Position
of
Signals:
The
Chemical
Shift
Basic
Vocabulary:
Chemical
Shift—the
position
of
a
signal
in
an
NMR
spectrum.
Deshielded
(downfield)—a
nucleus
whose
chemical
shift
is
increased,
feels
stronger
magnetic
field
due
to
the
removal
of
electron
density,
magnetic
induction,
etc
in
neighboring
atoms
or
groups
Integration—In
NMR
spectroscopy,
the
process
of
measuring
the
area
of
an
NMR
signal.
It
is
used
to
determine
the
relative
number
of
hydrogens
that
correspond
to
each
signal.
Magnetic
Induction—In
NMR
spectroscopy,
the
phenomenon
that
an
external
magnetic
field
causes
pi
electrons
in
an
electron
cloud
to
circulate.
As
these
electrons
move
around,
their
own
electric
charge
causes
them
to
create
their
own
magnetic
field.
Ppm—In
NMR
spectroscopy,
the
x­axis
units
of
the
chemical
shift
scale.
The
ppm
scale
increases
from
right
to
left.
Shielded
(upfield)—a
nucleus
whose
chemical
shift
is
decreased,
feels
weaker
magnetic
field
due
to
the
addition
of
electron
density,
magnetic
induction,
etc
in
neighboring
atoms
or
groups
Tetramethylsilane
(TMS)—a
chemical
shift
reference
used
in
NMR
spectroscopy
because
it
has
a
chemical
shift
of
0.00
ppm.
chemical
shift
=
0.00
ppm
by
definition
What
is
a
chemical
shift?
It
is
controlled
by
the
ΔE
(spin‐flip
energy),
which
is
determined
by
the
magnitude
of
the
magnetic
field
at
the
nucleus.
For
instance:
Low
magnetic
field

small
ΔE
(low
chemical
shift)
High
magnetic
field
large
ΔE
(high
chemical
shift)
Chemical
shift, ,is
the
difference
between
the
frequency
of
a
nuclear
spin
flip
of
the
nucleus
in
question
and
the
nuclear
spin
flip
of
a
reference
molecule,
divided
by
the
operating
frequency
of
the
NMR
spectrometer.
In
other
words,
it
is
the
resonant
frequency
of
a
nucleus
compared
to
a
standard.
The
position
and
number
(intensity)
of
chemical
shifts
can
help
us
determine
a
molecule’s
structure.
A
chemical
shift
graph:
Different
groups
within
in
a
molecule
can
have
different
chemical
shifts,
depending
on
their
bonds
and
the
atoms
attached.
What
factors
cause
chemical
shifts?
1.
Electron
Density:
In
general,
higher
electron
density

lower
chemical
shift
lower
electron
density

higher
chemical
shift
High
electron
density
of
neighboring
atoms
may
shield
the
nucleus
from
an
external
magnetic
field.
If
there
is
little
or
no
electron
density
in
nearby
atoms
or
groups,
the
nucleus
can
also
be
deshielded.
What
is
shielding
and
deshielding?
Shielding
is
when
the
nucleus
experiences
a
weaker
magnetic
field
around
it.
This
can
be
caused
by
other
atoms
“getting
in
the
way”
of
the
nucleus
and
the
magnetic
field,
or
the
nucleus
itself
having
a
low
spin‐flip
energy.
Due
to
the
weaker
magnetic
field
it
experiences,
a
nucleus
with
more
shielding
will
have
a
lower
ppm
and
therefore
lie
on
the
right
side
of
the
chemical
shift
scale.
Deshielding
is
when
the
nucleus
experiences
a
higher
magnetic
field
around
it.
This
can
be
due
to
its
proximity
to
a
strong
magnetic
field
or
having
itself
a
high
spin‐flip
energy.
Because
of
its
high
magnetic
field,
a
more
deshielded
nucleus
will
have
a
higher
ppm
and
therefore
lie
on
the
left
side
of
the
chemical
shift
scale.
Chemical
Shift
Scale
(ppm)
More
Deshielded
More
Shielded
2.
Electronegativity:
The
higher
the
electronegativity
difference
between
H
and
its
surrounding
atoms,
the
higher
the
chemical
shift.
This
is
because
the
higher
the
electronegativity
difference
between
H
and
another
atom,
the
lower
the
electron
density
is
around
the
H
(proton),
and
thus
the
more
deshielding
the
proton
experiences.
As
a
result,
the
proton
will
have
a
higher
chemical
shift.
For
example:
Iodine
has
an
EN
of
2.5
Bromine
has
an
EN
of
2.8
Chlorine
has
an
EN
of
3.0
Fluorine
has
an
EN
of
4.0
Hydrogen
has
an
EN
of
2.1
The
H—I
bond
is
the
least
electronegative
and
the
H—F
bond
is
the
most
electronegative.
So,
in
terms
of
chemical
shifts,
H—I
<
H—Br
<
H—Cl
<
H—F.
Less
EN
difference,
so
lower
chemical
shift
More
EN
difference,
so
higher
chemical
shift
Other
Trends:
• Electronegativity
effects
decrease
with
increasing
distance:
As
the
hydrogens
move
farther
away
from
chlorine,
the
most
electronegative
atom
in
the
molecule,
they
experience
more
shielding
due
to
the
other
hydrogen’s
“in
the
way.”
As
a
result,
the
Ha’s
have
a
significantly
higher
chemical
shift
(3.42
ppm),
than
the
Hd’s
(0.92
ppm).
•
In
a
methane
molecule,
the
more
“R”
groups
that
are
attached
to
the
central
carbon
atom,
the
more
electronegative
the
molecule
is,
and
therefore
the
higher
the
chemical
shift.
R3CH
>
RCH2R
>
RCH3
because
the
EN
of
the
“R”
group
is
greater
than
the
EN
of
hydrogen.
(Whatever
“R”
is,
it
will
have
a
greater
electronegativity
than
H).
Extra
Example:
Visual
representation
of
C‐H
chemical
shifts
in
different
molecules:
cyclo‐
hexane
aldehyde
alkene
nitro
benzene
Increasing
chemical
shift
PPM
TMS,
0
ppm
Decreasing
chemical
shift
It
is
important
to
note
that
the
position
of
signals
is
different
from
the
relative
intensity
of
the
signals
or
peaks.
The
position
is
what
has
been
discussed
here:
it
is
dependent
upon
shielding
and
electronegativity
of
nearby
atoms.
The
intensity
is
proportional
to
the
relative
number
of
equivalent
protons
there
are
in
one
position—it
is
related
to
the
measurement
of
the
area
under
the
peak,
calculated
by
integration.
Practice
Problem:
Which
of
the
following
has
the
highest
chemical
shift?
Which
has
the
lowest?
Solution:
1.
CH3F
Fluorine
is
the
most
electronegative
atom
(EN=4.0).
Therefore,
in
the
C—F
bond,
the
area
of
electron
density
is
around
the
F,
so
there
is
less
proton
density
around
C
and
surrounding
H’s
(protons).
As
a
result,
there
is
more
deshielding
(or
less
shielding)
around
the
protons,
causing
there
to
be
a
great
chemical
shift.
Oxygen
has
an
EN
of
3.5,
and
bromine
has
an
EN
of
2.8.
Oxygen
has
a
greater
EN
Works
Cited:
"The
Basis
for
Differences
in
Chemical
Shifts."
UC
Davis
ChemWiki.
3
Aug.
2011.
Web.
11
May
2012.
<http://chemwiki.ucdavis.edu>.
"Chemical
Shift."
Wikipedia.
Wikimedia
Foundation,
05
Nov.
2011.
Web.
11
May
2012.
<http://en.wikipedia.org/wiki/Chemical_shift>.
Hardinger,
Steven.
Chemistry
14C
Lecture
Supplement.
Fifth
ed.
Plymouth,
MI:
Hayden‐McNeil,
2011.
Hardinger,
Steven.
"UCLA
Chemistry
and
Biochemistry."
Organic
Chemistry
at
UCLA.
UCLA
Chemistry
and
Biochemistry.
Web.
11
May
2012.
<http://www.chem.ucla.edu/harding/index.html>.
"Nuclear
Magnetic
Resonance
Spectroscopy."
NMR
Spectroscopy.
Web.
12
May
2012.
<http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nm
r/nmr1.htm>.