Intermolecular Interactions: Coulomb (Ion-Ion) Interactions

2014-09-30

Intermolecular interactions: the programming language for construction of molecular assemblies Intermolecular interactions determine bulk properties and structure and properties of multi- molecular assemblies

S. M. Lindsay Introduction to Nanoscience pp 210-216

Coulomb (ion-ion) interactions Bond energy = 100 – 300 kJ/mol

Non-directional and may be both attractive and repulsive

12 1 = ∝ 40

1 2014-09-30

Is there a fundamental difference between the ionic and the covalent bond?

Li F Van der Waals 1.82 1.47 radius (Å) Covalent radius (Å) 1.34 0.71 LiF Ionic radius (Å) 0.90 1.19

RLiF: 1.56 Å A chemical bond forms when electrons can be simultaneously near two or more nuclei!

Dipole−dipole interacons Bond energy = 5 – 50 kJ/mol

−12212 −12 1 = ∝ 40

2 2014-09-30

Orbital interactions 2

• The extent of stabilization (or destabilization) is inversely Δ proportional to the energy difference EOs between the orbitals (Os) involved.

• For a given energy difference between the interacting Os, the magnitude of stabilization (or destabilization) is proportional to the extent of overlap between these orbitals.

Bonding between atoms of different electronegativity (atom number)

HH H F

H1s H1s

HH H F + - HH HF

3 2014-09-30

Formaldehyde MOs

H H C

O H H C

H H C

CH2

H H C

O ABE

Liquid crystals

N C5H11 C

C5H11 N

4-Cyano-4’-pentylbiphenyl is used in liquid crystal displays. Its orientation can be changed by applying a voltage

4 2014-09-30

Hydrogen bonding Bond energy = 4 – 120 kJ/mol

Hydrogen bond Hydrogen bond donor acceptor - + A H B Electronegative atom, An atom with nonbonding usually O or N. electron pair, usually O or N. Highly directional

Hydrogen bonding

Strong Moderate Weak A–H···B interaction Mainly covalent Mainly electrostatic Electrostatic Bond energy 60-120 16-60 <12 (kJ/mol) Bond lengths (Å) H···B 1.2-1.5 1.5-2.2 2.2-3.2 A···B 2.2-2.5 2.5-3.2 3.2-4.0 Bond angles (°) 175-180 130-180 90-150 Examples Gas phase Acids C–H hydrogen bonds Designed hydrogen Alcohols O–H···π hydrogen acceptors Biological molecules bonds HF complexes

5 2014-09-30

Four-electron-three-centre bond Strong symmetric hydrogen bonds

Non- bonding

Bonding

Bond energy 167 kJ/mol

Hydrogen bond in FH-F-

6 2014-09-30

DNA Nanotechnology

E (eV) 20 C* Ethene MOs

H H C 10 C H H

0 2pz 2pz

H H C -10 B E C H H H H C -20 2 C H H 2 -30

-270 H H C ABE 1 and 1 C H H -280

7 2014-09-30

π-interactions Bond energy = 0 – 80 kJ/mol

~ 3.5 Å

Face-to -face

- -

H - - - + + + - - - Edge-to-face

Crystal strycture of benzene

8 2014-09-30

CH3 Self-assembled monolayers (SAM) with herring-bone patterns on flat gold (111) surfaces SH

+ - + - - + Au ------+ + - - - + - - - + + - + - - + + + ------+ -- - + - + - - - - + + + - - -

+ - + - - + - - - - + + - - - + - - - + + - - + + + ------+ -- - + - + - - - - - + + + - - -

5Å 5Å 5Å 5Å 6.5Å 9.0Å

3.4 Å 3.4 Å

Increased adsorbate size and inclination

R1 R2

Au(111)

9 2014-09-30

Van der Waals interactions Bond energy = <5 kJ/mol van der Waals interactions can be divided into dispersion (London) interactions and exchange (Pauli)–repulsion.

Dispersion forces •A weak intermolecular force arising from quantum induced instantaneous polarization multipoles in molecules. •The London force becomes stronger as the size of the interacting atoms or molecules increases. –The polarizability increases with atom and molecular size. = •The London force also becomes stronger as the interacting surface increases. α α 3 1 ≈− 6 4 +

disp μA is the induced dipole moment, αA is the polarizability, E is the applied electric field, EAB is the dispersion interaction energy, and IA is the ionization potential.

Exchange–repulsion or steric hindrance Pauli-repulsion and exclusion principle repulsion

Exchange–repulsion is a repulsiv interaction that arises when two neighboring molecules are close enough so that their electron clouds overlap and, as a consequence of the Pauli exclusion principle, repel each other.

ℎ 1 ∝ 12

10 2014-09-30

Exchange–repulsion and conformation

Filled Empty Bonding Antibonding orbitals orbital σσ

H H H H Eclipsed Exchange- Weakened conformation repulsion hyperconjugation

σ σ*

Less exchange- Strengthened Staggered repulsion hyperconjugation conformation (attractive) Of minor importance Of major importance

Self-assembled monolayers (SAMs) of alkane thiols

11 2014-09-30

Hydrophobic effects NB not a force!

Can be divided into two energetic components: enthalpic and entropic. It is not an attractive nor a repulsive force.

The non-polar solute hampers polar interactions and interrupts the hydrogen bonding network. The solvation shell has restricted mobility, i.e. lower entropy. By aggregation, non-polar molecules reduces their surface area and minimize the disruptive effect.

Shape dependent chemical potential

• Packing effects depend on geometry

• Non-spherical micelles v < 1 a0 c 3 • Spherical micelles 1 < v < 1 3 a0 c 2 • Vesicles/bilayers 1 v < <1 v 2 a • cones >1 0 c  c a0 c a0

v lc is the length of the hydrocarbon chain ν is the volume occupied by the hydrocarbon chain

A0 is the area of the head group (From Molecular Cell Biology, 4th ed. By H. Lodish, A. Berk, S.L. Zipursky, P. Matsudara, D. Baltimore, J. Darnell. © 2000, W.H. Freeman and Company. Used with permission) Copyright Stuart Lindsay 2008