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