what is/ how do you calculate the chemical shift in an NMR?

[Courtney]

I have read the technical definitions but I still do not understand what the chemical shift means and how you calculate it given an NMR peak. If someone could kind of dumb it down for me but still give me the important things to know/ how it helps you determine the structure of a molecule with the NMR I would appreciate it.

[SuperNashwan]

If you are mainly interested in structure determination then I wouldn't worry too much about how chemical shifts are calclulated and concentrate on how you can apply the data. The best way of learning NMR is to get examples and practice. Eventually you will see familiar patterns and use this information.You really want to consider 3/4 pieces of information. One, the chemical shift will tell you the kind of connectivity your protons have, for example methyl protons generally have chem shifts around the 1ppm mark. Aromatics you can find around 6.5-7. Aldehydes around 10ppm. This indicates the type of C-H bonds you have.Secondly, look at the multiplicities. Are there any patterns? If you see a splitting pattern that is say 2 distinct even lines then you can use the n+1 rule which tells you that there is n=1 hydrogens on the adjacent atom. Coupling occurs in many different ways and you can refer to pascals triangle to help determine your multiplicities and you should learn these (IE singlets, doublets, triplets, doublet of doublets etc.)Thirdly look at the integration on the peaks. In modern NMR these will be given below the peaks, and tell you how many hydrogens are in that environment. Again using methyl protons as an example, you'd have a peak at about 1ppm, with 3 below. This 3 indicates 3H attached to your C atom, IE -CH3.Finally, coupling constants can be calulated from the difference between 2 peaks. This value (called the J value) can help you to assign the stereochemistry of your molecule. For example you have deduced from chemical shifts and you'd 13C nmr that you have a alkene group in your molecule with 2 subsitituents. Is it the E or Z alkene? Well you calculate your J value and its 16Hz. You know now its the E-alkene. Really this is a basic run through of what is a powerful technique in chemistry. Are you interested in organic synthesis? Well this technique has to be mastered. As an organic chemist I'd recommend not getting too bogged down in calculations (leave that to the phys guys), and just use the information you have in front of you to determine what you're lovely smelling molecule is! I always remember the first "unknown" NMR I figured out. Its a great feeling when you nail it.SO get some spectras, and get cracking!http://wwwchem.csustan.edu/Tutorials/NMRTABLE.HTM - se this as a guide to chemical shifts for protons in certain environments.http://www.wfu.edu/~ylwong/chem/nmr/h1/index.html - Guide to splitting patterns and some practice.

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