PART 4 – Molecular Orbital Method of Chemical Bonding, and ...
Transcript of PART 4 – Molecular Orbital Method of Chemical Bonding, and ...
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PART 4 – Molecular Orbital Method of Chemical Bonding, and Molecular
Interactions
Reference: Chapter 10 in textbook
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Molecular Orbital (MO) Method
MO method Molecular orbitals are formed by combination of atomic
orbitals of bonding atoms;
# of total MOs = # of total atomic orbitals;
All e’s belong to the molecule, not original atoms.
Electrons are filled based on the same criteria as in
atomic orbitals: (i) Pauli exclusion principle; (ii) Filled
from the lowest energy first, (Ground State).
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Molecular Orbital (MO) Method
Bonding, Antibonding, Nonbonding MOs Bonding MO: w/ energy lower than origianl AO;
Antibonding MO: w/ energy higher than original AO;
Nonbonding MO: w/ energy equal to orignal AO;
# of bonding MO = # of antibonding MO.
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Homonuclear Diatomic Molecules Bond Order = 0.5 * (# bonding e – # antibonding e)
Larger bond order ↔ More stable molecule
When bond order = 0, it means this molecule cannot exist.
Example: 1st period elements
Q: Explain (1) why H2-, He2
+ ions can exist? (2) What is the bond order of these ions? 4
H H2 H He He2 He
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MO of Li2 and Be2
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Ground State Electron Configuration in MO Li2 :(σ1s)2(σ1s
*)2 (σ2s)2 Be2 :(σ1s)2 (σ1s*)2 (σ2s)2 (σ2s
*)2
Li2 : KK(σ2s)2 Be2 : KK(σ2s)2 (σ2s*)2
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2nd Period Homonuclear Molecules
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Molecules Electron configuration Bond order Bond length Bond energy (pm) (kJ⋅mol-1)
Li2 KK(σ2s)2 1 267 108
Be2 KK(σ2s)2 (σ2s*)2 0 245 9
B2 KK (σ2s)2 (σ2s*)2 (π2p)2 1 159 289
C2 KK (σ2s)2 (σ2s*)2 (π2p)4 2 124 599
N2 KK (σ2s)2 (σ2s*)2 (π2p)4( σ 2p)2 3 110 942
O2 KK (σ2s)2 (σ2s*)2 ( σ 2p)2 (π2p)4 (π2p
*)2 2 121 494
F2 KK (σ2s)2 (σ2s*)2 ( σ 2p)2 (π2p)4 (π2p
*)4 1 141 154
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MO Energy
Q: Among the molecules above, (1) Which one is the most stable? (2) Which ones have paramagnetism? 7
Energy crossing between σ2p and π2p MOs
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Heteronuclear Diatomic Molecules
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CO (ground state):
KK(σ2s)2 (σ2s*)2 (π2p)4(σ 2p)2
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Heteronuclear Diatomic Molecules
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NO (ground state):
KK (σ2s)2(σ2s*)2 (σ 2p)2 (π2p)4 (π2p
*)1
N NO O
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Heteronuclear Diatomic Molecules
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LiF (ground state):
(1sLi)2 (1sF)2 (2sF)2 (σ )2 (πnb)4
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Delocalization of Electrons
Conjugated double bond: switching between single-bond and double bond.
π-bond electrons in the conjugated system are delocalized.
Example: C6H6 (benzene)
CH2=CH-CH=CH2
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Conjugated π Bond Conjugated π bond molecules/ions:
All conjugated atoms are in a same plane, each giving
a p-orbital to form the conjugated π bond;
Total π electron numbers < 2 * p-orbitals
Πnm (formed by n p-orbitals and m p-electrons)
m=n: Most of the organic conjugated molecules;
m>n: double bond next to an atom with lone pair e
(e.g. N, O, Cl, S)
m<n: cations, e.g. (CH2=CH-CH3)+, (C6H5)3C+
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Molecular Orbital Method Textbook, page 386, 10.90:
For the compound H2C=CHCOOCH3, (a) draw a complete structure; (b) How many pi-bonds and how many sigma bonds are in
this molecule? (c) What hybridizations are applied for each carbon? (d) How many electrons are de-localized? (e) How many atoms lie in the same plane? (f) What are the bond angles between each bond?
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HOMO and LUMO HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital
HOMO & LOMO are very useful in predicting chemical reactivity, optical/electrical properties. 14
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Band Theory of Bonding in Solids
Changing from “Bond” to “Band” In solid where huge number of atoms exist (& closely
pack), discrete MOs become continuous bands.
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Band Structures
Valence Band (Bonding orbitals in MO) Conduction Band (Nonbonding orbitals in MO) Band gap (Eg) Fermi energy (in metal)
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SUN
• Photoelectrochemical (PEC) cells are solar cells which generate electrical energy from light. Each cell consists of a semiconducting photoanode and a metal counter electrode immersed in an electrolyte.
• These semiconductors can drive the hydrogen and oxygen evolution using electrons/holes generated under solar illumination.
Photoelectrochemical Water Splitting
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van der Waals Interaction Weak interaction: 10-100 times lower
energy than chemical bonds (2-20 kJ/mol) Dipole-dipole interaction
Dipole-induced dipole interaction
Induced dipole-induced dipole
Higher MW (or larger molecule) higher induced-induced interaction
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van der Waals Interaction
Energy: Dipole-diple > Dipole-Induced > Induced-Induced 19
Polar Polar & Nonpolar Nonpolar
Dipole-Dipole √ × ×
Dipole-Induced √ √ ×
Induced-Induced √ √ √
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Hydrogen Bond
Hydrogen Bond consisting of a hydrogen atom between two high
electronegative atoms (e.g., N, O, F), with one side
being a covalent bond and the other being an
electronic interaction
Bond energy: Chemical bond (100—600 kJ/mol) Hydrogen bond (20—40 kJ/mol) van der Waals interaction (2-20 kJ/mol)
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Hydrogen Bond
Examples: Intermolecule Hydrogen bond
e.g. H2O, NH3, HF,
Intramolecule Hydrogen bond e.g. DNA, proteins
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Hydrophobic & Hydrophilic Groups
Hydrophobic (oil-like) Only containing non-polar or weakly polar groups,
e.g. nonpolar molecules, CH chains/rings, …
Hydrophilic (water-like) Containing highly polar group(s), e.g. –OH, –NH2,
–COOH, –X (X= F, Cl, Br, I), –SH, … 22
fat molecule
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Protein Structure – Amino Acids
Amino Acids Amino acids have a –
COOH and a –NH2 in
the same C atom;
Isoelectric point (pI)
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Protein Structure – Peptides
Amino Acids Peptides A peptide bond is formed by linked a –COOH in one
amino acid and a –NH2 in another amino acid.
Recall: What is “peptide plane”? Why it is formed? 24
Peptide plane
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Introduction of Proteins
Amino Acids Peptides Proteins A protein is formed of multiple peptide chains, via multiple
interactions including hydrogen bond, hydrophobicity, etc.
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Protein α-helix
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Oligonucleotide & DNA Structures Oligonucleotide:
including: phosphate group + sugar ring + base group
DNA: formed by oligonucleotide linkage & interaction
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Types of Crystals
Ionic Crystal Formed between an active metal and an active
nonmetal elements
e.g NaCl: Na Na+ + e-, Cl + e- Cl-
Usually, high melting temperature, high boiling
temperature; Hard.
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Types of Crystals Molecular Crystal
Formed between one or more nonmetal elements.
e.g. H2, HCl, CO2, Ar, H2O …
Low melting temperature, low boiling temperature; soft.
Melting, boiling temperatures depend on their
intermolecule interaction (e.g. van de Waals’ interaction,
hydrogen bond)
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Types of Crystals
Atomic Crystal Formed between one or more IVA group elements, or
with an IVA element and oxygen.
e.g. C, Si, SiC, SiO2 …
Extremely high melting and boiling temperature; Very
hard.
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Unit Cell
An important concept in describing crystal structures.
Reproduces the whole when stacked together
repeatedly
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The Close Packing of Spheres
Two Major Types of Close Packing
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Two Major Types of Close Packing
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Hexagonal Close Packing (hcp) Pattern: ABABAB… Number of balls in a unit cell = ?
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Cubic Close Packing (ccp) Also called face centered close packing (fcc) Pattern: ABCABCABC… Number of balls in a unit cell = ?
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NaCl (MX) structure
Net 4 Oh sites/unit cell
4 atoms per unit cell
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ZnS structure
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CsCl structure CaF2 (MX2) structure
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Relationship between structures