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The Molecular Modeling Workbook for Organic Chemistry - Hehre J.W.

Hehre J.W., Shusterman J.A. The Molecular Modeling Workbook for Organic Chemistry - Wavefunction, 1998. - 307 p.
Download (direct link): molecularmodelingworkbook1998.djvu
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alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis
involves a cationic metallocene (typically zirconocene) alkyl complex. An
alkene coordinates to the complex and then inserts into the metal alkyl
bond. This leads to a new metal! inaoa in which the "polymer" is extended
by two carbons, i.e.
H2C==CH2
H2C=CH2 I
cp2Zr+R -----------"- cp2Zr+R ----------"-
H2C-CH2
n?a^a1-^
cp2Zr+-CH2CH2R
Examine the sequence of structures corresponding to Ziegler-Natta
polymerization of ethene, or more specifically, one addition step
starting from a zirconocene-ethene complex where R=CH3. Plot energy
(vertical axis) vs. frame number (horizontal axis). Sketch Lewis
structures for the initial complex, the "final" adduct and the transition
state. Indicate "weak or partial" bonding by using dotted lines.
Is the reaction as written exothermic, i.e., is there a thermodynamic
driving force? Rationalize your result. Is there an activation barrier to
the reaction? If so, is it typical of that of a thermal reaction (.04 to.
10 au or approximately 40-60 kcal/mol), much smaller or much larger?
Stereoregularity of Polypropylene
The properties of polymers depend not only on overall chain length, but
also on the degree to which the monomers are ordered along the chain.
Different methods of preparation lead to vastly different degrees of
ordering. A good example is found in the polymerization of propylene.
This polymerizes predominantly "head-to-tail", and leads to a
stereocenter (*) at every other atom in the polymer chain.
Polypropylene made by free-radical polymerization is generally "atactic",
that is to say, there is no pattern to the stereochemistry. On the other
hand, both "isotactic" polypropylene (in which all the stereocenters are
the same) and "syndiotactic" polypropylene (in which the stereocenters
alternate) may be made via the Ziegler-Natta process (see Chapter 18,
Problem 4). Experimentally, both isotactic and syndiotactic polypropylene
generally have higher melting points than atactic polypropylene.
Examine three different strands ofpolypropylene. For each strand, assign
R/S stereochemistry to each stereocenter. (All three strands have as
their "terminal monomer" perfluoropropane in order to facilitate
assignment of stereochemistry.) Which of the three strands corresponds to
atactic polypropylene, isotactic polypropylene and syndiotactic
polypropylene?
Spectroscopy
1 Vibrational Spectrum of
Water........................................254
2 Infrared Spectra of Carbonyl
Compounds...............................255
3 Concentration Effects on Infrared
Spectra............................256
4 Vibrational Spectrum of 1 -
Octyne....................................257
5 Spectral Identification of Short-Lived
Molecules.....................258
6 Electronic Spectra of Conjugated
Alkenes.............................259
7 Solvent Effects on Electronic
Spectra................................260
8 Singlet and Triplet
Anthrone.........................................261
9 Magnetic Anisotropy and Chemical
Shifts..............................262
10 Vicinal H-H Coupling and the Karplus
Equation........................263
11 Long-Range "W"
Coupling..............................................264
12 Substituent Effects on 13C Chemical
Shifts...........................265
1,3,5-hexatriene (see problem 6)
1
Vibrational Spectrum of Water
The magnitudes of vibrational motions shown are greatly exaggerated. At
room temperature atoms move only about 10% away from their equilibrium
positions.
Molecules vibrate at characteristic frequencies, which depend both on the
"difficulty" of the motion (the so-called force constant) and on the
masses of the atoms involved. The more difficult the motion and the
lighter the atomic masses, the higher the vibrational frequency. For a
diatomic molecule the vibrational frequency is proportional to:
e is the force constant which is related to the "stiffness" of the bond,
and which is independent of the masses of the atoms involved, and jj, is
the so-called reduced mass.
Display water as a ball-and-spoke model. How many different vibrations
are there? Explain. One after the other, animate these vibrations. For
each, record the vibrational frequency and provide a description of the
atomic motions. What appears to be easier (lower frequency), motions
primarily associated with bond stretching or with angle bending?
Repeat the analysis with deuterium oxide (D20). Are the vibrational
frequencies the same, larger or smaller than those in water? Rationalize
your observations. Are the changes in vibrational frequencies greatest
for bond stretching or angle bending motions?
254 Chapter 19 Spectroscopy
Infrared Spectra of Carbonyl Compounds
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