Geometry predicting, with VSEPR

Figure 7 ELF isosurfaces for some molecules with typical geometries predicted by VSEPR...

If the central atom is surrounded only by single bonds, the process of determining the molecular geometry is fairly simple. We count the number of single bonds emanating from the central atom and use the information in Table 7.3 to correlate that number of electron pairs with the geometry predicted by VSEPR, and this describes the shape of the molecule. 

Figure 7.5 (page 177) shows the geometries predicted by the VSEPR model for molecules of the types AX2 to AX. The geometries for two and three electron pairs are those associated with species in which the central atom has less than an octet of electrons. Molecules of this type include BeF2 (in the gas state) and BF3, which have the Lewis structures shown below ... 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

AIM theory provides a physical basis for the theory of Lewis electron pairs and the VSEPR model of molecular geometry. Equipped with computers and computer-generated, three-dimensional electron density maps, scientists are able to view molecules and predict molecular phenomena without even having to get off their chairs ... 

The most stable structures of alkyl and alkenyl anions predicted with the VSEPR theory are supported by reliable calculations. There are no known experimental structural data. In fact, up to recently, one would have cited the many known geometries of the lithium derivatives of these carbanions as evidence for the structure. One would simply have dropped the C—Li bond(s) from these geometries. However, it is now known that the considerable covalent character of most C—Li bonds makes organo-lithium compounds unsuitable models for carbanions. 

Explain the main postulate of the VSEPR model. Fist the five base geometries (along with bond angles) that most molecules or ions adopt to minimize electron-pair repulsions. Why are bond angles sometimes slightly less than predicted in actual molecules as compared to what is predicted by the VSEPR model ... 

In 1968 Bartell published an article on the use of molecular models in the curriculum. In this paper the qualitative valence shell electron pair repulsion (VSEPR) model and the relative role of bonded and nonbonded interaaion in directed valence is discussed. The author correctly predicted the increasing importance of model force fields for geometry prediction. An early discussion of the use of molecular mechanics in teaching can be found in a paper by Cox. 07 A cursory description of the methodology of force field calculations is presented, along with computational results on the relative energy of the rotamers of butane and the conformers of cyclohexane. 

Valence shell electron pair repulsion theory (VSEPR) can be used to predict the shapes of molecules. According to this theory, the geometry of a molecule is such that the valence-electron pairs of the central atom are kept farthest apart to minimize the electron repulsions. Again, you have to view molecules in terms of Lewis structure so that the shape of the molecules can be predicted with the VSEPR theory. 

The isomers described to this point have had octahedral or square-planar geometry. In this section, we describe other geometries. Explanations for some of the shapes are consistent with VSEPR predictions (Chapter 3), with the general assumption that the metal d electrons are stereochemically inactive. In these cases, 3-coordinate complexes have a trigonal-planar shape, 4-coordinate complexes are tetrahedral, and so forth, assuming that... 

Table 14.2 summarizes the electron domain geometry and molecular geometry of molecules and ions with five electron domains as predicted by VSEPR theory. A is the central atom, the... 

The observed Raman spectrum shows five lines including two polarized ones. Since polarized lines originate from totally symmetric normal modes, and the number of observed fundamentals exceeds those deduced for point group Tj and D411, the BU4 ion must have C2v symmetry. Note that this geometry is consistent with the prediction of VSEPR theory. The normal modes that give rise to polarized Raman lines are sketched below. 

N2O, also known as dinitrogen oxide, is a colorless gas with a sweet odor and taste. The molecule has a linear geometry as predicted with simple valence shell electron pair repulsion (VSEPR) theory. Formal charge considerations suggest that the most important two resonance structures are ... 

Explain why it is not necessary to find the Lewis structure with the smallest formal charges to make a successful prediction of molecular geometry in the VSEPR theory. For example, write Lewis structures for SO2 having different formal charges, and predict the molecular geometry based on these structures. 

With VSEPR theory, we predict a trigonal-planar electron-group geometry (the measured bond angle is 117°). The hybridization scheme chosen for the central O atom is sjp-, and although we normally do not need to invoke hybridization for terminal atoms, this case is simplified if we assume sjP hybridization for the terminal O atoms as well. Thus, each O atom uses the orbital set + p. 

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