Ressonância Magnética Nuclear

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Keys to the Chapter 193 10-9 Carbon-13 Nuclear Magnetic Resonance The utility of another magnetic nucleus in NMR. Keys to the Chapter 10-2. Defining Spectroscopy Spectroscopy is mainly physics. In spite of that, the material in this section is really very basic and quite understandable: Spectroscopy detects the absorption of energy by molecules. The stake that the organic chemist has in this is also simple. Determining the structure of a molecule requires that the chemist be able to interpret spectroscopically observed energy absorptions in terms of structural features of an unknown molecule. This chapter describes the physical phenomena associated with nuclear magnetic resonance (NMR). It then describes their logical implications as applied to identifying structural features in a molecule from a nu- clear magnetic resonance "spectrum." Don't be frightened. The end result-the ability to interpret spectro- scopic data in terms of molecular structure-is actually one of the easiest skills to acquire in this course. It's almost fun! 10-3. Hydrogen Nuclear Magnetic Resonance In the upcoming text sections the physical basis for the utility of nuclear magnetic resonance in organic chem- istry is presented. This text section describes a couple of concepts that are likely to be unfamiliar to you. The first is the idea that a magnet can align with an external magnetic field in more than one way. Most of us are familiar with bar magnet compasses, which orient themselves along the Earth's magnetic field in one direction only. Nuclear magnets are actually similar, except for the fact that the reorientation energies are so small that energy quantization becomes significant. Like the compass, the nuclear magnet does have one energetically pre- ferred alignment (the α spin for a proton). Less favored alignments (spin states) are very close in energy to the preferred one on a nuclear scale, however. So, unlike the macroscopic compass, the microscopic nuclei can com- monly be observed in less favorable, higher energy orientations in a magnetic field. The second new idea is that of resonance. This is the term that describes the absorption of exactly the correct amount of quantized energy to cause a species in a lower energy state to move to a higher energy state. For nuclear magnets, this is commonly described as a spin flip (α spin state to ß spin state for a proton, the nucleus of the hydrogen atom). The amount of energy involved depends on the identity of the nucleus and the size of the external magnet. The text section describes these relationships in detail. It is the observation of resonance energy absorptions by magnetic nuclei at certain values of magnetic field strength and energy input (in the form of radio waves) that constitutes the physical basis for nuclear magnetic resonance spectroscopy. 10-4. The Hydrogen Chemical Shift The normal NMR spectrum can provide four important pieces of structural information about an unknown mol- ecule. The first two pieces of information are derived from the fact that hydrogens in different chemical environments display separate resonance lines in a high resolution NMR spectrum. Thus, first, by counting the number of resonance signals in the spectrum, one knows the number of sets of hydrogen atoms in dif- ferent chemical environments contained in the molecule. Second. the actual position of each resonance sig- nal is characteristic of certain kinds of chemical environments, e.g., it can imply proximity to a specific type of functional group, or attachment to a certain type of atom. The phenomenon responsible for this is the shield- ing of a magnetic nucleus under observation by the nearby electrons in the molecule. How this comes about physically is described in detail. On a typical NMR spectrum, resonance signals to the left of the chart result from less strongly shielded (deshielded) hydrogens, whereas signals to the right are representative of more strongly shielded ones. Deshielded hydrogens require lower external magnetic fields for resonance, whereas shielded ones require higher fields. So we have NMR spectra that have the following qualitative relationships: