WALSH DIAGRAM- SEMINAR TOPIC

CONTENTS
 INTRODUCTION
 CONCEPTS OF WALSH DIAGRAM
 APPLICATION IN TRIATOMIC MOLECULES
[IN AH₂ TYPE OF MOLECULES(BeH₂,BH₂,H₂O)]

INTRODUCTION
Arthur Donald Walsh FRS The introducer of walsh diagram (8 August 1916-23 April 1977) was a British chemist, professor of chemistry at the University of Dundee . He was elected FRS in 1964. He was educated at Loughborough Grammar School.
Walsh diagrams were first introduced in a series of ten papers in one issue of the Journal of the Chemical Society . Here, he aimed to rationalize the shapes adopted by polyatomic molecules in the ground state as well as in excited states, by applying theoretical contributions made by Mulliken .

1. WALSH DIAGRAM BASUDEV BARAL Regd.no-1311106007 M.Sc. (APPLIEDCHEMISTY )

2. CONTENTS INTRODUCTION CONCEPTS OF WALSH DIAGRAM APPLICATION IN TRIATOMIC MOLECULES [IN AH ₂ TYPE OF MOLECULES( BeH ₂,BH₂,H₂O)]

3. INTRODUCTION Arthur Donald Walsh   FRS The introducer of walsh diagram (8 August 1916-23 April 1977) was a British chemist, professor of chemistry at the  University of Dundee . He was elected FRS in 1964. He was educated at  Loughborough Grammar School . Walsh diagrams were first introduced in a series of ten papers in one issue of the Journal of the Chemical Society . Here, he aimed to rationalize the shapes adopted by polyatomic molecules in the  ground state  as well as in  excited states , by applying theoretical contributions made by  Mulliken  

4. Specifically, Walsh calculated and explained the effect of changes in the shape of a molecule on the energy of molecular orbitals . Walsh diagrams are an illustration of such dependency, and his conclusions are what are referred to as the "rules of Walsh”. In his publications, Walsh showed through multiple examples that the geometry adopted by a molecule in its ground state primarily depends on the number of its valence electrons. He himself acknowledged that this general concept was not novel, but explained that the new data available to him allowed the previous generalizations to be expanded upon and honed.

5. CONCEPTS OF WALSH DIAGRAM Normally we explain the experimentally observed geometries of molecules by certain devised rules and concepts such as VSEPR theory , hybridisation , etc . But Walsh actually Calculated the energies of molecular orbitals of a reference Geometry and its several distortion for simple tri atomic molecules . he correctly predicted the actual geometries of a number of simple tri atomic molecules like H⁺ 3 , H ₂ O , BeH ₂ etc by plotting a graph between the calculated energies of molecular orbitals of reference geometry and those of the distorted geometries of a molecule v\s the distortion parameter i.e , bond angle .

6. The position of the minimum energy of filled molecular orbitals in the graph indicates the most stable geometry of the molecule . Such graphs are known as “ WALSH DIAGRAM ” .

7. WALSH DIAGRAM FOR AH ₂ TYPE OF MOLECULE

8. WALSH DIAGRAM FOR AH ₂ TYPE MOLECULE

9. Here there are three tri atomic molecules w e take in example BeH2,BH2,H2O , the number of H atoms present is 2 , let them H a , H b . thus the outer shell of these atoms are 1s H a , 1s H b . In case of Be , B , O the outer most shell are 2s , 2p . Here , 2s has one orbital 2p has three orbital ( px , py , pz ) . So total of 6 numbers of outer most orbitals are present here namely 1sHa , 1sHb , 2s , 2px , 2py , 2pz . 6 atomic orbitals combines to form 6 molecular orbitals out of which , two BMOs = , two ABMOs = 1  u two are NB MOs = 3  g , 2  u 2  g 1  u

10. The electrons are filling in the orbitals on account of the lowest energy of the orbitals . So the orbitals with lowest energy filled first . Atomic orbitals represents with a wave function related to Hamiltonian operator ,  Ψ = C1 1sHa + C2 1sHb + C3 2sH + C4 2pxA + C5 2pyA +C6 2pzA

11. FOR BeH 2 MOLECULE For Be atom the electronic configuration is -: 1s² 2s² For H atom -: 1s² So Be atom has 2e-s in its outer most 2s orbital and 2H has 2e-s in its outer 1s orbital . So total of 4 electrons in outer shell . Now , For linear structure electronic configuration is , ² ² Angle HAH = 180⁰ So in diagram it corresponds to minimum energy , so it is the perfect structure . 2  g 1  u

12. For bent structure , angle HAH = 90⁰ or 90⁰-180⁰ electronic configuration are , (2a₁)² (3a₁)² | (2a₁)² (1b₂)² It corresponds to Emax so unstable not perfect one .

13. FOR BH 2 MOLECULE The electronic configuration of B atom -: 1s² 2s² 2p¹ for H atom -: 1s¹ B atom has 3e-s in outer most 2s and 2p obitals 2H atoms have 2e-s in outer most 1s obital so total of 5 e-s in outer shell . Now for linear structure angle HAH =180° where configuration is , ( 2  g )² (1  u )² ( 1  u )¹ here energy is maximum , so unstable not preferred one .

14. for bent structure , angle is 90⁰ or 90⁰-180⁰ electronic configuration is , (2a₁)² (3a₁)² (1b₂)¹ | (2a₁)² (1b₂)² (3a₁)¹ Stabilisation & destabilisation of energy( Emin ) the electronic configuration is preferred one . The angle is HAH= in between 90⁰-180⁰ i.e , 131⁰ .

15. FOR H 2 O MOLECULE Electronic configuration of O atom -: 1s² 2s² 2p⁴ for H atom -: 1s¹ O has 6 outer most e-s in 2s and 2p sub shells . 2H atoms have 2 outer most e-s in 1s sub shell . so total of 8e-s in outer most sub shell . FOR linear structure angle HAH = 180⁰ electronic configuration is , ( 2  g )² (2  u )² ( 1  u )⁴ This structure corresponds to maximum energy so destabilize and not perfect one .

16. Now for bent structure angle is 90 degree or 90-180 degree . electronic configuration is , (2a₁)² (3a₁)² (1b₂)² (1b₁)² | (2a₁)² (1b₂)² (3a₁)² (1b₁)² stabilisation & destabilisation constant( Emin .) The second electronic configuration is preferred , angle is HAH = 90⁰-180⁰ , very close to 90⁰ (104⁰.28˝).

17. This same concept can be applied to other species including non-hydride AB 2  and BAC molecules, HAB and HAAH molecules, tetra atomic hydride molecules (AH 3 ), tetra atomic non hydride molecules (AB), H 2 AB molecules,  acetaldehyde , penta atomic molecules (CH3I), hexa atomic molecules ( ethylene ), and  benzene .

18. WALSH DIAGRAM IN AH 3 TYPE OF MOLECULE

19. THANK YOU