Lecture VESPR by mikesanye


									        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:
   – Different atomic linkages: Structural Isomers
   – Same atomic linkages, different bonding: Resonance
   Resonance Structures (cont.)
• The classic example: O3.

                   O        O        O

     O    O O

     O    O    O

              Both structures are correct!
   Resonance Structures (cont.)
• In this example, O3 has two resonance structures:

    O    O   O

• 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.
            Structural Isomers
• What if different sets of atomic linkages can be
  used to construct correct LDSs:

             Cl O Cl            Cl Cl O

            Cl O    Cl         Cl Cl O

• Both are correct, but which is “more” correct?
                    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”.

               Cl O Cl                 Cl Cl O

#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.
                     Formal Charge
     • Example: CO2

        O    C   O       O   O C       O   O C

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
         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
   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.

              F                      F
          F   B   F             F    B   F

• Experiments demonstrate that the B-F bond
  strength is consistent with single bonds only.
   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                  F obey octet rule

       F    Cl   F                 Cl has 10e-
   Beyond the Octet Rule (cont.)
• Finally, one can encounter odd electron systems
  where full pairs will not exist.

• Example: Chlorine Dioxide.

                   O Cl O           Unpaired electron
• 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
   – Third Row elements and beyond can use valence shell
     expansion to exceed the octet rule.

• In the end, you have to practice…..a lot!
           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.
     VESPR Background (cont.)
• Must consider both bonding and lone pairs in
  minimizing repulsion.

• Example: CH4

     H C H

    Lewis Structure                VESPR Structure
     VESPR Background (cont.)

• Example: NH3 (both bonding and lone pairs).


Lewis Structure             VESPR Structure
           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.
          VESPR Applications
• Linear Structures: angle between bonds is 180°

• Example: BeF2

    F Be F

    F Be F
           VESPR Applications
• Trigonal Planar Structures: angle between
bonds is 120°

• Example: BF3

            F                           F
       F    B    F               F      B   F
     VESPR Background (cont.)
• Pyramidal: Bond angles are <120°, and structure
  is nonplanar:

• Example: NH3

  H                                                 107°
          VESPR Applications
• Tetrahedral: angle between bonds is ~109.5°

• Example: CH4

          H                             109.5°
      H C H
          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
          VESPR Applications
• Classic example of tetrahedral angle shift from
      109.5° is water:
         VESPR Applications
• Comparison of CH4, NH3, and H2O:
           VESPR Applications
• Trigonal Bipyramidal, 120° in plane, and two
      orbitals at 90° to plane:
• Example, PCl5:

           Cl                   90°
      Cl   P                                     120°
          VESPR Applications
• Octahedral: all angles are 90°:

• Example, PCl6:

         Cl        Cl
              P                90°
         Cl        Cl
 Advanced VESPR Applications
• Square Planar versus “See Saw”

                                    See Saw

                                   No dipole moment

                                    Square Planar
  Advanced VESPR Applications
• Driving force for last structure was to
  maximize the angular separation of the lone pairs.
   Advanced VESPR Applications
• VESPR and resonance structures. Must look at
  VESPR structures for all resonance species to
  predict molecular properties.

      O    O   O

     O          O                           O
         VESPR Applications
• Provide the Lewis dot and VESPR structures for
  CF2Cl2. Does it have a dipole moment?


      32 e-     Cl   C    F
                     Cl                               Cl


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