Nmr Spectroscopy

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Nmr Spectroscopy

• Chemistry 330

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Objectives

• Nmr basics
• chemical shifts
• relaxation times
• 2-Dimensional Nmr experiments
• COSY
• NOESY
• What kind of information do we obtain?

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Nmr Basics

• The signal in a nmr spectrum arises from
  transitions between nuclear spin states.
• 1H, 13C, 31P all have a nuclear spin quantum
  number, I 1/2.
• The total number of spin states
• 2I 1 2

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Energy Levels in the Spin 1/2 System

b -1/2
a 1/2
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The Nmr Signal

n (Hz)
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The Magnetisation Vector

• We can represent the slight excess of spins in
  the ? state by use of the nuclear magnetization
  vector, Mo

z
y
x
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Shielding

• The presence of the bonding electrons about the
  nucleus gives rise to electron shields around
  the nucleus
• Shielding constant ?

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The Nmr Spectrum with Shielding

s
n (Hz)
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The Chemical Shift

• Resonance frequencies are field dependent
• Define a field-independent parameter - the
  chemical shift (?)
• ?o - magnet strength in MHz

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The Chemical Shift Spectrum

s
d (ppm)
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J-Coupling (spin-spin splitting)

• What happens when we have non-equivalent protons
  on adjacent C atoms?

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The Coupling Constant
Jab (Hz)

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The COSY Experiment

• COrrelation SpectroscopY
• What happens when we project the following spin
  system in two-dimensions?

Jab ? Jbc
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The COSY Spectrum

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The NOESY Experiment

• In the 2-D NOESY (nuclear Overhauser enhancement)
  experiment, we look for through space dipolar
  couplings.

Hb
Jab gt 0
Jac 0
Ha
Hc
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The NOESY Spectrum

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The Origin of Spin-Lattice Relaxation

• By using a selective pulse (a 180? pulse), we can
  invert the populations of the nuclear spin states

z
y
x
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Spin-Lattice Relaxation

• The spin system attempts to re-establish the
  equilibrium magnetisation vector. We observe the
  magnitude of the magnetisation vector as a
  function of time after the inversion pulse is
  applied.
• (180? - ?n - 90? - acquire)n

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• ?1

z
y
x
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• ?2

z
y
x
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• ?3

z
y
x
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• ?4

z
y
x
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• ?5

z
y
x
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• ?6

z
y
x
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• ?7

z
y
x
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• ?8

z
y
x
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Magnetisation Intensity vs. Time

M?
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The Spin Lattice Relaxation Time

• The time constant governing the decay process is
  the spin-lattice relaxation time, T1
• M? limiting value of magnetisation intensity
• M? magnetisation intensity at t ?

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Nuclear Overhauser Enhancement (NOESY) Experiments

• 2-D Nmr NOESY experiments have been used
  extensively in the determination of the
  conformations of nucleic acids, proteins, and
  membranes.
• The presence of the cross peak in the 2-D NOESY
  spectrum indicates the presence of intermolecular
  or intramolecular dipolar interactions
  (representing a spatial proximity of lt 0.50 nm).

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NOESY Spectrum for SDS/C4OH
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NOESY Spectrum for SDS/Bz
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NOESY Spectrum for DTAB/Bz