WALSH DIAGRAM- SEMINAR TOPIC

WALSH DIAGRAM

BASUDEV BARALRegd.no-1311106007M.Sc. (APPLIEDCHEMISTY)

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

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.

CONCEPTS OF WALSH DIAGRAM

Normally we explain the experimentally observedgeometries of molecules by certain devised rules and concepts such as VSEPR theory , hybridisation , etc . But Walsh actuallyCalculated 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 , H₂O , BeH₂ 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 .

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” .

WALSH DIAGRAM FOR AH₂ TYPE OF MOLECULE

WALSH DIAGRAM FOR AH₂ TYPE MOLECULE

Here there are three tri atomic molecules we take in example BeH2,BH2,H2O , the number of H atoms present is 2 , let them Ha , Hb . thus the outer shell of these atoms are 1sHa , 1sHb .

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 = 1u

two are NB MOs = 3g , 2u

2g 1u

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

FOR BeH2 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 = 180⁰So in diagram it corresponds to minimum energy , so it is the perfect structure .

2g 1u

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 .

FOR BH2 MOLECULE The electronic configuration of B atom -: 1s² 2s²

2p¹

for H atom -: 1s¹B 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 ,

(2g )² (1u )² (1u )¹

here energy is maximum , so unstable not preferred one .

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 .

FOR H2O MOLECULE Electronic configuration of O atom -: 1s² 2s² 2p⁴

for H atom -: 1s¹O 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 , (2g )² (2u )²

(1u )⁴

This structure corresponds to maximum energy so destabilize and not perfect one .

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˝).

This same concept can be applied to other species including non-hydride AB2 and BAC molecules, HAB and HAAH molecules, tetra

atomic hydride molecules (AH3), tetra atomic non hydride molecules (AB), H2AB molecules, acetaldehyde, penta atomic molecules (CH3I),

hexa atomic molecules (ethylene), and benzene.

WALSH DIAGRAM IN AH3 TYPE OF MOLECULE

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