Tài liệu

Thanh N. Truong

Science and Technology

Tóm tắt nội dung

Using the Valence Shell Electron Pair Repulsion (VSEPR) model to predict the structure of SF4 then confirming or validating the model by using molecular orbital theory.

VSEPR and MO theories on the structure of SF4

Nội dung

VSEPR model provides a qualitative theoretical model to understand 3D structure of a stable molecule.  This model is based on a physical concept that bonds and lone pairs around a central atom repel each others and thus will arrange in space to minimize such repulsions and lone pairs take more space than bonds.  S has 6 electrons in the valence shell, four to form 4 single sigma bonds with F atoms in SF4 and thus has one lone pair.  According to to the VSEPR model, S in SF4 is surrounded by 4 bonds and a lone pair and thus it should have a triagonal bipyramid structure with the lone pair on the basal plane.  However, assuming that we don't know anything about VSEPR, SF4 may have these possible structures.

This experiment is to use molecular orbital theory to confirm VSEPR model.

Procedure:  Using tools in Avisto.  You can download Avisto and its tools at Astonis.

1. Use MolDesign to build the four 3D structures of SF4 above.
2. Use Basic QChem Edu, Basic QChem, Mopac GUI Cloud or Pro tool to search for a nearby stable structure from these four starting points.   Record the structures and heats of formation of the stable structures found by the program.
3. Use the result of these runs to perform vibrational analyses to confirm that these structures are in fact equilibrium structure and not the transition state.  In PsiViewer, click on the IR tab and look in the table of frequencies to see if there is any imaginary (shown as negative values in the table) frequency.  If there is then it is a transition state.  If not then it is in fact an equilibrium structure.

From the results, what can you say about the validity of the VSEPR model.