1. Intermolecular Interactions

2. Covalent 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

3. Intermolecular Interactions
Driving Forces for the Formation of Supramolecular Structures
 hydrophobic interaction <10 kcal/mol
 electrostatic interaction ~5 kcal/mol
 hydrogen bond interaction kcal/mol
 p-p aromatic interaction kcal/mol
 van der Waals interaction kcal/mol
The total intermolecular force acting between two molecules is the sum of all the forces they exert on each other.

4. Covalent bonding
Sharing of electrons to achieve stable electron configuration
Small difference in electronegativity of elements
Bond energy – 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.

5. Atomic Orbitals of Carbon

6. d-orbitals
f-orbitals

7. C sp3 hybridization and bond directionality
Shown together (large lobes only)
sp3
109.5o
Hybridizing s and three p orbitals form 4 identical sp3 orbitals C

8. sp3 hybridization of carbon orbitals

9. sp2 hybridization of carbon orbitals

10. sp hybridization of carbon orbitals

11. A. Ion–Ion Interaction + -
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

12. A. Ion–Ion Interaction Energy = (k . z1 . z2 . e2) / (.r12)
k = 1 / 4πo= Coulomb constant = N.m2/C2
e = elementary charge = C
 = 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.

13. A. Ion–Ion Interaction
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)
= (-1) . ( )2 /
= /
= J
= kcal/mol

14. A. Ion–Ion Interaction 1 nm? Energy = (k . z1 . z2 . e2) / (.r12)
= (-1) . ( )2 /
= /
= J
= kcal/mol
1 nm in Chloroform?
= (-1) . ( )2 /
= /
= J
= kcal/mol  8 % of a C-C bond

15. B. Ion-Dipole Interaction
Non-directional forces
Can be attractive or repulsive
Medium range interactions (1/r2)
Significantly weaker then ion-ion interactions
Example: crown ether complex
with alkali metal ions

16. B. Ion-Dipole Interaction
Energy = -(k . Q . u . cosq / e . r2)
Maximum when q = 0 or 180 degrees
Zero when q = 90 degrees
u = q . l
u = dipole moment
l = length of the dipole
q = partial charge on dipole
r = distance from charge to center of dipole
Q = charge on ion

17. B. Ion-Dipole Interaction
Example: Acetone pointing directly at Na+ ion (q = zero) at a distance of 1 nm (in chloroform)
Energy = -(k . Q . u . cosq / e . r2)
If q = zero
= -k . Q . u / e . r2
= / e . r2
= / (10-9)2
= J
= kcal/mol

18. Intermolecular Interactions p-p interactions
p – p 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 p-stacking interactions are responsible for the slippery feel of graphite. Similar p-stacking interactions help stabilize DNA double helix.

19. Intermolecular Interactions p-p interactions

20. Intermolecular Interactions p-p interactions
Distribution of electron density in benzene molecule

21. Intermolecular Interactions p-p StackingH d + d + H H d + d + - - d - H H + d + d + H H H
Edge-to-face

22. Intermolecular Interactions p-p interactions
Offset, face-to-face
Face-to-face, not favorable