Browsing by Author "NORİ-SHARGH, Davood"
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Item Conformational behaviors of trans-2,3- and trans-2,5-dihalo 1,4-diselenanes. A complete basis set, hybrid-density functional theory study and natural bond orbital interpretations(Journal of Molecular Modeling, 2014-05-14) NORİ-SHARGH, Davood; MOUSAVI, Seiedeh Negar; KAYI, HakanComplete basis set CBS-4, hybrid-density func tional theory (hybrid-DFT: B3LYP/6-311+G**) based methods and natural bond orbital (NBO) interpretations have been used to examine the contributions of the hyperconjugative, electrostatic, and steric effects on the con formational behaviors of trans-2,3-dihalo-1,4-diselenane [halo=F (1), Cl (2), Br (3)] and trans-2,5-dihalo-1,4- diselenane [halo = F (4), Cl (5), Br (6)]. Both levels of theory showed that the axial conformation stability, compared to its corresponding equatorial conformation, decreases from com pounds 1→3 and 4→6. Based on the results obtained from the NBO analysis, there are significant anomeric effects for compounds 1-6. The anomeric effect associated with the electron delocalization is in favor of the axial conformation and increases from compounds 1→3 and 4→6. On the other hand, dipole moment differences between the axial and equa torial conformations [Δ(μeq - μax)] decrease from compounds 1→3. Although Δ(μeq-μax) parameter decreases from com pound 1 to compound 3, the dipole moment values of the axial conformations are smaller than those of their corresponding equatorial conformations. Therefore, the anomeric effect as sociated with the electron delocalizations (for halogen-C-Se segments) and the electrostatic model associated with the dipole-dipole interactions fail to account for the increase of the equatorial conformations stability on going from com pound 1 to compound 3. Since there is no dipole moment for the axial and equatorial conformations of compounds 4-6, consequently, the conformational preferences in compounds 1-6 is in general dictated by the steric hindrance factor asso ciated with the 1,3-syn-axial repulsions. Importantly, the CBS-4 results show that the entropy difference (ΔS) between the equatorial axial conformations increases from compounds 1→3 and 4→6. This fact can be explained by the anomeric effect associated with the electron delocalization which affects the C2-Se bond orders and increase the rigidity of the corre sponding rings. The Gibbs free energy difference values be tween the axial and equatorial conformations (i.e. ΔGax-ax and ΔGeq-eq) of compounds 1 and 4, 2 and 5 and also 3 and 6 have been calculated. The correlations between the anomeric effect, electrostatic model, ΔGeq-ax, ΔGax-ax, ΔGeq-eq, bond orders, dipole-dipole interactions, structural parameters and confor mational behaviors of compounds 1-6 have been investigated.Item Correlations between hardness, electrostatic interactions, and thermodynamic parameters in the decomposition reactions of 3-buten-1-ol, 3-methoxy-1-propene, and ethoxyethene(Structural Chemistry, 2014-08-11) HASANZADEH, Neda; NORİ-SHARGH, Davood; KAYI, Hakan; JAVID, Nargess RezaiDecomposition of the three isomeric com pounds, 3-buten-1-ol (1), 3-methoxy-1-propene (2), and ethoxyethene (3), at two different (300 and 550 K) tem peratures has been investigated by means of ab initio molecular orbital theory (MP2/6-311?G**//B3LYP/6- 311?G**), hybrid-density functional theory (B3LYP/6- 311?G**), the complete basis set, nuclear magnetic reso nance analysis, and the electrostatic model associated with the dipole–dipole interactions. All three levels of theory showed that the calculated Gibbs free energy differences between the transition and ground state structures (DG=) increase from compound 1 to compound 3. The variations of the calculated DG= values can not be justified by the decrease of the calculated global hardness (g) differences between the ground and transition states structures (i.e., D[g(GS)-g(TS)]). Based on the synchronicity indices, the transition state structures of compounds 1–3 involve syn chronous aromatic transition structures, but there is no significant difference between their calculated synchronicity indices. The optimized geometries for the transition state structures of the decomposition reactions of compounds 1–3 consist in chair-like six-membered rings. The variation of the calculated activation entropy (DS=) values can not be justified by the decrease of D[g(GS)- g(TS)] parameter from compound 1 to compound 3. On the other hand, dipole moment differences between the ground and transition state structures [D(lTS-lGS)] decrease from compound 1 to compound 3. Therefore, the electrostatic model associated with the dipole–dipole interactions jus tifies the increase of the calculated DG= values from compound 1 to compound 3. The correlations between DG=, D[g(GS)-g(TS)], (DS=), k(T), electrostatic model, and structural parameters have been investigated.