Title: Qualitative introduction to quantum theory
1Qualitative introduction to quantum theory
BS1030, Chemistry for Life Scientists November
25, 2008
- Prof. Judith Klein-Seetharaman
- School of Biological Sciences
- j.klein-seetharaman_at_rhul.ac.uk
2From last lecture
3Schroedinger Equation
- Born interpretation gives spatial distribution of
electron - Quantum numbers
- Principal Quantum Number, n, (1,2,3,)
- Orbital Angular Momentum Quantum Number, l,
(0,1,n-1), (s,p,d,f,g..) - Magnetic Quantum Number, ml , (l,l-1,-l)
- Spin Magnetic Quantum Number, ms , (-1/2,1/2)
H? E?
Atkins Jones, Chapter 1
4Atomic Orbitals
- Electron configuration
- Building up principle (Aufbauprinzip)
Atkins Jones, Chapter 1
5Overview
- Last 2 weeks From classical mechanics to quantum
theory description of atomic structure - Energy is quantized
- Wave-particle duality
- The uncertainty principle
- The Schroedinger equation
- Quantum numbers
- Atomic orbitals
- This week Bonding and shapes of molecules
- Types of bonds
- MO theory
- Next week (Dr. Cloarec) Delocalized electrons
- Resonance structures
- Aromaticity
Atkins Jones Chapter 1
Atkins Jones Chapter 23
6What is a Chemical Bond?
7What will decide if two atoms form a bond?
8Example NaCl
- Na(g) ? Na(g) e-(g)
- energy required 494 kJ/mol (ionization energy)
- Cl(g) e-(g) ? Cl-(g)
- energy released 349 kJ/mol (electron affinity)
- Net change in energy
- energy required-energy released 494-349145
- Gas of Na and Cl is more energetically favourable
than gas of Na and Cl- - ???
9What happens when Na and Cl- come together?
- Na(g) Cl-(g) ? NaCl(s)
- Their mutual attraction releases energy 787
kJ/mol - Na(g) Cl(g) ? NaCl(s)
- Energy required energy released 145-787-642
- ? The attraction between the two opposite charges
provides the driving force for bond formation
10Relevance to Biomedicine
11Enzyme Binding Pocket
-
Nature Reviews Drug Discovery 2 879-890 (2003)
doi10.1038/nrd1225
12Relevance to Biomedicine
http//www.nist.gov/public_affairs/images/06MSEL01
1_BoneCementCellsAttachP_HR.jpg
13Bone is rigid due to ion pairs
PO43-
Ca2
http//humor.plein.nl/files/images/thanksgiving_do
g_gets_bone.jpg
14Types of Bonds
15Types of Bonds
- Ionic bond
- Covalent bond
- Metallic bond
16Covalent Bond Theories
- Lewis Theory (before quantum theory)
- Octet rule atoms share electrons until they
reach a noble-gas configuration
F F ? F F (F-F)
17Covalent Bond Theories
- Lewis Theory (before quantum theory)
- Octet rule atoms share electrons until they
reach a noble-gas configuration
?
F F ? F F (F-F)
18Covalent Bond Theories
- Valence bond theory (quantum based)
s
s
s-bond
19Covalent Bond Theories
- Valence bond theory (quantum based)
s
pz
s-bond
20Covalent Bond Theories
- Valence bond theory (quantum based)
pz
pz
s-bond
21Covalent Bond Theories
- Valence bond theory (quantum based)
px
px
p-bond
22Hybrid orbitals
- VB theory in CH4
- Promote the electron
- sp3 hybrids
- h1spxpypz
- h2s-px-pypz
- h3s-pxpy-pz
- h4spx-py-pz
- Other hybrids
- sp
- sp2
- sp3d2
23The problem.
O O
http//www.superphysics.netfirms.com/t047413a.jpg
24Covalent Bond Theories
- The molecular orbital, ?, is formed by a linear
combination of atomic orbitals (LCAO) and is
called an LCAO-MO. - ? ?A1s ? B1s
- ?A1s is a 1s-orbital centred on atom (A)
- ? B1s is a 1s-orbital centred on atom (B)
-
25Constructive interference bonding MO
? ?A1s ? B1s Between the nuclei the waves
interfere constructively so the total amplitude
increases. In this region the electrons interact
with both nuclei, so there is a consequent
lowering of the energy. ? is a bonding molecular
orbital
26Destructive interference antibonding MO
- ? ?A1s - ? B1s
- Between the nuclei the waves can interfere
destructively so the total amplitude decreases.
There is a nodal surface between the atoms where
the electrons are excluded from the region where
they would have a strong interaction with the
nuclei and hence the energy is higher than in the
atomic orbitals. - ? is an antibonding molecular orbital
27Hydrogen MOs
28Where shall we put the electrons?
29Hydrogen MOs
- Building-up principle
- Start with lowest energy
- Pauli exclusion principle
- Hund rule
30What do the MOs look like?
31What do the MOs look like?
32Li2 to N2 p2p lower than s2p
O2 and F2 p2p higher than s2p
33N2?
F2?
O2?
34N2 configuration s2s2s2s2p2p4s2p2
Atkins Jones
35- F2 s2s2s2s2s2p2 p2p4 p2p4
-
Atkins Jones
36O2 s2s2s2s2s2p2 p2p4 p2p1 p2p1
Atkins Jones
37The problem is solved!
O O
O O
http//www.superphysics.netfirms.com/t047413a.jpg
38Bond Order
- BOnet number of bonds allowing for the
cancellation of bonds by antibonds. - Bond order ½ x (number of electrons in bonding
orbitals the number of electrons in
antibonding orbitals) - BO ½(N N)
39Heteronuclear Diatomic Molecules
Y cAYA cBYB
- The electron is shared unequally between the two
atoms - Born interpretation
40Heteronuclear Diatomic Molecules
Y cAYA cBYB
- The electron is shared unequally between the two
atoms - Born interpretation
- If cA2 is large, then Y looks more like YA, the
electron density is greater near A and vice versa - Scenarios
41Heteronuclear Diatomic Molecules
Y cAYA cBYB
- The electron is shared unequally between the two
atoms - Born interpretation
- If cA2 is large, then Y looks more like YA, the
electron density is greater near A and vice versa - Scenarios
- Nonpolar covalent bond cAcB
- Ionic bond cA0 (or cB0)
- Polar covalent bond cA gt cB
42Types of Bonds
- Ionic bond
- Covalent bond
- Metallic bond
- Polar covalent bond
- Electronegativity
- Partial charges
- Electric dipole moment
- Ionic character
- Hydrogen bond
43Examples in Biomolecules
http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/B/BasePairing.gif
http//tainano.com/Molecular20Biology20Glossary.
files/image006.gif
44Summary
- What is a chemical bond?
- Bonding types
- Ionic
- Covalent
- Metallic
- In between
- Bonding theories
- Lewis
- Valence bond
- Molecular orbital
- Bonding and antibonding orbitals
- Building up principle
- Bond order
- Diatoms