Chem 373- Lecture 24: Applications of Valence Bond Theory

8/3/2019 Chem 373- Lecture 24: Applications of Valence Bond Theory

http://slidepdf.com/reader/full/chem-373-lecture-24-applications-of-valence-bond-theory 1/22

Lecture 24: Applications of Valence Bond

Theory

The material in this lecture covers the following in Atkins.

14 Molecular structure

Valence-bond theory

14.2 Homonuclear Diatomic Molecules14.3 Polyatomic Molecules

Lecture on-line

Applications of Valence Bond Theory (PowerPoint)

Applications of valence Bond Theory (PDF)

Handout for this lecture



H H

Valence Bond Theory Applications

A 1sH; B 1sH

-bond:invarient to rotation

I. Diatomics

A B

(1,2) = [A(1)B(2)+ A(2)B(1)] ×[ (1) (2)− (1) (2)]

In general we write (re,RN)as the product of electron paifunctions i(r2i−1, r2i) as

(re ,RN) 1(r1, r2 ) 2(r3, r4 )

.. i(r2i 1, r2i) j(r2 j 1, r2j ).. n(r2n 1, r2n)

Pair 1

Pair 2

Pair i Pair j

Pair n

A li i



H Cl

Applications

(1,2) = [A(1)B(2)+ A(2)B(1)] ×[ (1) (2)− (1) (2)]

+

sp(B):

1

2[3s + 3pz ]

Valence Bond Theory

Atomic orbitals on H(Hydrogen)

1sH

Atomic orbitals on C(Chlorine)

3sCl 3pzCl

3pxCl 3py

Cl

Hybride orbitals on H

(Hydrogen)

1sA

Hybride orbitals on Cl(Chlorine)

−sp(B) :

1

2[3s − 3pz ]

3px

Cl

3py

Cl



Valence Bond Theory

A = 1sH;B = +sp(Cl)for -bond

Electron pairingand formation of bonds

(1,2) = [A(1)B(2)+ A(2)B(1)] ×[ (1) (2)− (1) (2)]

Electron pairingand formation oflone-pairs

A=

−

sp(Cl)

;B=

−

sp(Cl)

for lone-pair

H Cl

H Cl

3pxCl

A = 3pxCl

;B = 3pxCl

for lone-pair

H Cl

A = 3pyCl;B = 3py

Cl

for lone-pair

3pyCl

H Cl



Atomic orbitals on Cl # 2

3sCl 3pzCl

3pxCl 3py

Cl

Cl Cl


(1,2) = [A(1)B(2)+ A(2)B(1)] ×[ (1) (2)− (1) (2)]

Atomic orbitals on Cl # 1

3sCl 3pzCl 3pxCl 3pyCl

Hybride orbitals on Cl # 1

3pxCl 3py

Cl

+sp(Cl −

sp(Cl

Hybride orbitals on Cl # 2

3pxCl 3py

Cl+

sp(Cl−

sp(Cl



Electron pairingand formation of bonds

Cl Cl


(1,2) = [A(1)B(2)+ A(2)B(1)] ×[ (1) (2)− (1) (2)]

A = +

sp(Cl1);B = +

sp(Cl2

Cl Cl

−bond

Electron pairingand formation of

lone-pairs

A = −sp(Cl1) ;B = −

sp(Cl1)

for lone-pair

ClCl

3pxCl

A = 3pxCl1

;B= 3pxCl1

for lone-pair

Cl Cl

A = 3pyCl;B = 3py

Cl

for lone-pair

3pyCl

Cl Cl

Same for Cl # 2



The orbital overlap

and spin-pairing

between electrons

in two collinear porbitals that result

in the formation

of a ( bond.


(1,2 ) = [A(1)B(2)+A(2)B(1)] ×[ (1) (2)− (1) (2)]





(1,2 ) = [A(1)B(2)+A(2)B(1)]×[ (1) (2)− (1) (2)]

I. Diatomics

N N C O

A =+

sp(1);B =

+

sp(2)

for -bond

+

sp(1):

1

2[2s + 2pz ]

+

sp(1):

1

2

[2s − 2pz ]

A = −sp(1) ;B = −

sp(1)

for lone-pairs

A = 2px1;B = 2px

1

A = 2py1;B = 2py

1

−

bonds

Orbitals change signon reflexation in plancontaining 1- 2 bond

vector





(1,2 ) = [A(1)B(2)+A(2)B(1)]×[ (1) (2)− (1) (2)]

I. Diatomics

The structure of bonds in a nitrogen molecule,

which consists of one band and two bands.

The electron density has cylindrical symmetry

around the internuclear axis.




CH C H

2. Linear molecules

A representation of the structure of a triple bond

in ethyne; only the bonds are shown explicitly.The overall electron density has cylindrical symmetry

around the axis of the molecule.




Valence Bond Theory Applications3.Trigonal planar

C

H

H

C

H

HC2H4

C

H

H

O

CH2O

tr1

tr2

x

y

tr3

tr11

3 [s px ]

tr21

3[s

1

2px

3

2py ]

tr31

3 [s1

2 px3

2py ]

C2H4

CH2O





C

H

H

C

H

HC2H4

C2H4

(a) An s orbital and two p orbitals can be

hybridized to form three equivalent orbitals

that point towards the corners of an equilateraltriangle. (b) The remaining, unhybridized p

orbital is perpendicular to the plane.

3.Trigonal planar




CH

H

H

H

X

y

z

t1

t2

t3

t4


4.Tetrahedral

t11

2[s px py pz ]

along (x,y,z)

t21

2[s px py pz ]

along (-x,-y,z)

t31

2[s px py pz ]

along (-x,y,-z)

t41

2[s px py pz]

along (x,-y,-z)

sp

3

hybrides




An sp3 hybrid orbital formed from thesuperposition of s and p orbitals on the same

atom. There are four such hybrids: each one

points towards the corner of a regular

tetrahedron. The overall electron densityremains spherically symmetrical.

CH

H

H

H


4.Tetrahedral sp3 hybrides




A more detailed representation of theformation of an sp

3 hybrid by interference

between wavefunctions centred on the same

atomic nucleus. (To simplify the

representation, we have ignored the radial

node of the 2 s orbital.)

CH

H

H

H


4.Tetrahedral

sp3 hybrides

= +

V l B d Th Applications



OH

H

NH

H

H

(1,2 ) = [A(1)B(2)+A(2)B(1)] ×[ (1) (2)− (1) (2)]


4.Tetrahedral

sp3 hybrides

X

y

z

t1

t2

t3

t4

CH

H

H

H

X

y

z

t1

t2

t3

t4

X

y

z

t1

t2

t3

t4




A first approximation to the valence-bond

description of bonding in an H2O molecule.Each bond arises from the overlap of an H1s

orbital with one of the O2 p orbitals. This

model suggests that the bond angle should be

90°, which is significantly different from the

experimental value.

(1,2 ) = [A(1)B(2)+A(2)B(1)] ×[ (1) (2)− (1) (2)]


4.Tetrahedral sp3 hybrides

OH

H

X

y

z

t1

t2

t3

t4

use of sp3 hybrides

Use of

p - orbitals





5.Bipyramidal d2sp2 hybrides

PF

F

F

F

F

tr1

1

3[s p

x

]

tr21

3[s

1

2px

3

2py ]

tr3

1

3[s

1

2p

x

3

2p

y

]

tr1

tr2

x

y

tr3

d412

[pz dz2 ]

d51

2[pz d

z2 ]

d4

d5

z

SF

F

F

F





5.Bipyramidal

d2sp2 hybrides

PF

F

F

F

F S

F

F

F

F

(1,2 ) = [A(1)B(2)+A(2)B(1)] ×[ (1) (2)− (1) (2)]




y pp

6. Octahedral

d2sp3 hybrides

(1,2 ) = [A(1)B(2)+A(2)B(1)] ×[ (1) (2)− (1) (2)]

1

2

34

56

x

y

z oc116

[s 2dz2 3pz ]

oc2

1

6[s

1

2d

z2

3

2d

x2 y2 3px ]

oc3

1

6[s

1

2d

z2

3

2d

x2 y2 3py ]

oc4

1

6[s

1

2d

z2

3

2d

x2 y2 3px ]

oc5

1

6[s

1

2d

z2

3

2d

x2 y2 3py ]

oc6

1

6[s 2d

z2 3p

z

]




y pp

6. Octahedral

d2sp3 hybrides

(1,2 ) = [A(1)B(2)+A(2)B(1)] ×[ (1) (2)− (1) (2)]

x

y

z

SF

F

F

F

F

F

What you should learn from this lecture



What you should learn from this lecture

1. You are not required to know the mathematicalform of the s and p atomic orbitals as well as

the sp,sp2, sp3, sp2d2, sp3d2 hybrides. However youshould be able to draw their shapes

2. You should be able to convert Lewis structuresbased on bonds and lone- pairs into valencebond pair functions(1,2 ) = [A(1)B(2)+A(2)B(1)] × [ (1) (2)− (1) (2)]

where A and B are atomic orbitals (or hybrides)

on different centersfor bonds ,and orbitals on the same centerfor lone-pairs

Date post:	06-Apr-2018
Category:	Documents
Upload:	nuansak3
View:	219 times
Download:	0 times

Chem 373- Lecture 24: Applications of Valence Bond Theory

Documents