Bonding in Molecules

Question 1

What rules must be taken into account for filling MO diagrams?

Answer 1

Aufbau principle
Pauli exclusion principle
Hund’s rule

All of which underpinned by orbital approx.

Question 2

When does s-p mixing occur?

Answer 2

Beggining of second period as 2s close in energy to 2p

Question 3

What is the change in orbitals in sp mixing?

Answer 3

Question 4

What is the overlap in heteronuclear diatomics?

Answer 4

2p_x,y doesn’t overlap with MOs in H
2p_z has the same symmetry

Question 5

What is the relation between overlap and ΔE?

Answer 5

ΔE α S²_AB / ΔE

Where S_AB is an overlap integral

From this, orbitals don’t overlap when v different in E

Question 6

What happens across the first row for mol diagrams with H?

Answer 6

Li 2s interacts with H 1s
F 2p interacts with H 1s

B 2p and 2s interacts with H 1s

Question 7

How do you contstruct MO diagrams for AH_n?

Answer 7

Identify symmetry of equivalent H atoms
Generate SALCS on identical atoms
Combine SALCS with orbitals on A, when same symmetry

When more than 2 symmetry types then MO diagrams constructed from orbitals of 2x chemically reasonable fragments

Question 8

What is the reducible representation of a molecule?

Answer 8

How things change under a symmetry operation of a point group
1 for each atom when no movement, 0 for when both change

Question 9

How do you find irrep of a molecule?

Answer 9

Look at point group symmetry groups for the orbitals
Those with corresponding symmetry is the irrep

Question 10

What occurs in photoelectron spectroscopy?

Answer 10

Photon absorbed by e- which is then ejected

KE = hv - IE
so IE can be calculated and count vs IE can be found

Question 11

What is expected from PES?

Answer 11

1 progression of peaks per MO
Progression as explains change in structure during ionisation (Franck-Codon Principle)

Question 12

What is the Franck-Cordon Principle?

Answer 12

Principle explaining PES progression of peaks

Can populate from ground state of original PE curve multiple vibrational state of excited state

Question 13

What is the PES of HF?

Answer 13

Question 14

What are Σ and Π in PES?

Answer 14

Σ - e^- removed from s orbital
Π - e^- removed from p orbital
² - doublet so 1 unpaired e-

Question 14

What are Σ and Π in PES?

Answer 14

Σ - e^- removed from s orbital
Π - e^- removed from p orbital
² - doublet so 1 unpaired e-

Question 15

What do progression types in PES describe?
(examples)

Answer 15

1st peak most intense then decays -> v.little change in structure when ionised

2 peaks of similar intensity - bonding in orbital, elongation occured

1 peak - non bonding

Question 16

What is Walsh’s principle?

Answer 16

Molecule adopts structure which stabilises HOMO
So maximises HOMO-LUMO gap

Question 17

What is the difference between 1st and 2nd order effects?

Answer 17

1st orders are one which can be predicted by the change in shape

Question 18

What are 2nd order effects in Walsh diagrams?

Answer 18

Bending allows for symmetries to change and so mixing occurs

π gains s character -> stabilised
σ gains p character -> destabilised

Only relevant if ΔE not large between two orbitals

Question 19

What does the broadness and space between progressions in PES?

Answer 19

Indicates difference in vibrational states in ionised states

Question 20

What is the 1st order theorem?

Answer 20

Nnon-linear molecule in orbitally degen state will distort to remove degen

Question 21

What are the substituents in hypervalent molecules?

Answer 21

ΔE between HOMO & LUMO determines stability

To max ΔE then requires highly electronegative atom as a ligand

In resonance form then more electronegative atom required to stabilise -ve charge

Question 22

What is the Wade Mingos rule?

Answer 22

In a borane cluster:
closo - full borane cluster
nido - removes 1 vertex from closo
arachno - removes 2 vertex from closo

Question 23

What is the equation for skeletal electron count in borane clusters?

Answer 23

SEC = Total valence electron count - 2n
Where n = number of vertex atoms

TVEC is number of B x 4 and + any charge

Question 24

What is the skeletal electron counts in different borane clusters?

Answer 24

Closo: SEC = 2n+2
Nido: SEC = 2n+4
Arachno: SEC = 2n+6

Question 25

What are isolobal analogies?

Answer 25

Used to rationalize structures of closely related species

Question 26

What are isolobal analogies of borane clusters?

Answer 26

When remove BH²⁺ it removes 3 orbitals
Can replace with any isoelectronic species - e.g. CH³⁺

Question 27

What are the characteristics of isolobal fragments?

Answer 27

Same # of frontier orbitals
Fronteir orbitals have same symmetry
Same # of e- in these fronteir orbitals
Fronteir orbitals of similar energy

lp on each atom plays same role as B-H

Question 27

What are the characteristics of isolobal fragments?

Answer 27

Same # of frontier orbitals
Fronteir orbitals have same symmetry
Same # of e- in these fronteir orbitals
Fronteir orbitals of similar energy

Question 28

What are the orbital energies in TMs?

Answer 28

nd < (n+1)s < (n+1)p

Question 29

What is the radial distribution function of a lanthanide?

Answer 29

Principle number of valence orbitals is 1 less than principle quantum number of core orbitals

Closer to nucleus than core

Question 30

How does bonding change from O -> S -> Se?

Answer 30

Bond strength decreases as poorer overlap down group

Question 31

How does bonding change from Cr -> Mo -> W?

Answer 31

Better overlap down group so bond strength increases
e- spends more time in core down the group

Question 32

What is the pairing energy in main group and TMs?

Answer 32

1st row TM -> large pairing energy as 3d has no radial nodes so e- in same orbital closer and repel more
2nd/3rd row -> smaller as 4d has 1 radial node, and 5d has 2 radial nodes, both of which reduces e-e- repulsion