David James Chem 6304 March 31 st , 2010

1. “An In-Depth AB Initio Study of Thermodynamics and Stabilization Energies of Mono- and Di-substituted Methyl Halides” David James Chem 6304 March 31st, 2010

2. Outline • Introduction to compounds of interest • Stabilization energies • Anomeric Effect (bond separation reactions) • Geometry optimization • Substitution Reactions • Trends in Mulliken Charges • Problems at the MP4 basis set • Conclusions

3. Molecules of Interest Methane: CH4 Methyl Halides: CH3X Methylene Halides: CH2X2 Di-substitued Methyl Halides: CH2YX

4. Computational Details Optimizations and Frequencies calculated at: Levels of Theory: Basis Sets: HFSTO-3G MP26-31G B3LYP6-31+G (MP4)**6-31++G(f2d,p)

5. The Anomeric Effect X π - donor Y C σ - acceptor -Most stable systems will consist of a good π – donor and a good σ – acceptor substituents -As shown, C is a good π – acceptor and good σ – donor -If both X and Y have only accepting or donating properties, the compound will be destabilized -Acceptor/acceptor  pull too much electron density away from C center -Donor/donor  will give too much electron density to C center

6. Bond Separation Reactions X-CH2-Y + CH4 CH3-X + CH3-Y X π - donor Y C F: Very good σ acceptor and fairly weak π donor. Cl: good σ acceptor and very weak π donor. Br: good σ acceptor and very weak π donor. σ - acceptor X, Y - Isodesmic Reaction: Should be able to calculate accurately at low levels of theory and basis sets

7. Predicting Stabilization using Geometry VS. Calculated at the HF/6-31++G(f2d,p) level. Can give insight into π – donating ability of substituents

8. > > Anomeric Stabilization Energies F: Very good σ acceptor and fairly weak π donor. Cl: good σ acceptor and very weak π donor. Br: good σ acceptor and very weak π donor. • Expectations: • CH2FCl CH2FBr CH2ClBr • All levels of theory and basis sets will give similar values (isodesmic reactions)

9. Anomeric Stabilization Energies CH2F2 > CH2Cl2 ~ CH2Br2 **AB Initio Molecular Orbital Theory: CH2F2 = 60 kJ/mol (HF/3-21G)** CH2Cl2 = -17 kJ/mol (HF/3-21G(*)) 10 kJ/mol difference My calculations of CH2Cl2 with HF/3-21G = -27 kJ/mol Shows that second row elements are very sensitive to polarization function in these ISODESMIC reactions!

10. Substitution Reactions 2X-CH2 + Y2 2CH3-Y +X2 Polarization functions must be used to fully describe the process when 2nd and 3rd row elements are present!

11. Trends in Mulliken Charges • STO-3G shows Cl to carry a larger negative charge then F • 6-31G properly assigns charge in order of electronegativity • Charge on C increases with increasing electronegativity of substituent. • Charge of H decreases with increasing electronegativity of substituent.

12. Problems with MP4 • At the MP4 level of theory, the initial geometry is used for all calculations throughout, meaning that redundant internal coordinates (RIC) are not used. • Probably do to cost of computing. • Symmetry of compound is lost. • Optimized structures must be used. • Even CH4 did not use RIC! • ie: Predicts CH4 to be CV3 symmetry

13. Conclusions • Geometry Optimizations  comparing bond lengths can give insight into π-donating abilities of substituents. • Not all isodesmic reactions can be fully described by low levels of theory and small basis sets. • d polorizable functions are necessary to fully describe second row elements. • Mulliken charges can be very random. Trends in basis sets can be observed, but the magnitude of the charges must be investigated using additional information for complete understanding. • MP4 level of theory does not use RIC; therefore, giving incorrect values when optimized structures are not initially used.