Nuclear Magnetic Resonance Spectrometry

Chap 19

• Classical Description of NMRClassical Description of NMR

• Absorption ProcessAbsorption Process

• Relaxation Processes (to thermal equil.)Relaxation Processes (to thermal equil.)

• Spin-LatticeSpin-Lattice

• Spin-SpinSpin-Spin

Circularly-polarizedCircularly-polarized

radio frequency mag.radio frequency mag.

field Bfield B11 is applied: is applied:

When applied rf frequencyWhen applied rf frequency

coincides with coincides with ννLarmorLarmor

magnetic vector begins to magnetic vector begins to

rotate around Brotate around B11

Component absorbed (d or l)Component absorbed (d or l)

is same as direction of is same as direction of

precessionprecession

Fig. 19-6Fig. 19-6

Behavior of Magnetic Moments of NucleiBehavior of Magnetic Moments of Nuclei

Bo

Clockwise

rotation

Spin-Lattice (Longitudinal) RelaxationSpin-Lattice (Longitudinal) Relaxation

• Precessional cones representing

spin ½ angular momenta:

• number β spinsspins > number α spins

• After time T1 :

• Populations return to

Boltzmann distribution

• Momenta become random

• T1 ≡ spin-lattice relaxation time

• Tends to broaden NMR lines

Spin-Spin (Transverse) RelaxationSpin-Spin (Transverse) Relaxation

• Occurs between 2 nuclei havingOccurs between 2 nuclei having same precessional frequencysame precessional frequency

• Loss of “phase coherence”Loss of “phase coherence”

• Orderly spins to disorderly spinsOrderly spins to disorderly spins

• TT22 ≡ spin-spin relaxation time≡ spin-spin relaxation time

• No net change in populationsNo net change in populations

• Result is broadening Result is broadening

Fourier Transform NMRFourier Transform NMR

• Nuclei placed in strong magnetic field, Bo

• Nuclei precess around z-axis with momenta, M

• Intense brief rf pulse (with B1) applied at 90° to M

• Magnetic vector, M, rotates 90° into xy-plane

• M relaxes back to z-axis: called free-induction decay

• FID emits signal in time domain

Fig 19-6 Behavior of Mag Moments with 90° Pulse

Free Inductive Decay

Fig 19-7 Two Nuclear Relaxation Processes

Spin-Lattice(Longitudinal)

Spin-Spin(Transverse)M

Mz

• FID emits signal in time domain

• As relaxation proceeds, the FID decreases exponentially

θ

Brief rf pulse applied FID is detected at νLarmor

Behavior of Mag Moments with 90° Pulse

Not necessary to know νLarmor ; short pulse is analog of ahammer striking a bell exciting a range of frequencies.

Simple FID of a sample of spins with a single frequencySimple FID of a sample of spins with a single frequency

Fourier TransformNMR SpectrumNMR Spectrum

Simple FID of species with two frequenciesSimple FID of species with two frequencies

Fig 19-8 13C FID Signal for Dioxane

νRF = νLarmor

Fourier transform of (a)

Fig 19-9 13C FID Signal for Dioxane

νRF ≠ νLarmor

Fourier transform of (a)

Fig 19-10 13C FID Signal for Cyclohexane

Fig 19-11 Wide-Line NMR Spectrum of H2O

In Glass Tube

= 10-4 T

νRF = 5 MHz

Proton NMR Spectrumof Ethanol at 60 MHz

Fig 19-12

Remainder of Chapter:High Resolution

NMR Spectra

Environmental EffectsEnvironmental Effects

(1) Chemical Shift

• Nearby electrons and nuclei generate small B fields which tends to oppose Bapplied:

Bo = Bapplied – σBapplied

where σ ≡ screening constant

It is the local field Bo that interacts with magnetic moments!

• Now, resonance condition:

Common to hold νRF constant (e.g., 100 MHz) and sweep Bo

)1(2

oLarmor B

Abscissa Scales for NMR Spectra

• In terms of chemical shift, δ

• Almost impossible to measure absolute Bo

• Measure change in Bo relative to internal standard: Tetramethylsilane (TMS)

ppm10 x ν

ννδ 6

ref

sampleref

High Resolution NMR Spectrum of High Resolution NMR Spectrum of EthanolEthanolFig. 19-12Fig. 19-12

Bo

High field

High shieldLow field Low shield

in ppm

Fig 19-13 Effect of Field Strength on Chemical Shift

1.41 T

2.35 T