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Chemistry 2
Lecture 1
Quantum Mechanics in Chemistry
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Your lecturers
12pm Assoc. Prof. Adam J Bridgeman
Room 222
93512731
8am Assoc. Prof Timothy Schmidt
Room 315
93512781
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Learning outcomes
•Be able to recognize a valid wavefunction in terms of its being single valued, continuous, and differentiable (where potential is).
•Be able to recognize the Schrödinger equation. Recognize that atomic orbitals are solutions to this equation which are exact for Hydrogen.
•Apply the knowledge that solutions to the Schrödinger equation are the “observable energy levels” of a molecule.
•Use the principle that the mixing between orbitals depends on the energy difference, and the resonance integral.
•Rationalize differences in orbital energy levels of diatomic molecules in terms of s-p mixing.
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How quantum particles behave
• Uncertainty principle: DxDp >
• Wave-particle duality
• Need a representation for a quantum particle: the wavefunction (ψ)
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How quantum particles behave
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The Wavefunction
• The quantum particle is described by a function – the wavefunction – which may be interpreted as a indicative of probability density
• P(x) = Ψ(x)2
• All the information about the particle is contained in the wavefunction
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The Wavefunction
Must be single valued Must be continuous Should be differentiable (where potential is)
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The Schrödinger equation
• Observable energy levels of a quantum
particle obey this equation
Energy
wavefunction
Hamiltonian operator
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Hydrogen atom
• You already know the solutions to the Schrödinger equation!
• For hydrogen, they are orbitals…
1s
2s
2p
3s
3p No nodes
1 node 2 nodes
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Hydrogen energy levels
1s
2s 2p
3s
No nodes
1 node
2 nodes
3p
2
2
n
ZEn
energy level
Rydberg constant
Principal quantum number
Nuclear charge
(also 3d orbitals…)
E
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1s
2s 2p
3s 3p
4s 4p
ionization potential
UV (can’t see)
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Solutions to Schrödinger equation
• Hamiltonian operates on wavefunction and gets function back, multiplied by energy.
• More nodes, more energy (1s, 2s &c..)
• Solutions are observable energy levels.
• Lowest energy solution is ground state.
• Others are excited states.
H
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Solutions to Schrödinger equation
Exactly soluble for various model problems
• You know the hydrogen atom (orbitals)
In these lectures we will deal with
• Particle-in-a-box
• Harmonic Oscillator
• Particle-on-a-ring
But first, let’s look at approximate solutions.
H
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Approximate solutions to Schrödinger equation
• For molecules, we solve the problem approximately.
• We use known solutions to the Schrödinger equation to guide our construction of approximate solutions.
• The simplest problem to solve is H2+
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Revision – H2+
• Near each nucleus, electron should behave as a 1s electron.
• At dissociation, 1s orbital will be exact solution at each nucleus
r
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Revision – H2+
• At equilibrium, we have to make the lowest energy possible using the 1s functions available
r
r ?
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Revision – H2+
1sA 1sB
1sA 1sB
= 1sA – 1sB
1sA 1sB
E
bonding
anti-bonding
= 1sA + 1sB
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Revision – H2
1sA 1sB
1sA 1sB
= 1sA – 1sB
1sA 1sB
E
bonding
anti-bonding
= 1sA + 1sB
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Revision – He2
1sA 1sB
1sA 1sB
= 1sA – 1sB
1sA 1sB
E
bonding
anti-bonding
= 1sA + 1sB
NOT BOUND!!
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2nd row homonuclear diatomics
• Now what do we do? So many orbitals!
1s 1s
2s 2s
2p 2p
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Interacting orbitals Orbitals can interact and combine to make new approximate solutions to the Schrödinger equation. There are two considerations:
1. Orbitals interact inversely proportionally to their energy difference. Orbitals of the same energy interact completely, yielding completely mixed linear combinations.
2. The extent of orbital mixing is given by the integral
dH 12ˆ
orbital wavefunctions
integral over all space
total energy operator (Hamiltonian)
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Interacting orbitals 1. Orbitals interact inversely proportionally to their energy
difference. Orbitals of the same energy interact completely, yielding completely mixed linear combinations.
1s 1s
2s 2s
2p 2p
( )sBsA 222
1
( )sBsA 222
1
sA2 sB2
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Interacting orbitals 1. The extent of orbital mixing is given by the integral
somethingˆ12 dH
1s 1s
2s 2s
2p 2p
s2 p2
The 2s orbital on one atom can interact with the 2p from the other atom, but since they have different energies this is a smaller interaction than the 2s-2s interaction. We will deal
with this later.
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Interacting orbitals 1. The extent of orbital mixing is given by the integral
0ˆ12 dH
1s 1s
2s 2s
2p 2p
s2 p2
cancels
There is no net interaction between these orbitals. The positive-positive term is cancelled by the positive-negative term
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(First year) MO diagram Orbitals interact most with the corresponding orbital on the other atom
to make perfectly mixed linear combinations. (we ignore core).
2s 2s
2p 2p
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Molecular Orbital Theory - Revision
•Molecular orbitals may be classified according to their symmetry
•Looking end-on at a diatomic molecule, a molecular orbital may
resemble an s-orbital, or a p-orbital.
•Those without a node in the plane containing both nuclei resemble an
s-orbital and are denoted -orbitals.
•Those with a node in the plane containing both nuclei resemble a p-
orbital and are denoted -orbitals.
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Molecular Orbital Theory - Revision
•Molecular orbitals may be classified according to their contribution to
bonding
•Those without a node between the nuclei resemble are bonding.
•Those with a node between the nuclei resemble are anti-bonding,
denoted with an asterisk, e.g. *.
* *
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Molecular Orbital Theory - Revision
• Can predict bond strengths qualitatively
N2 Bond Order = 3
*2 p
*2 p
p2
p2
s2
*2 s
diamagnetic
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More refined MO diagram orbitals can now interact
*2 p
*2 p
p2
p2
s2
*2 s
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More refined MO diagram * orbitals can interact
*2 p
*2 p
p2
p2
s2
*2 s
*
*
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More refined MO diagram orbitals do not interact
*2 p
*2 p
p2
p2
s2
*2 s
*
*
*
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More refined MO diagram sp mixing
*2 p
*2 p
p2
p2
s2
*2 s
*
*
*
This new interaction energy Depends on the energy spacing between the 2s and the 2p
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sp mixing
2
2
n
ZEn
c.f.
2p
2s
Smallest energy gap, and thus largest mixing between 2s and 2p is for Boron.
Largest energy gap, and thus smallest mixing between 2s and 2p is for Fluorine.
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sp mixing
*
*
*
Be2
*
*
*
B2
*
*
*
C2
*
*
*
N2
weakly bound paramagnetic diamagnetic
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sp mixing in N2
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sp mixing in N2
-bonding orbital Linear combinations of lone pairs
:N≡N:
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Summary • A valid wavefunction is single valued, continuous, and
differentiable (where potential is).
• Solving the Schrödinger equation gives the observable energy levels and the wavefunctions describing a sysyem:
• Atomic orbitals are solutions to the Schrödinger equation for atoms and are exact for hydrogen.
• Molecular orbitals are solutions to the Schrödinger equation for molecules and are exact for H2
+.
• Molecular orbitals are made by combining atomic orbitals and are labelled by their symmetry with σ and π used for diatomics
• Mixing between atomic orbitals depends on the energy difference, and the resonance integral.
• The orbital energy levels of diatomic molecules are affected by the extent of s-p mixing.
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Next lecture
• Particle in a box approximation
– solving the Schrödinger equation.
Week 10 tutorials
• Particle in a box approximation
– you solve the Schrödinger equation.
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Practice Questions 1. Which of the wavefunctions (a) – (d) is
acceptable as a solution to the Schrödinger
equation?
2. Why is s-p mixing more important in Li2
than in F2?
3. The ionization energy of NO is 9.25 eV
and corresponds to removal of an
antibonding electron
(a) Why does ionization of an antibonding
electron require energy?
(b) Predict the effect of ionization on the
bond length and vibrational frequency
of NO
(c) The ionization energies of N2, NO, CO
and O2 are 15.6, 9.25, 14.0 and 12.1
eV respectively. Explain.