Types of interactions in supramolecular chemistry Flashcards
Ion-ion interactions
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
Ion-dipole
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]+
Dipole-dipole interactions
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
Cation-pi interactions
(similar to ion-dipole interactions) 80 kJ mol-1 Diverse energetic landscape Directional Strength of interaction depends on dielectric constant of the medium
Anion-pi interactions
(technically anion-pi* interactions)
Very weak
Only present with electron-deficient aromatic systems (but F- may interact with electron-rich aromatics)
Pi-pi interactions
(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
Van der Waals interactions
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
Closed shell interactions
Very weak
Heavy atoms
Aurophilic interactions and halogen bonds
Aurophilic interactions
Aggregation of gold complexes via formation of weak gold-gold bonds
Unusually strong due to relativistic effects, comparable strength to H-bonds
Halogen bonds
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
Hydrogen bonding
(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
Strong H-bond
Mainly covalent interaction
60-120 kJmol-1 bond energy
1.2-1.5 A H-bond length
175-180 degree bond angle
Examples of strong H-bonds
Gas phase dimers between strong acids/bases
Proton sponges
HF complexes
Moderate H-bond
Mainly electrostatic interaction
16-60 kJmol-1 bond energy
1.5-2.2 A bond length
130-180 degree bond angle
Examples of moderate H-bonds
Acids
Alcohols
Biological molecules
Weak H-bond
Electrostatic interaction
<12 kJmol-1 bond energy
2.2-3.2 A bong length
90-150 degree bond angle
Examples of weak H-bonds
Minor components of bifurcated bonds
C-H H-bonds e.g. in pyridine
O-H—pi H-bonds
H-bond geometries
Linear Bent Donating bifurcated Accepting bifurcated Trifurcated 3-centre bifurcated
Primary H-bond interactions
Direct interaction between the donor and acceptor group
Secondary H-bond interactions
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
Why do mixed donor/acceptor arrays suffer from repulsion?
Because partial charges of the same sign are being brought into close proximity by the primary interactions
Solvophobic effects
Very weak
Ubiquitous (all ions/molecules/supramolecules are solvated at all times, except in gas phase)
Affected by enthalpies and entropic factors
What must the energetics and kinetics of a complexation process take into account?
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
The formation of a host-guest complex is…
..always entropically favourable, but can by enthalpically favourable or unfavourable
Why do large atoms displace more solvent when they form a complex than small atoms?
Large atoms form more solvent contacts
logK
= C1a2Hb2H + C2
C1, C2
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
a2H
Positive charge on H-bond donor
b2H
Negative charge on H-bond acceptor
How to treat interactions in solution
There is competition between solute-solute, solute-solvent and solvent-solvent interactions
Equation for predicting the free energy of H-bonding interactions in any solvent
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
