Spectroscopy: Part B

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Spectroscopy Part B
Chapter 19
Nuclear Magnetic Resonance (NMR) Spectroscopy,
Mass Spectrometry
Please read Chapter 19 and study figures and
examples!
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Nuclear Magnetic Resonance Spectroscopy

• Powerful, but theoretically complex, analytical
  tool.
• Involves the nuclei of atoms.
• Structural environment about specific nuclei is
  deduced from
• information obtained about the nuclei.

• NMR requires nuclear spin I
• protons and neutrons - possess spin.
• In many atoms, individual spins in nucleus are
  paired e.g.12C nucleus has no overall spin.
• In some atoms nucleus possesses net spin
• If no. neutrons no. of protons odd no., then
  the nucleus has a half-integer spin (i.e. 1/2,
  3/2, 5/2) (e.g. 1H, 13C)
• If no. neutrons and no. of protons are each odd,
  then the nucleus has an integer spin (i.e. 1, 2,
  3) (e.g. 14N)

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NMR Spectroscopy Nuclear Spin
Nuclear Spin I most common useful nuclei are -
1H, 13C, 15N, 19F, 31P (I ½)
If a nucleus has a spin I gt 0, then its rotation
generates a magnetic field
H vector quantity
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Nuclear Spin (cont.)
i. At zero external magnetic field, spins are
degenerate! ii. Apply an external magnetic
field spins states will differ in energy
depending upon relative orientation with respect
to external field.
-nuclei with I ½ will adopt two specific
orientations with respect to an
externally-applied magnetic field...
iv. to convert lower energy spin state into
higher energy spin state, require external energy
source .........irradiate with radiofrequency
(rf) radiation!
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Nuclear Spin (cont.)
i. At zero external magnetic field, nuclear
spins are degenerate!
ii. In external magnetic field spins states
will differ in energy depending upon relative
orientation with respect to external field.
iii. To convert lower energy spin to higher
energy spin, irradiate with radiofrequency (rf)
radiation.

• Radiofrequency (rf) radiation provides energy h?
  for spin flip to higher energy orientation to
  occur
• 'Spin flip' is called 'resonance' hence
  'nuclear magnetic resonance'

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Nuclear Spin (cont.)

• Difference in energy between the two nuclear spin
  states depends on strength of external magnetic
  field

• For nucleus of H atom (proton), spin energy
  differences

-1/2
? (MHz)
100
200
300
360
500
1/2
4.73
6.35
8.46
11.75
2.34
0
H0 (Tesla)
Thus, at H0 4.7 T (Tesla), use rf radiation of
200 MHz etc.
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Effect of Electrons

• Nuclei are surrounded by electrons

• In external magnetic field, the electrons
  generate an induced magnetic field which shields
  the nucleus from the external field.

Net magnetic field at nucleus H0 - Hi
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Effect of Electrons (cont.)
Net effect field experienced at the proton (H
atom nucleus) will be LESS than the external field
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Effect of Electrons (cont.)
Consider proton (nucleus of H atom)
Increasing magnetic field strength ?
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NMR Spectroscopy Instrumentation

• Sample is normally dissolved in a solvent which
  has no H-atoms in
• a small tube which is spun at high speed!

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NMR Spectroscopy Spectrum X-Axis
X-axis relates to field strength, but frequency
(Hz) is used (n gH0/2p ? magnetogyric
ratio)
For field-independent recording of signal
position i. use internal standard
tetramethylsilane (CH3)4Si (TMS) for 1H NMR
spectra and arbitrarily assign ? 0. ii. use
chemical shift d relative to the internal
standard
signal frequency (from TMS)
d (signal)
applied frequency
Thus, peak at 200 Hz from TMS at applied
frequency of 200 MHz becomes
200
d (peak)
0.000001
200 x 106
d values are very small - e.g. 0.000001
therefore express as ? 1 ppm (1 part per
million)
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Spectrum X-Axis (cont.)
1H NMR spectrum of chloroform at 200 MHz, proton
absorbs at 1456.667 Hz downfield from reference
standard TMS
? ppm
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1H NMR Spectroscopy The NMR Spectrum
Plot absorption vs. external field strength or
frequency
High Field (highly shielded nuclei appear here)
Low Field (deshielded nuclei appear here)
Si(CH3)4 (TMS) reference (? 0)
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1H NMR Spectroscopy Features of Spectra
1. Chemical Shift (chemical environment of
proton) 2. Number of Signals (symmetry of
molecule) 3. Signal Integration (relative number
of different types of
protons) 4. Coupling (information on
neighbouring protons)
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1H NMR Spectroscopy 1. Chemical Shift
Electronegative substituents withdraw electron
density and thereby reduce shielding, increasing ?
Compound CH4 CH3Cl CH2Cl2 CHCl3
Compound CH3I CH3Br CH3Cl CH3F
d 0.2 3.0 5.3 7.3
d 2.2 2.7 3.0 4.1
Compound CH3Si(CH3)3 CH3CH3
d 0.0 0.9
Compound CH3N(CH3)2 CH3OCH3
d 2.2 3.2b
However, increasing electronegativity is not the
only factor to cause increase in ?!
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1. Chemical Shift (cont.)
2.3-2.9
0.9-1.8
1.6-2.6
4.5-6.5
2.0-2.6
6.5-8.5
9-11
2.3-2.8
Approximate ranges - ? affected by nature of
other groups attached!
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1. Chemical Shift (cont.)
Correlation Chart
?
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1H NMR Spectroscopy 2. No. of Signals
How many different types of protons?
Protons in identical environment chemically
equivalent - give one signal
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2. No. of Signals (cont.)
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2. No. of Signals (cont.)
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2. No. of Signals (cont.)
Equivalence of nuclei
Two nuclei are equivalent if they can be
interchanged by i. a symmetry operation
two Ha are the same interchange by a 180
rotation or reflection
two Hb are the same interchange by a 180
rotation or reflection
ii. by a rapid conformational change
Heq and Hax interchange by a rapid ring-flip
Cyclohexane one signal in 1H NMR spectrum at
room temperature!
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1H NMR Spectroscopy 3. Signal Integration
Integration of area under signal - gives
relative number of protons
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3. Signal Integration (cont.)
Integration of area under signal - gives
relative number of protons in this case,
multiply values by 2 to get actual numbers
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3. Signal Integration (cont.)
Multiply the integrals by 2 to get all integer
numbers
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1H NMR Spectroscopy 4. Spin Coupling

• If two protons, or sets of protons, on the same
  or adjacent carbon atoms are non-equivalent, they
  will undergo spin coupling with each other!

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4. Spin Coupling (cont.)
Hb is split by the two Ha into a (21)-let
triplet Ha is split by the one Hb into a
(11)-let doublet
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4. Spin Coupling (cont.)
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4. Spin Coupling (cont.)
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4. Spin Coupling (cont.)
For coupling to equivalent protons on adjacent
C-atom
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4. Spin Coupling (cont.)
For coupling to non-equivalent protons on
adjacent C-atom each coupling will be different
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Mass Spectrometry

• Ionization of organic molecules use 'electron
  impact' (ei)-high-energy electrons at 70 eV
  (6500 kJ!)

• Radical cation of organic molecule produced -
  molecular ion
• mass mass of parent molecule!

• Molecular ion unstable breaks up to generate
  fragment or daughter ions fragmentation.

CH4. ? CH3 H. CH4. ? CH3. H
CH3 ? CH2 H. CH2. ? CH H.

• Masses of ions can be measured according to mass
  to charge (m/z) ratio!

other methods apart from ei can be used
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Mass Spectrometry Instrumentation
STEP 2 Sort according to m/z (mass/charge ratio)
STEP 1 Ionize molecule M e ? M 2e
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Mass Spectrometry The Mass Spectrum

• Plot of ion current (proportional to number of
  ions) vs. m/z
• the intact molecule gives the molecular ion M
• other peaks are due to fragment ions ( incl.
  isotopic ions)
• tallest peak is the base peak (may not be M!)

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The Mass Spectrum (cont.)
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The Mass Spectrum (cont.)

• Fragmentation complicated mechanism, but can
  correlate with stabilities of cations!

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Mass Spectrometry Isotopes

• At low resolution, MS measures integral masses
• Isotopic ions easily detected and measured.

Therefore, peaks will appear to the right of M
for 12C, 1H due to M for ions containing 13C,
2H, other isotopes.
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Isotopes (cont.)

• Consider chlorobenzene
• two M peaks at m/z 112, m/z 114, intensity
  ratio 7426
• Due to isotopes 35Cl and 37Cl

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Isotopes (cont.)
Dichloromethane CH2Cl2 M (parent ion) m/z is 84
(CH2 35Cl 35Cl), 86 (CH2 35Cl 37Cl), 88
(CH2 37Cl 37Cl) base peak at m/z 49
(CH235Cl) is formed by loss of 35Cl from 84
and loss of 37Cl from M 86 m/z 51 (CH237Cl)
formed by loss of 35Cl from M 86 or loss of 37Cl
from M 88.
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Isotopes (cont.)
2-Bromopropane CH3CHBrCH3 M m/z 122 (CH3CHCH3
79Br) 124 (CH3CHCH3 81Br) base peak due to
fragment ion m/z 43 (CH3CHCH3) is formed by
loss of 79Br from M 122 and 81Br from M 124
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Isotopes (cont.)
Bromobutane CH3CH2CH2CH2Br M m/z is 136
(CH3CH2CH2CH2 79Br) 138 (CH3CH2CH2CH2 81Br)
base peak m/z 57 formed by loss of 79Br from M
136 and loss of 81Br from M 138.
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Isotopes (cont.)
Iodoethane CH3CH2I major isotope for I is 127I
M m/z 156 (which is also base peak), 29 formed
by loss of 127I from M 156