FT-NMR

Fundamentals

• Nuclear spin

• Spin quantum number – ½

• Nuclei with spin state ½ are like little bar magnets and align with a B field.

• Can align with (++) or against (+-) B

• Small energy gap between + and – spin alignment (NMR insensitive/Boltzman dist)

• Can probe difference with RW

(NMR insensitive/Boltzman dist)

• Small population difference between +1/2 and -1/2 state

• It is the small excess of nuclei in the -1/2 that produce NMR signal

Common NMR nuclei

• Protons, 1H• 13C• 15N• 19F• 31P• Sensitivity depends on natural isotopic

abundance and E = ћB0 , bigger magnet, greater sensitivity

Precession of nuclear dipoles

The basis of the NMR experiment

• Chemical Shift; Nuclei in different bonding environments have different Es (electron density).

• Spin-Spin Splitting; Adjacent nuclei split the signal into multiplets in a predictable fashion.

Chemical Shift

• Shielding – Electrons have spin, produce local B

environments– Protons in different electronic environments

experience different B (Bm +Be), different precessing frequencies, E = h

– Chemical shift proportional to size of magnet

– ppm {(-0)/0}*106

Spin-Spin Coupling

• Adjacent nuclei have a 50/50 chance of being spin up (+1/2) or spin down (-1/2)

• Each produce a small magnetic field that is either with or against B0

• 1 adjacent proton CHOCH3– CH3 is a doublet at frequencies

-a, +a (equal intensity), 1:1– CH is a quadruplet

Splitting Patterns

• J values• Quadruplet • Triplet • Multiplets

1 3 3 11 2 1

¼ ½ ¼ ¾ 1 ½ ¾ ¾ 1 ½ ¾ ¼ ½ ¼

1 2 1 3 6 3 3 6 3 1 2 1

Precession of nuclear dipoles

FT pulse

• Radiofrequency generator– A short, intense pulse generates a magnetic

field in the x-y plane (excites all nuclei)

– M0 of the nuclei interacts with the magnetic field produced by the pulse.

– Tips M0 off axis

Θ = B1p

p – length of pulse, 90 pulse

Vector illustration

Relaxation

• T1 spin-lattice (relaxing back to precessing about the z axis)

• T2 spin-spin (fanning out)

Induced current in coil

• After pulse, nuclei begin to precess in phase in the x-y plane

• Packet of nuclei induce current in RF coil

• Relaxation is measured by monitoring the induced coil

• → FID (→ FT) NMR spectrum

FID

Noise reduction and increasing resolution

• Apodization: Multiply the free-induction decay (FID) by a decreasing exponential function which mathematically suppresses the noise at long times. Other forms of apodization functions can be used to improve resolution or lineshape.

• Zero filling

13C NMR

• 13C frequency

• Different tuning folk

• Broadband Decoupling of 1H

• No spin-spin coupling

• NOE effect

• Assignments based on chemical shift

• Wider frequency range

Obtaining a 13C NMR Spectrum

• 1H Broadband decoupling– Gives singlet 13C peaks, provided no F, P, or

D present in the molecule)– Continuous sequence of pulses at the 1H

frequency causes a rapid reversal of spin orientation relative to the B0, causing coupling to 13C to disappear

1H channel

13C channel

Broadband Decoupling

H3C4-C3H=C2H-C1OOH

180 10

C-1

C-3C-2

solventC-4

13C Chemical Shifts

• Reference is TMS, sets 0 ppm• A range of 200 ppm• Chemical shifts can be predicted

– Empirical correlations– Ex. Alkanes

i = -2.3 + 9.1n + 9.4n – 2.5n + 0.3n + 0.1n + Sij

2-methylbutane

i = -2.3 + 9.1*1 + 9.4*2 – 2.5*1 - 1.1 = 22.0 (22.3)

1

Signal averaging

• 13C experiment generally take longer than 1H experiments because many more FIDs need to be acquired and averaged to obtain adequate sensitivity.

• NOE effect (enhancement/reduction in signal as a result of decoupling)

1H

1H 13C

13C

N1

N4

N3N2 N2

N1

N3

N4

W2

W1

NOE effect

• W2 (Enhancement) dominates in small molecules

• Relevant for all decoupling experiments

Other more complex 1D Experiments

• 1H NOE experiment

• Inversion Recovery Experiment; Determination of T1

• J modulated Spin Echo

• INEPT Experiment

• DEPT Experiment

Targeted 1H Spin Decoupling

• Continuous irradiation at a frequency (2) that corresponds to a specific proton in the molecule during the 1H NMR experiment

• All coupling associated with the protons corresponding to 2 disappears from the spectrum

channel

1H channel

1H targeted decoupling (NOE)

O

O

1

2

3

TMS

3

1

2

2

NOE- nuclear Overhauser effect

• Saturation of one spin system changes the equilibrium populations of another spin system

• NOE effect can be positive or negative. In small molecules it is usually positive

Selective Heteronuclear Decoupling

• Saturate at a specific frequency

• Multiplets collapse reveal connectivity

More Complex NMR Pulse Sequences

• J-Modulated Spin Echo experiment– Cq and CH2 down and CH3 and CH up

• DEPT experiment = 45, 90, 135– CH3 [DEPT(90)], CH2 [DEPT(45)-DEPT(135)], CH

[DEPT(45)+DEPT(135)-0.707DEPT(90)]

• 2D-NMR– Het. 2D J resolved/Homo 2D J resolved– 1H-1H COSY– 1H/13C HETCOR

13C decoupled spectrum

0

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050100150200

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13C J modulated spin echo pulse experiment

-100

-80

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050100150200

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Inte

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ityCH and CH3

Cq and CH2

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020406080100

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020406080100

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020406080100

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020406080100

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Inte

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DEPT

DEPT(90)CH3

DEPT(45) – DEPT(135)CH2

DEPT(45)+DEPT(135)-0.707DEPT(90)CH

13C decoupled spectra

2D-HETERNUCLEAR J-RESOLVED EXPERIMENT

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020406080100120

F2 13C ppm

F1

{J V

alu

e (C

-H)}