chapter 10 p2 Flashcards
what does molecular orbital theory apply to?
Applies quantum mechanics to molecules, similar to
how it was applied to atoms
how does VB picture molecules?
VB pictures a molecule as a group of atoms bonded
through localized overlap of valence or hybrid
atomic orbitals
what does MO assume about orbitals?
MO assumes that orbitals are delocalized over the
whole molecule and occupied by electrons
how are MOS generated
You cannot solve Schroedinger’s equation exactly for
molecular orbitals
• Through a series of trial and errors we start with an
educated guess (trial function) and a computer program
tries to find the function that minimizes the energy of the
molecules orbitals
• MO’s can be mathematically approximated by the linear
combination of atomic orbitals (LCAO)
what are MOs similarities as AO?
Similar characteristics as AO
!Each can hold two electrons
!The square of the wavefunction gives the probability
density (shape)
what happens when two waves align and are in phase?
When two waves align and are in phase, the resulting wave is sum of the two waves !Constructive interference
what happens when two waves align out of phase?
• When two waves align out of phase, the resulting wave is the difference of the two wave !Destructive interference
how many phases do s orbitals have?
• s-orbitals only have
one phase
!Either (+) or (-)
how many phases do p oribtals have?
p-orbitals have two phases !(+) and (-) !Each lobe is a different phase
what do we use to approximate molecular orbitals?
We use a linear combination of atomic orbitals (LCAO) to
approximate molecular orbitals - combine atomic
orbitals of similar energy
when is a bonding oribtal formed? energy?
When the wave functions overlap constructively they
form a bonding orbital
• They are lower in energy than the original atomic orbitals
constructive interference
what happens when wave functions overlap destructively?
• When wave functions overlap destructively they form
anti-bonding orbitals
! A node is created between two atoms (no electrons)
! Energy of the orbital is higher than that of the original
atomic orbitals
look at mo diagram. what do the number of AOs equal?
The number of AOs combined always equals the
number of MOs formed
what is bond order?
Bond Order = difference between number of
electrons in bonding and antibonding orbitals
which electrons are considered for bond order? what does a higher bond order mean? bond order of 0?
- only need to consider valence electrons
- higher bond order = stronger and shorter bonds
- if bond order = 0, then bond is unstable compared to
individual atoms and no bond will form
formula for bond order
Bond Order = (# bonding electrons) - (# antibonding electrons) divided by
2
BO of He2? does it exist?
= 0
how many p orbitals can overlap end on end?
one, the other two cant
interactions of p orbitals: constructive end to end
The constructive end-to-end overlap of p-orbitals results
in the formation of a bonding σ2p MO
interactions of p orbitals: destructive end to end
The destructive end-to-end overlap of p-orbitals results in
the formation of an antibonding σ*
2p MO
interactions of p orbitals: constructive side by side
Constructive side-by-side
overlap of p-orbitals results in
the formation of π2p bonding
orbitals
interactions of p orbitals: destructive side by side
• Destructive side-by-side overlap
of p-orbitals results in the
formation of π*2p bonding
orbitals
energy of MOs
• MOs formed from 2s orbitals are lower in energy than MOs from from 2p orbitals !s orbitals are lower in energy than p orbitals
which orbitals overlap more extensively
Atomic p orbitals overlap
more extensively end to end
than side by side
theta vs pi energy
!σ2p MO is usually lower in energy than π2p !Destabilizing effect of σ2p* MO is greater than π2p *
look at diagrams for energy
ok
mixing assumption
We might assume that the 2s and 2p do not mix when
we combine AO to form MO
• True LCAO treatment would combine all AO that are
relatively close in energy
which elements have mixing?
leads to?
It occurs in some elements (B2,C2,N2), mixing between
the 2s and 2p (σ bond)
• Larger, less electronegative atoms have σ2s & σ2p
interaction
• Leads to reversal in the σ2p and π2p orbitals
diagram with heteronuclear molecules?
Asymmetric MO diagram due to different
electronegativities
• Element with higher electronegativity is lower in energy
• If the molecule contains O or F, no s-p mixing occurs
is oxygen magnetic? how do we know? know diamagnetic and paramagnetic
• Molecular oxygen is magnetic (O2)
Based on VB theory we have paired up all of the
electrons
• But if we look at the MO diagram we see there are two
unpaired electrons
similarities between vb and mo
• In most cases involving σ bonding, MO theory produces
results very similar to the predictions in VB theory
• VB theory is simpler to apply to large molecules
• The failures of the valence bond theory usually involve π
bonding
!If you need to use resonance when drawing the Lewis
structure, valence bond theory will fail
• It is common to see MO theory applied to only π bonds
in qualitative descriptions of bonding in many molecules
ex benzene
• Valence bond theory does a good job of describing the sigma bonding in a molecule of benzene • MO theory is needed to properly describe the π bonds !All of the C-C bonds in benzene are equivalent
chart for bond angles and shapes
Electron groups Molecular geometries Angle (degrees) Hybrid orbitals 2 linear 180 sp 3 trigonal planar, bent 120 sp2 4 tetrahedral, trigonal pyramidal, bent 109.5 sp3 5 trigonal bipyramidal, seesaw, T-shaped 90, 120, 180 sp3d 6 octahedral, square pyramidal, square planar 90, 180 sp3d2
what combo gives us linear electron shape? molecular geometry? angles?
electron group 2, bonding groups 2, lone pairs 0
- linear
- 180
what combo gives us trigonal planar electron shape? molecular geometry? angles?
either EG 3, BG 3, LP O or EG 3, BG 2, LP 1
330 is trigonal planar and 120
321 is bent and <120
3 combos for tetrahedral electron shape
440, 431, 422
-440 is tetrahedral, 109.5
-431 is trigonal pyramidal, <109.5
422 is bent, <109.5
4 combos for trigonal bipyramidal electron shape
550, 541, 532, 523
- 550 trigonal bipyramidal 120 equatorial, 90 axial
- 541 seesaw <120 (equatorial) <90 axial
- 532 t shaped <90
- 523 linear 180
3 combos for octahedral electron group
- 660 octahedral 90
- 651 square pyramidal <90
- 642 square planar 90
describe MO diagram with mixing
sigma star
pi star
sigma
pi
describe mo diagram without mixing
sigma star
pi star
pi
sigma
