How can I read a #C^13 NMR# spectrum?

Question

Paisley Boney · Accepted Answer

How many signals do you see?

If you see #"2,3,4,5"# signals, there are #"2,3,4,5"# different #""^13"C"# environments. This may seem glib, but you can usually use symmetry to tell you how many signals you should observe in the #""^13"C{"""^1"H}"# #"NMR"# spectrum, and thereby decide a likely molecular structure. Methane (when dissolved in a suitable solvent) should reasonably gives rise to one signal in its #""^13"C"{""^1"H"}# #"NMR"# spectrum. Why? Because there is only one chemical environment. Ethane, again should give rise to the ONE signal; this will occur at a different chemical shift to that of methane. Clearly, the methyl groups are symmetric, and should respond equally to radiation. When we go to propane, however, #"H"_3"CCH"_2"CH"_3#, the spectrum should give rise to TWO signals. Why? Because there are 2 chemical environments: #"(i) methyl"#, #"CH"_3#; and # "(ii) methylene"#, #"CH"_2.# #"Butane"#, again should give rise to 2 signals. You will have to tell me why. #"Pentane,"# #"H"_3"C""{(CH"_2")"_3"}CH"_3# on the other hand, should give rise to 3 signals. I cannot hope to tell you all you need to know about #"NMR spectroscopy"# in a few sentences (for a start, I don't know everything), but for the moment forget about WHERE the absorptions come (this is something the experiment tells you), and consider HOW MANY absorptions there should be. Aspects of symmetry will help you here; it will also help you in relation to the #""^1"H"# #"NMR spectrum"#. So how many signals in the #""^13"C"{""^1"H"}# #"NMR"# spectrum should be seen for: #(i)# #"H"_3"CCO"_2"H"#, #(ii)#, #"H"_3"CCH"_2"Cl"#, #(iii)# #"H"_3"CCH"_2"C"-="N"#, and #(iv)#, #"H"_3"CCH"_2"CH"_2"CH"_2"CH"_3#?

Emilia Alvarez · Answer

undefined Carbon-13 NMR spectra display chemical shifts of carbon atoms in a molecule. The number of peaks indicates the different types of carbons, and the integration provides the relative abundance. Chemical shifts are measured in parts per million (ppm) downfield from tetramethylsilane (TMS).