Describing molecular structures using quantum chemistry

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Lecture 2

• Describing molecular structures using quantum
  chemistry

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Methane

• Major component of natural gas
• Simplest hydrocarbon
• CH4
• Draw Lewis and Kekulé structures for methane
• How can we draw the molecular bonding orbitals?

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Methane Structure

• VESPR theory predicts a tetrahedral structure.
  Why?
• Experimental evidence confirms this stucture.
• Molecular orbitals are mathematical functions
  dependant on the exact position of nuclei.
• We are not computers, so there is a simple method
  for estimating shapes of simple molecular
  orbitals in simple molecules.

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Orbitals Needed

• Carbon
• 2S, 2Px, Py and Pz

• 4 Hydrogens
• 4 x 1S

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Geometry Conundrum

• The H atoms are 104? apart
• The p-orbitals are 90? apart
• How can I avoid trigonometry?
• Answer Do all trigonometry in advance.

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Dependency on Angle

• The 1S orbital of the hydrogen with overlap with
  the 2S and the 2Px,y,z orbitals of carbon

• Bonding Orbital is

AX Cos(fx)
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Dependency on Angle
Orbital combination is dependant on the angle of
the two orbitals w.r.t. their axes of mutual
symmetry
Cosine (0?) 1 or maximum
Cosine (90?) 0 or minimum
Cosine (some other angle) some number between 1
and 0 (in between)
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Thats a Lot of Math

• Lets simplify.
• Let us combine all the terms for the carbon
  atomic orbitals in advance.
• We will create a new orbital from the combination
  of fractions of the 4 atomic orbitals in play.
• This new atomic orbital will be called an SP3
  orbital
• Made from fractions of an S and 3 P orbitals

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Combining an SP3 Orbital
A1
A2
A3
A4

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The SP3 Orbital

• 4 atomic orbitals
• S, Px ,Py ,Pz
• Combine to make 4 hybrid atomic orbitals
• 4 x SP3
• Each SP3 alignes with axes of C-H covalent bonds
• We created them for that very purpose

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Hybridization Summary
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Addition and Subtraction
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Orbital Combination
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Combining Orbitals
C-H antibonding MOs
Hydrogen 1S AOs
CarbonS and Px,y,z AOs
C-H bonding MOs
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Combining Orbitals
C-H antibonding MOs
CarbonSP3 AOs
Hydrogen 1S AOs
C-H bonding MOs
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Exercise

• Draw the molecular orbital diagram for acetlyene
  (C2H2)
• Sketch all the atomic orbitals that you would be
  combining to make bonding molecular orbitals
• Sketch all the molecular bonding orbitals created
  from the combinations of the above
• Sketch the antibonding orbitals
• Bonus. - Rank the orbitals in energy
• Hint ? molecular orbitals will be the lowest and
  highest energy orbitals. ? molecular orbitals
  will be in between these two extremes.

• Hand in your work at the start of the next class
  period

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The C-C Covalent Bond
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The C-H Covalent Bond
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The C-O Polar Covalent Bond
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Consider Ethanol

• Red - More e density
• Blue - Less e density
• Observe the C-C, C-O and C-H bonds
• Is the O-H bond polar? Why or why not? Is it
  more or less polar than the C-O bond?

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The Na-Cl Ionic Bond
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Ionic Bonds

• Bonds with little orbital overlap
• Very little stabilization from sharing electrons
• Stabilization comes from electrostatic attraction
• Occur between atoms of greatly different
  electronegativity