Introduction to Molecular Materials
Homework assignment No 2
due May 26
1. True of Fals:
a. The smectic phase is more thermodynamically stable than a nematic phase T F
b. Tilted LC phases of chiral molecules exhibit spontaneous polatization T F
c. Nematic liquid crystals exhibit positional order of molecules T F
d. Bragg reflection is observed in discotic liquid crystals T F
e. SmC-I transition has lower enthalpy than melting T F
f. Paramagnetic susceptibility has smaller absolute magnitude than Çdia T F
1. Give an example of a pi and a sigma radical
pi: sigma:
2. 4-Phenyl[1,2,3,5]dithiadiazolyl radical dimerize in solution and the measured
thermodynamic parameters for the process are: "H = - 8.4Ä…0.5 kcal/mol and "S = - 27.5
cal/molK. Calculate concentration of free radical in 0.1 M solution in benzene at ambient
temperature.
3. Most radicals containing Ä…-hydrogens undergo disproportionation. However nitroxyl
radical shown below does not. Explain why (hint: consider the orinetation of the MOs)
O
N
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4. Some nitroxyl radicals, such as the one shown below, can be used as pH
sensors. Explain how such a probe might work.
NMe2
N
N
O
5. The SOMO of the [1,2,3,5]dithiadiazolyl radical has a node on the carbon
atom.
a) sketch the SOMO (hint: count the pi electrons and the numner of pi MOs)
N
S
H
S
N
structure SOMO contour
b) suggest orientation of the two radicals that would be likely to result in
ferromagnetic spin alignment (high spin system). Use the McConnell model.
6. The EPR spectrum of the benzyl radical in solution shows the following
absolute hfcc values aH for Ä… (16.4 G), ortho (5.17 G), meta (1.77 G), and para
(6.19 G) hydrogen atoms. Calculate spin concentration at each position using the
McConnell relationship and Q = 23G. Interpret these results using appropriate
resonance forms. What is the sign of spin density at the C(1), C(3) and C(5)
atoms? (Remember that the total spin density equals 1.0).
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7. Predict whether the following systems are high-spin or low-spin using the star, non-
star method.
circle one: high/low high/low high/low
8. Briefly describe the following terms:
a) chiral nematic& ..
b) SmC phase& ..
c) B2 phase& ..
d) lyotropic liquid crystal& .
9. Briefly explain the following notation and sketch the DSC trace consistent with it.
Cr1 65 Cr2 90 (G 85) SmC 120 N 142 I
10.0 20.1 8.2 1.0 0.5
heat
temperature /oC
Cr1& & & & & .& & Cr2& & & & & ......... G& & & & & & & &
SmC& & & & & & & & & & N& & & & & & & ....
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10. For the following molecule predict the sign of magnetic anisotropy "Ç and its
orientation in magnetic filed in the nematic phase.
C5H11 COO
C5H11
"Ç > 0 or "Ç < 0 (circle one)
11. Define dielectric anisotropy "µ and list 4 parameters that it depends on.
definition: & & & & & & &
paramaters: a)& & & & & & & & & & .. b)& & & & .& & & & & ..
c)& & & & & & & & & & & d)& & & & & & & & & & &
12. An electro-optical cell is shown below with the molecules depicted as lines. Design a
material that would undergo the Freedericksz transition and show electro-optical switching
effect as shown below. Why the optical effect is observed when the cell is switched?
E = 0
E > 0
13. What type of liquid crystal and which of its characteristics make it useful for
thermography?
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