Lecture36Notes - Chem 120A Spring 2006 Diatomic Molecules...

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Chem 120A Diatomic Molecules 04.24.06 Spring 2006 Lecture 36 Reading: Engel: Chapter 13 1 Homonuclear Diatomics Molecular Orbitals come from combinations of atomic orbitals (AOs). In order for atomic orbitals to com- bine to form a molecular orbital they must meet the following requirements: i) The AOs must be similar in energy ii) The AOs can be neither too diffuse nor too compact iii) The AOs must have the same symmetry with respect to rotation about the molecular axis When we combine the 1s orbitals of two hydrogen atoms, we get a σ g ( 1 s ) bonding and σ * u ( 1 s ) antibonding orbital. Here g refers to orbitals which are symmetric with inversion and u to MOs which are antisymmetric to inversion. The two electrons from each of the H-atoms both go in the σ g ( 1 s ) bonding orbital (see Figures 1 and 2). The bond order is 1, where the bond order is 1/2(# of electrons in bonding orbitals - # of electrons in antibonding orbitals. Figure 1: Molecular orbitals formed by the 1 s atomic orbitals of Hydrogen Chem 120A, Spring 2006, Lecture 36 1
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2 Why are there only weak (van der Waals) interactions between He atoms? If we try to make diatomic Helium, He 2 , we have to put 2 electrons in the σ g ( 1 s ) bonding orbital and two electrons in the σ * u ( 1 s ) anti- bonding orbital, so the bond order is 0. Thus there is no energetic advantage for Helium atoms to share their electrons (see Figure 3). Figure 3: Molecular orbital diagram for He 2 What about diatomics beyond the 1st row? Now we can combine both s and p orbitals. We will take the internuclear axis to be the z-axis. Thus two 2 p z orbitals can combine head on to form a σ g ( 2 p z ) bonding and σ * u ( 2 p z ) antibonding orbitals. The 2 p x and 2 p y which are perpendicular to the internuclear axis combine on their sides to form two π u ( 2 p x , y ) and two π * u ( 2 p x , y ) orbitals, which will be degenerate (see Figure 4). Chem 120A, Spring 2006, Lecture 36
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This note was uploaded on 09/29/2009 for the course CHEM 120A taught by Professor Whaley during the Spring '07 term at University of California, Berkeley.

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Lecture36Notes - Chem 120A Spring 2006 Diatomic Molecules...

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