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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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Compare the dipole moment and the electrostatic potential map for the
ground state of acetone to those of the n to pistar state of acetone.
Which molecule is more polar? Rationalize the differences by appealing to
the shape of the orbitals (in ground-state acetone) whose electron
populations are changed by excitation.
The i->e* transition appears at 279 nm in hexane. Calculate the energy
necessary for this transition (use equation 1). Is this enough energy to
cause a chemical change in the molecule, i.e., is this enough energy to
break a chemical bond? n bonds typically have strengths of approximately
0.1 au (60 kcal/mol), and i bonds typically have strengths in the range
of 0.1 to 0.15 au (60 to 95 kcal/mol).
Acetone is a moderately polar molecule that can hydrogen-bond with water.
Which electronic state of acetone would be stabilized more by moving the
molecule from hexane to water? Will this shift the n->7t* transition to
longer or shorter UV wavelengths? Explain.
260 Chapter 19 Spectroscopy
Singlet and Triplet An throne
I
I
Anthrone absorbs visible light leading to an excited singlet state. This
state is short-lived and can either return to the ground singlet state by
light emission ("fluorescence"). It can also relax to a triplet state in
a process known as intersystem crossing. The triplet state is much longer
lived than the excited singlet and has ample time to undergo a chemical
reaction. It can also return to the ground singlet state by emission of
light ("phosphorescence").
First, try to draw resonance contributors for both ground state and
triplet anthrone. Then display a spin density surface for the triplet
state of anthrone. (Note that the spin density surface shows the location
of both unpaired electrons, one of which may be in a e orbital and one of
which may be in a i orbital.) Where are the two unpaired electrons? Are
they localized or delocalized? Given that spin delocalization generally
leads to stabilization, would you expect the triplet state of anthrone to
be stable?
Compare the geometries of triplet and ground state singlet anthrone.
Where do they differ the most? Focus on the carbonyl group. Has the CO
bond distance altered? Does the molecule incorporate a fully-developed CO
7t bond (as in ground state singlet anthrone), or a single bond (as in
phenol)? Is the carbonyl carbon planar or puckered? Rationalize your
observations.
t I

Spin density surface for triplet anthrone locates the two unpaired
electrons.
(singlet)
Chapter 19 Spectroscopy 261
9
Magnetic Anisotropy and Chemical Shifts
edge-on
view of benzene ring
\
shielding /
\ region / deshielding deshielding
region region
/
shielding4', region
Molecules that contain a benzene ring generate local magnetic fields when
placed in a strong external field (in an NMR spectrometer). The local
field is anisotropic. Protons that lie in the ring plane are deshielded
(signal moves to higher chemical shift), while those located above and
below the ring plane are shielded (signal moves to lower chemical shift).
These shifts can be used to help assign NMR spectra and establish
molecular configuration.
The 'H NMR spectrum of the cyclophane shows three signals at 8 4.27,
6.97, and 7.24 (1:2:1 ratio) due to the benzene ring hydrogens. Examine
cyclophane and identify which hydrogens are responsible for each signal.
The labeled hydrogens in the spiroketone fall into two symmetry-related
pairs. Ha and I/ produce signals at 8 2.4, and Hb and Hb' produce signals
at 8 3.48. Examine spiroketone and determine which pair of hydrogens is
responsible for which signals. Incorporate a sketch of the molecule
showing the geometry of the rings and the orientation of the labeled
hydrogens as part of your answer.
The NMR spectra of cis and ?rans-diphenylbenzo-cyclobutene are nearly the
same except for the chemical shifts of the hydrogens on the four-membered
ring. The hydrogens in one compound produce a singlet at 8 4.44, while
those in the other compound produce a singlet at 8 5.2. Examine the
geometries of cis and trans diphenylbenzo-cyclobutane. Which compound is
responsible for which NMR spectrum? Explain your reasoning.
262 Chapter 19 Spectroscopy
Vicinal H-H Coupling and the Karplus Equation
The coupling constant, J, between vicinal protons varies with dihedral
angle, 0. The relationship between J and 0 is given by the "Karplus
equation".
J = (7 - cos0 + 5cos20) Hz
The Karplus equation predicts that J is largest when the two CH bonds are
staggered anti (0=180) or eclipsed (0=0), and J is smallest when the
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