NMR Spectroscopy Part I: Basic concepts of 1H and 13C NMR

1
NMR SpectroscopyPart I Basic concepts of 1H and
13C NMR
CHEM 430 Organic Spectral Analysis
2
NMR Spectroscopy
Introduction

• General overview
• NMR spectroscopy has emerged as the penultimate
  spectroscopic method for organic structural
  analysis
• Currently, the development of novel NMR methods
  is in its golden age with some of the 2-D
  methods entering their maturation period as
  routine spectroscopic methods
• A typical NMR sample consists of 5-10 mg of
  sample, with which a full analysis of 1H, 13C,
  ADEPT, 1H-1H COSY, 1H-13C COSY, NOESY could be
  done in a few hours on a high-field instrument
• Important spin-offs of NMR spectroscopy include a
  host of medical and security imaging equipment

3
NMR Spectroscopy
General Theory

• Nuclear Spin States
• The sub-atomic particles within atomic nuclei
  possess a spin quantum number just like electrons
• As with electrons, the nucleons are organized in
  energy levels
• Just as when using Hunds rules to fill atomic
  orbitals with electrons, nucleons must each have
  a unique set of quantum numbers
• The total spin quantum number of a nucleus is a
  physical constant, I
• For each nucleus, the total number of spin states
  allowed is given by the equation
• 2I 1

4
NMR Spectroscopy
General Theory

• Nuclear Spin States
• Observe that for atoms with no net nuclear spin,
  there are zero allowed spin states
• All the spin states of a given nucleus are
  degenerate in energy

5
NMR Spectroscopy
General Theory

• Nuclear Magnetic Moments
• A nucleus contains protons, which each bear a 1
  charge
• If the nucleus has a net nuclear spin, and an odd
  number of protons, the rotation of the nucleus
  will generate a magnetic field along the axis of
  rotation
• Thus, a nucleus has a magnetic moment, m,
  generated by its charge and spin
• A hydrogen atom with its lone proton making up
  the nucleus, can have two possible spin states,
  degenerate in energy

m
m
H
H
½
- ½
6
NMR Spectroscopy
General Theory

• Nuclear Spin States
• In the presence of an externally applied magnetic
  field, these two spin states are no longer
  degenerate in energy
• The spin opposed orientation is slightly higher
  in energy than the spin aligned orientation

m
H
- ½
B0 externally applied magnetic field
DE
m
H
½
7
NMR Spectroscopy
General Theory

• Absorption of Energy
• The energy difference between the two
  non-degenerate spin states in the presence of an
  applied magnetic field is quantized
• At low B0 is easy to surmise that the potential
  energy of the spin opposed state would be low,
  and as B0 grows in strength, so would the
  potential energy
• Thus, with increasing strength of B0, DE between
  the two spin states also increases

DE
B0 increasing
8
NMR Spectroscopy
General Theory

• Absorption of Energy
• From theory we have already discussed, we say
  that a quantum mechanical particle can absorb a
  photon of energy equal to DE and become promoted
  to the higher state
• This energy is proportional to the frequency of
  the photon absorbed, and in the case of nuclear
  spin, is a function of the magnetic field
  applied
• DE hn f (B0)
• Every nucleus has a different ratio of m to
  angular momentum (each has a different charge and
  mass) this is referred to as the magnetogyric
  ratio, g
• DE hn f (gB0)
• Angular momentum is quantized in units of h/2p,
  thus
• DE hn g (h/2p)B0

9
NMR Spectroscopy
General Theory

• Absorption of Energy
• Solving for the frequency of EM radiation we are
  observing
• DE n (g/2p) B0
• For a bare hydrogen nucleus (H), g 267.53
  (106 radians/Tsec)
• In a field strength of 1 Tesla, DE 42.5 MHz
  (for our discussion, at 1.41 T, DE 60 MHz)
• DE hn f (B0)
• This energy difference corresponds to the highly
  weak radio frequency region of the EM spectrum
  with wavelengths of gt5 meters equal to lt 0.02
  calmol-1

UV
X-rays
IR
g-rays
Radio
Microwave
10
NMR Spectroscopy
General Theory

• Mechanism of absorption nuclear magnetic
  resonance
• What we are actually observing for DE is the
  precessional or Larmor frequency (w) of the
  spinning nucleus this is analogous to a
  spinning toy top precessing as a result of the
  influence of the earths magnetic field

w
H
B0
11
NMR Spectroscopy
General Theory

• Mechanism of absorption Nuclear Magnetic
  Resonance
• When a photon of n 60 MHz encounters this
  spinning charged system (a bare proton) the two
  can couple and change the spin state of the
  proton

w
DE
This state is called nuclear magnetic resonance,
and the nucleus is said to be in resonance with
the incoming radio wave
H
B0
n w
w
H
12
NMR Spectroscopy
General Theory

• Mechanism of absorption Nuclear Magnetic
  Resonance
• The energy difference corresponding to 60 MHz (DE
  hn) is 2.39 x 10-5 kJ mol-1 (tiny) thermal
  energy at room temperature (298 oK) is sufficient
  to populate both energy levels
• The energy difference is small, so rapid exchange
  is occurring between the two populations, but
  there is always a net excess of protons in the
  lower energy state
• From the Boltzman distribution equation we can
  calculate the population of each energy state
• Nupper/Nlower e-DE/kT e-hn/kT
• _at_ 298 oK the ratio is 1,000,000 / 1,000,009 !
• There is an excess population of 9 nuclei in the
  lower energy state!

13
NMR Spectroscopy
General Theory

• Mechanism of absorption Nuclear Magnetic
  Resonance
• As the applied B0 increases, exchange becomes
  more difficult and the excess increases
• In each case, it is these few nuclei that allow
  us to observe NMR
• When radio radiation is applied to a sample both
  transitions upward and downward are stimulated
  if too much radiation is applied both states
  completely equilibrate a state called
  saturation no NMR signal can be observed

14
NMR Spectroscopy
General Theory

• Chemical Shift
• Spectroscopic observation of the NMR phenomenon
  would be of little use if all protons resonated
  at the same frequency
• The protons in organic compounds are not bare
  nuclei, they are surrounded by an s -orbital of
  containing an electron shared with an electron in
  a hybridized orbital of another atom to form a
  covalent bond
• In the presence of an external magnetic field, an
  induced circulation of electrons opposite to that
  of a proton is observed since the two are of
  opposite charges
• This induced circulation generates a magnetic
  field in opposition to the applied magnetic field
  a local diamagnetic current

15
NMR Spectroscopy
General Theory

• Chemical Shift
• Since the magnetic field felt by the proton
  within this electron cloud is lowered, the
  resonance condition frequency is also lowered
• This effect of lowering the energy of transition
  by a cloud of electrons is called diamagnetic
  shielding or shielding
• The opposite effect if electron density is
  removed from the vicinity of the proton is called
  deshielding

DE
B0
B0
16
NMR Spectroscopy
General Theory

• Chemical Shift
• The effect of electrons on a 1.41 T magnetic
  field is negligible, but measurable
• Compare the resonance frequencies for the protons
  in fluromethane vs. chloromethane
• CH3F CH3Cl
• The stronger inductive w/d of electrons by
  fluorine reduces the resonance frequency by 72 Hz
  (not MHz) compared to an operating frequency of
  the instrument at 60 MHz _at_ 1.41 T barely 1 part
  per million (ppm)
• There needs to be a reference proton by which
  these chemical shifts can be related - the
  best candidate would be a completely deshielded
  proton (H) which does not exist in the solution
  phase

17
NMR Spectroscopy
General Theory

• Chemical Shift
• NMR spectroscopists chose the other end of the
  spectrum- a proton that was more shielded than
  any other known proton (at the time) those in
  tetramethylsilane (TMS)
• The 12 chemically identical protons in TMS were
  used as the standard zero for an NMR spectrum
• The resonance frequency of any proton to be
  studied (since all were less shielded) would be
  at parts per million of the operating frequency
  of the instrument greater than this zero
• This allowed NMR instruments of varying field
  (and thus operating frequency) strengths to use
  the same scale
• Heres how

18
NMR Spectroscopy
General Theory

• Chemical Shift
• In an applied field of 1.41 T, the resonance
  frequency for a typical proton is 60 MHz, at 2.35
  T it is at 100 MHz a ratio of 5/3
• Thus, for a given proton, the shift in Hz from
  the TMS standard should be 5/3 greater in the 100
  MHz instrument compared to the 60 MHz
• Since these are simple ratios, we can simply
  factor out the effect of field strength by
  defining d, or chemical shift to be
• d (shift from TMS in Hz)
• (spectrometer frequency in MHz)
• or ppm of the instruments operating frequency

19
NMR Spectroscopy
Spectrometer Design

• Continuous-Wave (CW) Instrument
• An NMR spectrometer needs to perform several
  functions
• Generate a high (gt1 Tesla) magnetic field to
  split the energy levels of the spin states enough
  to
• Create an excess nuclei population large enough
  to observe
• Make the radio n that correspond to the
  transition be observable
• Ensure that the field is homogeneous (shimming)
• Vary either the applied field or the
  radiofrequency (RF) to observe different nuclei
  at their various energies of transition
• Receive the faint signal of the relaxation of the
  excited nuclei to their ground state
• Process the signal into a usable spectrum vs. a
  reference

20
NMR Spectroscopy
Spectrometer Design

• Continuous-Wave (CW) Instrument

RF Detector
RF (60 MHz) oscillator
Permanent Magnet
Variable magnetic field 1.41 T few millionths
of T
21
NMR Spectroscopy
Spectrometer Design

• How it works (CW NMR)
• The sample is placed in a 5 mm solution cell or
  tube (experimental aspects we will cover shortly)
  in the center of a large permanent or
  electromagnet
• A RF oscillator coil at 90 to the sample
  generates a radio signal at the operating
  frequency of the instrument (60 MHz for a 1.41 T
  field)
• The overall magnetic field is varied by a small
  electromagnet capping the poles of the larger
  field magnet
• Remember DE n (g/2p) B0, so variations of
  either magnetic field or frequency will cover the
  observed spectral width if the other is held
  constant
• As with older dispersive IR instruments, the
  sweep of magnetic fields is simultaneous with the
  movement of the chart paper

22
NMR Spectroscopy
Spectrometer Design

• How it works (CW NMR)
• As a particular proton population comes into
  resonance, a second receiver coil at 90 to the
  transmitter coil will pick up the change in
  orientation of nuclear spin
• This is recorded by the chart as a voltage
  response, proportional to the size of the proton
  population that generated the resonance
• One artifact of CW instruments is that the
  relaxation of the protons is slower than the
  movement (sweep) of the chart paper
• This causes the ringing effect a decreasing
  oscillation of the signal after the spectrometer
  has moved past a given resonance
• CW instruments operate by bringing each
  individual population of protons into resonance
  individually.

23
NMR Spectroscopy
Spectrometer Design

• Limitations- CW NMR
• Since the spectrum is collected once, the sample
  must possess enough protons to give a suitable
  excess population that can be observed need a
  concentrated sample
• Due to the limitations of the relatively low
  magnetic field (CW instruments top out at 60-90
  MHz) the coupling constants for JHH are
  relatively large compared to the spectral width
  so only simple molecules can be observed and
  their structures elucidated
• For nuclei of lower magnetogyric ratios, g, or
  natural abundance (13C most specifically) the
  ratio of radio noise to signal is high

24
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• First, what is a Fourier Transform?
• Fourier transforms interconvert mathematical
  functions in the frequency domain to the time
  domain
• For purposes of this discussion, we will black
  box the actual calculations and derivations of
  these functions, but we need to understand what
  they do

?
f(n) ? f(t) e-int dt f(t) ½ p ? f(n) eint dt
-?
?
-?
25
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• If we feed two simple oscillating equations into
  a FT, here are the results

f (t) cos (nt)
FT
-n
n
f (t) sin (nt)
FT
n
-n
26
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• In the FT instrument, all proton populations are
  excited simultaneously by a short, intense burst
  of RF energy
• Due to a variation of the Heisenberg Uncertainty
  Principle, even if the RF generator is set at 90
  MHz, if the duration of the pulse is short, the
  radio waves do not have time to establish a solid
  fundamental frequency
• This can be illustrated by the following cartoon,
  showing the combination of a short pulse being
  added to a step function

on
on
on


off
off
off
off
tp pulse duration
27
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• If this short pulse is converted into the
  frequency domain by a FT
• Observe that we have a continuum of frequency
  content centered at the operating frequency of
  the instrument
• We will talk more about the effects of pulse time
  and width when we discuss advanced 1-D and 2-D NMR

on
FT
off
off
tp
n
28
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• For now, if a sample containing one unique
  population of hydrogens was excited over tp by a
  pulse, it would then relax back to its original
  spin state
• As each nuclei relaxes it will emit RF radiation
  of a given frequency since different nuclei will
  relax at different rates, the signal decays over
  time
• This emission is recorded by the spectrometer as
  a free-induction decay or FID

off
off
29
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• The actual frequency of the FID is the
  interference signal of the relaxing protons
  superimposed with the frequency of the RF source
• Conversion of this decay signal by FT back into
  the frequency domain gives us the actual n of
  resonance for the proton being observed
• Again to due Heisenburg and other factors, NMR
  signals are not single lines, but a Lorentzian
  shaped continuum of lines centered at the n of
  the signal

Proton signal
Pulse n
FT
n
time
30
NMR Spectroscopy
Spectrometer Design

• Pulsed Fourier Transform (FT) Instrument
• Advantages
• Since all nuclei are excited and observed
  simultaneously, the pulse can be repeated after
  each relaxation period (for 1H, about 10 seconds)
  and the resulting signals added together
• Because we are observing weak radiofrequency
  signals in a sea of RF noise for dilute samples
  (or those observed once as in CW NMR) noise
  becomes an issue
• If several to hundreds of FIDs are added
  together, signals will tend to constructively add
  together and become more pronounced since noise
  is random, it will tend to destructively add and
  become less pronounced
• Signal to noise ratio improves as a function of
  the square root of the scans (FIDs) performed
  S/N f (n)

31
NMR Spectroscopy
The NMR Spectrum - 1H

• A typical 1H NMR is recorded from -2 to 15 d
  (ppm) what is typically reported is the region
  from 0 to 10 d
• Remember, if a proton is shielded (e- circulation
  reduces felt magnetic field) DE for the
  transition is lowered and the signal is near the
  high field or upfield region of the spectrum
  (right)
• If the proton is deshielded (e- circulation
  doesnt reduce the felt magnetic field) DE for
  the transition is raised and the signal is near
  the low field or downfield region of the spectrum
  (left)

low DE shielded 1H reduces B0 upfield
high DE deshielded 1H sees full B0 downfield
0
10
d or ppm
32
NMR Spectroscopy
The NMR Spectrum - 1H

• The number of signals observed will be equal to
  the number of unique populations of chemically
  equivalent protons
• To determine if two protons are chemically
  equivalent, substitute X for that each
  respective hydrogen in the compound and compare
  the structures
• If the two structures are fully superimposible
  (identical) the two hydrogens are chemically
  equivalent if the two structures are different
  the two hydrogens were not chemically equivalent
• A simple example p-xylene

Same structure
33
NMR Spectroscopy
The NMR Spectrum - 1H

• The position (v) of each resonance is dependant
  on the electronic environment around the proton
  chemical shift as a result of local diamagnetic
  shielding
• There are three principle effects that
  contribute to local diamagnetic shielding
• Electronegativity
• Hybridization
• Proton acidity/exchange

34
NMR Spectroscopy
The NMR Spectrum - 1H

• Local Diamagnetic Shielding - Electronegativity
  
• Electronegative groups comprise most organic
  functionalities
• -F -Cl -Br -I -OH -OR -NH2
• -NHR -NR2 -NH3 -CO -NO2 -NO -SO3H
• -PO3H2 -SH -Ph -CC and most others
• In all cases, the inductive w/d of electrons of
  these groups decreases the electron density in
  the C-H covalent bond proton is deshielded
  higher DE of transition

35
NMR Spectroscopy
The NMR Spectrum - 1H

• Local Diamagnetic Shielding - Electronegativity
  
• Protons bound to carbons bearing electron
  withdrawing groups are deshielded based on the
  magnitude of the withdrawing effect Pauling
  electronegativity

36
NMR Spectroscopy
The NMR Spectrum - 1H

• Local Diamagnetic Shielding - Electronegativity
  
• The magnitude of the withdrawing effect is
  cumulative
• The magnitude of the withdrawing effect is
  reduced by distance, as the inductive model
  suggests

37
NMR Spectroscopy
The NMR Spectrum - 1H

• Local Diamagnetic Shielding - Hybridization
• The hybridization of the carbon the proton is
  bound exerts a strong electronic effect
• The greater the s-character, the more tightly
  bound the electrons are to carbon, raising its
  effective electronegativity (sp 50 s, sp2, 33
  s and sp3 25 s)

Something odd is happening here, as we will
discuss
38
NMR Spectroscopy
The NMR Spectrum - 1H

• Local Diamagnetic Shielding - Proton
  Acidity/Exchange
• If an organic molecule that possesses hydrogen
  atoms of low pKA are dissolved in a deuterated
  solvent that also has a low pKA, the visible
  protons will exchange with deuterium from
  solvent and become invisible to the NMR
  spectrometer
• Such studies are useful, if it is desired to see
  which H-atoms on an organic are acidic!

39
NMR Spectroscopy
The NMR Spectrum - 1H

• Local Diamagnetic Shielding - Proton
  Acidity/Exchange
• Due to H-bonding effects, the resonance for
  certain functional groups (esp. OH and NH2) can
  change drastically dependent on concentration and
  the extent of the H-bonding
• Just as in IR spectroscopy, peaks corresponding
  to these resonances are broad and often undefined
  observing a continuum of bond
  strengths/electron densities about the observed
  proton
• The correlation tables for the position of such
  protons tend to be broad and unreliable
• Acid OH 10.5-12.0 d
• Phenol OH 4.0-12.0 d
• Alcohol OH 0.5-5.0 d
• Amine NH2 0.5-5.0 d
• Amide NH2 5.0-8.0 d
• Enol CHCH-OH gt15 d

40
NMR Spectroscopy
The NMR Spectrum - 1H

• Some observed 1H resonances can not be fully
  explained by local diamagnetic shielding effects
• Magnetic Anisotropy literally magnetic
  dissimilarity
• For example, by our hybridization model, a proton
  bound to an sp2 C should be observed at lower d
  than a proton bound to an sp C

41
NMR Spectroscopy
The NMR Spectrum - 1H

• Magnetic Anisotropy
• This effect is primary due to the fact that there
  is an additional effect of circulating electrons,
  observed in p-systems
• In benzene, the 6-p-orbitals overlap to allow
  full circulation of electrons as these electrons
  circulate in the applied magnetic field they
  oppose the applied magnetic field at the center
  just like the circulation of electrons in the 1-s
  orbital about hydrogen at the middle!

B0
42
NMR Spectroscopy
The NMR Spectrum - 1H

• Magnetic Anisotropy
• On the periphery of the ring, the effect is
  opposite the magnetic effect reinforces the
  applied B0, and DE becomes greater deshielding
  effect

43
NMR Spectroscopy
The NMR Spectrum - 1H

• Magnetic Anisotropy
• This theory can easily be tested by the
  observation of large aromatic systems that
  possess protons inside the ring (now a shielding
  effect)
• Or over a ring system

-1.8 d
8.9 d
2.0 d
-1.0 d
44
NMR Spectroscopy
The NMR Spectrum - 1H

• Magnetic Anisotropy
• In alkynes, a similar situation (to the central
  protons in large aromatic systems) arises where
  the terminal proton is in the region of maximum
  shielding

45
NMR Spectroscopy
The NMR Spectrum - 1H

• General Correlation Chart 1H NMR
• Due to the three effects on local diamagnetic
  shielding, in conjunction with the effect of
  magnetic anisotropy 1H NMR chemical shifts are
  variable
• Avoid using hard and fast rules (tables of
  numbers)
• Instead, start from the general correlation table
  and deduce structural features based on the
  effects just discussed
• After a structural inference has been made, then
  use the more specific correlation tables to
  confirm the analysis

46
NMR Spectroscopy
The NMR Spectrum - 1H

• General Correlation Chart 1H NMR
• Here are the general regions for 1H chemical
  shifts

0.0
10
9
8
7
6
5
4
3
2
1
15
downfield d (ppm)
upfield deshielded shielded higher DE
lower DE
47
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• The magnetic effects of nuclei in close proximity
  to those being observed have an effect on the
  local magnetic field, and therefore DE
• Specifically, when proton is close enough to
  another proton, typically by being on an adjacent
  carbon (vicinal), it can feel the magnetic
  effects generated by that proton
• On any one of the 108 of these molecules in a
  typical NMR sample, there is an equal statistical
  probability that the adjacent (vicinal) proton is
  either in the ½ or ½ spin state
• If there is more than one proton on an adjacent
  carbon all the statistical probabilities exist
  that each one is either ½ or ½ in spin
• The summation of these effects over all of the
  observed nuclei in the sample is observed as the
  spin-spin splitting of resonances

48
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observe what effect this has on an isolated ethyl
  group
• The two methylene Ha protons have three
  neighbors, Hb, on the adjacent methyl carbon
• Each one of these hydrogens can be ½ or ½ ,
  and since we are not looking at one molecule, but
  billions, we will observe all combinations

49
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observing the resonance for the Ha protons
• The first possibility that exists is that all
  three Hb protons have a ½ spin in this case
  the three protons combine to generate three small
  magnetic fields that aid B0 and deshield the
  protons pushing the resonance for Ha slightly
  downfield (the magnetic field of a proton is tiny
  compared to B0)

All 3 Hb protons ½
resonance for Ha in absence of spin-spin splitting
50
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observing the resonance for the Ha protons
• The second possibility is that two Hb protons
  have a ½ spin and the third a - ½ in this
  case the two protons combine to enhance B0 and
  the other against it, a net deshielding there
  are 3 different combinations that generate this
  state

resonance for Ha in absence of spin-spin splitting
51
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observing the resonance for the Ha protons
• The third possibility is that two Hb protons
  have a ½ spin and the third
• ½ here, the two protons combine to reduce B0
  and the other enforce it, a net shielding
  effect there are 3 different combinations that
  generate this state

resonance for Ha in absence of spin-spin splitting
52
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observing the resonance for the Ha protons
• The last possibility is that all three Hb
  protons have a ½ spin in this case the three
  protons combine to oppose B0, a net shielding
  effect there is one combination that generates
  this state

resonance for Ha in absence of spin-spin splitting
53
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observing the resonance for the Ha protons
• The result is instead of one resonance (peak)
  for Ha, the peak is split into four, a quartet,
  with the constituent peaks having a ratio of
  1331 centered at the d (n) for the resonance

resonance for Ha in absence of spin-spin splitting
54
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Observing the resonance for the Hb protons
• Similarly, the Hb protons having two protons, on
  the adjacent carbon each producing a magnetic
  field, cause the Hb resonance to be split into a
  triplet

resonance for Ha in absence of spin-spin splitting
55
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Rather than having to do this exercise for every
  situation, it is quickly recognized that a given
  family of equivalent protons (in the absence of
  other spin-coupling) will have its resonance
  split into a multiplet containing n1 peaks,
  where n is the number of hydrogens on carbons
  adjacent to the carbon bearing the proton giving
  the resonance this is the n 1 rule

56
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• You will soon recognize that there are certain
  recognizable patterns for the simple alkyl
  fragments

tert-butyl - singlet
methyl - singlet
ethyl quartet - triplet
n-propyl triplet - quintet - triplet
iso-propyl septet - doublet
57
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Consider this the basis for spin-spin splitting
  is that protons on adjacent carbons exert their
  own magnetic fields with or opposite the applied
  magnetic field because all alkyl protons are in
  roughly the same chemical environment (sp3
  orbital 1s H orbital) their magnetic influence
  is similar
• Due to free rotation in open chain compounds,
  the distance effect of the magnetic influence is
  averaged
• The n1 rule works in these simple alkyl cases
  ONLY!
• propyl -CH2- triplet -CH2- sextet
  -CH3 triplet

58
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• The amount of influence exerted by a proton on
  an adjacent carbon is observed as the difference
  (in Hz) between component peaks within the
  multiplet it generates. This influence is
  quantified as the coupling constant, J
• In complex spectra, you can determine which
  groups of protons are exerting magnetic influence
  on another group by comparing J values.
• For the ethyl group this is easily observed

This J
Is equal to this J
-CH2-
-CH3
59
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• For alkyl chains, typical 3JHH values are on the
  order of 6-8 Hz
• Since Js are generated by the magnetic
  influence within a molecule they are independent
  of the instrument frequency. This leads to the
  observation on high-field FT-NMR spectrum of the
  multiplets appearing narrower cleaning up the
  appearance of the spectrum
• Remember 1 d (ppm) on a 60 MHz spectrum is 60
  Hz, whereas 1 d is 300 Hz on a 300 MHz spectrum

60 MHz propyl bromide
300 MHz propyl bromide
60
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• The next level of complexity (which we will
  cover in detail in Chapter 5) is when protons on
  adjacent carbons exert different Js than one
  another.
• Consider the ethylene fragment

The influence of the geminal-relationship is over
the shortest distance
The magnetic influence of the trans- relationship
is over the longest distance
The cis-relationship, is over an intermediate
distance
61
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• For ethylene we would then observe three
  chemically distinct resonances with spin-spin
  splitting exerted by the other two protons
• J couplings

The observed multiplet for Ha is a doublet of
doublets
2Jgem 0 1 Hz
3JAC
3Jtrans 11- 18 Hz
3JAB
3JAB
3Jcis 6 - 15 Hz
62
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• Similar behavior is observed with aromatic
  rings since the ring structure is fairly rigid
  and electronic effects are conducted over a
  longer distance, J couplings are observed
  across the ring system

In low-field 1H NMR the signal for this proton
would be split into a doublet by the proton ortho
to it. On a high field instrument one finds
this 3Jortho as well as a 4Jmeta and a 5J para
from the effect of the protons meta and para to
it Typically 3Jortho 7-10 Hz 4Jmeta 1-3
Hz 5J para 0-1 Hz
3Jortho
4Jmeta
5Jpara
63
NMR Spectroscopy
The NMR Spectrum - 1H

• Spin-spin splitting 1H NMR
• For our initial treatment of 1H NMR the alkenyl,
  aromatic and the following J values should be
  learned

3J 6-8
3Jtrans 11-18
3J 8-11
3Jtrans 4-8 3Jcis 6-12
3Jcis 6-15
3Ja,a 8-14 3Ja,e 0-7 3Je,e 0-5
3J 5-7
3Jortho 7-10 Hz 4Jmeta 1-3 Hz 5J para 0-1 Hz
3Jallyl 4-10
3Jtrans 4-8 3Jcis 6-12
64
NMR Spectroscopy
The NMR Spectrum - 1H

• Integration 1H NMR
• Like instrumental chromatography, in NMR
  spectroscopy, the area under a peak (or
  multiplet) is proportional to the number of
  protons in the sample that generated that
  particular resonance
• The NMR spectrometer typically will print this
  information on the spectrum as an integral line
  (stepped line on the spectrum below)
• The height of the integral is proportional to
  that proton population by comparing the ratios
  of the integrals on an NMR spectrum you can
  determine the number of protons as a least common
  multiple of these ratios

65
NMR Spectroscopy
The NMR Spectrum - 1H

• Integration 1H NMR
• For example observe the integration of the
  ethanol spectrum below

-CH3
3.75 units high
-OH
1.25 units high
2.5 units high
66
NMR Spectroscopy
The NMR Spectrum - 1H

• Problem Set I Problems from text 2-28
• You should find these problems to be fairly easy!