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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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two bonds are perpendicular (0=90). Because of J's dependence on
dihedral angle, it can be used to distinguish between isomers such as
artemisin acetate and 6-epiartemisin acetate.
J(vicinal)
Ha,Hb artemisin acetate 6-epiartemisin acetate
6.7
7.8 7,11
8,9axial
11.6
10.9 11.5
10.9
5.7
10.5 0
10.5
For each molecule (isomer A and isomer A), obtain dihedral angles for the
following pairs of vicinal hydrogens: Ia-I7, Io-I8, H7-Hlb and H8-
H9axial. Use the Karplus equation to estimate coupling constants for each
pair, and then compare your predictions to the experimental coupling
constants (see above). Which molecule is artemisin acetate and which is
6-epiartemisin acetate?
Long-Range "W" Coupling
Observable proton-proton coupling is generally limited to protons
separated by two bonds (geminal coupling) or three bonds (vicinal
coupling). Longer-range coupUng is much weaker, but significant
exceptions occur when the intervening CH and CC bonds form a planar "zig-
zag" or "W" pattern, as HaCCCHa in bicyclo[2.1.1]hexane.
Examine the structure of bicyclohexane and note the spatial relationships
of the various CH and CC bonds. Do all the bonds separating Ha and Ha lie
in the same plane?
Examine the structures of the norcamphor and camphor derivatives (A and
B).
Note the relationship between Ha and the other hydrogens. Long-range
coupling to Ha (J ~ 4 Hz) is observed in only one of these molecules.
Which molecule do you think this is? Identify the proton that is coupled
to Ha.
The NMR signal due to Ha in the dihydropyran derivative (C) appears as a
doublet (J = 1.3 Hz), indicating coupling to either Hb or Hb-, but not
both. Make a sketch of this molecule showing the orientation of Ha, Hb,
and Hb. Which proton, Hb or U, is responsible for the long-range
coupling? Why can't long-range coupling to CH3 account for the Ha
doublet?
Substituent Effects on 13C Chemical Shifts
13C chemical shifts depend, in part, on the amount of electron density
around the 13C nucleus. Since benzene ring substituents perturb the
electron density at selected carbons around the ring, one might expect
these substituents to exert a noticeable effect on the chemical shifts of
these nuclei.
First compare electrostatic potential maps for phenol, toluene, benzene,
trifluorotoluene, benzonitrile, benzaldehyde and nitrobenzene. Relative
to hydrogen (in benzene), which substituent leads to the greatest
reduction in negative charge in the n system? Which leads to the greatest
increase in negative charge? Next, obtain atomic charges for the meta and
para carbons for each system. (Define the meta charge as the average of
the atomic charges on the two meta carbons.) Which set of charges is more
sensitive to substituent structure? Is it the same set for which the
chemical shifts show the greater sensitivity (see table at right)?
Plot para chemical shift (vertical axis) vs. para atomic charge
(horizontal axis). Are the two properties correlated? If they are, what
is their relationship? Does chemical shift increase (13C becomes
deshielded) or decrease (13C becomes shielded) with increasing negative
charge at carbon?
Draw the most important resonance contributors for nitrobenzene (include
all of the contributors needed to explain for the variation in
electrostatic potential, charge and chemical shift relative to benzene).
Do these resonance contributors account for the different behavior of
8meta and 8para? Explain.
chemical shifts in PhX
X ^meln ^para
OH 130.1 121.2
CH3 128.4 125.6
H 128.5 128.5
CF3 128.9 131.9
CN 129.1 132.8
CHO 129.1 134.3
no2 129.4 134.5

Electrostatic potential map for benzonitrile shows negatively-charged
regions in red and positively-charged regions (in blue).
Chapter 19 Spectroscopy 265
a
Mass Spectrometry
1 Mass Spectra of
Alcohols..........................................268
2 Mass Spectra of Alkenes and Arenes. Resonance Stabilized

Cations............................................................269
3 McLafferty
Rearrangement............................................270
transition state for McLafferty rearrangement of butanal radical
cation
(see ?aiUaoC)
Electrostatic potential map for 2-methyl-2-butanol radical cation shows
most positively-charged regions (in blue) and less positively-charged
regions (in red).
Spin density for 2-methyl-2-butanol radical cation shows location of
unpaired electron.
Mass Spectra of Alcohols
The mass spectra of alcohols often completely lack a peak corresponding
to the parent ion. This is due to extremely rapid loss of neutral
fragments following initial ionization. For example, the mass spectrum of
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