علوم پایه (دانشگاه آزاد اسلامی)
سال:1390 | دوره:21 | شماره:80 (ویژه نامه شیمی) |تعداد مقالات:16

Introduction: QSRR study is suggested for the prediction of GC retention times of complex petroleum compounds. Modeling of the GC retention times as a function of molecular structures was established by means of the chemometrics methods such as PLS, OSC-PLS and LS-SVM.Aim: These models were applied for the prediction of the GC retention times of these compounds, which were not in the modeling procedure.Material and Method: Descriptors were calculated utilizing Dragon software. These descriptors are calculated using two-dimensional representation of the molecules and therefore geometry optimization is not essential for calculating these types of descriptors.Results: Good results were achieved in LS-SVM model with percentage error ranges from -0.14 to 0.12 for retention times. The resulted model showed high prediction ability with RMSEP of prediction of 0.0152 for LS-SVM.Conclusion: OSC-PLS and LS-SVM models were established to predict the GC retention time of some organic compounds in petroleum sample. A proper model with high statistical quality and low prediction errors was obtained.

Introduction: The conformational behavior of dihydroxymethane has been analyzed by means of hybrid-density functional theory (B3LYP/6-311++G**) based method and NBO interpretation.Aim: The correlation between the energy differences between the various conformations of dihydroxymethane, the anomeric effect associated with the electron delocalization, diploe-dipole interactions (i.e. the electrostatic mode) and steric repulsions have been investigated. Results: Based on the potential energy surface obtained at the B3LYP/6-311++G** level of theory, the most stable conformation of dihydroxymethane is found to be an axial symmetrical (C2 symmetry) conformation (i.e. gauche-gauche, g.g). The calculated energy for the second lowest energy-minimum conformation, viz. gauche-gauche* (g.g*, Cs symmetry) is 2.44 kcal mol-1. Third lowest energy-minimum structure of dihydroxymethane is the unsymmetrical quasi-gauche-anti conformation (i.e. qa.g, by about 2.58 kcal mol-1 higher than g.g conformation). Based on the obtained NBO results, the calculated generalized anomeric effect (GAE) value decreases from of the g.g conformation of dihydroxymethane to its qa.g conformation but decreases from qa.g conformation to g.g* conformation. Therefore, the GAE can not explain the more stability of the g.g conformation of dihydroxymethane compared to the corresponding g.g* conformation. On the other hand, the calculated dipole moment value of the g.g conformation of dihydroxymethane is smaller than that of g.g* conformation.Conclusion: Interestingly, these findings led to the proposal that the electrostatic model is more significant for the explanation of the conformational preference of dihydroxymethane than the GAE and steric interactions.