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Lecture 24: VESPR

Lecture 24: VESPR. Reading: Zumdahl 13.13 Outline Concept behind VESPR Molecular geometries. Resonance Structures. We have assumed up to this point that there is one correct Lewis structure. There are systems for which more than one Lewis structure is possible:

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Lecture 24: VESPR

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  1. Lecture 24: VESPR • Reading: Zumdahl 13.13 • Outline • Concept behind VESPR • Molecular geometries

  2. Resonance Structures • We have assumed up to this point that there is one correct Lewis structure. • There are systems for which more than one Lewis structure is possible: • Different atomic linkages: Structural Isomers • Same atomic linkages, different bonding: Resonance

  3. Resonance Structures (cont.) • The classic example: O3. Both structures are correct!

  4. Resonance Structures (cont.) • In this example, O3 has two resonance structures: • Conceptually, we think of the bonding being an average of these two structures. • Electrons are delocalized between the oxygens such that on average the bond strength is equivalent to 1.5 O-O bonds.

  5. Structural Isomers • What if different sets of atomic linkages can be used to construct correct LDSs: • Both are correct, but which is “more” correct?

  6. Formal Charge • Formal Charge: Compare the nuclear charge (+Z) to the number of electrons (dividing bonding electron pairs by 2). Difference is known as the “formal charge”. #e- 7 6 7 7 6 7 Z+ 7 6 7 7 7 6 Formal C. 0 0 0 0 +1 -1 • Structure with less F. C. is more correct.

  7. Formal Charge • Example: CO2 e- 6 4 6 6 4 6 7 4 5 Z+ 6 4 6 6 6 4 6 6 4 FC 0 0 0 0 +2 -2 -1 +2 -1 More Correct

  8. Beyond the Octet Rule • There are numerous exceptions to the octet rule. • We’ll deal with three classes of violation here: • Sub-octet systems • Valence shell expansion • Odd-electron systems

  9. Beyond the Octet Rule (cont.) • Some atoms (Be and B in particular) undergo bonding, but will form stable molecules that do not fulfill the octet rule. • Experiments demonstrate that the B-F bond strength is consistent with single bonds only.

  10. Beyond the Octet Rule (cont.) • For third-row elements (“Period 3”), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding. • When this occurs, more than 8 electrons can surround a third-row element. • Example: ClF3 (a 28 e- system) F obey octet rule Cl has 10e-

  11. Beyond the Octet Rule (cont.) • Finally, one can encounter odd electron systems where full pairs will not exist. • Example: Chlorine Dioxide. Unpaired electron

  12. Summary • Remember the following: • C, N, O, and F almost always obey the octet rule. • B and Be are often sub-octet • Second row (Period 2) elements never exceed the octet rule • Third Row elements and beyond can use valence shell expansion to exceed the octet rule. • In the end, you have to practice…..a lot!

  13. VESPR Background • The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry? • Recall from last lecture that we had two types of electron pairs: bonding and lone. • Valence Electron Shell Pair Repulsion (VESPR). 3D structure is determined by minimizing repulsion of electron pairs.

  14. VESPR Background (cont.) • Must consider both bonding and lone pairs in minimizing repulsion. • Example: CH4 Lewis Structure VESPR Structure

  15. VESPR Background (cont.) • Example: NH3 (both bonding and lone pairs). Lewis Structure VESPR Structure

  16. VESPR Applications • The previous examples illustrate the strategy for applying VESPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arranging bonding/lone electron pairs in space such that repulsions are minimized.

  17. VESPR Applications • Linear Structures: angle between bonds is 180° • Example: BeF2 180°

  18. VESPR Applications • Trigonal Planar Structures: angle between bonds is 120° • Example: BF3 120°

  19. VESPR Background (cont.) • Pyramidal: Bond angles are <120°, and structure is nonplanar: • Example: NH3 107°

  20. VESPR Applications • Tetrahedral: angle between bonds is ~109.5° • Example: CH4 109.5°

  21. VESPR Applications • Tetrahedral: angle may vary from 109.5° exactly due to size differences between bonding and lone pair electron densities bonding pair lone pair

  22. VESPR Applications • Classic example of tetrahedral angle shift from 109.5° is water:

  23. VESPR Applications • Comparison of CH4, NH3, and H2O:

  24. VESPR Applications • Trigonal Bipyramidal, 120° in plane, and two orbitals at 90° to plane: • Example, PCl5: 90° 120°

  25. VESPR Applications • Octahedral: all angles are 90°: • Example, PCl6: 90°

  26. Advanced VESPR Applications • Square Planar versus “See Saw” See Saw No dipole moment Square Planar

  27. Advanced VESPR Applications • Driving force for last structure was to maximize the angular separation of the lone pairs.

  28. Advanced VESPR Applications • VESPR and resonance structures. Must look at VESPR structures for all resonance species to predict molecular properties.

  29. VESPR Applications • Provide the Lewis dot and VESPR structures for CF2Cl2. Does it have a dipole moment? 32 e- Tetrahedral

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