What is Molecular Orbital Theory?

Transcription

1 What is Molecular Orbital Theory? Scientists choose the model that best helps them answer a particular question If we want to know shape, use VSEPR along with Valence-Bond and Hybrid- Orbital Theories But, VB doesn t explain magnetic & spectral properties and only considers localized electron density MO theory considers the TOTAL electron densities and bonding properties according to symmetry (constructive and destructive orbital overlaps) In MO theory, the molecule is described as a collection of nuclei with the electron orbitals delocalized over the entire molecule

2 How does it work? Key ideas: formation of MOs from AOs, energy and shape of MOs and how they are populated by electrons Approximations are used: Mathematically combine (add or subtract) the atomic orbitals (specifically, the atomic wave functions) of nearby atoms to form MOs (molecular wave functions) Addition of two wavefunctions gives a bonding MO (increases probability of finding electrons between the nuclei giving a bond) Subtraction of two wavefunctions gives an antibonding MO (decreases probability of finding electrons between two nuclei to zero, a node) This means number of MOs will always be the same as the number of AOs Bonding MOs are lower in energy than their antibonding MOs AOs must be of similar energy and orientation (the symmetry) Bonding and antibonding MOs most commonly combine in two types of symmetry, σ (sigma) and π (pi) bonds Electrons fill Mos just as they do AOs, following Aufbau, Pauli and Hund MOs are best represented in a Molecular Orbital Diagram The MOs are labelled according to their bonding symmetry (σ or π) and the orbitals from which they are made (, 2p 3d etc.) Antibonding orbitals are denoted by a superscript star (*) Eg = (σ ) = (σ *) 2

3 MO Diagrams of 2 nd Period Diatomics Do not memorize these

4 MO diagram for He 2+ and He 2. σ* σ* Energy Energy σ σ AO of He MO of He + AO of He + AO of He MO of He 2 AO of He He 2+ bond order = He 2 bond order =

5 SAMPLE PROBLEM Predicting Stability of Species Using MO Diagrams PROBLEM: Use MO diagrams to predict whether H 2+ and H 2- exist. Determine their bond orders and electron configurations. PLAN: Use H 2 as a model and accommodate the number of electrons in bonding and antibonding orbitals. Find the bond order. SOLUTION: bond order: bond order: σ σ AO of H σ MO of H 2 + AO of H configuration is: AO of H AO of H - σ configuration is: MO of H - 2

6 Bonding in s-block homonuclear diatomic molecules. Energy σ* σ* σ σ Be 2 Li 2 Be 2 bond order = Li 2 bond order =

7 Contours and energies of s and p MOs through combinations of 2p atomic orbitals.

8 Relative MO energy levels for Period 2 homonuclear diatomic molecules. without -2p mixing with -2p mixing MO energy levels for O 2, F 2, and Ne 2 MO energy levels for B 2, C 2, and N 2

9 MO occupancy and molecular properties for B 2 through Ne 2

10 The paramagnetic properties of O2

11 SAMPLE PROBLEM Using MO Theory to Explain Bond Properties PROBLEM: As the following data show, removing an electron from N 2 forms an ion with a weaker, longer bond than in the parent molecules, whereas the ion formed from O 2 has a stronger, shorter bond: N 2 N 2 + O 2 O 2 + Bond energy (kj/mol) Bond length (pm) Explain these facts with diagrams that show the sequence and occupancy of MOs. PLAN: Find the number of valence electrons for each species, draw the MO diagrams, calculate bond orders, and then compare the results. SOLUTION: N 2 has 10 valence electrons, so N 2+ has 9. O 2 has 12 valence electrons, so O 2+ has 11.

12 SAMPLE PROBLEM Using MO Theory to Explain Bond Properties N 2 N 2 + O 2 O 2 + σ 2p σ 2p π 2p π 2p σ 2p π 2p π 2p σ 2p σ σ bond orders: σ σ

13 σ The MO diagram for HF Energy 2p x 2p y 2p σ AO of H MO of HF AO of F

14 σ* 2p The MO diagram for CO 2p π* 2p Energy σ 2p 2p possible Lewis structures π 2p C O σ* AO of C MO of CO σ AO of O

15 σ* 2p The MO diagram for NO π* 2p Energy 2p σ 2p 2p possible Lewis structures π 2p 0 0 N O σ* AO of N MO of NO σ AO of O N O