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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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