Nuclear Magnetic Resonance Spectroscopy
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Transcript of Nuclear Magnetic Resonance Spectroscopy
Nuclear Magnetic Resonance Spectroscopy
Dr. TodebushChemistry 2412L
IntroductionNMR 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 SpinA 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 FieldWhen placed in an external field,
spinning protons act like bar magnets
Two Energy StatesThe 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 proton to flip
Spin flip = resonance◦ Detected and recorded by the
spectrometer as a signal
Magnetic ShieldingIf all protons absorbed the same
amount of energy in a given magnetic field, not much information could be obtained
But protons 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 ProtonsMagnetic field strength must be increased
for a shielded proton to flip at the same frequency
Differences detected by machine, cause differences in signals (chemical shift, d)
Protons in a MoleculeDepending on their chemical environment, protons in a
molecule are shielded by different amountsChemically equivalent nuclei
◦ Interchanged through bond rotation or element of symmetry◦ Have same absorption
Chemically different nuclei have different absorption
NMR SignalsThe number of signals shows how many
different kinds of protons are presentThe location of the signals shows how
shielded or deshielded the proton isThe intensity of the signal shows the
number of protons of that typeSignal splitting shows the number of
protons on adjacent atoms
The NMR Spectrometer
The NMR Graph
Tetramethylsilane
TMS is added to the sampleSince silicon is less electronegative than
carbon, TMS protons are highly shieldedSignal defined as zeroOrganic protons absorb downfield (to
the left) of the TMS signalDeuterated solvent signal
Si
CH3
CH3
CH3
H3C
Chemical ShiftMeasured in parts per millionRatio of shift downfield from TMS
(Hz) to total spectrometer frequency (Hz)
Same value for 60, 100, or 300 MHz machine
Called the delta (d) scale
Delta Scale
downfield upfield
Location of SignalsMore 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
13C-NMR
12C has no magnetic spin13C has a magnetic spin, but is only
1% of the carbon in a sampleSignals are weak, get lost in noiseHundreds of spectra are taken,
averagedSignal = one sharp line for each
different type of carbon
3-PentanoneHow many signals?Chemical shifts:
◦sp3 C upfield◦sp, sp2 C downfield◦C adjacent to en atom downfield
O
O
2-ButanoneHow many signals?Chemical shifts?
O
O
How is 13C useful for reactions we have studied?Zaitsev vs. non-Zaitsev
CH3Br
Base
E2vs.
CH3 CH2
7 signals 5 signals
1H-NMRMore info than 13C-NMRGiven 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-NMRInfo about type of H giving rise to
signalStrongly shielded = upfield (to the
right)Less shielded = downfield (to the
left)Most common shifts:
◦Wade Appendix 1A◦Wade, Table 13-3◦-CH2-O-C(O)- ranged from 3.7-4.7 ppm
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 d3.5 for N-H and d4.5 for O-H
Using chemical shifts
Given a structure, predict dUse to distinguish between two
structuresExample:
◦Constitutional isomers◦Each with 2 sets of H’s
HOC
CH3
O
HC
O
O
CH3
Which isomer best fits this spectrum?
orHO
CCH3
O
HC
O
O
CH3
Which isomer best fits this spectrum?
orHO
CCH3
O
HC
O
O
CH3
Intensity of SignalsThe area under each peak is
proportional to the number of protonsShown by integration line
◦ Height a area under peak a # H’s in set◦ Measure height with ruler or look at graph
paper◦ Ratio of height = ratio of hydrogens
HOC
CH3
O
HC
O
O
CH3