Question

What is the difference between a regular 13C NMR and a 13C NMR registered in a proton coupled mode?

Answer 1

The¹³C NMR is generated in the same fundamental was as proton NMR spectrum.Only 1.1 % of naturally occurring carbon is ¹³C and actually an advantage because of less coupling.

Requirement for NMR: Spin quantum :(I) ≠0 .Meaning must be an odd number and/or neutrons. Ex.¹H,²H, ¹³C, ¹⁹F, etc

¹³C NMR :-

The ¹³C NMR is directly about the carbon skeleton not just the proton attached to it.

a) The number of signals tell us how many different carbons or set of equivalent carbons

b)The splitting of a signal tells us how many hydrogens are attached to each carbon. (N+1 rule)

c)The chemical shift tells us the hybridization (sp³, sp², sp) of each carbon.

d)Integration: Not useful for ¹³C NMR

¹³C NMR registered in a proton coupled mode :-

Proton –coupled spectrum shows splitting of the carbon signal only by protons attached to that carbon itself. ¹³C _H coupling not ¹³C _¹³C H or not ¹³C _¹³C _ ¹³C _H or not¹³C _¹³C.

Thus, for each carbon the multiplicity of the signal depends upon how many protons are attached to it.

Note: Due to low natural abundance, ¹³C NMR spectra do not ordinarily show carbon-carbon splitting two ¹³C being next to other is 1.1 %x 1.1%=0.012 %(because ¹²C does not have a magnetic moment, it cannot split the signal of an adjacent ¹³C), and are thus enormously simplified.