# Coupling Constants

Electronics - Devices

Oct 7, 2013 (4 years and 9 months ago)

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Coupling Constants

Review of Splitting

Caused by shift due to magnetic fields of

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

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