Mohamed F. Shibl

Electronic absorption spectra of some 2-thiouracil derivatives

Abstract The electronic absorption spectra of 2-thiouracil and some of its derivatives were investigated using polar and nonpolar solvents. The present analysis is facilitated via computer deconvolution of the observed spectra and molecular orbital (MO) computations. Comparison between the experimentally observed and theoretically computed spectra in addition to a quantitative assignment of all transitions observed were undertaken. The computed dipole moments are used to indicate the polarity of the excited state and hence predict its solvent dependence. The spectra are, in general, very well predicted and assigned using INDO/S computational results. The spectrum of the trifluoro derivative is much more complicated and the corresponding states show much more solvent dependence than those observed for other thiouracil studied.

Optimization of selection of chain amine scrubbers for CO2 capture

In order to optimize the selection of a suitable amine molecule for CO2 scrubbers, a series of ab initio calculations were performed at the B3LYP/6-31+G(d,p) level of theory. Diethylenetriamine was used as a simple chain amine. Methyl and hydroxyl groups served as examples of electron donors, and electron withdrawing groups like trifluoromethyl and nitro substituents were also evaluated. Interaction distances and binding energies were employed as comparison operators. Moreover, natural bond orbital (NBO) analysis, namely the second order perturbation approach, was applied to determine whether the amine–CO2 interaction is chemical or physical. Different sizes of substituents affect the capture ability of diethylenetriamine. For instance, trifluoromethyl shields the nitrogen atom to which it attaches from the interaction with CO2. The results presented here provide a means of optimizing the choice of amine molecules for developing new amine scrubbers.

Electronic Absorption Spectra of Some Triazolopyrimidine Derivatives

The electronic absorption spectra of triazolo pyrimidine and some of its derivatives were measured in polar as well as nonpolar solvents. Assignment of the observed transitions is facilitated via molecular orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. The observed transitions are assigned as charge transfer (CT), localized, and delocalized according to the contribution of the various configurations in the CI-states. The correspondence between the calculated and experimental transition energies is satisfactory.

Regioselective synthesis and ab initio calculations of fused heterocycles thermally and under microwave irradiation.

Pyrazolo[1,5-a]pyrimidine, triazolo[1,5-a]pyrimidine, and pyrimido[1,2-a]benzimidazole, pyrido[1,2-a]benzimidazole ring systems incorporating phenylsulfonyl moiety were synthesized via the reaction of 3-(N,N-dimethylamino)-1-(thiophen-2-yl)-2-(phenylsulfonyl)prop-2-en-1-one derivatives with the appropriate aminoazoles as 1,3-binucleophiles and 1H-benzimidazol-2-ylacetonitrile using conventional methods as well as microwave irradiation. The regioselectivity of the cyclocondensation reactions was confirmed both experimentally by alternative synthesis of reaction products and theoretically using ab initio quantum chemical calculations namely the Density Functional Theory (DFT). The theoretical work was carried out using the Becke, three parameter, Lee-Yang-Parr hybrid functional (B3LYP) combined with the 6-311++G(d,p) basis set. It was found that the final cyclocondensation reaction product depends mainly on the initial addition to the activated double bond by the nitrogen atom of the 1,3-binucleophiles that has the higher electron density.

Hydrogen bond coupling in sodium dihydrogen triacetate

The coupling of hydrogen bonds is central to structures and functions of biological systems. Hydrogen bond coupling in sodium dihydrogen triacetate (SDHTA) is investigated as a model for the hydrogen bonded systems of the type O-H…O. The two-dimensional potential energy surface is derived from the full-dimensional one by selecting the relevant vibrational modes of the hydrogen bonds. The potential energy surfaces in terms of normal modes describing the anharmonic motion in the vicinity of the equilibrium geometry of SDHTA are calculated for the different species, namely, HH, HD, DH, and DD isotopomers. The ground state wave functions and their relation to the hydrogen bond structural parameters are discussed. It has been found that the hydrogen bonds in SDHTA are uncoupled, that is elongation of the deuterated hydrogen bond does not affect the non-deuterated one.

Solvent and substituent effects on the electronic structures of triazoles: computational study

The tautomerisation reaction of 1,2,3-triazole and some of its derivatives has been studied and the transition states were located. The analysis of results indicates that 2H-triazole is more stable than the 1H-tautomer in all studied compounds in gas phase and in solution. The proton detachment energies and the geometries of the compounds studied were analysed using density functional theory (DFT) and Hartree–Fock (HF) methods in gas phase and solution. Deprotonation takes place in all cases studied by the detachment of N–H proton. The solvent effect on the stability the tautomeric and anion forms of studied compounds has been examined using B3LYP/6-31G* level of theory by applying the polarisable continuum model.

Electronic Structure of some A3 Adenosine-Receptor Antagonist——A Structure Activity Relationship

DFT quantum chemical computations have been carried out at the B3LYP/6-31G (d) level. Full geometry optimization has been performed and equilibrium geometries for a new series of phenyl thiazoles have been located. Ground state electronic properties, charge density distributions, dipole moments and its components have been calculated and reported. Effect of substituents on the geometry and on the polarization of the studied series of compounds are analyzed and discussed. Some structural features have been pinpointed to underline the affinity and selectivity of the studied compounds as adenosine A3-receptor antagonists. Results of the present work indicate that activity towards A3 receptor sites is directly correlated with both of the polarity and the co-planarity of the thiazole.