Nuclear Magnetic Resonance Spectroscopy
Dr. Sheppard
Chemistry 2412L
Introduction NMR is the most powerful technique for organic
structure determination Number and type of atoms in a molecule
Connectivity of atoms
Used to study a wide variety of nuclei: 1H
13C
15N, 19F, 31P
Radio-frequency radiation used to transition between energy states 30 – 900 MHz
Transition = nuclear spin
Nuclear Spin
A nucleus with an odd atomic number or an odd mass number has a nuclear spin
The spinning charged nucleus generates a magnetic field
External Magnetic Field
When placed in an external field, spinning nuclei act like bar magnets
Two Energy States
The magnetic fields of the
spinning nuclei will align
either with the external field,
or against the field
A photon with the right
amount of energy can be
absorbed and cause the
spinning nucleus to flip
Spin flip = resonance Detected and recorded by the
spectrometer as a signal
The NMR Spectrometer
Magnetic Shielding If all nuclei absorbed the same amount
of energy in a given magnetic field, not much information could be obtained
But nuclei are surrounded by electrons that shield them from the external field
Circulating electrons create an induced magnetic field that opposes the external magnetic field Effective magnetic field
Shielded Nuclei Magnetic field strength must be increased for a
shielded nucleus to flip at the same frequency Differences detected by machine, cause
differences in signals (chemical shift, )
Nuclei in a Molecule Depending on their chemical environment, atoms in a
molecule are shielded by different amounts
Chemically equivalent nuclei Interchanged through bond rotation or element of symmetry
Have same absorption
Chemically different nuclei have different absorption
1H-NMR Spectrum for Methanol
Tetramethylsilane
TMS is added to the sample
Since silicon is less electronegative than
carbon, TMS protons are highly shielded
Signal defined as zero
Organic protons absorb downfield (to the left)
of the TMS signal
Deuterated solvent signal
Si
CH3
CH3
CH3
H3C
Chemical Shift
Measured in parts per million
Ratio of shift downfield from TMS (Hz)
to total spectrometer frequency (Hz)
Same value for 60, 100, or 300 MHz
machine
Called the delta () scale
Delta Scale
downfield upfield
Location of Signals
More electronegative atoms
deshield more and give
larger shift values (downfield)
Effect decreases with
distance
Additional electronegative
atoms cause increase in
chemical shift
Hydrogen and Carbon Chemical Shifts
NMR Spectra
13C-NMR
12C has no magnetic spin 13C has a magnetic spin, but is only 1% of
the carbon in a sample Signals are weak, get lost in noise Hundreds of spectra are taken, averaged Signal = one sharp line for each different
type of carbon
3-Pentanone
How many signals? Chemical shifts:
sp3 C upfield sp, sp2 C downfield C adjacent to en atom downfield
O
O
2-Butanone
How many signals? Chemical shifts?
O
O
How is 13C-NMR useful for reactions we have studied?
Zaitsev vs. non-ZaitsevCH3Br
Base
E2vs.
CH3 CH2
7 signals 5 signals
Interpreting 13C-NMR
The number of different signals indicates the number of different kinds of carbon
The chemical shift indicates the functional group
Use to support 1H-NMR analysis
1H-NMR
More info than 13C-NMR The number of signals shows how many different
kinds of protons are present
The location of the signals shows how shielded or
deshielded the proton is
The intensity of the signal shows the number of
protons of that type
Signal splitting shows the number of protons on
adjacent atoms
1H-NMR
Given a structure, how many signals are expected?
How many sets of H in each molecule?
CH3 CH
CH3
CH3 CH2 CH
CH3
CH3 CH3 CH2 CH2CH3 CH2 CH3
Isomers• Same molecular formula• Same IR stretches• Different NMR
Another example: CH3 C
O
CH2 C
O
O C
CH3
CH3
CH3
Chemical shifts in 1H-NMR Info about type of H giving rise to signal Strongly shielded = upfield (to the right) Less shielded = downfield (to the left) Most common shifts:
McMurry, Table 13-3
Typical Values
O-H and N-H Signals
Chemical shift depends on concentration Hydrogen bonding in concentrated
solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H
Using chemical shifts
Given a structure, predict Use to distinguish between two structures Example:
Constitutional isomers Each with 2 sets of H’s
HO
C
CH3
O
H
C
O
O
CH3
Which isomer best fits this spectrum?
orHO
C
CH3
O
H
C
O
O
CH3
Which isomer best fits this spectrum?
orHO
C
CH3
O
H
C
O
O
CH3
Intensity of Signals The area under each peak is proportional to
the number of protons Shown by integration line
Height area under peak # H’s in set Measure height with ruler or look at graph paper Ratio of height = ratio of hydrogens
HO
C
CH3
O
H
C
O
O
CH3
So far… Determine the number of sets of equivalent
hydrogen atomsNumber of signals on spectrum
Determine the number of hydrogen atoms in
each setIntegration line
Determine general information about adjacent
groupsChemical shift ()
Next… Determine specific information about
adjacent groups
In particular, how many H atoms on the
adjacent atomsSignal splitting