NMR Lecture Part 1

NMR Lecture Part 1 - Biophysical Magnetic Resonance Theory...

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Biophysical Magnetic Resonance Theory and Applications
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NMR- Why is it useful? Unparalleled chemical specificity. Quantitatively interpretable at molecular level. Structures without crystals. Diverse dynamic probes from 10 -12 to 10 2 s. Nondestructive. Penetrates nearly all samples.
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Spin and Intrinsic Magnetism Fundamental particles have intrinsic angular momentum which gives rise to a magnetic moment. Spin angular momentum and the magnetic moment are proportionally related by the gyromagnetic ratio, which can be positive or negative. The sign of γ can be thought of as the direction of precession. . μS
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NMR Active Nuclei There are a number of biologically relevant nuclei with spin The most sensitive is 1 H Sensitivity is determined both by γ and natural abundance
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Spin States are Quantized Spin is a quantum mechanical phenomenon, so it is quantized. For a given spin quantum number I , the spin state m I can only take values I , I -1, I - 2, …, - I . The number of states available for a given spin is given by 2 I +1. So, for a spin ½ particle, there are two spin states: m I = ½, -½.
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Spin in a Magnetic Field At zero field, the spin states m I = I , I - 1, …, - I all have the same energy. (I.e., they are degenerate.) When spins experience an applied magnetic field, the spin states separate. 2 hB E hv   5 exp ~ 10 2 N N kT   
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Vector Interpretation This can also be visualized as spin “vectors” partially aligning with the magnetic field. Note: the net alignment with the field is highly exaggerated.
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NMR Spectroscopy Like any other spectroscopy, we need to detect the absorption/emission of radiation to obtain a signal. Originally this was done by irradiating the sample with a fixed radiowave frequency, then sweeping the applied field until resonance was observed (continuous wave NMR). Now we use pulsed Fourier transform NMR to detect a broader set of frequencies simultaneously.
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NMR Signal When a 90 radiofrequency pulse is applied to a sample, the net magnetization vector is rotated into the x-y plane. Once the net magnetization is no longer aligned with the applied field, it begins to precess around it at the Larmor frequency.  B B 0
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Precession According to the right hand rule, nuclei with a positive gyromagnetic ratio have negative precession, and nuclei with a negative gyromagnetic ratio have positive precession.
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Signal Detection Signal precession is detected by the receiver coil, which is often the same coil used to pulse the sample. The signal is recorded as a function of time, and is called the Free Induction Decay (FID).
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This document was uploaded on 01/03/2012.

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NMR Lecture Part 1 - Biophysical Magnetic Resonance Theory...

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