Rafie H. Abu-Eittah

The electronic absorption spectra of pyridine azides, solvent-solute interaction.

The electronic absorption spectra of: 2-, 3-, and 4-azidopyridines have been investigated in a wide variety of polar and non-polar solvents. According to Onsager model, the studied spectra indicate that the orientation polarization of solvent dipoles affects the electronic spectrum much stronger than the induction polarization of solvent dipoles. The effect of solvent dipole moment predominates that of solvent refractive index in determining the values of band maxima of an electronic spectrum. The spectra of azidopyridines differ basically from these of pyridine or mono-substituted pyridine. Results at hand indicate that the azide group perturbs the pyridine ring in the case of 3-azidopyridine much more than it does in the case of 2-azidopyridine. This result agrees with the predictions of the resonance theory. Although the equilibrium <==> azide tetrazole is well known, yet the observed spectra prove that such an equilibrium does not exist at the studied conditions. The spectra of the studied azidopyridines are characterized by the existence of overlapping transitions. Gaussian analysis is used to obtain nice, resolved spectra. All the observed bands correspond to pi-->pi* transitions, n-->pi* may be overlapped with the stronger pi-->pi* ones.

A study on the complex formation between copper(II) and some ephedrines

Abstract Complex formation between the copper(II) ion and ephedrine and norephedrine has been studied potentiometrically and spectrophotometrically. A spectral-structural correlation has been investigated for the complexes. Copper ephedrine and norephedrine complexes possessed a geometry which varied from octahedral to tetragonal to square planer depending on the experimental conditions. Semi-empirical MO calculations were performed on a model “copper-ephedrine complex” with different geometries, with satisfactory results.

A theoretical DFT study on the structural parameters and azide–tetrazole equilibrium in substituted azidothiazole systems

Azido-tetrazole equilibrium is sensitive to: substitution, solvent, temperature and phase. In this work, the effects of the type and position of substitution on the thiazole ring of azidothiazoles on its structural parameters and on the azido-tetrazole equilibrium have been theoretically investigated using the density functional procedures at the B3LYP/6-311G(∗∗) level of theory. This study includes the investigation of the equilibrium geometry, the transformation of the trans-conformer to the cis one then the ring closure to the tetrazole isomer. The transition states of the two steps were located, confirmed and the structural parameters were calculated. In all the steps of calculations, geometry optimization was considered. The results obtained indicate that substitution by: -NO(2) and -CN group shifts the equilibrium to the azide side and in some cases the tetrazole isomer is not obtained. On the other hand, substitution by: -NH(2) and -OH groups shifts the equilibrium to the tetrazole side and in some cases the azide isomer is not obtained and if formed changes spontaneously to the tetrazole isomer. The decisive parameters which determine the position of the equilibrium are: charge density on atoms N3 and N8, rearrangement of bond length and bond angles during the process of cyclization and variation of dipole moment as a result of cyclization. Results of this work indicate that substitution on C5 is more efficient than substitution on C4 of the thiazole ring.

Spectroscopic studies on styryl-, thienylethenyl- and furylethenyl-pyridines: molecular orbital treatment and effect of positional isomerism

The electronic absorption spectra of 2-, 3- and 4-styrylpyridines and 2- and 4-thienylethenyl and furylethenylpyridines in polar and non-polar solvents have been investigated. Different spectra were obtained for the different positional isomers. Solvent–solute interactions caused a shift in band maxima with solvent polarity that was dependent on the specific isomer under investigation. Spectral investigation indicated an extensive electronic interaction between the two heterocyclic rings joined through an ethylenic linkage.Molecular orbital calculations, using the INDO/S-CI procedure, were performed. The nature of the MO, whether localized or delocalized was a function of the specific isomer. The contribution of CT configuration to the electronic transition was different for different isomers. The solvent–solute interaction was calculated at the MO level. Satisfactory correspondence was found between calculated and experimental results.

Spectroscopic studies on some complexes of nickel(II) and palladium(II) with thiosemicarbazides

SummaryThe electronic absorption spectra of the complexes M(L)Cl2 and M(L)2Cl2 [M=NiII or PdII; L=thiosemicarbazide (tsc), 1-phenylthiosemicarbazide (1-phtsc), or 4-phenylthiosemicarbazide (4-phtsc)] were investigated in a number of solvents. The complexes Ni(tsc)Cl2, Ni(tsc)2Cl2, Ni(4-phtsc)Cl2 and Ni(4-phtsc)2Cl2 exist in a distorted Oh geometry when dissolved in any of the solvents used. On the other hand, the complex Ni(1-phtsc)2Cl2, although weakly paramagnetic, proved to be planar. Theoretical and experimental results proved that the complex has neither Oh nor Td geometry. Molecular orbital calculations were performed on some selected complex ions assuming a local point group symmetry of D2h.

A theoretical study of the thermal Curtius rearrangement of some cinnamoyl azides using the DFT approach

The thermal Curtius rearrangement of cinnamoyl azide, 1-azido-3-phenylprop-2-ene-1-one, and the reactions of some of its derivatives is studied theoretically using the DFT-B3LYP/6-31G(d,p) approach. The potential energy surface profiles of the rearrangement are calculated. The transition state was located and confirmed. The Curtius rearrangement of the studied compounds is a one-stage, discrete reaction. A weak effect of substitution on the reaction rate is due to the unique, localized π system of the studied molecules; strong opposing dipoles span the whole molecule.

A kinetic study on the formation of some carbon-centered and silicon-centered free radicals

Abstract The specific rate constants for formation of methyl, aminomethyl, hydroxymethyl and cyanomethyl free radicals were calculated using the transition state theory. The activation energies were calculated using the UHF in the MNDO level with complete geometry optimization. Force field calculations were performed on the reactants as well as on the activated complex. The position and geometry of the active complex were determined through a total energy as well as vibration frequency calculation. Similar calculations were performed on the corresponding silicon-centered radicals and the results were rationalized.

Internal rotation in relatively complicated systems: benzoic, nicotinic, isonicotinic and picolinic acids and their thio derivatives

The internal rotation of an OH group around a single CO bond has been investigated for benzoic, nicotinic and isonicotinic acids and two conformers of picolinic acid. The rotation (torsion) angle was varied from 0°(syn conformer) to 180°(anti conformer) and total energy was calculated at the MNDO level with complete geometry optimization. The height of the potential barrier for internal rotation was calculated with reference to the energy of the syn conformer which was scaled to zero energy. For comparison, the internal rotation of an SH group around a single CS bond was investigated for the molecules thiobenzoic acid, thionicotinic acid and thioisonicotinic acid. Some molecules are ‘free-rotators’ while others are ‘rigid’. The height of the rotational barrier was dependent on the ‘rotamer’, OH or SH, as well as on the specific isomer of the framework.

Electron donor–electron acceptor complexes of carbonyldi-imidazoles and thiocarbonyldi-imidazoles with iodine. Effect of solvent polarity

The formation of charge-transfer complexes between carbonyl and thiocarbonyldi-imidazole with iodine has been investigated spectrophotometrically. The thermodynamic functions of formation, resonance energy, dipole moment and ionic character of the charge-transfer complex were calculated. The ionization potential of the donor was calculated from the charge-transfer results. The study has been conducted in different solvents and the results were analysed statistically.

A Theoretical Study on the Structures and Spectra of Ni(II) Aquo Azides, Thiocyanates and Isothiocyanates

A theoretical treatment of the formation and stability of some aquated nickel II azides, thiocyanates and isothiocyanates has been carried out. Ab initio molecular orbital calculations have been carried out using the DZ basis set and 3 F ground state for nickel and using the 3-21 G split valence basis set for H, C, N and O-atoms. For S-atom the 3-21 G basis set was augmented with 3 d -function. Geometry optimization was carried out for each of the studied species; charge distribution and electron overlap population were calculated. The results of this work show that azide ion is a stronger ligand than isothiocyanate ion and the latter is stronger than thiocyanate ion with respect to bonding to nickel ion. The results of calculations indicated that the triplet state was the stable ground state for all the studied species. Calculations using ROHF procedure led to better results than calculations using UHF method.