Types of interactions in supramolecular chemistry

Question 1

Ion-ion interactions

Answer 1

Strongest (100-350 kJmol-1)
Even stronger than covalent bonds in some cases
Can be attractive or repulsive
Non-directional
Long range (1/r)
Strength of interaction depends on dielectric constant of the medium i.e. stronger in chloroform than water

Question 2

Ion-dipole

Answer 2

50-200 kJmol-1, significantly weaker than ion-ion interactions
Can be attractive or repulsive
(Somewhat) directional
Medium range (1/r^2) - dipole not strong enough to attract opposite charge over a large distance
Strength of interaction depends on dielectric constant of the medium
e.g Na+/crown ether complexes, [Na(H2O)6]+

Question 3

Dipole-dipole interactions

Answer 3

5-50 kJmol-1, relatively weak
Directional - dipoles must align correctly
Strength of interaction depends on dielectric constant of the medium
Observed in carbonyl compounds in the solid state

Question 4

Cation-pi interactions

Answer 4

(similar to ion-dipole interactions)
80 kJ mol-1
Diverse energetic landscape
Directional
Strength of interaction depends on dielectric constant of the medium

Question 5

Anion-pi interactions

Answer 5

(technically anion-pi* interactions)
Very weak
Only present with electron-deficient aromatic systems (but F- may interact with electron-rich aromatics)

Question 6

Pi-pi interactions

Answer 6

(Not exclusive to aromatics - also alkenes and alkynes)
Relatively weak (1-50 kJmol-1)
Directional (see revision card)
Strength of interaction depends on dielectric constant of the medium
Heavily influenced by the nature of the pi-system - larger pi-system = stronger interaction; heteroaromaticity

Question 7

Van der Waals interactions

Answer 7

Very weak (0.4-4 kJmol-1)
Non-directional
Appear due to temporary polarisations of the electron clouds - depends on the polarisability of the molecule (larger molecules are more easily polarised)
Implicated in crystal close packing to minimise void spaces in crystals

Question 8

Closed shell interactions

Answer 8

Very weak
Heavy atoms
Aurophilic interactions and halogen bonds

Question 9

Aurophilic interactions

Answer 9

Aggregation of gold complexes via formation of weak gold-gold bonds
Unusually strong due to relativistic effects, comparable strength to H-bonds

Question 10

Halogen bonds

Answer 10

Attractive interaction between an electrophilic region associated with a halogen atom in a molecule and a nucleophile region in another/the same molecule
Generally occur between the polarisable halogens (I, Br) and electronegative atoms with a lone pair

Question 11

Hydrogen bonding

Answer 11

(subset of dipole-dipole interactions)
Ubiquitous in nature
6-120 kJmol-1
Diverse energetic landscape - some H-bonds are stronger than covalent bonds, some are as weak as vdW forces
Directional
Strength of interaction depends on dielectric constant of the medium

Question 12

Strong H-bond

Answer 12

Mainly covalent interaction
60-120 kJmol-1 bond energy
1.2-1.5 A H-bond length
175-180 degree bond angle

Question 13

Examples of strong H-bonds

Answer 13

Gas phase dimers between strong acids/bases
Proton sponges
HF complexes

Question 14

Moderate H-bond

Answer 14

Mainly electrostatic interaction
16-60 kJmol-1 bond energy
1.5-2.2 A bond length
130-180 degree bond angle

Question 15

Examples of moderate H-bonds

Answer 15

Acids
Alcohols
Biological molecules

Question 16

Weak H-bond

Answer 16

Electrostatic interaction
<12 kJmol-1 bond energy
2.2-3.2 A bong length
90-150 degree bond angle

Question 17

Examples of weak H-bonds

Answer 17

Minor components of bifurcated bonds
C-H H-bonds e.g. in pyridine
O-H—pi H-bonds

Question 18

H-bond geometries

Answer 18

Linear
Bent
Donating bifurcated
Accepting bifurcated
Trifurcated
3-centre bifurcated

Question 19

Primary H-bond interactions

Answer 19

Direct interaction between the donor and acceptor group

Question 20

Secondary H-bond interactions

Answer 20

Interactions between neighbouring groups when H-bond donor and acceptor are in close proximity
Partial charges on adjacent atoms can either increase the binding strength through attraction between opposite charges or decrease the affinity due to repulsion between like charges

Question 21

Why do mixed donor/acceptor arrays suffer from repulsion?

Answer 21

Because partial charges of the same sign are being brought into close proximity by the primary interactions

Question 22

Solvophobic effects

Answer 22

Very weak
Ubiquitous (all ions/molecules/supramolecules are solvated at all times, except in gas phase)
Affected by enthalpies and entropic factors

Question 23

What must the energetics and kinetics of a complexation process take into account?

Answer 23

The desolvation of both host and guest upon binding
The resolvation of the resulting complex
Often occurs with release of free solvent and consequence formation of solvent-solvent interactions

Question 24

The formation of a host-guest complex is…

Answer 24

..always entropically favourable, but can by enthalpically favourable or unfavourable

Question 25

Why do large atoms displace more solvent when they form a complex than small atoms?

Answer 25

Large atoms form more solvent contacts

Question 26

logK

Answer 26

= C1a2Hb2H + C2

Question 27

C1, C2

Answer 27

Solvent-dependent constants
C1 increases as the polarity of the medium decreases
C2 is relatively insensitive to solvent - implying it is a fundamental property of the interaction between any 2 molecules

Question 28

a2H

Answer 28

Positive charge on H-bond donor

Question 29

b2H

Answer 29

Negative charge on H-bond acceptor

Question 30

How to treat interactions in solution

Answer 30

There is competition between solute-solute, solute-solvent and solvent-solvent interactions

Question 31

Equation for predicting the free energy of H-bonding interactions in any solvent

Answer 31

DeltaDeltaDeltaG(H-bond) = - (ab + asbs) + (abs + asb)
= -(a-as)(b-bs)

Where a and b are the H-bond donor and acceptor constant for the solute molecules and as and bs rare the corresponding H-bond donor and acceptor constants for the solvent