CHEM 141 – Organic Chemistry I
W, 4-29, 2009
NMR Spectroscopy
• Resonance
• Instrument & solvents
• Diamagnetic currents and shielding
• Tetramethylsilane (TMS) as a reference
13C NMR Spectroscopy
ƒ 13C-1H splitting & the (n + 1) rule
ƒ Chemical shifts regions
ƒ DEPT experiments
Nuclear Spin in B0
• the energy difference between allowed spin states
increases linearly with applied field strength
• values shown here are for 1H nuclei
Nuclear Magnetic Resonance
• when nuclei with a spin quantum number of 1/2 are
placed in an applied field, a small majority of nuclear
spins are aligned with the applied field in the lower
energy state
• the nucleus begins to precess and traces out a coneshaped surface, in much the same way a spinning top
or gyroscope traces out a cone-shaped surface as it
precesses in the earth’s gravitational field
• we express the rate of precession as a frequency in
hertz
Nuclear Magnetic Resonance
• Figure 13.3 the origin of nuclear magnetic “resonance
Nuclear Magnetic Resonance
‹ If
the precessing nucleus is irradiated with
electromagnetic radiation of the same frequency
as the rate of precession,
• the two frequencies couple,
• energy is absorbed, and
• the nuclear spin is flipped from spin state +1/2 (with
the applied field) to -1/2 (against the applied field)
Nuclear Magnetic Resonance
• Figure 13.3 the origin of nuclear magnetic “resonance
Nuclear Magnetic Resonance
‹ Resonance:
in NMR spectroscopy, resonance is
the absorption of electromagnetic radiation by a
precessing nucleus and the resulting “flip” of its
nuclear spin from a lower energy state to a
higher energy state
‹ The instrument used to detect this coupling of
precession frequency and electromagnetic
radiation records it as a signal
• signal: a recording in an NMR spectrum of a nuclear
magnetic resonance
NMR Spectrometer
NMR Spectrometer
‹ Essentials
of an NMR spectrometer are a
powerful magnet, a radio-frequency generator,
and a radio-frequency detector
‹ The sample is dissolved in a solvent, most
commonly CDCl3 or D2O, and placed in a sample
tube which is then suspended in the magnetic
field and set spinning
‹ Using a Fourier transform NMR (FT-NMR)
spectrometer, a spectrum can be recorded in
about 2 seconds
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
22.4
.4
2 .6
2.8
3.0
3 .2
3 .4
Nuclear Magnetic Resonance
• if we were dealing with 1H nuclei isolated from all other
atoms and electrons, any combination of applied field
and radiation that produces a signal for one 1H would
produce a signal for all 1H. The same is true of 13C
nuclei
• but hydrogens in organic molecules are not isolated
from all other atoms; they are surrounded by
electrons, which are caused to circulate by the
presence of the applied field
• the circulation of electrons around a nucleus in an
applied field is called diamagnetic current and the
nuclear shielding resulting from it is called
diamagnetic shielding
Nuclear Magnetic Resonance
• the difference in resonance frequencies among the
various hydrogen nuclei within a molecule due to
shielding/deshielding is generally very small
• the difference in resonance frequencies for hydrogens
in CH3Cl compared to CH3F under an applied field of
7.05T is only 360 Hz, which is 1.2 parts per million
(ppm) compared with the irradiating frequency
360 Hz
300 x 106 Hz
=
1.2
106
= 1.2 p pm
Nuclear Magnetic Resonance
• signals are measured relative to the signal of the
reference compound tetramethylsilane (TMS)
CH3
CH3
Si CH3
CH3
Tetrameth yls ilane (TMS)
• for a 1H-NMR spectrum, signals are reported by their
shift from the 12 H signal in TMS
• for a 13C-NMR spectrum, signals are reported by their
shift from the 4 C signal in TMS
• Chemical shift (δ): the shift in ppm of an NMR signal
from the signal of TMS
13C-NMR
‹ Each
Spectroscopy
nonequivalent 13C gives a different signal
• a 13C signal is split by the 1H bonded to it according to
the (n + 1) rule
• coupling constants of 100-250 Hz are common, which
means that there is often significant overlap between
signals, and splitting patterns can be very difficult to
determine
most common mode of operation of a 13CNMR spectrometer is a hydrogen-decoupled
mode
‹ The
13C-NMR
Spectroscopy
‹ In
a hydrogen-decoupled mode, a sample is
irradiated with two different radio frequencies
• one to excite all 13C nuclei
• a second broad spectrum of frequencies to cause all
hydrogens in the molecule to undergo rapid
transitions between their nuclear spin states
the time scale of a 13C-NMR spectrum, each
hydrogen is in an average or effectively constant
nuclear spin state, with the result that 1H-13C
spin-spin interactions are not observed; they are
decoupled
‹ On
Chemical Shift - 13C-NMR
Typ e of
Carbon
Chemical
S hift (δ)
RCH3
RCH2 R
R3 CH
10-40
15-55
20-60
RCH2 I
RCH2 Br
0-40
25-65
RCH2 Cl
35-80
R3 COH
40-80
R3 COR
40-80
65-85
RC CR
R2 C=CR2
100-150
Type of
Carb on
C R
Chemical
Sh ift (δ)
110-160
O
RCOR
160 - 180
O
RCNR2
165 - 180
O
RCCOH
165 - 185
O
O
RCH, RCR
180 - 215
Chemical Shift - 13C-NMR
The DEPT Method
‹ In
the hydrogen-decoupled mode, information on
spin-spin coupling between 13C and hydrogens
bonded to it is lost
‹ The DEPT method is an instrumental mode that
provides a way to acquire this information
• Distortionless Enhancement by Polarization Transfer
(DEPT): an NMR technique for distinguishing among
13C signals for CH , CH , CH, and quaternary carbons
3
2
The DEPT Method
‹ The
DEPT methods uses a complex series of
pulses in both the 1H and 13C ranges, with the
result that CH3, CH2, and CH signals exhibit
different phases;
• signals for CH3 and CH carbons are recorded as
positive signals
• signals for CH2 carbons are recorded as negative
signals
• quaternary carbons give no signal in the DEPT method
Isopentyl acetate
• 13C-NMR: (a) proton decoupled and (b) DEPT
13
C-NMR
• Much lower sensitivity than 1H NMR (13C
abundance 1.1%)
• Don’t see 13C-13C couplings (most 13C are
next to a 12C)
• Since 13C-1H couplings are large & overlap
with other signals, we usually run “protondecoupled spectra.
• # of signals (lines) gives # of different
carbons
• Chemical shifts suggest functional groups
present
• Range of chemical shifts (δ) much wider than
for H’s (0 to approx. 200 ppm)
• Integrations not reliable
• DEPT experiments performed to determine #
of attached protons