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CHE219_Fall2007-NMR LECTURE NOTES - CHE 219 NMR LECTURE...

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µ CHE 219 NMR LECTURE NOTES FOR Dr. de Ropp Copyright 2007 Jeffrey S. de Ropp v
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TABLE OF CONTENTS 3-14 Introduction 15-19 Rotating Frame 19-20 Nyquist Frequency 21 Quadrature Phase Detection 23 Acquisition Time 24 Fourier Transformation 25-27 Data Processing 28-29 Probe Tuning 29-30 Field Locking 31-33 Field Shimming 33-34 Optimizing ADC Conditions 35-36 Pulse Recycle Considerations 37-38 Calibrating the 90 o Pulse 39-66 Nuclear Spin Interactions 40-53 Nuclear Spin Relaxation 54-60 Nuclear Spin Coupling 61-66 NOE 66-91 2D NMR 66-70 Introduction 71-75 2D 13 C Shift - J CH 76-79 Summary of Principles 80-91 2D NMR Applications 92-93 3D NMR 94 Reading List 95 List of Abbreviations 2
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Nuclear Magnetic Resonance (NMR) Spectroscopy Most nuclei have the property of spin. 3 The easiest way to understand this is to remember that electrons also have the property of spin. Electron spin is the fourth quantum number (n, l, m L , m S ) that specifies the energy state of an electron. Electrons have a spin quantum number of 1/2 and pair up with antiparallel spins in atomic orbitals: Protons and neutrons (nucleons) are also spin 1/2 particles and arrange themselves in energy states somewhat similar to atomic orbitals. If a nucleus has both an even number of protons (Z) and an even number of neutrons (N), then all nucleon spins will be paired and there is no net spin. Examples are 12 C and 16 O. All other nuclei possess spin, and a spinning charged particle generates a magnetic dipole: 6 8 Spinning nucleus Axis of rotation Electrons flowing in a loop e- µ Since the nucleon spin is quantized at 1/2, so too the net nuclear spin is quantized: I = n/2 , where n = 1, 2, 3 .... Thus I = 1/2, 1, 3/2, 2, .... + - + Magnetic dipole - V V
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4 Roughly speaking we can predict I by the following rules: # of neutrons #of protons I even even 0 even odd 1/2 or 3/2 or 5/2 .... odd even 1/2 or 3/2 or 5/2 .... odd odd 1 or 2 or 3 .... About 1/4 of all NMR active nuclei have spin I = 1/2, and this includes all the most commonly observed nuclei ( 1 H, 13 C, 19 F, 31 P). The spinning nucleus generates a magnetic dipole µ of magnitude µ = γ h I 2 π where is h is Planck's constant and γ is the magnetogyric ratio, a unique constant for each type of nucleus. Since h/2 π = h, we have that: µ = γ h I The magnitude of µ thus depends on γ and will be different for each nucleus. For example, γ of 13 C is 1/4 that of 1 H so the 13 C dipole will be 1/4 that of 1 H. _ Now in the absence of a magnetic field all the nuclear dipoles are oriented randomly: When an external field B o is applied a wondrous thing occurs: the dipoles "align" with the field in two seperate, quantized energy states and precess around B 0 at a frequency ω L , termed the Larmor frequency. _ V V V V V
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The energy required for a nuclear spin transition is very small; about 1x10 -25 Joules. This corresponds to low frequencies in the electromagnetic spectrum.
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