Molecular Modeling

1. Molecular Modeling

2. Molecular Modeling: Visualizations & Predictions • Numerical Methods • Integral Method • Semi-Empirical MO-SCF Methods • Approximate MO Methods  • Ab Initio Methods

4. HΨ = EΨ H = Hamiltonian His an operator corresponding to the total energy of the system. Kinetic + potential energy

5. LCAO: linear combination of atomic orbitals NDDO: neglect of diatomic differential overlap

6. The cost of calculations (cpu time required) increases rapidly as the basis set size is increased and as the amount of electron correlation increases.

7. Web MOhttp://butane.cabrillo.edu/Username: your last namepassword: DVC id number • Web MO: undergraduate molecular modeling college consortium • Web-based, on-line accessible molecular modeling software, Graphic User Interface (GUI), free to DVC students, no cpu charges • Uses MOPAC 7, GAMESS 2000, and GAUSSIAN

8. Web MOhttp://butane.cabrillo.edu/Username: your last namepassword: DVC id# Output: • Dipole moment • Bond Orders • Partial Charges • Vibrational Modes • Molecular Orbitals • Ultraviolet-Visible-Infrared Graphics • NMR Chemical Shifts

9. Molecular Orbital Theory (Review) http://chemconnections.org/general/movies/html_swf/MolecularOrbitalTheory/MolecularOrbitalTheory.html

10. Molecular Orbital Theory • Valence electrons are delocalized in molecular orbitals spread over the entire molecule. • First Principle: The total # of molecular orbitals equals the number of atomic orbitals contributed by the atoms that have combined. • Second Principle: The bonding molecular orbitals are lower in energy than the parents orbitals and are more symmetrical (fewer nodes) than the antibonding orbitals which are higher in energy. • Third Principle: The electrons of the molecule are assigned to orbitals of successively higher energy according to the Pauli exclusion principle and Hund’s rule.

11. Molecular Orbital Theory 1. # MO’s = # atomic orbitals used. 2. Bonding MO is lower in energy than atomic orbitals. Antibonding MO is higher. 3. Electrons assigned to MO’s of higher and higher energy. Bond order = 1/2 [# e- in bonding MOs - # e- in antibonding MOs]

12. Analogy between Light Waves and Atomic Wave Functions

14. QUESTION When comparing the M.O. theory of bonding to the Localized Electron model which of the following would be an incorrect claim? For a molecule of H2; MO1 = 1s A + 1s B; MO2 = 1s A – 1s B The molecular orbitals (both bonding and antibonding) still have a maximum electron occupancy of two just as the localized orbitals. In H2, the bonding orbital (MO1) is lower in energy than the 1s orbital of hydrogen. Although not used in molecular bonding, the 1s orbital of hydrogen is present in H2.

16. Homo: Highest Occupied Molecular Orbital Sigma Bond

17. Sigma Bond

18. Single bond: C-C Bond Order =1 Bond Length: C-C > C=C > CΞC Bond Energy: C-C < C=C < CΞC

19. Pi Bond

20. sand π Bonding inC2H4Double bond: C=C Bond Order =2 Bond Length: C-C > C=C > CΞC Bond Energy: C-C < C=C < CΞC

21. Pi Bond

22. sand π Bonding inC2H2Triple bond: CΞC Bond Order =3 Bond Length: C-C > C=C > CΞC Bond Energy: C-C < C=C < CΞC

23. Consequences of Multiple Bonding There is restricted rotation around C=C bond.

24. Anti-bonding Orbital Lumo: Lowest Unoccupied Molecular Orbital

26. QUESTION Consider the two molecular orbitals of benzene that follow. Which statement is correct? MO1 MO2 MO1 and MO2 are likely bonding orbitals. MO1 and MO2 are likely antibonding orbitals. MO1 is likely a bonding orbital and MO2 is likely an antibonding orbital. MO1 is likely an antibonding orbital and MO2 is likely a bonding orbital