1 2.2 charge-dipole interaction review (isr2011, sec 4.1) what is a dipole? how are polar molecules...
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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
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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
qu
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Two Types of Polar Molecules
Polar molecules Molecules carrying no net charge but possessing an el
ectric 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/HCl, http://en.wikipedia.org/wiki/Water, 3/3/2009
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Two Types of Polar Molecules
Environment-dependent polar molecules “The dipoles of some molecules depend on their envir
onment 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
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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
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Order of Magnitude of Molecular Dipoles
contd
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Order of Magnitude of Molecular Dipoles
contd
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Order of Magnitude of Molecular Dipoles
contd Use bond moments to estimate molecular di
poles as shown in previous table E.g., using bond moment O-H to estimate the dip
ole of H2O Exercise. Try other molecular dipoles.
CH3OH (methanol, 甲醇 ) CH3COOH (assuming COOH are on the same plane) CH3Cl
Hint:120o
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Order of Magnitude of Molecular Dipoles
contd A case where simple addition of individual bon
d moments does not give right prediction Chloroform: CHCl3
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, CHCl3” (from wikipedia http://en.wikipedia.org/wiki/Hydrogen_bonding, 20131022)
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Charge-Dipole Interaction in Isolation
Dipole u = ql Assume r >> l The interaction energy
uE
r
ur
4
Q
r
1
4
cosQu),r(w
30
20
-q
+q
r+Q
(derive it)
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Charge-Dipole Interaction in Isolation
Example Estimate max. interaction energy between a wat
er 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 kBT at 300 K)
contd
* kB: Boltzmann constant = 1.38 x 10-23 J/K
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Charge-Dipole Interaction in Medium
Q. How to determine the intermolecular potential? If we know the probability distribution of , we can
+Q
+Qr = +
randomly oriented
r = finite
most likely to point around 0o
r),r(w)r(w
Q1. What is the probability distribution of ?
To appendix on Boltzmann distribution
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Charge-Dipole Interaction in Medium
Boltzmann distribution theorem predicts the probability density function p(,) using w(,r)
contd
+Qz dir
ddsin2
eC
dAeC
dA),(p))d,[),d,([Prob
2Tk
),r(w
0
Tk
),r(w
0
theoremdist. Boltzmann
B
B
where
0
2
0
2Tk
),r(w
0 ddsin2
eC1 B
i.e. dA),(p1
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Charge-Dipole Interaction in Medium
Average potential w(r) becomescontd
ddsin2
e),r(wC
ddsin2
),(p),r(w),r(w)r(w
2Tk
),r(w
0
2
00
0
2
0
2
r
B
r>>1
22
2
20
rEu
kT3
1
r4
Qu
kT3
1),r(w)r(w
4r
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.
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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 <w(r,)> Unfortunately, because |w(r,)| << kT is no longer val
id, no simple formula as in previous case is possible.