NMR Theory Hopefully Explained

NMR Theory Hopefully Explained - NMR Theory Hopefully...

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NMR, n uclear m agnetic r esonance, is important because it provides a powerful way to deduce the structures of organic molecules. In addition, the same principle is used in MRI medical imaging. Unfortunately, the physics behind NMR is extremely complicated. What follows is an attempt to provide all the information you need to understand the basic principles underlying the NMR technique. 1.0 Moving charge-magnetic field interactions: 1.1 Moving charges (electrons, protons) create a magnetic field. 1.2 Charges will interact with an external magnetic field (created by a big magnet in the laboratory), causing them to move in response to the field. This, in turn, induces an additional magnetic field because of 1.1 . 1.3 This is how electromagnets and other cool things like generators work, but also describes how charged subatomic particles, namely electrons and protons in molecules, interact with a magnetic field and also how they interact with each other. 2.0 Quantum mechanical description of nuclear spin. These are the essential facts you need before we describe NMR. 2.1 Atomic nuclei with an odd atomic mass or an odd atomic number have a quantum mechanical property called “spin” that is designated by a spin quantum number such as 1/2 or 1 . For NMR experiments, we are only concerned with nuclei having a spin quantum number of 1/2. In particular we are interested in 1H (most common isotope of hydrogen by far) and 13C (rare but useful isotope of carbon, the most common isotope being 12C). Recall from general chemistry that the number 1,12, 13 etc. is the atomic mass, that is, the total number of protons and neutrons in the given isotope. 2.2 Nuclear spin can be thought of as the positive charge of protons “circulating”. In other words you should think about nuclear spin as circulating charge. I know this sounds weird, but just accept it and realize this is best way to think about spin. There are two consequences here: 2.2A Nuclear spin can interact with an external magnetic field. 2.2 B
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This note was uploaded on 09/21/2010 for the course CH 53200 taught by Professor Bocknack during the Spring '10 term at University of Texas.

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NMR Theory Hopefully Explained - NMR Theory Hopefully...

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