Post on 21-Jan-2016
Intermolecular InteractionsIntermolecular Interactions
Covalent Bond EnergiesCovalent Bond Energies
C-O bond 81 kcal/mol 1.43 Å
C-C bond 86 kcal/mol 1.54 Å
C-H bond 103 kcal/mol 1.11 Å
C=C bond 143 kcal/mol 1.33 Å
C=O bond 165 kcal/mol 1.21 Å
Compared to most non-covalent interactions these are:
• Very high energies
• Very short distances
• Highly dependant on orientation
Intermolecular InteractionsIntermolecular InteractionsDriving Forces for the Formation of Supramolecular Structures
hydrophobic interaction <10 kcal/mol
electrostatic interaction ~5 kcal/mol
hydrogen bond interaction 2-10 kcal/mol
aromatic interaction 0-10 kcal/mol
van der Waals interaction 0.1-1 kcal/mol
The total intermolecular force acting between two molecules
is the sum of all the forces they exert on each other.
Covalent bonding
Sharing of electrons to achieve stable electron configuration – Small difference in electronegativity of elements
– Bond energy – 50-100 kcal/mol
– Directional bond; between specific atoms in a specific direction, normally along the line connecting the two atoms that share a pair of electrons.
Atomic Orbitals of Carbon
d-orbitals f-orbitals
sp3 hybridization and bond directionality
Shown together (large lobes only)
sp3
sp3
sp3
sp3
109.5o
Hybridizing s and three p orbitals form 4 identical sp3 orbitals
C
sp3 hybridization of carbon orbitals
sp2 hybridization of carbon orbitals
sp hybridization of carbon orbitals
Can be a very strong bond - even stronger then covalent bonds in some cases.
Can be an attractive or a repulsive force.
Non-directional force
Long range (1/r)
Highly dependant on the dielectric constant of the medium
+ -
A.A. Ion–Ion InteractionIon–Ion Interaction
A.A. Ion–Ion InteractionIon–Ion Interaction
Energy = (k . z1 . z2 . e2) / (r12)
k = 1 / 4πo= Coulomb constant = 9 .109 N.m2/C2
e = elementary charge = 1.6 .10-19C
= dielectric constant
r12 = distance between the charges
The energy of an ion-ion interaction only decreases at a rate proportional to 1 / r. Therefore these are very long range forces.
When designing a host / guest complex, what will be the energetic incentive for bringing two oppositely charged species to a distance of 3 nm of one another in water?
Energy = (k . z1 . z2 . e2) / (r12)
= 9 .109 . 1 . (-1) . (1.6 .10-19)2 / 78.5 . 3 . 10-9
= -2.3 . 10 -28 / 2.4 . 10 -7
= -9.8 . 10-22 J= -0.14 kcal/mol
A.A. Ion–Ion InteractionIon–Ion Interaction
1 nm?
1 nm in Chloroform?
Energy = (k . z1 . z2 . e2) / (r12)
= 9 .109 . 1 . (-1) . (1.6 .10-19)2 / 78.5 . 1 . 10-9
= -2.3 . 10 -28 / 0.8 . 10 -7
= -29.4 . 10-22 J= -0.42 kcal/mol
= 9 .109 . 1 . (-1) . (1.6 .10-19)2 / 4.8 . 1 . 10-9
= -2.3 . 10 -28 / 4.8 . 10-9
= -4.79 . 10-20 J= -6.89 kcal/mol 8 % of a C-C bond
A.A. Ion–Ion InteractionIon–Ion Interaction
B.B. Ion-Dipole InteractionIon-Dipole Interaction
Non-directional forces
Can be attractive or repulsive
Medium range interactions (1/r2)
Significantly weaker then ion-ion interactions
O
Na+
Example: crown ether complex with alkali metal ions
Energy = -(k . Q . u . cos/ r2)
Maximum when = 0 or 180 degreesZero when = 90 degrees
u = q . l
u = dipole momentl = length of the dipoleq = partial charge on dipoler = distance from charge to center of dipoleQ = charge on ion
O
Na+
B.B. Ion-Dipole InteractionIon-Dipole Interaction
Energy = -(k . Q . u . cos/ r2) If = zero= -k . Q . u / r2
= -9 .109 . 1.6 .10-19 . 2.9 . 3.336 .10-30 / r2
= -1.39 . 10-38 / 4.8 . (10-9)2
= -2.9 . 10-21 J= -0.42 kcal/mol
Example: Acetone pointing directly at Na+ ion ( = zero) at a distance of 1 nm (in chloroform)
B.B. Ion-Dipole InteractionIon-Dipole Interaction
Intermolecular InteractionsIntermolecular Interactions interactions
– stacking (0 – 10 kcal/mol). Weak electrostatic interaction
between aromatic rings. There are two general types: face-to-face
and edge-to-face:
Face-to-face -stacking
interactions are responsible
for the slippery feel of
graphite. Similar -stacking
interactions help stabilize
DNA double helix.
Intermolecular InteractionsIntermolecular Interactions interactions
Intermolecular InteractionsIntermolecular Interactions interactions
Distribution of electron density in benzene molecule
Edge-to-face
Intermolecular InteractionsIntermolecular Interactions Stacking
H
HH
H
H H
H
H
- -+
Offset, face-to-face
HH
HH
HH
HH
Face-to-face, not favorable
Intermolecular InteractionsIntermolecular Interactions interactions