week 3 Flashcards
what are molecular orbitals made of
atomic orbitals
adding wavefunctions of AOs across the molecule
number of AOs = …
number of MOs
what 2 things must AOs fulfil to form MOs
AOs must match interns of symmetry
AOs must be close enough in energy to overlap
- the closer in energy, the larger the interaction
AOs must be of similar energies
in which two ways can AOs overlap
constructively/in-phase
- peaks and troughs of wave line up perfectly
- leads to a wave that is the sum of both waves
destructively/out-of-phase
- waves are oppositely aligned
- positive peaks of one coincide with negative peaks of the other
- waves cancel out, leads to a node
see w3 vid13 for examples
what does a sigma orbital suggest about an orbital
sigma means the orbital is symmetric about the internuclear axis
what is used to denote an anti-bonding orbital
star - *
describe the orbital formed wen two in-phase 1s AOs overlap
a sigma gerade orbital
- sigma bc symmetric about internuclear axis
- gerade because symmetric wrt inversion centre
what is a nodal plane in an orbital
region where there is 0 probability of finding electron density
what is more energetically stable, bonding or anti-bonding MOs
bonding MOs
what does bond order indicate
how is it calculated
overall strength of bonding interaction
= 1/2(no. of bonding e⁻- no. of anti-bonding e⁻)
describe the in phase and out of phase overlap of the 2pz orbital
2pz orbitals will overlap in a horizontal manner with the heads of the lobes overlapping into each other ∞↔︎∞like that
in phase
- sigma gerade orbital σg
out of phase
- sigma ungerade σu*
describe the in-phase and out of phase overlap of the 2px/y orbitals
why are they π orbitals
px and py form π orbitals because
- the resulting MOs change sign when rotated 180° across internuclear axis
- pi-orbitals are anti-symmetric wrt rotation around internuclear axis
they overlap like this = 8↔︎8 with the lobes vertical
in phase
- lobes at the top and at the bottom are the same sign
- forms πu orbital
out of phase
- pi gerade anti bonding orbital πg*
what does increased overlap between AOs mean for the energy of MOs
more overlap between AOs = bigger energy separation between bonding MO and anti-bonding MO
which have more overlap, pz or px/py
pz have greater overlap
so larger energy separation between bonding and anti-bonding MOs
see w3 vid14 for examples fo MO diagrams
what is sp mixing
when MOs of the same symmetry label interact with each other