Journal of Physical and Theoretical Chemistry
Volume:4 Issue: 1, Spring 2007

In this paper,the molecular geometry for three tautomers of uracil and four tautomers of cytosine has been analyzed. vibrational IR spectra of the tautomers were   investigated at HF and B3LYP level using the AB initio 6-31G* and LANL2DZ basis sets from the program package Gaussian 98 (A.7 Public Domain version).
The physico-chemical and biochemical properties of uracil and cytosine are one of the principal criteria for the selection of these compounds. Calculated results were compared with the corresponding experimental data if available. The harmonic wave numbers for the main tautomers of uracil (U1) and cytosine (C2) will compare at the 25ºC, 37ºC, and 40ºC.

The fact that the properties of a molecule are tightly connected to its structural  characteristics  is one of the fundamental concepts in chemistry. In this connection,  graph theory has been successfully applied in developing some relationships between topological indices and some thermodynamic properties. So ,  a novel method for computing the new descriptors to construct a quantitative relation between structure and properties is presented. At first, a brief review on the classical graph theories introduced and, then, the link with molecular similarity is drawn. In the applications section, molecular topological indices are calculated. Afterwards, the molecular descriptors, that include the necessary structural information for properly describtion of system  are employed to derive a numerical correlation with thermodynamic properties. Finally, some useful topological indices for examination of the structure-property relationship are presented. In addition, the relationship between the Randic, Wiener,  Hosoya ,  Balaban and Schultz indices and Harary numbers and Distance matrix to the enthalpies of formation, heat capacities, (Cp) , enthalpies of combustion, enthalpies of vaporization  and normal boiling points for  normal alcohols is established.

The mono-metallic and bi-metallic catalysts have been prepared by impregnating with solutions containing a compound of H2PtCl6,WO3 and 1ml HCl (0.1 mol). It should be noted that the catalysts’ activity and selectivity have been determined under these conditions : 450-5000C ,and 15-25atm by H2. For converting n-heptane , the molar ratio H2/C7H16 is 5 , and LHSV is 1.5ml/h. It has been proved that , under these experimental conditions , the conversion of n-heptane into branched aliphatic isomers is catalyzed by a bifunctional mechanism. The results indicate that in the presence of W , the catalysts have much higher selectivity and stability for isomerization reaction. At a given conversion , the bi-metallic catalysts produce much lower concentrations of cracked products. According to the tests , W modifies the support acidity resulting to the higher selectivity for isomerization and the lower one for cracking , and it also modifies the Pt properties

The Potential Energy Surface PES of (H2)2 dimer has been investigated, using five simple rigid rotor models. These models are called: head to head, symmetric side to side, L , steplike and T model. All calculations were done at two levels of ab initio  MP2(Full) and QCISD (T,Full) using cc-pVTZ basis set at singlet state of spin multiplicity. The results of scanning PES were then fitted to Taylor series up to 15 terms to obtain the equilibrium distances and interaction energies between  pairs of H2 molecules.The standard deviation of the residuals in fitting procedure  is well  below 10-9 in all cases.L  model has been found to be the  most  stable  one instead of previously reported T model and 7 times more stable than other models.The validity of  hard sphere approximation was also tested by solving simple mathematical equation and found to be no longer valid in these interactions.

Quaternary ammonium compounds (QACS) are employed both as disinfectants for manual processing lines and surfaces in the food industry, and in human medicine area. QACS also cause cell death by protein denaturation . One of the QAC is cetyl trimethyl ammonium bromide (CTAB) that appears to rapture the cell membrane. The primary site of action of CTAB has been suggested to be the lipid components of the membrane, causing cell lysis as secondary effect.
For these important roles of CTAB as a cationic surfactant, CTA (cetyl trimethyl ammonium) was studied as the main active site of CTAB. After optimization the values of thermodynamic functions, chemical shifts and Mulliken charges were obtained by calculation in Gaussian 98 program

Using Hartree–Fock (HF) and Density Functional Theory (DFT) calculations the thermodynamic properties such as thermal energy , , thermal enthalpy , , thermal entropy , , thermal Gibbs free energy , , heat capacity ,Cv, and molecular structures of several species involving in keto↔enol tautomerism related to acetaldehyde (A), 5,5-dimethyl-1,3-cyclohexanedione (dimedone) and  acetylacetone (AA) have been investigated. In addition, the geometric structure of all stages intervening in enol ↔ keto conversion related to three above mentioned compounds have been investigated on ab initio calculation and DFT level by using  STO-3G, 6-31G, 6-31++G and 6-31++G** basis sets. The results showed that the enol structure related to acetylacetone is more stable because of intramolecular hydrogen bond formation in the enol form while the keto structure is more stable in acetaldehyde and dimedone because of strong C=O bond existence in the keto tautomers. The computed HF/DFT results are in a good agreement with the experimental studies. Moreover,the values of , , and thermodynamic equilibrium constant, Kth, of tautomerization phenomenon related to acetaldehyde, dimedone and acetylacetone are calculated on the basis of DFT-IR method.

We have studied the adsorption processes of H2 on the V (100) surface of Vanadium using self consistent field theory.Dissociative adsorptions of H2 are significantly favored compared to molecular adsorptions. There is a significant charge transfer from the first layer of the vanadium surface to the Hydrogen atoms. Three possible adsorption sites, top, bridge and center site, were considered in the calculations. The geometry of V (100) surface has also been optimized theoretically with Hartree-Fock method and the results show that the prediction from the HF/LANL2DZ, 6-31G* and HF/LANL2DZ, 6-31G** reproduce the experimental observation. These results indicate that two models of vertical top site and horizontal bridge site have short distances from surface and are  more stable than other models, also the 6-31G* basis set give more optimized structures than 6-31G** basis set in calculations.

In order to study the Cu(II) ion selective electrode, an optimized membrane was prepared by mixing of 30 mg of PVC powdered, 5 mg of considered ionophore, 60 mg of plasticizer and 5 mg of additive. The sensor showed a near Nernstian response for Cu(II) ions over a concentration range from 1.0×0-6 –1.0×0-2 M with a slope of 28.90.5 mV per concentration decade with a working pH range of 3.5-6.3. The limit of detection was 1.010-6 M. The response time of the electrode is about 15s and can be used for more than 2 month without any divergence in potential.
   The proposed sensor showed fairly a good discriminating ability towards Cu(II) ion in comparison to other ions. The practical application of the electrode was tested by using it as an indicator electrode to determine the end point in the potentiometric titration of Cu(II) with EDTA solution.