chem 241 final- 2
What convention is used when discussing bonding in O2 with p orbitals?
By convention, place the nuclei along the z axis.
Why do we place the nuclei along the z axis?
So we can define the relative positions of the two sets of p orbitals from each atom and identify which give σ bonding and which give π bonding.
Which O atomic orbitals are used to construct σ-bonding molecular orbitals in O2?
The 2s and 2pz atomic orbitals on O are used to construct σ-bonding MOs.
Which p orbital is used for σ bonding when the internuclear axis is the z axis?
pz.
Which p orbitals are used for π bonding when the internuclear axis is the z axis?
px and py.
How do 2pz orbitals combine in O2?
They combine to form a 2pz σ-bonding MO and a 2pz* σ-antibonding MO.
How are σ MOs formed from p orbitals?
By end-on overlap along the internuclear axis.
How are π MOs formed from p orbitals?
By sideways overlap of p orbitals perpendicular to the internuclear axis.
What is important about the phases of the p lobes in σ bonding from 2pz orbitals?
Same-phase lobes pointing toward each other combine to make the σ-bonding MO.
What happens in the σ antibonding combination of 2pz orbitals?
Opposite-phase lobes face each other and give a σ-antibonding MO.
In the 2pz antibonding MO, which lobes are smaller?
The opposite-phase lobes pointing toward each other are smaller.
In the 2pz antibonding MO, which lobes are larger?
The opposite-phase lobes pointing away from the nucleus–nucleus bond are larger.
What nodal feature is associated with a p atomic orbital?
One nodal plane.
What nodal feature is associated with π bonding formed from px or py orbitals?
A planar node containing the internuclear axis.
How do px orbitals combine in O2?
They form π-bonding and π* antibonding molecular orbitals by sideways overlap in the x-z plane.
How do py orbitals combine in O2?
They form π-bonding and π* antibonding molecular orbitals in the analogous way, perpendicular to the px set.
What is the electron density pattern in a π-bonding MO?
Electron density is above and below, or on either side of, the internuclear axis, with bonding overlap between the nuclei.
What is the electron density pattern in a π* antibonding MO?
Electron density is mainly outside the nucleus–nucleus bonding region.
What extra nodal feature does a π* antibonding MO have?
Another planar node between the nuclei, in addition to the nodal plane containing the internuclear axis.
What MO diagram result is shown for O2?
The 2s bonding and antibonding MOs are both full, so there is no net 2s σ bonding, and the 2p-derived MOs determine the bonding.
What Lewis structure does the O2 MO diagram agree with?
O=O.
What does MO theory predict for the bonding in O2?
One σ bond and one net π bond, so overall bond order 2.
Why is there one net π bond in O2?
Because the π bonding set is filled, but the π* antibonding set contains two electrons, reducing the net π bond order to 1.
Why is O2 paramagnetic?
Because it has 2 unpaired electrons in the π* orbitals.
Where are the unpaired electrons in O2 located?
In the degenerate π*(2px) and π*(2py) orbitals.
What spin state is ordinary O2 according to the notes?
Triplet O2.
Why is ordinary O2 called triplet O2?
Because it has two unpaired electrons with parallel spins, giving S = 1 and multiplicity 2S + 1 = 3.
What is the spin multiplicity formula?
2S + 1.
What does S represent in the multiplicity formula?
The total spin quantum number, obtained from the sum of the electron spins.
What is the total spin S for triplet O2?
S = 1.
What is the multiplicity of triplet O2?
2(1) + 1 = 3.
What is singlet O2?
An excited form of O2 in which the electron spins are paired overall to give S = 0.
What is the multiplicity of singlet O2?
2(0) + 1 = 1.
Why are triplets lower in energy than singlets in the notes?
Because of spin exchange stabilization energy.
Why is triplet O2 less reactive than singlet O2?
Triplet O2 is lower in energy and spin-restricted, so it is not as reactive as singlet O2.
How can singlet O2 be formed from triplet O2?
Using light and a photocatalyst.
What medical example was given for singlet O2 formation?
Photodynamic cancer therapy.
How does photodynamic cancer therapy work in the notes?
A photocatalyst is added near a tumour, light is applied, and triplet O2 in blood is converted locally into singlet O2 in the tumour region.
Why is singlet O2 useful in photodynamic therapy?
Because it is more reactive and forms locally in the tumour.
Why do humans glow faintly in the visible according to the notes?
Because traces of singlet O2 react with molecules in the body and give off faint amounts of light.
What does σ mean in the summary orbital descriptors?
Rotation of the orbital about the internuclear axis generates no phase change.
What is the key symmetry property of a σ orbital?
If you rotate it around the internuclear axis, the phase does not change.
What does σ* mean?
There is a nodal plane between the nuclei, and the plane is orthogonal to the internuclear axis.
What is the key feature of a σ* orbital?
A nodal plane between the nuclei perpendicular to the atom–atom axis.
What does π mean in the summary orbital descriptors?
Rotation around the internuclear axis gives phase change.
What is the key symmetry property of a π orbital?
If you rotate it 180° around the internuclear axis, the phase changes.
What does π* mean?
Rotation around the internuclear axis gives phase change and there is a nodal plane between the nuclei.
What are the two defining features of a π* orbital?
It has phase change on rotation around the internuclear axis and a nodal plane between the nuclei.
What is meant by “small s-p gaps: σ-π crossover”?
The relative energy levels of the 2pz σ MO and the 2px, 2py π MOs change and depend on the s-p gap.
Across B2 to F2, what changes in the MO diagram?
The relative ordering of σ(2p) and π(2p) changes as Zeff increases.
What happens to the σ 2p MO as Zeff increases?
The σ 2p MO drops below the energy of the π 2p bonding MOs as Zeff increases.
For which molecules in the diagram is π(2p) below σ(2p)?
B2, C2, and N2.
For which molecules in the diagram is σ(2p) below π(2p)?
O2 and F2.
What does MO theory indicate for N2 bond order?
Bond order 3.
What Lewis structure does the N2 MO diagram support?
:N≡N:
Why is N2 extremely stable?
Because MO theory indicates a bond order of 3, consistent with a very strong triple bond.
What biological/environmental point was made from N2 stability?
Much of nature is N-starved because N2 is so stable and hard to react.
What natural processes were mentioned that can react with N2?
Lightning and a few nitrogenase enzymes.
What historical chemistry point was made about early life and N2?
Early life on the planet was based on enzymes and systems that reacted with N2 when very little O2 was present.
What atmospheric change was described after cyanobacteria evolved?
Cyanobacteria produced O2 from water and sunlight.
What reaction was written for cyanobacterial O2 production?
H2O + sunlight → O2 + “H2”
What consequence of rising O2 was mentioned?
O2 is more reactive than N2, and increasing O2 in the early atmosphere destroyed many N2-consuming life forms and converted much Fe(s) into Fe2O3.
How does N2 bond to Fe in the Haber-process discussion?
Through σ donation from a lone pair on N to an empty σ orbital on Fe and π-back donation from filled Fe dπ orbitals into empty 2p π* orbitals on N2.
What Fe acceptor orbital type is mentioned for N2 σ donation?
An empty σ orbital on Fe, described as some lowest-energy combination such as s, pz, or dz2.
Where does the σ donation from N2 come from?
From a lone pair on N.
Where does the π-back donation to N2 go?
Into the empty 2p π* orbital on N2.
What Fe orbital was specifically named for π-back donation to N2?
Filled dxy on Fe.
What is the term used when σ donation and π-back donation occur together?
Synergistic bonding.
Why is the bonding called synergistic?
Because both σ donation and π-back donation occur simultaneously and reinforce the interaction.
How are heteronuclear diatomic molecular orbitals generated?
They are constructed in the same manner as homonuclear diatomics.
What heteronuclear example is used in the notes?
HF.
Why is HF discussed as a real-world example?
It is used to clean or etch Si wafers in making logic chips.
Why is HF useful for etching Si?
Because the Si–F bond is very strong.
What other strong bond was mentioned alongside Si–F?
P–F.
Why is HF dangerous according to the notes?
HF can go through skin and etch bone underneath.
Why are the valence orbital energies on F lower than those on H?
Because the electronegativity, or Zeff, for F is higher than for H.
Why does H 1s not interact significantly with F 2s?
The H 1s valence AO is too high in energy relative to F 2s to interact significantly.
Which F orbital does H 1s overlap with in HF?
F 2pz, when the z axis is taken along the H–F bond.
Why is pz the correct F p orbital for overlap with H 1s in HF?
Because the z axis is along the bond between the atoms, so pz points directly along the H–F bond.
Why is px overlap with H 1s small in HF?
Because px is oriented incorrectly for strong overlap along the bond axis.
Why is pz-s overlap large in HF?
Because both orbitals point along the bond axis and can overlap end-on.
What orbitals form the H–F σ-bonding MO?
H 1s and F 2pz.
What orbitals form the H–F σ* antibonding MO?
H 1s and F 2pz in the out-of-phase combination.
What nodal feature is shown for H–F σ bonding from pz and s overlap?
A planar node inherited from the p orbital character.
What nodal feature is shown for H–F σ* antibonding?
Planar nodes, including the antibonding node between the atoms.
Why do we usually need computations to generate MOs for molecules like HF?
Because detailed MO construction requires computational methods such as density functional theory.
Why does the professor say hybridization is often more useful?
Because it is simpler for routine bonding descriptions, while MO theory is more detailed.
Is HF largely covalent or ionic according to the notes?
Largely covalent.
Why is HF only a weak acid in water according to the notes?
It is a weak source of H+(aq), so Ka is small.
What equilibrium expression was written for HF in water?
HF(aq) ⇌ H+(aq) + F−(aq)
What hydronium note was written under H+ in the HF equilibrium?
H3O+(aq).
How can you quickly remember which p orbitals give σ vs π when the bond axis is z?
pz gives σ bonding; px and py give π bonding.
How can you quickly remember why O2 is paramagnetic?
Two unpaired electrons remain in the π* orbitals.
How can you quickly remember the difference between triplet and singlet O2?
Triplet O2 has two parallel unpaired electrons and is ordinary O2; singlet O2 is a higher-energy reactive form with paired spin overall.
How can you quickly remember the σ-π crossover trend?
As Zeff increases across the row, σ(2p) drops below π(2p).
How can you quickly remember the bonding of N2 to Fe?
N2 donates a lone pair to Fe by σ donation, and Fe back-donates from filled d orbitals into N2 π*; together this is synergistic bonding.
How can you quickly remember orbital descriptors?
σ = no phase change on rotation, σ* = σ with a node between nuclei, π = phase change on rotation, π* = π plus a node between nuclei.