Interactions between Dipolar Molecules

Intermolecular & Surface Forces Class 4

October 8th, 2013 Prof. Darren Lipomi

Dipolar Molecules

•  Requirements of polar molecules – Charges – Differences in electronegativities – Asymmetries

•  Examples: H–Cl, H2O, amino acids •  Aside: nomenclature

H3CC CH3

O O=(CH3)2CO =

Dipole Moment

•  Dipole moment, u

•  Mnemonic: vector opposite E-field

δ+ δ–

u

H–Cl

u = ql

partial charges distance between ±q For q = ±e, l = 0.1 nm, then u = 1.6 × 10–29 Cm

= 4.8 D (debye) 1 D = 3.3 × 10–30 Cm

Examples of Dipole Moments

Molecule n-Alkane (CnH2n+2) CO2 H2O CH3OH (CH3)2CO NaCl CsCl

Dipole moment 0 (why?)

0 1.85 1.70 2.90 8.50

10.40

Bond and Group Moments Bond C–H O–H

Bond moment 0.40 1.51

Group C–OH C–COOH

Bond moment 1.65 1.70

O H

H u

½ θ

θH2O = 104.5°

uH2O = 2uOH cos (½ θ) = 2(1.51 D) cos (52.25°) = 1.85 D

Dipole Self-Energy

•  Self-energy of the individual partial charges (±q), always positive, minus work done bringing charges together

•  For small, highly polar molecules, can be used to estimate energy due to partitioning between solvents…

Ion-Dipole Interactions

Sum of pair potentials of ion with ±q

When r >> l (point dipole approximation)

Strength of Ion-Dipole Energy

+!−!

θ = 0°!θ = 180°!δ+ δ–

δ– δ+

u

u Max attraction!Max repulsion!

•  Ions, small dipoles (Na+, H2O) interact w/ energy >10 kT at contact!

•  Energy is work done to orient the dipole, or orient polar solvent molecules around charged solute!

Increases!w/!decreasing!ionic!radius!

Be2+!too!small!to!support!6!H2O!

Anions!bigger!than!ca=ons!and!have!smaller!hydra=on!shells!

Fixed Dipole-Dipole Interactions

e"•  Dipoles in line: •  ~kT at contact!

•  Dipoles antiparallel: w(r, 90, 90, 180) = –u1u2/4πε0εr3

•  Antiparallel orientation dominant for highly anisotropic dipoles with charges at each end

Charge-Freely Rotating Dipole Interaction

•  With large enough ε, T, or r, dipoles rotate freely, but some orientations are more probable than others (derivation p. 84-85)

•  For ion in water at room temp, valid for r > 0.2 nm

Two Freely Rotating Dipoles (Keesom Energy)

•  Keesom energy is the dipole-dipole interaction, but when freely rotating (almost all of the time for solutions)

•  First of three 1/r6 potentials (VDW)

Continuum vs. Molecular Theories

•  Is dielectric constant a continuum or molecular concept?

•  What about dipole moment?

•  Explain the dissolution of salt in water using either a continuum or molecular theory

Dielectric Constant •  Ability of a medium to screen electric fields •  Or, to reduce self-energy of solutes •  Three factors determine ε for liquids:

1.  Dipole moment 2.  Density of dipoles (increases with decreasing

size) 3.  Intermolecular forces (that reinforce the

polarized state) •  What is the dielectric constant of a

nonpolar liquid?

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