The 1H spectral range is very narrow'

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Simplification of Complex Spectra

• The 1H spectral range is very narrow.
• Normally 0 to 10 ?.
• For most molecules of any size, the spectra
  quickly become congested.
• Due to many different chemical shifts.
• Due to overlap of multiplet splittings.

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Double Resonance

• Selectively radiate a particular type of H with
  high power.
• Causes rapid transitions between upper and lower
  spin states, causing saturation.
• The signal for that nucleus disappears from the
  spectrum
• ALL coupling to the irradiated nucleus also
  disappears from other peaks in the spectrum.
• spin decoupling

3
Double Resonance
4
Double Resonance

• Allows simplification of complex or overlapping
  multiplets.
• Allows identification of which Hs are coupled.

5
Double Resonance

• Allyl bromide
• Radiate at CH2Br frequency
• A series of doublets

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Heteronuclear spin decoupling
-CH3 decoupled
Aromatic H decoupled
19F spectrum
ALL H decoupled
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13C NMR Spectroscopy
8
13C NMR

• 12C is the most abundant natural isotope of
  carbon, but has a nuclear spin I 0, rendering
  it unobservable by NMR.
• Limited to the observation of the 13C nucleus
  which constitutes only 1.1 of naturally
  occurring carbon.

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13C Transition Energy

• The magnetogyric ratio, ?, for the 13C is 67.3
  compared to 267.5 for 1H.
• Remember the resonance condition for a nucleus is
  given by
• ? (?/2?)B0
• If the gyromagnetic ratio is lowered, the ?E is
  also lowered. Where a 1H spectrum using a 1.41 T
  magnet is observed at 60 MHz, a 13C spectrum is
  observed at 15 MHz roughly 4 times less
  energetic.
• Boltzmann Nupper/Nlower e-?E/kT e-h?/kT
• _at_ 298 K the ratio is 1,000,000 / 1,000,002

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13C NMR

• The combined effects of smaller excess
  populations in the lower energy state, low
  natural abundance, and slow relaxation rates
  result in a 13C signal that is typically 6000
  times weaker than that observed for 1H.
• With FT instruments, this is not a problem
  simply take more scans! (recall S/N increases as
  the square root of the number of scans).
• 16 scans on a 5-10 mg sample will give a good 1H
  spectrum,
• 512 scans on a 50 mg sample will give a good 13C
  spectrum.

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Fourier Transform NMR

• Radio-frequency pulse given.
• Nuclei absorb energy and precess (spin) like
  little tops.
• A complex signal is produced, then decays as the
  nuclei lose energy.
• Free induction decay is converted to spectrum.
  

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13C NMR

• low 13C abundance
• a single molecule will have at most only one 13C
  atom
• however, we are sampling a very large number of
  molecules, even in a 50 mg sample!
• thus our sampling will see a 13C at every C
  position in the molecule!

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13C Shielding

• 13C spectra are typically recorded from 0 220
  ppm with the zero being the methyl carbon in TMS
• (much wider range than 1H spectra!)
• 13C nuclei are shielded or deshielded (CHEMICAL
  SHIFT) due to the same factors as for 1H NMR.
• 1. Electron withdrawing ability (by inductance or
  resonance) of nearby groups.
• 2. Hybridization.
• 3. Electron current effects.

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13C NMR Chemical Shifts

• Several functionalities appear directly on 13C
  NMR which are not visible in 1H NMR
• - Quaternary carbons
• - ipso carbons
• - Carbonyl carbons

sp3 carbon
sp3-EWG
alkyne carbons
alkene carbons
aromatic carbons
carbonyl carbons
downfield d (ppm)
upfield deshielded shielded higher DE
lower DE
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Carbonyl Carbon Chemical Shifts
anhydrides
nitriles
acid chlorides
amides
esters
carboxylic acids
aldehydes
conj. ketones
ketones
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Spin-Spin Coupling in 13C NMR

• Homonuclear coupling of 13C-13C is possible in
  theory.
• However, due to the low natural abundance of 13C,
  it is rare to find two 13Cs in the same
  molecule, let alone adjacent to one another.
• No need to consider 13C-13C coupling except for
  enrichment studies!
• Heteronuclear coupling between 13C and the 1H
  atoms attached to them is observed (1H abundance
  99).
• Because the 1H atoms are directly attached, the
  coupling constants (1J)are large, typically
  100-250 Hz.
• When such spectra are observed, they are referred
  to as proton coupled spectra (or non-decoupled
  spectra).

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1H 13C Splitting

• The splitting follows the simple N1 rule
• The multiplet analysis gives useful information,
  but there are two major limitations
• 1) If the 13C signal is weak (common) the outer
  peaks of the multiplet may be lost in the noise
  of the spectrum.
• 2) Due to the large J-constants, the multiplets
  quickly begin to overlap and become congested.

quaternary singlet
methine doublet
methylene triplet
quaternary quartet
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13C NMR Spectrum
Proton-Coupled
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Effect of Coupling
Three equal intensity lines at 77 ppm CDCl3
solvent 13C- 2D coupling

• Coupling can cause 13C NMR spectra to become very
  complicated (convoluted) quite easily.

1H Coupled
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1H Decoupling

• To simplify the 13C spectrum, and to increase the
  intensity of the observed signals, a decoupler is
  used to remove the spin effects of the 1H
  nucleus.
• A second RF generator irradiates at the 1H
  resonance frequency causing the saturation
  effectively averaging all their spin states to
  zero.
• 1H channel-
• 13C channel

13C n pulse
13C FID
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13C Proton Decoupled Spectrum
13C1H
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Effect of Decoupling
1H Coupled
1H Decoupled
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13C NMR Spectra

• Due to signal enhancement and spectral
  simplification, 13C spectra are usually reported
  as 1H decoupled.
• Each chemically unique carbon in the molecule
  gives rise to a single peak.
• Of course chemically equivalent carbons
  contribute to the same peak!
• The number of different signals (peaks) indicates
  the number of different kinds of carbon.
• The location (chemical shift) indicates the type
  of functional group.

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13C NMR Intensities

• Peak areas (heights) are NOT proportional to
  number of carbons.
• Carbon atoms with more hydrogens give stronger
  signals, due to more efficient relaxation
  (transfer of spin to the hydrogens).
• However, peak areas (heights) can be compared
  within the same type of carbons (e.g. methyls)

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Example Ethanol
27
Example 1-bromohexane
28
Example cyclohexane
29
Example cyclohexene
30
Example 1,3-cyclohexadiene
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Example 1,4-cyclohexadiene
32
Example m-nitrotoluene
6
2
5
4
1
3
7
2
4
6
5
7
1
3
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Nuclear Overhauser Enhancement (NOE)

• A phenomenon observed with proton-decoupled
  13C-NMR is that the intensity of the signal for a
  given 13C increases versus the proton-coupled
  spectrum roughly proportional to the number of
  protons attached.
• The degree of this signal enhancement is called
  the Nuclear Overhauser Enhancement (NOE).
• This effect is general, and appears anytime when
  one of two types of atoms is irradiated, while
  the spectrum of the other is observed. In this
  case, while the 1H population is irradiated to
  saturation, the 13C is observed. Here a
  heteronuclear effect.

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NOE

• The effect can be a positive or negative one, but
  for the case of 1H-13C, the effect is positive
• The maximum enhancement is given by
• NOEmax 1 (? irradiated)
• 2 (? observed)
• This value is what is added to the observed
  intensity in the coupled spectrum to give the
  intensity observed in the decoupled spectrum
• total predicted intensity 1 NOEmax

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NOE

• For 1H 13C, NOE ½ (267.5/67.28) 1.988
• A maximum enhancement of almost 200 is possible.
• NOE operates in both directions 13C nuclei (if
  decoupled) would enhance the signal of 1H
  however, this signal would be weak due to the low
  abundance of 13C.
• Because NOE for 13C 1H operates in the opposite
  direction (a rare nuclei always bound to an
  abundant one) it is a useful probe into
  structural assignments.
• The NOE effect is very short-range, falling off
  as 1/r3 the distance between the nuclei.

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Origin of NOE
An isolated two spin system between a single
carbon and single hydrogen atom The effects of
coupling are left out for simplicty Shown are
the four combinations of spin states of these two
nuclei, N1-4 The two energy states where both
are spin up or spin down are the lowest and
highest energy states The mixed states are
roughly degenerate in energy
C H
N4
C H
C H
N3
N2
C H
N1
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Origins of NOE
Quantum mechanics dictates that allowed
transitions involve only one change of spin at a
time single quantum transitions The allowed
transitions are shown in red
C H
N4
C H
C H
N3
N2
C H
N1
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Origins of NOE
Let the equilibrium population of the two
degenerate states be B The N1 level would be
higher than B by a small amount, ? The N4 level
would be lower than B by a the same amount,
? The signal for a 13C in this case would be
proportional to ? at equilibrium The two 13C
transitions are N1 N2 and N3 N4
C H
N4
C H
C H
N3
N2
C H
N1
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Origins of NOE
When a decoupler is used, the 1H populations are
disturbed from their equilibrium
values Relaxation processes restore these
disturbed populations to their equilibrium
values One such process is a double-quantum
transition, where both the C and H nuclei relax
simultaneously (blue line) This leak in the
upper state enhances the population of the lower
energy state for carbon the excess population
is larger and the signal intensifies
C H
N4
C H
double quantum transition
C H
N3
N2
C H
N1
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NOE

• NOE an example of cross-polarization,
  polarization of spin states of one type of
  nucleus causes a polarization of the spin states
  of another nucleus.
• A heteronuclear NOE effect is always observed in
  normal 1H decoupled 13C spectra.
• Total NOE for a given C increases with number of
  nearby Hs. Thus intensities of C signals are
  generally
• CH3 gt CH2 gt CH gt C
• NOE effect is quite general. Can also be applied
  in a homonuclear sense, i.e. 1H1H

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NOE
Difference
Difference
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NOE

• Depends on cross-polarization of spin states.
• Can tell us what nuclei are close together.
• In contrast to J-coupling (spin-spin) which
  operates through the bonding electrons, NOE is a
  through-space effect.
• Thus NOE can tell us about the proximity of atoms
  which are separated by many bonds, e.g. proteins,
  RNA, DNA

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Example m-nitrotoluene
6
2
5
4
1
3
7
2
4
6
5
7
1
3
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13C Chemical Shift Predictions

• Examining a large set of chemical shift data has
  allowed the development of empirical rules or
  substituent parameters to allow chemical shift
  predictions for most commonly encountered
  situations.
• Example the carbon atoms of a substituted
  benzene ring.
• Benzene itself ? single peak at 128.7 ppm
• Add to this value substituent increments which
  depend on the chemical nature of the substituent
  and where it is on the ring relative to the
  carbon whose shift is being predicted.

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13C Aromatic Substituent Parameters
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C1 128.7 (CH3)ipso (NO2)meta 128.7 8.9
0.8 138.9 ppm C2 128.7 (CH3)ortho
(NO2)ortho 128.7 0.7 (-5.3) 124.1 ppm
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Example m-nitrotoluene
6
5
4
2
7
1
3
48
Example p-Hydroxyacetophenone
1
6
2
5
3
4
2
3
6
5
1
4
49
Example benzonitrile
Very weak no attached Hs No NOE effect!
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13C Shift Predictions Alkyls

• Can also make predictions for alkyl groups

Base value use unsubstituted hydrocarbon
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Example bromocyclopentane
2
1
2
3
3
1