chem120A_s07_lecture 40

chem120A_s07_lecture 40 - Chem 120A 05/02/07 Spring 2007...

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Chem 120A Polyatomic molecules and Molecular geometry 05/02/07 Spring 2007 Lecture 40 READING: Atkins and Friedman: Ch. 8.8-8.11 Molecular orbitals are best formed from symmetry adapted atomic orbitals. This requires some knowledge of group theory and we will not cover this here. For those of you interested, read Ratner/Schatz Ch. 13, Atkins/Friedman Ch. 5. This material underlies the discussion in Atkins/Friedman Ch. 8.8. We shall make some qualitative arguments here that may be regarded as a zeroth order description and give insight into molecular geometries, but that are not quantitatively the most accurate description. Using LCAO theory to predict molecular shape in a qualitative manner As you have probably seen in general chemistry and organic chemistry, hybrid orbitals can be used to predict molecular geometry. Here we examine the basis of the theory of hybridization. Let’s consider the methane molecule, CH 4 , which has tetrahedral geometry. The carbon atom is located at the center of the tetrahedron and each of the hydrogen atoms is at a vertex of the tetrahedron. The electron configuration of carbon is 1 s 2 2 s 2 2 p 2 . We can use the 2 s and the 2 p orbitals to form linear combinations that have the appropriate geometry: ψ c = c 1 2 s + c 2 2 p x + c 3 2 p y + c 4 2 p z . (1) Because there are initially 4 orbitals available (one 2 s and 3 2 p orbitals), we can find 4 hybridized orbitals using the linear combination. The orbitals are sp 3 hybridized, and the angle between the orbitals is 109.5 o . The molecular orbitals in the methane molecule are then formed from a linear combination of each of these hybrid orbitals with the 1 s orbital of a hydrogen atom, which gives σ and * orbitals. Another example is SF 6 , which is octahedral. The electron configuration of the central sulfur atom is [Ne]3
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This note was uploaded on 02/19/2010 for the course CHEM 120A taught by Professor Whaley during the Spring '07 term at Berkeley.

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chem120A_s07_lecture 40 - Chem 120A 05/02/07 Spring 2007...

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