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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

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 Chemical Shift

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

J-Coupling (spin-spin splitting)

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

Ha

Hb

Hc

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

Jab Jbc

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

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

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).

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