Intermolecular Interactions pp interactions

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Intermolecular Interactionsp-p interactions
Distribution of electron density in benzene
molecule
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Intermolecular Interactionsp-p Stacking
H
H
H
-
-
H
H

H
H
H
Edge-to-face
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Intermolecular Interactionsp-p interactions
Offset, face-to-face
Face-to-face, not favorable
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Intermolecular InteractionsDipole-Dipole
Interactions
Non-directional forces Can be attractive or
repulsive Short range (1/r3) Significantly
weaker then ion-dipole interactions Occur
between molecules that have permanent net dipoles
(polar molecules). For example, dipole-dipole
interactions occur between SCl2 molecules, PCl3
molecules and (CH3)2CO molecules.
O
f
O
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Intermolecular InteractionsDipole-Dipole
Interactions

• Energy
• - (k u1 u2 / e r 3) 2 cosq1cosq2 -
  sinq1sinq2cosf
• Maximum in inline configuration where q 0
• Simplifies to
• - 2 (k u1 u2 / e r 3)

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Intermolecular InteractionsDipole-Dipole
Interactions

• Example Two acetone molecules in chloroform in
  head to tail arrangement separated by 0.5 nm
• - 2 (k u1 u2 / e r 3)
• -2(9109 (2.88 3.34 10-30 )2 / 4.8
  (0.510-9)3
• -28.08 10-22 J
• -1.68 kJ/mol

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Intermolecular Interactionsvan der Waals
(Dispersion) Forces

• Bond energy is very weak (0.1-3 kJ/mol)
• Exists between almost all atoms and molecules
• Arise from atomic or molecular dipoles
• Interactions between the fluctuating induced
  dipoles (due to instantaneous and short-lived
  vibrational distortions)

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Intermolecular Interactionsvan der Waals
(Dispersion) Forces
Original temporary dipole
This transient polarization will continue to
fluctuate because of the electron movement, but
the induced polarities are synchronized so that
the attraction from the polarity is maintained as
long as the molecules are close together.
Array of molecules, which have a temporary dipole
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Intermolecular Interactionsvan der Waals Forces
Often described as dispersion forces, i.e.
instantaneous dipole interactions. More general
definition includes ion-induced dipole, and
dipole-induced dipole interactions. Describes the
ubiquitous weak attraction between all atoms. E
-A/r6 Lennard-Jones potential combines vdw forces
with the hard sphere repulsion (B/r12) between
atoms at very close distance. E B/r12 -
A/r6 This equation allows us to find the van der
Waals radii of atoms. This is the size of an
atom or the preferred distance atoms will pack to
if there are no other significant interactions.
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Intermolecular Interactionsvan der Waals
(Dispersion) Forces
Strength of interaction is essentially a function
of the surface area of contact and the
polarizability of electron shells. The larger the
surface area the stronger the interaction will
be. Regardless of other interactions found
within a complex there will always be a
contribution from vdw. This is what drives
molecules to eliminate spaces or vacuums and
makes it difficult to engineer porous or hollow
structures and gives rise to the phrase Nature
abhors a vacuum.
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Intermolecular Interactionsvan der Waals Forces
Induced dipole-induced dipole
Three main types of VDW interactions
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Hydrogen bonding
special case of dipole-dipole interactions, which
have a certain amount of covalent character and
directionality can be significantly stronger
then typical dipole-dipole directional probably
the most important of all intermolecular
interactions short range (2.5-3.5 Å) Forms when
a hydrogen atom is positioned between two
electronegative atoms, i. e. O, N, F (but not C!)
D-H---A
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Hydrogen bonding
geometry of the hydrogen bond
Most stable
N
N
N

• D-H-A angle
• ? H-A-X angle

H
H
H
?
O
O
O
?180o
Ideal angles
?120o
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Hydrogen bonding
Distance
Van der Waals radius of H 1.1Å, O 1.5Å.
Therefore closest approach should be 2.6Å.
H
R
O
H
O
R
Actual separation is about 1Å less! Distance of
1.76Å. Intermediate between vdw distance and
typical O-H covalent bond of 0.96Å.
H
R
O
H
O
R
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Hydrogen bonding
O-H---O bond
Typical distances and angles Energy 20-40
kJ/mol NH---O 1.80 to 2.00 Å OH---O 1.60 to
1.80 Å ? (D-H-A) 150 160o ? (H-A-X) 120
130o
oriented toward the free electron lone pair on
the acceptor oxygen Water (2A/2D), alcohols
(2A/D), ketones (2A), ethers (2A), carboxylic
acids (3A/D), etc. Responsible for the unusual
properties of water (high melting boiling
point, high surface tension, etc.)
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Hydrogen bonding
N-H---O bond N-H---N bond
N-H---O distance is roughly 2.7Å
Amines, amides, heterocycles, etc.
hydrogen bonding in proteins
hydrogen bonding in DNA
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Hydrogen bonding
G-C
Guanine
Cytosine
A-T
Adenine
Thymine
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Hydrogen bonding
How much is a hydrogen bond worth? experimental
value is extremely variable values in the
literature from zero to 60 kJ/mol
Strength of an H-bond is related to the D-H---A
distance and the DHA angle. The shorter the
distance between D A the stronger the
interaction. Ideally, DHA angle should be 180.
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Hydrogen bonding
If we calculate the energy of a H-bond in
chloroform (dielectric 4.8) to be 50 kJ/mol,
if we change solvent to water (dielectric 78.8)
all else being the same we would expect the new
energy of interaction to be 50 4.8 / 78.8 3.0
kJ/mol based on our equation for dipole-dipole
interactions. However we find experimentally
that the H-bond is only worth 0.5 kJ/mol. Why
might this be?
H
R
R
2
O
H
O
O
H
R
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Hydrogen bonding in supramolecular synthesis
Hydrogen bond in supramolecular synthesis
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Catenanes
8 yield
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Rotaxanes
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Hydrogen bonding in supramolecular synthesis
Hydrogen bond in supramolecular synthesis
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Hydrogen bonding in supramolecular synthesis
Hydrogen bond in supramolecular synthesis
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An example of a cycloenantiomeric, chiral rotaxane