Structure and Bonding in Molecules by Molecular Orbital Approach

1. Structure and Bonding in Molecules by Molecular Orbital Approach Lecture 2 7-Aug-2015

2. The structure and bonding present in the molecules have a close relationship with its function.

3. Recap • Ionic bond is a result of coulombic attraction of opposite charges • Covalent bonds result from electron sharing between two atoms, electrons are shared to allow noble gas configuration • Bond length is the average distance between two covalently bonded atoms. Bond formation releases energy, which is called as bond energy.

4. Recap • Electronegativity is a measure of atom’s ability to attract electrons • Polar bonds are formed between atoms of differing electronegativity • The shape of molecules is strongly influenced by electron repulsion • Lewis Structures describe bonding by the use of valence electrons dots • When two or more Lewis structures differing only in the positions of the electrons are needed to describe a molecule, they are called resonance forms. None correctly describes the molecule.

5. Some unanswered questions so far • Why are nobel gases relatively stable ? • What is so good about two electron bond ? • How do multiple bond look like? • Why some Lewis structures are incorrect?

6. Classical Description of and Atom Intuitively Appealing Classical description of atomic structure is technically incorrect Electron moving in an Orbit will give rise to Electromagnetic radiation It violates Heisenberg’s uncertainty Principle

7. According to de Broglie, a particle of mass m that moves with velocity v has a wavelength λ As a consequence of this postulation orbiting electrons can be described by equation that are same as those used in classical mechanics to describe waves. Matter waves have amplitudes with alternating positive and negative signs

8. Constructive Interference Destructive Interference “The theory of electron motion is called quantum mechanics. The equations developed in this theory is called wave equations, it has a series of solutions and they are called as Wave functions usually described by a Greek letter psi, ψ. Square is wave functions ψ2 at a given point gives the probability of finding the electron

10. Atomic Orbitals have characteristic shapes 1S Orbital 2S Orbital

11. Shapes of Px, Py, Pz Orbitals A: in three dimensions B: in two dimensions For Animations: www.whfreeman.com/organic6e

12. Aufbau Principle assigns electrons to orbitals

13. Using Aufbau Principle Determination of Electronic configuration becomes simple

14. Using Aufbau Principle Determination of Electronic configuration becomes simple

15. Molecular Orbitals and Covalent Bonding

16. The overlap of atomic orbital gives rise to sigma and pi bonds Overlap of any ‘n’ atomic orbitals gives rise to ‘n’ molecular orbitals. The amount of energy by which bonding level drops is the same amount of energy by which antibonding level is raised.

17. Energy Splitting Greater the splitting greater is the strength of the bond Molecular Orbital Theory explains why He2 does not exist whereas H2 exist

18. Geometric factors and size of the orbitals affect the degree of overlap

19. Hybrid orbitals: Bonding in Complex Molecules Classical Theory of Bonding does not explain why both the bond in BeH2 is of same length

20. Orbital Hybridization: Hybrid Orbitals • Just as the mixing of atomic orbitals of different atoms forms molecular orbitals, the mixing of atomic orbitals of the same atom leads to hybrid orbitals

21. sp Hybrids produce linear structures Be Be

22. sp2 Hybrids creates trigonal structures

23. sp3 Hybridization explains the shapes of tetrahedral carbon compounds For Animations see: www.whfreeman.com/organic6e

24. Any combination of atomic orbitals may overlap to form bonds

25. Hybrid orbital may contain lone pair of electrons:Ammonia and Water

26. Pi bonds are present in ethylene and acetylene

27. Structures and Formulas of Organic Molecules • To establish the identity of a molecule, we determine its structure. • Elemental analysis reveals the emprical formula: For example emprical formulae for water is H2O • However by this method we cannot tell the structure: We cannot distinguish ethanol and Dimethyl ether, they are called structural isomers

28. X-Ray Diffraction: Ultimate proof of a Structure Drawback: Not all molecules could be made as a single crystal Common methods of structural elucidation is by using spectroscopic techniques such as NMR, IR, Mass spectrometer.

29. Several types of Drawings used to represent molecular structures

30. The pictures and contents in this presentation were taken from various internet sources and text book Organic Chemistry Structure and Function by Peter Vollhart and Neil Schore for the purpose of better understanding and effective propagation of scientific knowledge.Wherever possible due references was given to the original source, I do not take any credit whatsoever.There is no commercial interest attached with this document, it is strictly for internal use only.