1 / 22

Lecture 2

Lecture 2. Describing molecular structures using quantum chemistry. Methane. Major component of natural gas Simplest hydrocarbon CH 4 Draw Lewis and Kekulé structures for methane How can we draw the molecular bonding orbitals?. Methane Structure.

quasim
Download Presentation

Lecture 2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 2 Describing molecular structures using quantum chemistry Chemistry 243 - Lecture 1

  2. Methane • Major component of natural gas • Simplest hydrocarbon • CH4 • Draw Lewis and Kekulé structures for methane • How can we draw the molecular bonding orbitals? Chemistry 243 - Lecture 1

  3. 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. Chemistry 243 - Lecture 1

  4. Orbitals Needed • Carbon • 2S, 2Px, Py and Pz • 4 Hydrogens • 4 x 1S Chemistry 243 - Lecture 1

  5. 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. Chemistry 243 - Lecture 1

  6. 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) Chemistry 243 - Lecture 1

  7. 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) Chemistry 243 - Lecture 1

  8. That’s 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 Chemistry 243 - Lecture 1

  9. Combining an SP3 Orbital A1 + A2 + A3 + A4 = Chemistry 243 - Lecture 1

  10. 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 Chemistry 243 - Lecture 1

  11. Hybridization Summary Chemistry 243 - Lecture 1

  12. Addition and Subtraction Chemistry 243 - Lecture 1

  13. Orbital Combination Chemistry 243 - Lecture 1

  14. Combining Orbitals C-H antibonding MOs Hydrogen 1S AOs CarbonS and Px,y,zAOs C-H bonding MOs Chemistry 243 - Lecture 1

  15. Exercise • Fill the MOs with electrons Combining Orbitals C-H antibonding MOs CarbonSP3 AOs Hydrogen 1S AOs C-H bonding MOs Chemistry 243 - Lecture 1

  16. 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 Chemistry 243 - Lecture 1

  17. The C-C Covalent Bond Chemistry 243 - Lecture 1

  18. The C-H Covalent Bond Chemistry 243 - Lecture 1

  19. The C-O Polar Covalent Bond Chemistry 243 - Lecture 1

  20. 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? Chemistry 243 - Lecture 1

  21. The Na-Cl Ionic Bond Chemistry 243 - Lecture 1

  22. 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 Chemistry 243 - Lecture 1

More Related