spectroscopy 3: magnetic resonance chapter 15. pulse techniques in nmr the “new technique”...
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Pulse Techniques in NMR
• The “new technique”
• Rather than search for and detect each individual resonance,the pulsed technique detects all resonances simultaneously
• Analogous to hitting a bell with a hammer and recording allfrequencies, then separating each individual frequency
• The resulting Fourier-transform NMR gives much greatersensitivity and freedom from noise
• Classical Description of NMR
• Absorption Process
• Relaxation Processes to thermal equilibirum
• Spin-Lattice
• Spin-Spin
Fig 15.29(a) Circularly-polarized mag field B1 (from rf
pulse) is applied perpendicular to z-axis
Bo
Component absorbed (d or l)
is same as direction of
precession
Counter
Clockwise
rotation
Fig 15.29(b) Circularly-polarized mag field B1 (from rf
pulse) is applied perpendicular to z-axis
When applied rf frequency
coincides with νLarmor
magnetic vector begins to
rotate around B1
Spin-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
Fig 15.34
Spin-Spin (Transverse) Relaxation
• Occurs between 2 nuclei having same precessional frequency
• Loss of “phase coherence”
• Orderly spins to disorderly spins
• T2 ≡ spin-spin relaxation time
• No net change in populations
• Result is broadening
Fig 15.36
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
Fourier TransformNMR Spectrum
Time domain Frequency domainFT
Fig 15.31 A free-induction decay (FID) signal of
a single resonance frequency
Fig 15.32 A simple free-induction decay (FID)signal of
a sample with two FID frequencies
Fourier Transform