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Unformatted text preview: Chemistry 124 Second Examination
October 20, 2008 Name ________________ The exam budgets 50 minutes, but you may have 60 minutes to finish it. Good answers can fit in the space provided. Question values correspond to alloted time. Don't waste too much time on cheap questions. Read each question carefully to see what it asks for (bold face is used to help highlight questions). Make sure you are answering the question, not just saying something vaguely relevant to its topic. 1. (4 min) Label the symmetric atomic orbital pairs that would give the following four profiles for overlap integral versus C-C distance 2. (4 min) Explain whether Correlation Energy should raise or lower a molecular energy calculated by the SCF procedure. 3. (2 min) N ame two accurate QUANTITATIVE tools that would have been available to you, if you were a chemistry student in Yale’s Sheffield Scientific School in the class of 1901. Chem 124 Second Hour Exam 10/20/08 Page 2 4. (4 min) A ssuming, perhaps unrealistically, that the atom- atom distances and overlap integrals are identical, explain, in terms of electron energies, which of the following bonds should be stronger. (A diagram would help.) the 1- electron bond of H2
+ + the 3- electron bond of He2 5. (4 min) Use the idea of plum-pudding orbitals to rank these three one-node MOs of the water molecule (A,B,C) by their relative energy (low, medium, high). Each orbital is shown in both top and side view, as is the molecular skeleton in the first row. Explain your reasoning in a v ery few words. (You need not give names or signs) Top View Side View
Low Medium High ? A B C Chem 124 Second Hour Exam 10/20/08 Page 3 6. (6 min) Use hybridization theory to explain whether the F -C-F angles of the •CF3 radical should be larger or smaller than the H-C-H angles of the •CH3 r adical. Describe the experiment that justifies your prediction. 7. (5 min) How was the Bürgi-Dunitz angle for attack of N on C=O established experimentally? Chem 124 Second Hour Exam 10/20/08 Page 4 8. (6 min) Use proper curved arrows to draw the 3 steps in a mechanistic scheme for conversion of NH3 to NH2NH3Cl by Cl2. 9. (6 min) According to “resonance” the properties of an amide are different from those expected for a molecule with simple carbonyl and amine groups. Briefly explain three different ways in which resonance influences the properties of protein polymers that contain multiple amide groups. Chem 124 Second Hour Exam 10/20/08 Page 5 10. (9 min) Using a ll of the following four molecules SKETCH SIX (6) LOCALIZED “frontier” orbitals that would account for significant reactivity, and EXPLAIN the energy that makes each of them special. (Show orbital signs.) Li-CH3 R2C=N-H R-O-O-R C H2C H2C C CH2 ...
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