Transcript Coupling Constants

Coupling Constants
Review of Splitting
• Caused by shift due to magnetic fields of
adjacent protons
• We say that these protons are “coupled”
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Protons may be coupled to different degrees
Coupling constant
Typically 7 Hz for adjacent sp3 carbons
Tree diagram
Tree diagram
• Shortcut: N+1 if
all protons
coupled with
same constant
• Look at tree
• Coupling constant
is 7.1-7.2 Hz
Tree diagram
• Draw the tree
diagram that
shows why signal
A is a triplet
• What is the
coupling constant
for signal A
Coupling Constants
• Coupling
constants are
not all 7Hz
• In this class,
we will need
to know J
values
associated
with alkenes
Typical Constants
• Use the
table to
predict
typical
coupling
constants
Example: Cinnamic Acid
• In lab, you made trans cinnamic acid
• Can the trans and cis isomers be differentiated
using proton NMR?
• Yes—with coupling constants
400 MHz NMR:
Doublet at 6.310ppm and
6.355ppm
400 MHz NMR:
Doublet at 5.925ppm and
5.950ppm
Calculate the coupling constants for these doublets
Spectra that are not N+1
• Consider the allylic methyl
group
• Coupled to two protons—
but not with the same
coupling constant
• Not N+1
• Split into a doublet by Ha
• That doublet is split into
doublet by Hb
• It is doublet of doublets
Why not N+1?
• Jac = 1.7Hz (typical
0 Hz)
• Jbc = 6.9 Hz
(typical 4-10 Hz)
• If Jac = Jbc = 6.9Hz,
what would we
observe?
Predict the Splitting
• What signals would be
observed for proton A?
– Proton A is coupled to
one proton B (doublet)
– Proton A is coupled to
three proton C (quartet)
– Doublet of quartets with
J = 15.6 Hz and J = 1.7Hz
Predict the Splitting
• Do the same for
proton B
• How is signal
same/different than
proton A signal?
Proton B
• Also a doublet of
quartets
• But coupling
constants are 15.6
and 6.9Hz
Exercise
• Explain these
two observed
signals
– How would
you describe
them?
– Which
proton(s) do
they belong
to?
Types of problems
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Know typical coupling constants
Describe expected signal (dd, dt, etc)
Draw expected proton NMR
Interpret proton NMR given coupling
constants
Predict Structure
• C4H8O ether
• IR: 1650 cm-1