Molecular Orbital Theory Flashcards
Constructive interference
Waves in phase so reinforce each other
Destructive interference
Waves out of phase so cancel out
In phase combination of AO
Positive overlap
Out of phase combination of atomic orbitals
Negative overlap
Consequences of positive overlap
Increase in electron density that’s greatest at midpoint, electron experiences attraction of both nuclei so the electrons stabilised
Consequences of negative overlap
Decrease in electron density giving a node where electron density = 0, electron s destabilised
Calculating bond order
Electrons in BMO - electrons in ABMO / 2
No node when viewed down internuclear axis
σ
1 node when viewed down internuclear axis
π
2 nodes when viewed down internuclear axis
δ
symmetric wrt inversion
G
Antisymmetric wrt inversion
U
*
AMBO
Orthogonal AO
Positive and negative overlap cancel out
Stable BMO/unstable AMBO when
Large AO overlap, AO close in energy and low splitting
Paramagnetic
Has unpaired electrons
diamagnetic
No unpaired electrons
S/P mixing
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
Why is AO energy mismatch not 0 for heterodiatomics
Will have different electronegativities so valence electrons will be at different energies
Describe MO of heterodiatomics
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
Importance of frontier MO and reactivity
Control reactivity, electron in HOMO of Nu- attacks LUMO of E+
Walsh’s rule
Molecule adopts structure which best stabilises HOMO, if HOMO’s unaffected the next nearest occupied atomic orbital determines geometry
Application of Walsh’s rule
Bending of triatomic molecules
How does adding an electron to a NBMO affect bond length and strength
No significant changes
