Does anyone know how to assign H's or C's to an NMR spectra?
thanks anyone n everyone :)
Answer:
The proton (1H) variety of Nuclear Magnetic Resonance works as a means to identify chemical compounds because:
--the location of a peak has to do with how "shielded" (electronegative) or "deshielded" (electropositive) that particular location is with electrons. The carbon or other atom it is attached to will "pull" on the hydrogen's electron with varying intensity, so this alters the fundamental resonance frequency by a small amount, usually measured in parts per million off the main resonance frequency of a reference compound (usually tetramethylsilane). These variations are known and you can look up a table of NMR "Chemical Shifts" for various molecular functional groups (-CH2-, -CH3- in various environments, -COOH (carboxylic acid), etc.)
http://www.chem.uni-potsdam.de/~thomas/i...
--the intensity of a peak has to do with how many hydrogens of that sort is resonating. E.g. a -CH3 group in a specific spot of the molecule will have a peak height of 3, whereas compared to that, a -CH2- group will have a peak height of 2. Relative ratios of peak areas (not necessarily peak heights) gives you the exact hydrogen ratios.
--the peaks often "split" depending on "spin-coupling" (aka J-coupling) based on how many neighbouring protons there are. A single peak may be split into a "doublet" (double peak) if there is a single -H on the next carbon, because that -H can be in two different spin-states and the proton peak on the first carbon responds to each of those states. If you see a triple-peak (in a characteristic 1:2:1 ratio), it's because of a nearby -CH2- group (the protons could be both spin -1/2, one spin -1/2 and the other +1/2 (which is twice as likely) and finally both are spin +1/2, so that's 3 possibilities with intensities related to the binomial distribution). And if you see a quadruple-peak (in a characteristic 1:3:3:1 size ratio) that is the proton's peak splitting because of a neighbouring -CH3 group. Aromatic and non-alkane groups produce different patterns. The set of peaks even obligingly "tilt" in favour of what neighbouring proton is splitting them, the peaks getting larger for the ppm closer to the neighbouring H's.
With all these clues you can determine molecular structure of an organic compound. In practice, you might couple NMR info with some other method to get a complete picture. There is for example NMR/MS (mass spectrometry) machines. Or you might conduct more sophisticated NMR experiments with names like COSY and NOESY.
Medical hospitals use a different kind of NMR for imaging purposes but there they just call it "Magnetic Resonance". The public fears the word "nuclear" although in NMR it just means the nuclei of atoms are responding to the radio frequencies in a magnetic field. It has nothing to do with the nuclear radiation the public is afraid of. Strong magnetic fields and radio emissions are harmless, unless you have a heart pacemaker, in which case you are advised to keep your distance from the active coil of the NMR machine.
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