ANNOUNCEMENTS. If you have questions about your exam 2 grade, write to me or Chapter 8 homework due April. 13 th.

Transcription

1 ANNOUNCEMENTS If you have questions about your exam 2 grade, write to me or Chem200@mail.sdsu.edu. Chapter 8 homework due April. 13 th. Chapter 9 home work due April. 20th. Exam 3 is 4/14 at 2 pm.

2 LECTURE OBJECTIVES Chapter 8.2 Determine orbital hybridization from the VSEPR molecular structures. Chapter 8.3 Describe multiple bonds (i.e. double and triple bonds) in terms of atomic orbital overlap.

3 SIGMA BONDS Sigma (σ) bonds are the result of direct orbital overlap. Any two orbitals that overlap can form σ bonds (e.g. s-p, p-p, p-d, s-sp x, ) σ bonds have very high electron density along the axis of the bond.

4 DOUBLE BONDS H H C C H H If bonds arise from the overlap of atomic and/or hybrid orbitals, how are double and triple bonds formed? What orbitals are interacting and how do they interact for form multiple bonds?

5 MULTIPLE BONDS The heart of every multiple bond is a σ bond. The additional bonds in a multiple bond arise from pi (π) bonding. π bonds arise from two single (unpaired) electrons in p orbitals on atoms bound by a σ bond.

6 π-bonds π-bonds are formed by the side-to-side overlap of partially filled (one electron) p atomic orbitals on adjacent atoms. They are typically not as strong as corresponding σ-bonds - π-bonds overlap above and below the bond axis. π-bonds react differently than σ-bonds.

7 ETHYLENE (C2H4) BONDING Carbon hybridization: sp 2 Bonds: 2 single bonds (2 σ) and 1 double bond (1 σ + 1 π)

8 CARBON DIOXIDE BONDING O C O Each C=O bond is 1σ and 1π bond Carbon hybridization (sp) Example p p sp s Oxygen hybridization (sp 2 ) Example p p s sp 2

9 σ Bonds π Bonds Basic structure Basic structure σ-bonding σ-bonding π-bonding Electron density Electron density

10 MULTIPLE PI BONDS Acetylene forms a triple bond, with a single σ-bond and two π-bonds.

11 Bond Bond Types Hybridization Shape Single Bond σ-bond sp 3 Double Bond σ-bond & π-bond sp 2 Triple Bond σ-bond & 2 π-bonds sp

12 QUESTION Which hybrid orbital, and what number, and type of bonds are used by all the central atoms in C2F4? Answer: sp 2 ; 4 σ & 1 π bonds A sp 2 ; 4 σ & 2 π bonds B sp 2 ; 5 σ & 1 π bonds C sp 3 ; 4 σ & 1 π bonds D sp 3 ; 4 σ & 2 π bonds E

13 LIMITATIONS OF VALENCE BOND THEORY Does not adequately explain the magnetic and spectral properties of molecules. Understates the importance of delocalized electrons (resonance structures). Does not provide a satisfactory explanation of the bonding in hypervalent molecules.

14 LIMITATIONS OF VALENCE BOND THEORY The paramagnetic properties of O2 The color of tomatoes. Lycopene watch?v=kcgeev8qula

15 LECTURE OBJECTIVES Chapter 8.4 Describe bonding in terms of molecular orbital theory (MO theory). Use MO theory to determine bonding/bond order. Complete MO diagrams for selected diatomic molecules. Determine magnetic properties of diatomic molecules using MO theory/diagrams.

16 CENTRAL THEMES OF MOLECULAR ORBITAL THEORY A molecule is viewed on a quantum mechanical level as a collection of nuclei surrounded by delocalized molecular orbitals. Molecular wave functions are the summation of the atomic wave functions. A bonding molecular orbital (MO) is formed by wave functions that reinforce each other. This yields a region of high electron density between the nuclei. Wave functions that cancel each other are antibonding molecular orbitals. A node of zero electron density occurs between the nuclei when this happens.

17 BONDING & ANTIBONDING WAVE FUNCTIONS Bonding Amplitude of the two waves combined is higher than each individually. Antibonding Amplitude of the two waves combined cancels each wave to zero at a point.

18 Bonding Antibonding High e - density between nuclei Lower energy & more stable than the individual atomic orbitals that formed the MO. Low e - density between nuclei Higher energy & less stable than the individual atomic orbitals that formed the MO.

19 ENERGIES OF THE BONDING & ANTIBONDING MO S IN H 2 Bonding MO is lower in energy than the individual atomic orbitals Antibonding MO is higher in energy than the individual atomic orbitals

20 MOLECULAR ORBITAL DIAGRAM The filling of Energy 1s σ*1s 1s molecular orbitals follows the same concepts as were used to fill atomic orbitals. σ1s AO MO AO of H of H2 of H

21 BOND ORDER MO theory can be used to calculate the bond order. Bond Order = # of e- in bonding MOs - # of e- in antibonding MOs 2 bond order = ½(2-0) =1 single bond bond order = ½(2-2) =0 no bond

22 QUESTION Using MO theory, determine which option below is correct pertaining to the potential Code existence of He2 and He2 +. He2 may exist He2 + may exist A Yes Yes B No No Energy 1s σ*1s 1s C Yes No D No Yes σ1s AO MO AO

23 MO DIAGRAMS OF HE 2 & HE 2 + Energy 1s σ*1s 1s Energy 1s σ*1s 1s σ1s σ1s AO of He MO of He2 + AO of He + AO of He MO of He2 AO of He He2 + bond order = 1/2 He2 bond order = 0

24 BONDING IN S-BLOCK HOMONUCLEAR DIATOMIC MOLECULES Energy 2s σ*1s 2s Energy 2s σ*1s 2s Li2 σ1s Be2 σ1s Li2 bond order = 1 Be2 bond order = 0

25 CONTOURS & ENERGIES OF SIGMA & PI MO FORM COMBINATIONS OF 2P ATOMIC ORBITALS all p orbitals The pz orbital acts like the py orbital, forming a π2p MO.

26 RELATIVE MO ENERGY LEVELS FOR PERIOD 2 HOMONUCLEAR DIATOMIC MOLECULES MO energy levels for O2, F2 & Ne2. No 2s-2p mixing With 2s-2p mixing MO energy levels for B2, C2 & N2. Energy Mixing of 2s and 2p orbitals lowers the energy of σ2s & σ*2s, but raises energy of σ2p & σ*2p.

27 MO MIXING Orbital energies decrease across the period as the effective nuclear charge increases, and the atomic radius decreases.

28 QUESTION Predict the bond order for O2 and O2 +. Answer: 2 and 1.5 A 2 and 2 B 2 and 2.5 C 2 and 3 D None of the above E Energy

29 QUESTION Predict the bond order for N2 and N2 +. Answer: 3 and 2 A 3 and 2.5 B 3 and 3 C 3 and 3.5 D None of the above E Energy

30 MO occupancy and molecular properties for B2 through Ne2. Higher bond order results in greater bond energy and shorter bond length. The pairing of the electrons correlates with the magnetic properties.

31 Diamagnetic substance MAGNETISM Substance whose electrons are all paired ( ) Weakly repelled by magnetic fields. Paramagentic substance Substance with one or more unpaired electrons ( ) Attracted by magnetic fields.

32 QUESTION Which of the following species will be paramagnetic? C2, C2 +, N2 C2 C2 + N2 All are paramagnetic None are paramagnetic Answer: A B C D E Energy

33 MOLECULES WITH PI BONDS The energy difference between the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of molecules with multiple π-bonds is often in the UV or visible energy range. Lycopene in tomatoes has a small enough HOMO-LUMO gap that all visible light can be absorbed except red light. The more π-bonds involved in resonance structures, the lower the energy difference of the HOMO-LUMO gap. Lycopene

34 SUNBLOCK/SUNSCREEN Sunscreens are designed with HOMO-LUMO gaps that will allow the molecules to absorb UV light, but not visible light. OH H 2 N O para-aminobenzoic acid (PABA) Visible Light UV Light