Proton
NMR
Spectroscopy:
How
to
Identify
Proton
Equivalence
Basic
Vocabulary:
Equivalent
Protons:
protons
that
are
identical
in
every
way.
They
have
the
same
magnetic
environment,
which
gives
them
the
same
spin
flip
energy.
Nonequivalent
Protons:
protons
that
only
have
to
differ
in
a
single
way.
These
protons
do
not
have
the
same
magnetic
environment.
Why
do
we
need
to
identify
proton
equivalence
in
the
first
place?
The
number
of
signals
found
in
the
NMR
spectrum
corresponds
to
the
number
of
nonequivalent
proton
groups
in
the
molecule.
Knowing
how
to
identify
proton
equivalence
will
allow
you
to
identify
nonequivalent
proton
groups
and
identify
the
number
of
signals
that
will
show
up
in
the
NMR
spectrum
for
the
molecule!
3
NMR
Signals
=
3
different
Inte
(nonequivalent)
proton
sets
nsity
of
sign
al
(Pho
ton
Qual
ity)
How
to
determine
Equivalence:
Spin
Flip
Energy
(Photon
Energy)
There
are
a
few
different
ways
of
determining
equivalence
that
often
need
to
be
used
together!
1. Identify
a
mirror
plane
in
the
molecule.
If
there
is
symmetry
along
the
mirror
plane
then
the
protons
are
equivalent!
H
H
The
green
protons
and
the
red
protons
are
symmetrical
along
the
HO
C
C
OH
mirror
plane
H
H
2. Identify
an
axis
of
rotation.
If
the
molecule
looks
the
same
upon
rotation,
then
the
protons
are
equivalent!
Rotate
120
Rotate
120
Even
though
the
protons
of
the
methyl
are
rotating
around
the
carbon
(the
yellow,
orange,
and
pink
tabs
help
keep
track
of
the
placement!),
the
rotations
do
not
change
the
molecule
itself.
These
three
protons
are
equivalent.
Molecular
Model
Of:
In
general,
hydrogens
attached
with
a
single
bond
to
the
same
atom
in
an
acyclic
molecule
are
equivalent.
3. Lastly,
you
can
identify
what
atoms
the
protons
are
connected
to
and
compare
them.
If
the
atom
that
the
protons
are
directly
connected
are
the
same
move
on
to
identify
the
additional
bonded
to
assure
there
is
no
difference!
If
they
differ
by
even
one
other
atom,
then
they
are
nonequivalent!
Here,
we
already
know
the
hydrogens
of
the
methyl
group
are
equivalent
to
each
other,
and
the
hydrogens
of
the
methylene
group
are
equivalent
to
each
other
because
of
symmetry.
Are
all
the
hydrogens
n
the
molecule
equivalent
to
each
other?
H
Hydrogens
of
methyl
group
are
directly
bonded
to
a
carbon.
H
OH
H
H
Carbon
is
attached
to
a
carbon.
Hydrogen
of
the
alcohol
is
directly
bonded
to
oxygen.
Hydrogens
in
methylene
are
directly
bonded
to
a
carbon.
The
carbon
is
attached
to
oxygen
and
a
carbon.
Three
Non‐equivalent
groups=
3
signals
on
the
NMR
spectrum
●We
can
immediately
see
that
the
hydrogen
of
the
alcohol
is
non‐equivalent
to
the
rest
because
it
is
directly
attached
to
oxygen
while
the
other
hydrogens
are
attached
to
carbon.
●Since
the
hydrogens
of
the
methyl
and
methylene
group
are
both
attached
to
a
carbon,
we
need
to
look
at
the
atoms
each
carbon
is
bonded
to
in
order
to
correctly
identify
if
they
are
equivalent.
The
carbon
of
the
methyl
group
is
only
attached
to
a
carbon,
but
the
carbon
of
the
methylene
group
is
attached
to
a
carbon
and
oxygen.
Because
they
differ
by
the
addition
of
an
oxygen
atom,
they
are
non‐
equivalent.
Example
Problems:
Determine
the
number
of
signals
that
will
be
found
on
the
NMR
Spectrum
for
each
molecule.
1. H
CH3
CH3
F
H
CH3
The
protons
of
each
methyl
group
are
equivalent
to
each
other
because
of
the
axis
of
rotation.
The
protons
of
methylene
group
are
equivalent
because
of
their
symmetry.
However,
the
methyl
protons
and
the
methylene
protons
are
non‐equivalent
to
each
other
because
the
atoms
they
are
attached
to
are
not
identical.
The
methyl
protons
are
attached
to
a
carbon,
which
is
attached
to
a
carbon
with
three
additional
carbon
bonds.
The
methylene
protons
are
attached
to
a
carbon
that
is
attached
to
one
other
carbon
and
a
fluorine.
Conclusion:
2
groups
of
non‐equivalent
protons=2
signals
on
the
NMR
Spectrum.
2. All
hydrogens
attached
to
a
benzene
ring
are
equivalent.
There
are
three
mirror
planes
in
this
molecule,
and
all
the
hydrogens
are
symmetrical
along
each
one!
**Don’t
let
the
alternating
double
bonds
mess
you
up.
Since
a
benzene
ring
is
in
resonance,
all
the
bonds
have
partial
pi
bond
character,
and
are
therefore
equal!
Conclusion:
All
hydrogens
are
equivalent=
1
signal
on
the
NMR
Spectrum
3. F
Cl
The
addition
of
the
fluorine
and
chlorine
substituents
to
the
chlorine
rings
destroys
the
symmetry
and
identical
atom
attachment
the
molecule
had
before.
**You
may
be
tempted
to
say
that
the
top
and
bottom
hydrogens
are
symmetrical
and
therefore
equivalent.
However,
if
you
look
at
the
atoms
each
hydrogen
is
bonded
to
it
is
apparent
that
they
are
not
identical.
The
top
hydrogen
is
bonded
to
a
carbon,
which
is
bonded
to
a
carbon
with
a
fluorine
substituent.
The
bottom
hydrogen
is
bonded
to
a
carbon,
which
is
bonded
to
a
carbon
with
a
chlorine
substituent.
The
two
different
halogens
make
the
hydrogens
non‐equivalent.
Conclusion:
4
nonequivalent
hydrogens=4
signals
on
the
NMR
Spectrum
Works
Cited
Image
of
benzene
ring
from:
http://www.daviddarling.info/encyclopedia/B/benzene.html
All
other
information
from
Thinkbook,
pages
133‐139,
and
he
corresponding
lecture.