1. Spectroscopy 3: Magnetic Resonance CHAPTER 15

2. 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 all frequencies, then separating each individual frequency The resulting Fourier-transform NMR gives much greater sensitivity and freedom from noise

3. Classical Description of NMR Absorption Process Relaxation Processes to thermal equilibirum Spin-Lattice Spin-Spin

4. Fig 15.27 Vector model of angular momentum for a single spin-1/2 nucleus

5. Fig 15.28 Spin-½ nuclei in absence of a B 0 field B 0 = 0 B 0 ≠ 0

6. Fig 15.29(a) Circularly-polarized mag field B 1 (from rf pulse) is applied perpendicular to z-axis BoBo Component absorbed (d or l) is same as direction of precession

7. Counter Clockwise rotation Fig 15.29(b) Circularly-polarized mag field B 1 (from rf pulse) is applied perpendicular to z-axis When applied rf frequency coincides with ν Larmor magnetic vector begins to rotate around B 1

8. Fig 15.30 A 90° pulse is applied to rotate M vector into xy-plane

9. Spin-Lattice (Longitudinal) Relaxation Precessional cones representing spin ½ angular momenta: spins number β spins > number α spins After time T 1 : Populations return to Boltzmann distribution Momenta become random T 1 ≡ spin-lattice relaxation time Tends to broaden NMR lines Fig 15.34

10. Spin-Spin (Transverse) Relaxation Occurs between 2 nuclei having same precessional frequency Loss of “phase coherence” Orderly spins to disorderly spins T 2 ≡ spin-spin relaxation time No net change in populations Result is broadening Fig 15.36

11. Fig 15.35 Variation in the two relaxational processes

12. Fourier Transform NMR Nuclei placed in strong magnetic field, B o Nuclei precess around z-axis with momenta, M Intense brief rf pulse (with B 1 ) 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

13. Fourier Transform NMR Spectrum Time domainFrequency domain FT Fig 15.31 A free-induction decay (FID) signal of a single resonance frequency

14. Fig 15.32 A simple free-induction decay (FID)signal of a sample with two FID frequencies Fourier Transform

15. 13 C FID Signal for Dioxane ν RF = ν Larmor Fourier transform of (a)

16. 13 C FID Signal for Dioxane ν RF ≠ ν Larmor Fourier transform of (a)

17. 13 C FID Signal for Cyclohexane

18. Fig 15.33 A free-induction decay (FID) signal of a sample of ethanol

19. Enjoy a safe and blessed Christmas!!