exam_1b_2008_2_soln

exam_1b_2008_2_soln - NAME:_SOLUTIONS_ Chem 1b, 2008, 2nd....

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NAME:___________SOLUTIONS _______ Chem 1b, 2008, 2 nd . midterm exam 1) (50 points) This problem deals with two electronic states of diatomic helium, e.g. He 2 . The electronic configuration of each species is given below. Species I: [σ(1s)] 2 [σ(1s)*] 2 Species II: [σ(1s)] 2 [σ(1s)*][σ(2s)] a) Which species is an excited state of He 2 ? Provide the basis for your answer. Species II has a σ(2s) MO which is constructed using atomic orbitals outside the valence shell of ground state helium. b) Sketch the PES, i.e. a graph of electronic energy versus internuclear separation, of each species. Species I Species II Refer to Figure 1 at the end for the solution. c) Both species contain a molecular orbital with a node. Which molecular orbital? What is the location and physical significance of this node? The σ(1s)* MO is an antibonding orbital and contains a nodal surface between the two helium atoms and perpendicular to the interatomic axis. It decreases the electron density between the two helium nuclear and thereby increases the internuclear repulsion in the molecule. d) Simplissimus constructed the following molecular orbital for Ne 2 . His z axis is the line connecting the two Ne atoms. A [Ne(2s)] + φ B [Ne(2s)]} + A [Ne(2p x )] + φ B [Ne(2p x )]} i) Point out the flaw(s) in his molecular orbital. In constructing MO’s, one must use atomic orbitals of the same symmetry. Simplissimus has a used σ, e.g. Ne(2s), and π, e.g.,, Ne(2p x ), atomic orbitals in the same MO. ii) Modify his orbital and convert it into one that is acceptable. He could use Ne(2p
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exam_1b_2008_2_soln - NAME:_SOLUTIONS_ Chem 1b, 2008, 2nd....

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