Nuclear Magnetic Resonance Spectrometry Chap 19. Absorption in CW Experiments Energy of precessing...

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Transcript of Nuclear Magnetic Resonance Spectrometry Chap 19. Absorption in CW Experiments Energy of precessing...

  • Slide 1
  • Nuclear Magnetic Resonance Spectrometry Chap 19
  • Slide 2
  • Absorption in CW Experiments Energy of precessing particle E = - z B o = - B o cos When an RF photon is absorbed by a nucleus, must change direction magnetic moment z flips For z to flip, a B field must be applied B o in a circular path in phase with precessing dipole B is applied B o using circularly-polarized RF field
  • Slide 3
  • Fig 19-3 Model for the Absorption of Radiation by a Precessing Particle zz
  • Slide 4
  • Fig 19-3 Model for the Absorption of Radiation by a Precessing Particle When RF = v o absorption and spin flip can occur
  • Slide 5
  • Fig 19-4 Equivalency of a Plane-polarized Beam to Two (d, l) Circularly-polarized Beams Result is vector sum that vibrates in a single plane In instrument, RF oscillator coil is 90 to fixed B o field Only B rotating in precessional direction is absorbed
  • Slide 6
  • Classical Description of NMR Classical Description of NMR Absorption Process Absorption Process Relaxation Processes (to thermal equil.) Relaxation Processes (to thermal equil.) Spin-Lattice Spin-Lattice Spin-Spin Spin-Spin
  • Slide 7
  • Relaxation Processes (to thermal equilibrium) When absorption causes N 1/2 = N -1/2 system is saturated Fast decay is desirable Probability of radiative decay (fluorescence) v 3 Therefore in RF region, non-radiative decay predominates
  • Slide 8
  • B o field off: = at random angles Magnetization is zero B o field on: Spins precess around their cones at Larmor spins > spins Net magnetization, M
  • Slide 9
  • Circularly-polarized radio frequency mag. field B 1 is applied: When applied rf frequency coincides with coincides with Larmor magnetic vector begins to rotate around B 1 Behavior of Magnetic Moments of Nuclei
  • Slide 10
  • Spin-Lattice (Longitudinal) Relaxation Precessional cones representing spin angular momenta: spins number spins > number spins After time T 1 : Populations return to Boltzmann distribution Momenta become random T 1 spin-lattice relaxation time Tends to broaden NMR lines
  • Slide 11
  • Spin-Spin (Transverse) Relaxation Occurs between 2 nuclei having Occurs between 2 nuclei having same precessional frequency same precessional frequency Loss of phase coherence Loss of phase coherence Orderly spins to disorderly spins Orderly spins to disorderly spins T 2 spin-spin relaxation time T 2 spin-spin relaxation time No net change in populations No net change in populations Result is broadening Result is broadening
  • Slide 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
  • Slide 13
  • Simple FID of a sample of spins with a single frequency Fourier Transform NMR Spectrum
  • Slide 14
  • Simple FID of AX species with two frequencies
  • Slide 15
  • Vector Model of Angular Momentum Fig. 19-2 55