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

JCheng 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

JCheng 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) 3/3/2009http://en.wikipedia.org/wiki/Water

JCheng 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

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

JCheng Order of Magnitude of Molecular Dipoles contd

JCheng Order of Magnitude of Molecular Dipoles contd

JCheng 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

JCheng 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 )

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

JCheng 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 J/K

JCheng 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

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

JCheng 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 It is twice the value derived in Israelachvili 1991.

JCheng 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.