Coupling Constants

bahmotherElectronics - Devices

Oct 7, 2013 (3 years and 9 months ago)

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Coupling Constants

Review of Splitting


Caused by shift due to magnetic fields of
adjacent protons


We say that these protons are “coupled”


Protons may be coupled to different degrees


Coupling constant


Typically 7 Hz for adjacent sp
3

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 H
a


That doublet is split into
doublet by
H
b


It is doublet of doublets

Why not N+1?


J
ac

= 1.7Hz (typical
0 Hz)


J
bc

= 6.9 Hz
(typical 4
-
10 Hz)


If
J
ac

=
J
bc

= 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


Know typical coupling constants


Describe expected signal (
dd
,
dt
,
etc
)


Draw expected proton NMR


Interpret proton NMR given coupling
constants

Predict Structure


C
4
H
8
O ether


IR: 1650 cm
-
1