Molecular Orbital Theory

Question 1

Constructive interference

Answer 1

Waves in phase so reinforce each other

Question 2

Destructive interference

Answer 2

Waves out of phase so cancel out

Question 3

In phase combination of AO

Answer 3

Positive overlap

Question 4

Out of phase combination of atomic orbitals

Answer 4

Negative overlap

Question 5

Consequences of positive overlap

Answer 5

Increase in electron density that’s greatest at midpoint, electron experiences attraction of both nuclei so the electrons stabilised

Question 6

Consequences of negative overlap

Answer 6

Decrease in electron density giving a node where electron density = 0, electron s destabilised

Question 7

Calculating bond order

Answer 7

Electrons in BMO - electrons in ABMO / 2

Question 8

No node when viewed down internuclear axis

Answer 8

σ

Question 9

1 node when viewed down internuclear axis

Answer 9

π

Question 10

2 nodes when viewed down internuclear axis

Answer 10

δ

Question 11

symmetric wrt inversion

Answer 11

G

Question 12

Antisymmetric wrt inversion

Answer 12

U

Question 13

*

Answer 13

AMBO

Question 14

Orthogonal AO

Answer 14

Positive and negative overlap cancel out

Question 15

Stable BMO/unstable AMBO when

Answer 15

Large AO overlap, AO close in energy and low splitting

Question 16

Paramagnetic

Answer 16

Has unpaired electrons

Question 17

diamagnetic

Answer 17

No unpaired electrons

Question 18

S/P mixing

Answer 18

For B2/C2/N2 low 2S/2p mismatch so overlap is important
For O2/F2/Ne2 larger 2S/2p mismatch so effects of s/p mixing can be ignored
So 1σg and 2σu* MO stabilised, 3σg and 4σu* MO destabilised and 3σg lies above 1πu

Question 19

Why is AO energy mismatch not 0 for heterodiatomics

Answer 19

Will have different electronegativities so valence electrons will be at different energies

Question 20

Describe MO of heterodiatomics

Answer 20

MO splitting decreases with increasing energy mismatch
MO shaped no longer symmetrical, BMO has more character of more stable (more χ) AO and ABMO has more character of less stable AO

Question 21

Importance of frontier MO and reactivity

Answer 21

Control reactivity, electron in HOMO of Nu- attacks LUMO of E+

Question 22

Walsh’s rule

Answer 22

Molecule adopts structure which best stabilises HOMO, if HOMO’s unaffected the next nearest occupied atomic orbital determines geometry

Question 23

Application of Walsh’s rule

Answer 23

Bending of triatomic molecules

Question 24

How does adding an electron to a NBMO affect bond length and strength

Answer 24

No significant changes