Spectroscopy 3 Proton N objectives Understand how the features of a proton NMR s

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Spectroscopy 3 Proton N objectives Understand how the features of a proton NMR spectrum relate to chemical structure. Become more familiar with Spinworks software to process NMR FID data. Learn how to annotate proton NMR spectra. Reference Materials The PowerPoint presentation is provided on eCampus. Mohrig: pp 348-366,377-407. Chemical shift correlation charts can be found can be found on pages 361 and 362. See also: background information in the C-13 NMR procedure (Spectroscopy Background Now that you are familiar with the concepts of NMR signals, environments and chemical shift, it is time to venture into the added features of proton NMR that are not available in C-13 NMR. Protons appear over a much narrower chemical shift range, 0-12 ppm, and therefore overlap of signals is more common than with signals. A proton chemical shift correlation chart is given below. Recall that C-13 NMR makes use of the I.I% natural abundance of the 13C isotope found in nature. In proton NMR the signal arises from the most abundant isotope, IH, which has nuclear spin and therefore gives rise to NMR signals. This makes proton NMR a much more sensitive technique than C-13. Furthermore, due to the low abundance Catoms are very unlikely to be found next to each other and so do not interact in the course of the NMR Here, protons are indeed found on neighboring carbons. Because of this, there is an effect on the neighboring proton signals known as spin-spin coupling, or splitting. This is very powerful because it gives information about the relative positions of molecular features. Just remember nearest neighbor. A proton signal will split peaks, where n is the number of nearest neighbor protons on the adjacent carbon (s) that are within three bonds of the proton(s) giving rise to the signal Another difference between C-13 and proton NMR is that the time it takes for a nucleus in the proto encES n state to to the ground state relaxation time") is much shorter for under the curve)is consequence of this is that the size of a signal, measured as an integral (area proportional to the number of protons represented that signal

Explanation / Answer

1) Proton NMR signals are much stronger than C-13 signals. The major isotope of carbon, C-12 has a spin quantum number of 0 and is therefore magnetically inactive and cannot be detected by NMR. So we use the less abundant isotope, C-13 as it is magnetically active having a spin quantum number of half exactly like 1H. So, only a few nuclei of C-13 actually resonate in the magnetic field during NMR. Also, it is known, that the gyromagnetic ratio of C-13 is only one-fourth of H-1(proton) which reduces the signal strength of C-13 compared to proton NMR further.

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