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 CH302 Vanden Bout/LaBrake Fall 2013 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 ? Unit3Day4-LaBrake Page 1 a) polar
Quiz: CLICKER QUESTION 1 Is the molecule CH 2 Cl 2 ? a) polar b) nonpolar CH302 Vanden Bout/LaBrake Fall 2013 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…… CH302 Vanden Bout/LaBrake Fall 2013 Quiz: CLICKER QUESTION 3 The number of sigma and pi bonds, respectively, in the following is: a) 20, 5 b) 20, 4 c) 16, 9 d) 21, 5 Unit3Day4-LaBrake Page 2 e) 17, 4
The number of sigma and pi bonds, respectively, in the following is: a) 20, 5 b) 20, 4 c) 16, 9 d) 21, 5 e) 17, 4 CH302 Vanden Bout/LaBrake Fall 2013 REVIEW LEARNING MODULE 21 VB Unit3Day4-LaBrake Page 3
What are we going to learn today? EXPLORE MOLECULAR ORBITAL THEORY CH302 Vanden Bout/LaBrake Fall 2013 EXPLAIN THE VB HYBRIDIZATION IN ETHANOL Ethanol Diatomic Oxygen CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 4
http://www.youtube.com/watch?v=yJs5ENtilIo ANOTHER PHYSCIAL MODEL VB MO Valence Bond Molecular Orbital localized de-localized IT’S ABOUT THE MOLECULE NOT THE BOND……. CH302 Vanden Bout/LaBrake Fall 2013 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! CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 5 E
E CH302 Vanden Bout/LaBrake Fall 2013 BONDING AND ANTIBONDING ORBITALS E CH302 Vanden Bout/LaBrake Fall 2013 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. Unit3Day4-LaBrake Page 6
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 CH302 Vanden Bout/LaBrake Fall 2013 IDEAS ABOUT MO -Combine atomic orbitals, sometimes you get higher and sometimes you get lower energy -Remember electrons have wave functions -Constructive and destructive interference -Two possibilities http://winter.group.shef.ac.uk/orbitron/MOs/H2/1s1s-sigma/index.html CH302 Vanden Bout/LaBrake Fall 2013 What does the electron configuration for He 2 look like? HOW MANY ELECTRONS? WHERE DO THEY GO? BOND ORDER = ½(# BONDING - # ANTIBONDING) Unit3Day4-LaBrake Page 7
BOND ORDER = ½(# BONDING - # ANTIBONDING) CH302 Vanden Bout/LaBrake Fall 2013 POLL: CLICKER QUESTION 3 + ? What is the bond order for H 2 CAN YOU DRAW THE LEWIS STRUCTURE? CH302 Vanden Bout/LaBrake Fall 2013 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? CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 8
http://winter.group.shef.ac.uk/orbitron/MOs/ MO row 2 diatomic N2/2px2px-pi/index.html CH302 Vanden Bout/LaBrake Fall 2013 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* ) CH302 Vanden Bout/LaBrake Fall 2013 MOs can combine AO on dif atoms Diagram for CN - HOMO LUMO Unit3Day4-LaBrake Page 9
MOs can combine AO on dif atoms Diagram for CN - HOMO LUMO Bond Order? DIAMAGNETIC OR PARAMAGNETIC? CH302 Vanden Bout/LaBrake Fall 2013 Methane What if its not a diatomic? CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 10
What if it is a complicated molecule with lots of atoms It is hard to relate the MO to AO How is this tool used.. None the less the MOs are useful Calculate the lowest energy geometry Where is the electron density LUMO HOMO CH302 Vanden Bout/LaBrake Fall 2013 POLL: CLICKER QUESTION 4 Most molecules have LUMO A. An even number of electrons HOMO B. An odd number of electrons C. Equal chance of even or odd CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 11
POLL: CLICKER QUESTION 5 Most molecules are LUMO A. diamagnetic HOMO B. paramagnetic C. Evenly split between the two http://ch301.cm.utexas.edu/unit3.php CH302 Vanden Bout/LaBrake Fall 2013 Other way we use MO, just the pi electrons VB for sigma bonds CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 12
MO for the pi bond Where are the electrons? Delocalized around the molecule CH302 Vanden Bout/LaBrake Fall 2013 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 13
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 CH302 Vanden Bout/LaBrake Fall 2013 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 CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 14
Learning Outcomes Identify sigma and pi bonds 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) CH302 Vanden Bout/LaBrake Fall 2013 Unit3Day4-LaBrake Page 15
Recommend
More recommend
