enantiomerically pure. Mixtures of the diastereomers 12a,b and 14a,b were obtained by cyclization of compound 5b, with a 3-ethylmorpholinyl group, and of the acyclic amine 6b, respectively, in refluxing 1-butanol. The compounds 12a and 14a,b were proven enantiomerically pure. The configuration of the compounds 8, 9, 12, and 14 was determined by X-ray analysis [(±)-12a] and NMR 1H and 1H NOE difference spectroscopy. These results provide conclusive evidence for the mechanism of these cyclization reactions, which are further examples of the “tert-amino effect”. The effect of substituents on the enantio- and diastereoselectivity of the cyclization is discussed.

Optical constants of Zn1−xLixO films prepared by chemical bath deposition technique

Abstract: Zn1−xLixO films of different Li concentrations x = 0.0 – 0.5 were successfully deposited on glass substrates using chemical bath deposition (CBD) technique. Optical properties of the films were studied extensively in the wavelength range 190–800 nm from the measurements of the optical transmittance (T) and optical reflectance (R). The mechanism of the optical absorption follows the rule of direct allowed transition. Both the optical energy gap Eg and absorption coefficient ( α ) were found to be Li concentration-dependent. The refractive index values have been fitted to the single-oscillator models. The values obtained for the single-oscillator energy Eo are consistent with the optical gap results. The dependence of the optical constants of ZnO on the photon energy ( h ν ) at selected concentrations of Li were discussed. The exponential dependence of the absorption coefficient as a function of the incident photon energy suggests that the Urbach rule is obeyed and indicated the formation of a band tail. It was found that the optical energy gap decreases with increasing Li content; on the other hand the band tail width exhibits the opposite behaviour.

Influence of strontium doping on the indirect band gap and optical constants of ammonium zinc chloride crystals

Abstract: Optical transmittance measurements near the absorption edge in the energy range 3.4–6.4 eV have been carried out on (NH4)2ZnCl4:xSr2+ single crystals with x=0.000%, 0.020%, 0.039%, 0.087% or 0.144%. The introduction of Sr2+ in low concentration, x=0.02% or 0.039%, little bleach the intense charge transfer band observed for the undoped sample with a maximum at 5.30 eV. Doping with Sr2+ in a little bit higher concentration, x=0.087% or 0.144%, led to the disappearance of this band. While doping with Sr2+ has no effect on the indirect allowed type of optical transition in AZC, the optical energy gap decreased with increasing Sr2+ concentration. Measuring reflectance of AZC permitted the calculation of the refractive index n, the extinction coefficient K and both the real er and imaginary ei components of the dielectric permittivity as functions of photon energy. The validity of Cauchy–Sellimaier equation was checked in the wavelength range 5.45–5.8 eV and its parameters were calculated. Applying the single-effective-oscillator model, the moments of e(E) could be estimated. Electric susceptibility, as a measure of intraband transition, was calculated and its dependence on photon energy was considered.

Stereochemical Aspects of the “tert-Amino Effect” : 1. Regioselectivity in the Synthesis of Pyrrolo[l,2-a]quinolines and Benzo[c]quinolizines

Abstract: Substituted 2-vinyl-N,N-dialkylanilines cyclize in refluxing 1-butanol to give substituted pyrrolo[1,2-α] quinolines and benzo[c]quinolizines. This reaction proceeds via a 1,5-hydrogen transfer and subsequent C-C bond formation. When in the 2-vinyl-N,N-dialkylanilines 4, R1= H and R2 = H (4a,d), CH3(4b,e), or C2H5(4f), the cyclization products 5a,b,d-f are formed selectively, with the substituent R2at the bridgehead carbon atom. This regioselectivity is lost when R2 = CH2OCH3(4c,g), and a mixture of the regioisomers 5c,g, 6c,g, and 7c,g is formed. Reaction of compounds 4h-n (R1= CH3) yields the pyrrolo[l,2-α]quinolines 5-7(h-j) and benzo[c]quinolizines 5-7(k-n) selectively, in which the substituent at the bridgehead carbon atom is at the same face of the molecule (cis) as the hydrogen atom at C-5 [5-7(h-j)] or at C-6 [5-7(k-n)]. The configuration of these compounds was determined by1H NOE difference spectroscopy and single-crystal X-ray analysis (6n). Heating of 4o-q (R1= 4-C6H4CH3) in refluxing 1-butanol gives mixtures of the cis [5-7(o-q)] and trans [8—10(o—q)] compounds. The mechanism of these cyclizations, which are further examples of the “tert-amino effect”, and the effect of variation in substituents are discussed.

Novel Cavity Design Using Calix[n]arene Skeletons: Toward Molecular Recognition and Metal Binding.

Abstract: Calixarenes are macrocyclic molecules, like crown ethers and cyclodextrins.1-7 Calixarenes made up of phenol and methylene units have many conformational isomers because of two possible rotational modes of the phenol unit: the oxygen-through-theannulus rotation and the para-substituent-throughthe-annulus rotation (Figure 1). The conformational isomers thus yielded afford a great number of unique cavities with the different size and the different shape. Recently, a number of strategies have been exploited by which not only the conformation of calix[4]arenes, but also those of calix[6]arenes and calix[8]arenes, can be immobilized. This means that our group can now design various calixarene-based receptors that show high selectivity for guest molecules and metal cations. In this review article, our group describe novel strategies for cavity design using calix[n]arene skeletons, strategies that are intended to allow complexation of specific molecular targets or metal ions.

The Nature Of The Chemical Bond

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