Understanding Molecular Structures and Bonding in Chemistry: Methane, Hybridization, and Orbitals

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