Bonding in Molecules Flashcards
What rules must be taken into account for filling MO diagrams?
Aufbau principle
Pauli exclusion principle
Hund’s rule
All of which underpinned by orbital approx.
When does s-p mixing occur?
Beggining of second period as 2s close in energy to 2p
What is the change in orbitals in sp mixing?
What is the overlap in heteronuclear diatomics?
2px,y doesn’t overlap with MOs in H
2pz has the same symmetry
What is the relation between overlap and ΔE?
ΔE α S2AB / ΔE
Where SAB is an overlap integral
From this, orbitals don’t overlap when v different in E
What happens across the first row for mol diagrams with H?
Li 2s interacts with H 1s
F 2p interacts with H 1s
B 2p and 2s interacts with H 1s
How do you contstruct MO diagrams for AHn?
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
What is the reducible representation of a molecule?
How things change under a symmetry operation of a point group
1 for each atom when no movement, 0 for when both change
How do you find irrep of a molecule?
Look at point group symmetry groups for the orbitals
Those with corresponding symmetry is the irrep
What occurs in photoelectron spectroscopy?
Photon absorbed by e- which is then ejected
KE = hv - IE
so IE can be calculated and count vs IE can be found
What is expected from PES?
1 progression of peaks per MO
Progression as explains change in structure during ionisation (Franck-Codon Principle)
What is the Franck-Cordon Principle?
Principle explaining PES progression of peaks
Can populate from ground state of original PE curve multiple vibrational state of excited state
What is the PES of HF?
What are Σ and Π in PES?
Σ - e- removed from s orbital
Π - e- removed from p orbital
2 - doublet so 1 unpaired e-
What are Σ and Π in PES?
Σ - e- removed from s orbital
Π - e- removed from p orbital
2 - doublet so 1 unpaired e-
What do progression types in PES describe?
(examples)
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
What is Walsh’s principle?
Molecule adopts structure which stabilises HOMO
So maximises HOMO-LUMO gap
What is the difference between 1st and 2nd order effects?
1st orders are one which can be predicted by the change in shape
What are 2nd order effects in Walsh diagrams?
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
What does the broadness and space between progressions in PES?
Indicates difference in vibrational states in ionised states
What is the 1st order theorem?
Nnon-linear molecule in orbitally degen state will distort to remove degen
What are the substituents in hypervalent molecules?
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
What is the Wade Mingos rule?
In a borane cluster:
closo - full borane cluster
nido - removes 1 vertex from closo
arachno - removes 2 vertex from closo
What is the equation for skeletal electron count in borane clusters?
SEC = Total valence electron count - 2n
Where n = number of vertex atoms
TVEC is number of B x 4 and + any charge
What is the skeletal electron counts in different borane clusters?
Closo: SEC = 2n+2
Nido: SEC = 2n+4
Arachno: SEC = 2n+6
What are isolobal analogies?
Used to rationalize structures of closely related species
What are isolobal analogies of borane clusters?
When remove BH2+ it removes 3 orbitals
Can replace with any isoelectronic species - e.g. CH3+
What are the characteristics of isolobal fragments?
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
What are the characteristics of isolobal fragments?
Same # of frontier orbitals
Fronteir orbitals have same symmetry
Same # of e- in these fronteir orbitals
Fronteir orbitals of similar energy
What are the orbital energies in TMs?
nd < (n+1)s < (n+1)p
What is the radial distribution function of a lanthanide?
Principle number of valence orbitals is 1 less than principle quantum number of core orbitals
Closer to nucleus than core
How does bonding change from O -> S -> Se?
Bond strength decreases as poorer overlap down group
How does bonding change from Cr -> Mo -> W?
Better overlap down group so bond strength increases
e- spends more time in core down the group
What is the pairing energy in main group and TMs?
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
