VALENCE BOND THEORY & MOLECULAR ORBITAL THEORY

1. VALENCE BOND THEORY VALENCE BOND THEORY &MOLECULAR ORBITAL THEORY Chapter 3 Dr. Shuchita Agrawal BTIRT Sironja, Sagar

2. Valence Bond Theory • Extends the Lewis concept of electron pair bonds by introducing the notion of orbital overlap and hybridization. • Hybridization is necessary to account for molecular geometry predicted by VSEPR. • Hybrid atomic orbitals are constructed by making linear combinations (sums and differences) between hydrogen-like orbitals in the valence shell of each atom.

3. Hybridization • Example: BeH2 • Linear Molecule, equivalent Be-H bonds • Symmetry Adapted Linear Combination (SALC) of hydrogen-like atomic orbitals in the valence shell of Be • Basis set consists of: 2s, 2px, 2py, 2pz • Only 2s and 2px are mixed; 2py and 2pz retain their original form. • “ideal” hybrids ½ s, ½ p character • Energetic “cost” of hybridization (promotion energy) is offset by formation of stronger bonds

4. sp hybridization

5. Bonding Description • Valence electrons of each atom are distributed among the available orbitals. • In this case, the two valence electrons of Be are placed in the sp hybrid orbitals; the remaining p orbitals are vacant. • Each H atom has one electron in the 1s orbital. • Orbitals of each atom overlap to form electron pair bonds.

6. Sigma and Pi Bonding

7. sp3 hybridization

8. VBT description of H2O • Oxygen has sp3 hybridization. • Ideal tetrahedral angle is 109.5o • Observed bond angle in H2O is ~ 105o • This indicates that hybrids involved in O-H bonding orbitals have more p-character. • Lone-pair orbitals on oxygen have more s-character.

9. Molecular Orbital Theory • Fundamentally different than VBT • Valence atomic orbitals of all atoms in the molecule form the basis set. • For convenience, orbitals on symmetry-related atoms are combined into symmetry-adapted linear combinations calledligand group orbitals (LGOs). • Atomic orbitals on central atom are mixed with LGOs to generate molecular orbitals (MOs). • In the final step, valence electrons are placed into MOs starting at lowest energy.

10. Molecular Orbitals of H2 • Basis set of orbitals consists of only the 1s orbital on each H-atom • Linear combinations of these orbitals produce two molecular orbitals • + combination gives bonding orbitals(1s) (constructive interference between wave functions • - combination gives anti bonding orbital s*(1s) (destructive interference)

11. Molecular Orbitals of H2

12. Molecular Orbitals of Li2 NOTE: Both bonding and anti bonding orbitals arising from the 1s orbitals are filled – no net bonding results from core electrons!

13. p-Orbital overlap modes

14. Homo nuclear Diatomic with p-Orbitals

15. Effect of s-p interaction For atoms early in the 2nd period, energy separation between 2s and 2p orbitals is small. This allows mixing of s(2s) and s(2p) orbitals that raises the energy of the s(2p) orbital above that of the p(2p)

16. MO diagram for O2 and F2 Later in the period, the energy separation of the 2s and 2p orbitals Is greater (decreasing s-p interaction), so s(2p) drops below p(2p)

17. Molecular Orbital diagrams for 2nd row di atomics

18. Hetero nuclear Di atomics Strength of orbital interactions depends upon: • Symmetry match • Spatial overlap • Energy overlap

19. MO diagram for HF

20. Bonding in H2O

21. Polyatomic Molecules • Group orbital approach • Symmetry-Adapted Linear Combinations • Examples: H2O

22. Character Table for C2v

23. MO’s of H2O (A1 symmetry) O(2s) and O(2pz) AO’s mix with H(f1) LGO to form 3 MO’s: y(1a1) = (s) + (f1) y(2a1) = (pz) - (f1) Y(3a1) = (pz) + (f1) Strongly Bonding Weakly Bonding Strongly Anti bonding

24. MO’s of H2O (B2 symmetry) O(2py) AO mixes with H(f2) LGO to form 2 MO’s: y(1b2) = (py) + (f2) y(2b2) = (py) - (f2) Antibonding Bonding

25. Molecular Orbitals of Water

26. Bonding in SF6

27. Fluorine SALCs for SF6

28. MO diagram for SF6

29. Thanks Contact- mrs.shuchitaagrawal@yahoo.com 9977330500