Nmr spectroscopy
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Nmr Spectroscopy . Chem is try 330 . Objectives. Nmr basics chemical shifts relaxation times 2-Dimensional Nmr experiments COSY NOESY What kind of information do we obtain?. Nmr Basics. The signal in a nmr spectrum arises from transitions between nuclear spin states.

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Nmr Spectroscopy

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Nmr Spectroscopy

Chemistry 330


Objectives

  • Nmr basics

    • chemical shifts

    • relaxation times

  • 2-Dimensional Nmr experiments

    • COSY

    • NOESY

  • What kind of information do we obtain?


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


Energy Levels in the Spin 1/2 System

b -1/2

a +1/2


The Nmr Signal

n (Hz)


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


Shielding

  • The presence of the bonding electrons about the nucleus gives rise to ‘electron shields’ around the nucleus

    • Shielding constant 


The Nmr Spectrum with Shielding

s

n (Hz)


The Chemical Shift

  • Resonance frequencies are field dependent

  • Define a field-independent parameter - the chemical shift ()

    • o - magnet strength in MHz


The Chemical Shift Spectrum

s

d (ppm)


J-Coupling (spin-spin splitting)

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


d (ppm)

The Coupling Constant

Jab (Hz)


Ha

Hb

Hc

The COSY Experiment

  • COrrelation SpectroscopY

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

Jab Jbc


d (ppm)

d (ppm)

The COSY Spectrum


The NOESY Experiment

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

Hb

Jab > 0

Jac = 0

Ha

Hc


d (ppm)

d (ppm)

The NOESY Spectrum


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


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


1

z

y

x


2

z

y

x


3

z

y

x


4

z

y

x


5

z

y

x


6

z

y

x


7

z

y

x


8

z

y

x


Magnetisation Intensity vs. Time

M


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 = 


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 < 0.50 nm).


NOESY Spectrum for SDS/C4OH


NOESY Spectrum for SDS/Bz


NOESY Spectrum for DTAB/Bz


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