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PROTON NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (H-NMR) WHAT IS H-NMR SPECTROSCOPY? References: Bruice 14.1, 14.2 Introduction NMR or nuclear magnetic resonance spectroscopy is a technique used to determine a compound’s unique structure. It identifies the carbon-hydrogen framework of an organic compound. Using this method and other instrumental methods including infrared and mass spectrometry, scientists are able to determine the entire structure of a molecule. In this discussion, we will focus on H NMR or proton magnetic resonance. Even though there are many other spectrometers including C-NMR and N-NMR, hydrogen (H-NMR) was the first and is the most common atom used in nuclear magnetic resonance spectroscopy. How does it work? The atomic nucleus is a spinning charged particle, and it generates a magnetic field. Without an external applied magnetic field, the nuclear spins are random and spin in random directions. But, when an external magnetic field is present, the nuclei align themselves either with or against the field of the external magnet. References: Bruice, Figures 14.1 and 14.2 References: Thinkbook pg 54 and 55 No External Applied Magnetic Field External Magnetic Field is Applied _-spin state : Protons that align with the external magnetic field. They are in a lower energy state. _-spin state : Protons that align against the external magnetic field. They are in a higher energy state.
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The _E is the energy difference between the _ and _ spin states. This depends on the applied magnetic field. As shown by the graph above, the greater the strength of the applied magnetic field, the larger the energy difference between the two spin states. When radiation, that has the same energy as the _E, is placed upon the sample, the spin flips from _ to _ spin states. Then, the nuclei undergoes relaxation. Relaxation is when the nuclei return to their original state. In this process, they emit electromagnetic signals whose frequencies depend on _E as well. The H- NMR spectrometer reads these signals and plots them on a graph of signal frequency versus intensity. Resonance is when the nuclei flip back and forth between _ and _ spin states due to the radiation that is placed on them. To summarize, an NMR signal is observed when the radiation supplied matches the _E. And, the energy required to cause spin flip is dependent on the magnetic environment experienced by the nucleus. STRUCTURAL INFORMATION THAT THE NMR SPECTRUM TELLS US References: Bruice 14.4 References: Thinkbook pg 55 1. Number of Signals Each group of chemically equivalent protons gives rise to a signal. Chemically Equivalent Protons are protons that are in the same environment, and they must be identical in every way. They experience is the same magnetic force, and therefore, will create overlapping signals on the
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