Nuclear Magnetic Resonance (NMR) Spectroscopy

1
Nuclear Magnetic Resonance (NMR) Spectroscopy
Dr. Vincent J. Storhaug
2
Spectrometer Tuning
Lock
1
4
1H-19F
X Channel
Proton
X Channel
Proton
X Channel
3
Spectrometer Drift Locking
Time
Resonance Frequency

• We DO want to use one of the frequencies as a
  feedback loop to correct for any drift in the
  field with time.
• We DO NOT want to use one of the frequencies that
  we wish to observe Heisenberg Uncertainty
  Principle.

4
Spectrometer Drift Locking
Mass Natural Abundance () Spin NMR Frequency (Mhz) RelativeSensitivity
1.007825 99.985 1/2 400.000 1.0
2.0140 0.015 1 61.400 9.65 e-3
5
Using the Spectrometer Lock for Shimming

• What is Shimming? In order for signals to be
  symmetrical and sharp, the magnetic field must
  pass through the sample homogeneously however,
  there are problems
• NMR Tube never filled to EXACTLY the same level.
• Different deuterated solvents do not have the
  same physical properties, e.g. VISCOSITY
• NMR tube may never be inserted at EXACTLY the
  same orientation in the sample holder, i.e. -
  spinner.
• Microscopic particulates/colloidal particles.
• Tube quality may not be the same
• Wilmad
• Norell
• Kontes
• Aldrich (Fisher)
• Chemglass
• How do we correct for these problems?

6
Using the Spectrometer Lock for Shimming
Shim Stack
7
Using the Spectrometer Lock for Shimming
8
Purpose of the Phase Cycle

• Unwanted COHERENT signals can be received from
  a variety
• of sources
• Imbalance of spectrometer hardware
• Coherent noise (rf pickup)
• Artifacts generated by multipulse techniques
  (2D)
• Other sources

• The way to distinguish noise from signal is to
  phase cycle the pulse and the
• phase sensitive detector.
• Actual phase cycle depends on the nmr
  technique
• Typically for a 1D experiment, you have a
  phase cycle of 4 or 8
• Should always make the number of scans a
  multiple of the phase cycle

x
Pulse Phase
Receiver Phase
O (x)
O (x)
y
9O (y)
9O (y)
180 (-x)
180 (-x)
270 (-y)
270 (-y)
9
Purpose of the Phase Cycle
O (x)
180 (-x)
10
Terminology Chemical Shift (?)
Due to the presence of shielding electron(s), the
magnetic field strength effectively interacting
with the nucleus is always slightly weaker than
the applied magnetic field.
B0 magnitude of the resultant magnetic
field Bappl magnitude of the applied magnetic
field s screening constant
Revisiting the Resonance Frequency
11
Terminology Chemical Shift (?)
Step 1 Define a reference point Internal
reference used is tetramethyl silane (TMS)
methyl groups are isolated from one another, and
all hydrogen nuclei are magnetically equivalent.
In addition, the screening constant for TMS is
much larger than for most other hydrogen nuclei
in organic compounds. Step 2 Define an abscissa
scale We know that the resonance frequency is
dependent on the strength of the magnetic field.
What would be useful is a scale which allows us
to compare spectra directly from spectrometers
which use different applied field strengths
i.e. an abscissa value that is independent of
the applied field strength.
?r resonance frequency of reference (TMS for 1H
NMR) ?s resonance frequency of sample nucleus
12
Terminology Chemical Shift (?)
Since we know that (for a 1H NMR spectrum with
TMS as the internal reference) sr ltlt 1, we have
13
Terminology Chemical Shift (?)
Step 2 Define an abscissa scale (cont.) To make
the value more manageable, we apply the
following definition
Skoog, et al., defines the chemical shift
parameter d as
And provide it with the units ppm, i.e. - parts
per million.
14
Terminology Chemical Shift (?)
1.00
2.00
15
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts

1. The closest electrons to a nucleus are those that
   bond the nucleus to its neighboring atoms.
2. Under the influence of a magnetic field, bonding
   electrons tend to precess in a plane which is
   perpendicular to the applied field.
3. Any factor that effects the distribution of these
   bonding electrons will also effect the degree of
   shielding the nucleus experiences. Therefore, the
   electronegativities of the surrounding bonded
   atoms are an extremely important factor that will
   alter the distribution of bonding electrons.

Circulating Electrons
B0
Opposing Field of sB0
16
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
17
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
Compound CH3X CH3H CH3I CH3Br CH3Cl CH3F
Element X H I Br Cl F
Electronegativity of X 2.1 2.5 2.8 3.1 4.0
Chemical Shift (d) 0.23 2.16 2.68 3.05 4.26
18
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
19
Structural Effects on NMR Chemical Shifts
20
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
d
HC
2.9
CH
CH2
H2C
5.8
Increasing Electronegativity
H3C
CH3
0.9
Effect is caused by magnetic anisotropy. This is
a molecular level distortion of the applied
magnetic field caused by a ring current.
21
Shielding Effects Why We Obtain a Spectrum for
the Sample

• Circulation of electrons within molecular
  orbitals results in local magnetic fields that
  oppose the applied magnetic field.
• A higher shielding of the nuclear results in a
  lower Beff for the nucleus of interest. A smaller
  value for B means a slightly lower energy of
  transition.

22
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
Ring Current
H
H
B0
sB0
sB0
Opposing Field
Note the direction of the field for the hydrogen
nuclei. The magnetic field induced is
constructive for the hydrogen nuclei with respect
to B0, not destructive.
23
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
d
HC
2.9
CH
CH2
H2C
5.8
Increasing Electronegativity
H3C
CH3
0.9
Effect is caused by magnetic anisotropy. This is
a molecular level distortion of the applied
magnetic field caused by a ring current.
24
Origin of the Shielding Which Leads to
Differences in the Observed Chemical Shifts
H
H
C
C
B0
H
H
H
C
B0
C
H
25
Shielding Effects Why We Obtain a Spectrum for
the Sample

• Circulation of electrons within molecular
  orbitals results in local magnetic fields that
  oppose the applied magnetic field.
• A higher shielding of the nuclear results in a
  lower Beff for the nucleus of interest. A smaller
  value for B means a slightly lower energy of
  transition.

26
Origin of Spin-Spin Splitting
Spin-spin splitting occurs when the magnetic
moment of one nucleus (µA) interacts with the
magnetic moments of immediately adjacent nuclei
(µB, µC, etc.).
Polarization Interaction where the magnetic
coupling of nuclei is transmitted through bonding
electrons. The coupling can only travel so far
through bonds and still be effective/observable.
Because these effects are transmitted through
bonding electrons, they are independent of the
applied magnetic field. Thus, for a given (type
of) nucleus, the chemical shift remains the same,
and only the pattern of the signal changes.
27
Origin of Spin-Spin Splitting
CH3-CH2-OH
Energy
B0
28
Origin of Spin-Spin Splitting
CH3-CH2-OH
Because the neighboring nuclei oppose B0, the
magnetic field felt by the methyl protons is
lessened.
Energy
B0
29
Origin of Spin-Spin Splitting
CH3-CH2-OH
I
II
Energy
B0
III
30
Origin of Spin-Spin Splitting
Example ethanol
CH3-CH2-OH
Energy
B0
31
Origin of Spin-Spin Splitting
CH3-CH2-OH
I
II
III
Energy
B0
IV
32
Origin of Spin-Spin Splitting
CH3-CH2-OH
33
Patterns of Splitting Pascals Triangle
34
Patterns of Splitting(First Order Spectra)
35
Rules Governing the Interpretation of First Order
Spectra
In, first-order spectra, the chemical shift
(value) between interacting groups of nuclei is
large with respect to their coupling constants,
J. To guarantee first-order behavior, J/d
0.05 However, in most cases a larger value will
still allow approximation of first-order
responses.
?
Equivalent nuclei do not split each
other. Coupling constants decrease in magnitude
rapidly with physical separation of the groups of
nuclei. Coupling is rarely observed along more
than four bonds. The n 1 rule applies, where n
is the number of magnetically equivalent nuclei
attached to a neighboring atom. If coupling
exists for nuclei B with both nuclei A and C,
then the multiplicity of B is given by the
function (nA 1)(nC 1).
?
?
?
36
Rules Governing the Interpretation of First Order
Spectra
?
The approximate relative areas of the multiplet
are determined by the patterns found in Pascals
Triangle. Since coupling constants are
independent of the field strength, we can
distinguish a multiplet from individual closely
spaced signals by running the sample at a
different field strength. Reminder The chemical
shift is independent of the field strength, but
the resonance frequency is not.
?
37
Second Order Spectra
When J/d becomes greater than 0.10, the rules for
first order spectra do not apply.
?
As d approaches J in magnitude, the peaks on the
inner sides of the multiplets get enhanced at
the expense of the peaks on the outer sides.
(Remember that the total area must remain the
same.) More lines than might be expected by
applying the nearest neighbor rule may be
visible. (i.e. Coupling is observed along four
or more bonds.)
?
38
Spectra Involving Chemical Exchange Processes
Pure dry liquid ethanol.
Ethanol containing a very small amount of HCl.
Note in this case there is no change in chemical
shift(s), only in splitting pattern(s).
39
Spectra Involving Chemical Exchange Processes
?
The observed signal is the result of the weighted
average of the nucleus in its different magnetic
environments. Fast exchanges show up as sharp
signals. Exchanges on the NMR timescale
(intermediate) show up as broad signals.
(presence of acid/base catalyst, temperature,
nature of the solvent, etc.) Slow exchanges will
show two separate lines.
?
40
Terminology Chemical Shift (?)
1.00 ppm
400.00 Hz
399.978 Hz
1
3
5
7
41
Coupling Constants in Different Field Strengths
?0 399.978 MHz
42
Coupling Constants in Different Field Strengths
?0 399.978 MHz
HA
HB
JAB
JBA
43
Coupling Constants in Different Field Strengths
?0 399.978 MHz
HA
JAB
JAC
JAC
44
Coupling Constants in Different Field Strengths
?0 399.978 MHz
10 Hz
4 Hz
4 Hz
10 Hz
?0 499.983 MHz
4 Hz
4 Hz
45
Coupling Constants in Different Field Strengths
?0 399.978 MHz
10 Hz
4 Hz
4 Hz
12.5 Hz
?0 499.983 MHz
4 Hz
4 Hz