Intermolecular Interactions

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  • Created by: E.H13
  • Created on: 27-05-20 14:02

Types of Intermolecular Interactions

Electrostatic; interactions between charged atomc or molecula species, or between asymmetric charge distributions in neutral molecules.

Induction; an electric charge causing polarisation of neighbouring atoms/molecules and induced multipoles.

Induction energy; The attractive interaction between the original multipole and the induced multipole.

Dispersion; Attractive interactions between instantaneous dipoles arising from fluctuating charge distributions in atoms and molecules (only exist for a short period of time).

Hydrogen Bonding; attractive interactions in the form X-H----Y. A hydrogen atom cvalently bound to an electronegative atom, X, interacting with a second electronegative atom, Y.

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Electrostatic Interactions

Can be attractive or repulsive. Range of intraction is dependent on the type of interaction.

Van der Waals; a term used to cover electrostatic, induction and dispersion interactions between discrete neutral molecules or atoms. Most important contributions have 1/r3 or 1/r6 distance dependance.

Limit of compressibility of matter, caused by repulsive interactions due to overlap of electron densities, which dominate at short range. There are also electrostatic repulsions between atomic nuclei and between electrons (valence and core).

Pauli /Exchange Repulsion between electrons on neighbouring atoms (derived from Pauli Exclusion Principle).

Total short ranged interactions modelled using 1/rn, where n is usually 12 (dependent on interatomic distance), but exponential model more accurate. 

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Electrostatic Energy

Ion-Ion/Coulomb Potential energy; The interaction of 2 stationary point charges, with seperation distance r. Like charges repel, so Uc(r)>0, and opposite charges attract Uc(r)<0. Long ranged interaction.

Ion-Dipole energy; interaction energy betwwen stationary point charge, q, aand permanent dipole, u. Dependent on orientation of the dipole relative to the point charge. 

Dipole-Dipole energy; interaction energy of two coliniear dipoles, arranged in a head-to-tail fashion. the 2 polar molecules tend to align dipoles to lower the energy.

Alignment of molecular dipoles opposed by the effect of thermal energy, which leads to the rotaion of dipole. 

When kBT > UDD, rotationally averaged evergy can be obtained from a Boltzmann-weighted average. 

Multipole-Multipole interactions can be treated in the same way as for dipole-diople interactions. 

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Dielectric constant and Permittivity

In a medium, vacuum permittivity is replaed with permittivity of the medium. Permittivity is a screening constant.

Er = relative permittivity, aka the dielectric constant. 

Highly polar solvents -> high dielectric constant. Leads to a large reduction in coulombinc potential between ions and polar solvents.

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Induction and Dispersion Energy

Induction; A charge or asymmetric charge distibution gives rise to an electric field, E. This causes an induced dipole, uind = aE. (seen in the Stark effect).

Dispersion; The intermolecular forces between non polar molecules is dominated by 'London' or dispersion forces. 

Dynamic electron correltion; fluctuations in electron density give rise to instantaneous electronic dipole, which also induce dipole. These are long range, attractive dispersion forces. 

For small, highly polar molecules, dispersion energy is the largest contributions. Induction energies are small unless one of the molecules is charged. 

Small, non-polar moleules have low intermolecule binding energies, with larger molecules having higher interactions. 

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Lennard-Jones Potential

The Lennard-Jones potential gives an approximation to the total potential energy of interaction between two neutral atoms/non-polar molecules.

Equilibrium separation occurs at re = 21/6ro, ULJ(re) = -E

Increase in E going to heaveir gases atoms reflects the increase in the attractive dispersion forced due to higher polarisability. Polarisability is higher, as Zeff is lower. Tb and Tm for the rare gas elements follow the same trend as the E values for dimers.

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Hydrogen Bonding

Key propertes

  • Directional and short ranged
  • X-H---Y angle approx 180o
  • H----Y distance significantly longer than X-Y
  • lengthening of X-H bond, therefore shift of X-H stretching vibration  to lower energy
  • Broadening and gain of intensity of X-Y stretch

Contributions to H bonding; Major component is electrostatic forces. Other contributions from induction and dispersion forces, plus charge transfer as an incipient covalent bond is formed between H and Y. 

Consequences; high boiling points of water, ammonia and HF. 3d structure of ice. H-bonded dimers have significantly greater well depths (Lennard-Jones).

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Many-Body Energies

Moving to bigger collectections, accurate modelling of interatomic forces is impossible, even for Rare gas trimers. 

Non-pair-additivity; shows dispersion attraction between two atoms is effected by other, neighbouring atoms. 

Total potential energy for neutral molecules; Utot = Urep + UVdW + UHB.

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