Here we develop a molecular orbital theory treatment of the peptide group (10), which

Physical Chemistry | 8th Edition | ISBN: 9780716787594 | Authors: Peter Atkins, Julio de Paula

Problem 11.28 Chapter 11

Physical Chemistry | 8th Edition

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Physical Chemistry | 8th Edition | ISBN: 9780716787594 | Authors: Peter Atkins, Julio de Paula

Physical Chemistry | 8th Edition

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Problem 11.28

Here we develop a molecular orbital theory treatment of the peptide group (10), which links amino acids in proteins. Specifically, we shall describe the factors that stabilize the planar conformation of the peptide group. (a) It will be familiar from introductory chemistry that valence bond theory explains the planar conformation of the peptide group by invoking delocalization of the bond between the oxygen, carbon, and nitrogen atoms (11, 11): It follows that we can model the peptide group with molecular orbital theory by making LCAO-MOs from 2p orbitals perpendicular to the plane defined by the O, C, and N atoms. The three combinations have the form: 1 = aO + bC + cN 2 = dO eN 3 = fO gC + hN where the coefficients a through h are all positive. Sketch the orbitals 1,2, and3 and characterize them as bonding, non-bonding, or antibonding molecular orbitals. In a non-bonding molecular orbital, a pair of electrons resides in an orbital confined largely to one atom and not appreciably involved in bond formation. (b) Show that this treatment is consistent only with a planar conformation of the peptide link. (c) Draw a diagram showing the relative energies of these molecular orbitals and determine the occupancy of the orbitals. Hint. Convince yourself that there are four electrons to be distributed among the molecular orbitals. (d) Now consider a non-planar conformation of the peptide link, in which the O2p and C2p orbitals are perpendicular to the plane defined by the O, C, and N atoms, but the N2p orbital lies on that plane. The LCAO-MOs are given by 4 = aO + bC 5 = eN 6 = fO gC Just as before, sketch these molecular orbitals and characterize them as bonding, non-bonding, or antibonding. Also, draw an energy level diagram and determine the occupancy of the orbitals. (e) Why is this arrangement of atomic orbitals consistent with a non-planar conformation for the peptide link? (f) Does the bonding MO associated with the planar conformation have the same energy as the bonding MO associated with the non-planar conformation? If not, which bonding MO is lower in energy? Repeat the analysis for the non-bonding and anti-bonding molecular orbitals. (g) Use your results from parts (a) (f) to construct arguments that support the planar model for the peptide link.

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Gen Chem II Clausius­Clapeyron Equation: ln(P1/P2)=∆H /R*([1vap]­[1/T2]) R=8.31 J/mol K Hess’s Law: C=kP Pvap...

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Chapter 11, Problem 11.28 is Solved
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Textbook: Physical Chemistry
Edition: 8
Author: Peter Atkins, Julio de Paula
ISBN: 9780716787594

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