Unit3Day4-LaBrake Page 1 Unit3Day4-LaBrake Monday, October 21, 2013 11:32 AM Vanden Bout/LaBrake/Crawford CH301 WHY DOES A FROG FLOAT IN A MAGNETIC FIELS? MORE ON BONDING THEORIES UNIT 3 Day 4 Important Information LM22 DUE Th 9AM Laude LM Lecture 12 and 13 CH302 Vanden Bout/LaBrake Spring 2012 Quiz: CLICKER QUESTION 1 Is the molecule CH 2 Cl 2?
a) 20, 5 b) 20, 4 c) 16, 9 d) 21, 5 Unit3Day4-LaBrake Page 2 Quiz: CLICKER QUESTION 1 Is the molecule CH 2 Cl 2? a) polar b) nonpolar Quiz: CLICKER QUESTION 2 Given a tetrahedral electronic geometry, which of the following hybridizations is possible? a) sp b) sp 2 c) sp 3 d) sp 3 d TAKE A LOOK AT THE BALLONS AGAIN Quiz: CLICKER QUESTION 3 The number of sigma and pi bonds, respectively, in the following is:
The number of sigma and pi bonds, respectively, in the following is: Unit3Day4-LaBrake Page 3 a) 20, 5 b) 20, 4 c) 16, 9 d) 21, 5 e) 17, 4 REVIEW LEARNING MODULE 21 VB
Unit3Day4-LaBrake Page 4 What are we going to learn today? EXPLORE MOLECULAR ORBITAL THEORY EXPLAIN THE VB HYBRIDIZATION IN ETHANOL Ethanol Diatomic Oxygen
Unit3Day4-LaBrake Page 5 http://www.youtube.com/watch?v=yjs5entilio ANOTHER PHYSCIAL MODEL VB Valence Bond localized MO Molecular Orbital de-localized IT S ABOUT THE MOLECULE NOT THE BOND. What is bond strength and bond length in H 2? How can we get that? Quantum mechanics, oribitals for the whole molecule and energies for all electrons what distance? At every distance you calculate this and you get a radial function! For the whole molecule not just atomic orbital!
Unit3Day4-LaBrake Page 6 E BONDING AND ANTIBONDING ORBITALS E COMPARE MO TO VB FOR H 2 Very similar because the bond is the molecule If the molecule is stable, then electrons in MO are lower in energy than in AO.
BOND ORDER = ½(# BONDING - # ANTIBONDING) Unit3Day4-LaBrake Page 7 If the molecule is stable, then electrons in MO are lower in energy than in AO. Number of MO in molecule = total number of AO in atoms -Combine atomic orbitals, sometimes you get higher and sometimes you get lower energy -Remember electrons have wave functions -Constructive and destructive interference -Two possibilities IDEAS ABOUT MO http://winter.group.shef.ac.uk/orbitron/mos/h2/1s1s-sigma/index.html What does the electron configuration for He 2 look like? HOW MANY ELECTRONS? WHERE DO THEY GO?
Unit3Day4-LaBrake Page 8 BOND ORDER = ½(# BONDING - # ANTIBONDING) POLL: CLICKER QUESTION 3 What is the bond order for H 2+? CAN YOU DRAW THE LEWIS STRUCTURE? POLL: CLICKER QUESTION 4 Here are the MO s for N 2 The highest energy electrons are in what molecular orbital? A. σ 2s B. σ* 2s C. σ 2p D. σ* 2p E. π* 2p IS IT DIAMAGNETIC OR PARAMAGNETIC?
MO row 2 diatomic http://winter.group.shef.ac.uk/orbitron/mos/ N2/2px2px-pi/index.html Rules for Electron Configuration and Bond Order Calculation Identify all valence orbitals Use each matching pair of valence shell atomic orbitals to build a bonding and an antibonding MO and draw the resulting MO energy level diagram Note the total number of electrons present in the valence shells of the two atoms, adjust for ions Accommodate the electrons in the MO according to the building up principle To determine the bond order (BO), subtract the number of electrons in the antibonding orbitals from the number in bonding orbitals and divide the result by two. BO = ½ (N N* ) MOs can combine AO on dif atoms Diagram for CN - HOMO LUMO Unit3Day4-LaBrake Page 9
Unit3Day4-LaBrake Page 10 MOs can combine AO on dif atoms Diagram for CN - HOMO LUMO Bond Order? DIAMAGNETIC OR PARAMAGNETIC? What if its not a diatomic? Methane
Unit3Day4-LaBrake Page 11 What if it is a complicated molecule with lots of atoms How is this tool used.. Calculate the lowest energy geometry Where is the electron density It is hard to relate the MO to AO None the less the MOs are useful LUMO HOMO POLL: CLICKER QUESTION 4 LUMO HOMO Most molecules have A. An even number of electrons B. An odd number of electrons C. Equal chance of even or odd
Unit3Day4-LaBrake Page 12 POLL: CLICKER QUESTION 5 LUMO HOMO Most molecules are A. diamagnetic B. paramagnetic C. Evenly split between the two http://ch301.cm.utexas.edu/unit3.php Other way we use MO, just the pi electrons VB for sigma bonds
Unit3Day4-LaBrake Page 13 MO for the pi bond Where are the electrons? Delocalized around the molecule Typical MO for organic molecule Almost always diamagnetic HOMO/LUMO Energy gap Light absorption Dyes homo lumo gap is the color that is absorbed, gaps as a function of structure Organic photovoltaics CH302 Vanden Bout/LaBrake Fall 2012
Unit3Day4-LaBrake Page 14 POLL: CLICKER QUESTION 6 Currently, plastic photovoltaics absorb in the green regions. However, chemists would like to design plastic photovoltaics that absorb in the near IR region to increase the efficiency of the energy conversion. The chemists need to: A) Increase the HOMO LUMO gap B) Decrease the HOMO-LUMO gap WHAT HAVE WE LEARNED TODAY? The Molecular Shapes can be explained using the concept - Valence Bond Theory - Hybridized Orbitals. The bonds can be identified as sigma or pi. Bonding in molecules can be visualized using VB Bonding in molecules can be modeled mathematically using MO
Unit3Day4-LaBrake Page 15 Identify sigma and pi bonds Learning Outcomes Recognize localized vs. delocalized electrons within a structure Recognize that MO theory is a quantum mechanical method used by chemists to determine the energy of the elctrons in a molecule as well as its geometry Recognize that MO theory can be used to determine the energy of the light absorbed by a compound by exciting electrons between MOs (from HOMO to LUMO) Interpret a given MO diagram as well as fill in electrons into an MO diagram to predict bond order for a compound and predict whether it is paramagnetic or diamagnetic Recognize that constructive interference of atomic orbitals yields lower energy MO (bonding) while destructive interference lends to higher energy MO (anti-bonding)