Lecture 16 C1403 October 31, 2005

1. Lecture 16 C1403 October 31, 2005 18.1 Molecular orbital theory: molecular orbitals and diatomic molecules 18.2 Valence bond theory: hybridized orbitals and polyatomic molecules Bond order, bond lengths, connections of MO theory and VB theory with Lewis structures

2. + + + + _ _ + + + _ _ _ + + _

3. Making of a z and z* orbital from overlap of two 2pz orbitals Making of a x and x* orbital from overlap of two 2px orbitals Making of a y and y* orbital from overlap of two 2py orbitals

4. Potential energy curves for the and * orbitals of a diatomic molecule Distance dependence of the energy of a  and * orbital

5. Switch The reason for the “switch” in the s and p MOs Larger gap between 2s and 2p with increasing Z

6. Bond order? O2 Bond length = 1.21Å O2+ Bond length = 1.12 Å O2- Bond length = 1.26 Å O22- Bond length = 1.49 Å

7. Compare the Lewis and MO structures of diatomic molecules C2 N2 O2 F2

8. What is the bond order of NO in Lewis terms and MO theory?

9. 18.2 Polyatomic molecules Valence bond versus molecular orbital theory Hybridization of atomic orbitals to form molecular orbitals sp, sp2 and sp3 hybridized orbitals Hybridized orbitals and Lewis structures and molecular geometries Double bonds and triple bonds

10. 18.2Bonding in Methane andOrbital Hybridization

11. tetrahedral bond angles = 109.5° bond distances = 110 pm but structure seems inconsistent withelectron configuration of carbon Structure of Methane

12. only two unpaired electrons should form sbonds to only two hydrogen atoms bonds should be at right angles to one another Electron configuration of carbon 2p 2s

13. Promote an electron from the 2s to the 2p orbital sp3 Orbital Hybridization 2p 2s

14. sp3 Orbital Hybridization 2p 2p 2s 2s

15. Mix together (hybridize) the 2s orbital and the three 2p orbitals sp3 Orbital Hybridization 2p 2s

16. 4 equivalent half-filled orbitals are consistent with four bonds and tetrahedral geometry sp3 Orbital Hybridization 2p 2 sp3 2s

17. Shapes of orbitals p s

18. Nodal properties of orbitals p + – + s

19. take the s orbital and place it on top of the p orbital Shape of sp3 hybrid orbitals p + – + s

20. reinforcement of electron wave in regions where sign is the same destructive interference in regions of opposite sign + Shape of sp3 hybrid orbitals s + p + –

21. orbital shown is sp hybrid analogous procedure using three s orbitals and one p orbital gives sp3 hybrid shape of sp3 hybrid is similar – Shape of sp3 hybrid orbitals sp hybrid +

22. hybrid orbital is not symmetrical higher probability of finding an electron on one side of the nucleus than the other leads to stronger bonds + – Shape of sp3 hybrid orbitals sp hybrid

23. + – – The C—H s Bond in Methane In-phase overlap of a half-filled 1s orbital of hydrogen with a half-filled sp3 hybrid orbital of carbon: + sp3 s H C gives a s bond. + H—C s C H

24. consistent with structure of methane allows for formation of 4 bonds rather than 2 bonds involving sp3 hybrid orbitals are stronger than those involving s-s overlap or p-p overlap Justification for Orbital Hybridization

25. 18.2sp3 Hybridization and Bonding in Ethane

26. tetrahedral geometry at each carbon C—H bond distance = 110 pm C—C bond distance = 153 pm Structure of Ethane C2H6 CH3CH3

27. In-phase overlap of half-filled sp3 hybridorbital of one carbon with half-filled sp3hybrid orbital of another. Overlap is along internuclear axis to give a s bond. The C—C s Bond in Ethane

28. In-phase overlap of half-filled sp3 hybridorbital of one carbon with half-filled sp3hybrid orbital of another. Overlap is along internuclear axis to give a s bond. The C—C s Bond in Ethane

29. 18.2sp2 Hybridization and Bonding in Ethylene

30. planar bond angles: close to 120° bond distances: C—H = 110 pm C=C = 134 pm Structure of Ethylene C2H4 H2C=CH2

31. Promote an electron from the 2s to the 2p orbital sp2 Orbital Hybridization 2p 2s

32. sp2 Orbital Hybridization 2p 2p 2s 2s

33. Mix together (hybridize) the 2s orbital and two of the three 2p orbitals sp2 Orbital Hybridization 2p 2s

34. 3 equivalent half-filled sp2 hybrid orbitals plus 1 p orbital left unhybridized sp2 Orbital Hybridization 2p 2 sp2 2s

35. sp2 Orbital Hybridization p 2 of the 3 sp2 orbitalsare involved in s bondsto hydrogens; the otheris involved in a s bondto carbon 2 sp2

36. sp2 Orbital Hybridization s p s 2 sp2 s s s

37. p Bonding in Ethylene the unhybridized p orbital of carbon is involved in p bondingto the other carbon p 2 sp2

38. each carbon has an unhybridized 2p orbital axis of orbital is perpendicular to the plane of the s bonds p Bonding in Ethylene p 2 sp2

39. side-by-side overlap of half-filledp orbitals gives a p bond double bond in ethylene has a s component and a p component p Bonding in Ethylene p 2 sp2

40. Hybridization and methane: CH4

41. sp3 hybridization and ethylene: H2C=CH2

43. Ground state > excite one electron > mix orbitals: one s orbital and one p orbital = two sp orbitals Acetylene

45. Other examples of sp2 and sp hybridized carbon Formaldehyde: H2C=O Carbon dioxide: O=C=O

46. d2sp3 hybridization dsp3 hybridization

47. SN = 2 SN = 3 SN = 4 SN = 5 SN = 6 Hybrid orbitals are constructed on an atom to reproduce the electronic arrangement characteristics that will yield the experimental shape of a molecule

48. BeF2: SN = 2 = sp BF3: SN = 3 = sp2 CH4: SN = 4 = sp3 PF5: SN = 5 = sp3d SF6: SN = 6 = sp3d2

49. Describe the bonding for ethane, ethene and acetylene in terms of overlap of hybridized orbitals Ethane: Ethylene: Acetylene: