1. 1 2.2 Charge-Dipole Interaction Review (Isr2011, sec 4.1) What is a dipole? How are polar molecules formed? Order of magnitude of molecular dipole Charge-Dipole Interaction (Isr2011, sec 4.3) In isolation In medium

2. JCheng201209 2 What Is Dipole? Dipole Two equal charges, q, of opposite sign, separated by a distance l, constitutes an electric dipole. A dipole u is represented as a vector pointing from – q to +q and has a magnitude of q l -q +q

3. JCheng201209 3 Two Types of Polar Molecules Polar molecules Molecules carrying no net charge but possessing an electric dipole Inherent polar molecules E.g. in HCl, Cl atom tends to draw the hydrogen ’ s electron toward itself (as indicated in the electron cloud around the nuclei of Cl and H), forming a permanent dipole (indicated as blue arrow) http://en.wikipedia.org/wiki/HClhttp://en.wikipedia.org/wiki/HCl, http://en.wikipedia.org/wiki/Water, 3/3/2009http://en.wikipedia.org/wiki/Water

4. JCheng201209 4 Two Types of Polar Molecules Environment-dependent polar molecules “ The dipoles of some molecules depend on their environment and can change substantially when they are transferred from one medium to another, especially when molecules become ionized in a solvent. ” (Isr2011, p. 71) E.g. glycine (amino acetic acid) in water becomes a dipolar molecule contd

5. JCheng201209 5 Order of Magnitude of Molecular Dipoles Debye 1 Debye (1 D) = 3.336 x 10 -30 C-m E.g. A dipole of two charges,  e, separated by 1 Å = 4.8 D

6. JCheng201209 6 Order of Magnitude of Molecular Dipoles contd

7. JCheng201209 7 Order of Magnitude of Molecular Dipoles contd

8. JCheng201209 8 Order of Magnitude of Molecular Dipoles contd Use bond moments to estimate molecular dipoles as shown in previous table E.g., using bond moment O-H to estimate the dipole of H 2 O Exercise. Try other molecular dipoles. CH 3 OH (methanol, 甲醇 ) CH 3 COOH (assuming COOH are on the same plane) CH 3 Cl Hint: 120 o

9. JCheng201209 9 Order of Magnitude of Molecular Dipoles contd A case where simple addition of individual bond moments does not give right prediction Chloroform: CHCl 3 I guess the reason is the same one as that causing hydrogen bonding in chloroform “ A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl 3 ” (from wikipedia http://en.wikipedia.org/wiki/Hydrogen_bonding, 20131022) http://en.wikipedia.org/wiki/Hydrogen_bonding

10. JCheng201209 10 Charge-Dipole Interaction in Isolation Dipole u = q l Assume r >> l The interaction energy -q +q  r +Q (derive it)

11. JCheng201209 11 Charge-Dipole Interaction in Isolation Example Estimate max. interaction energy between a water molecule and Na + Assumptions Water: a spherical molecule, r = 1.4 Å, u = 1.85 D Na+: r = 0.95 Å (r: radius) (Ans: 96 kJ mol -1 = 39 k B T at 300 K) contd * k B : Boltzmann constant = 1.38 x 10 -23 J/K

12. JCheng201209 12 Charge-Dipole Interaction in Medium Q. How to determine the intermolecular potential? If we know the probability distribution of , we can +Q r = +  randomly oriented r = finite most likely to point around 0 o Q1. What is the probability distribution of  ? To appendix on Boltzmann distribution

13. JCheng201209 13 Charge-Dipole Interaction in Medium Boltzmann distribution theorem predicts the probability density function p( ,  ) using w( ,r) contd +Q z dir  where i.e.

14. JCheng201209 14 Charge-Dipole Interaction in Medium Average potential w(r) becomes contd r>>1  For r >> 1, w(r) becomes * Note this equation for charge-dipole interaction is from Atkin’s textbook on Physical Chemistry, 7th ed. 2000. It is twice the value derived in Israelachvili 1991.

15. JCheng201209 15 Interaction between Charge & Dipole in a Medium in Near Neighborhood Consider a charge +Q is placed right inside a fluid made of polar molecules Polar molecules distributed according to Boltzmann distribution What is free energy in this condition? The average free energy is still Unfortunately, because |w(r,  )| << kT is no longer valid, no simple formula as in previous case is possible.