TMS as a reference signal
'Each molecule of TMS has 12 equivalent protons and these give rise to a single, sharp NMR signal which can be identified easily. The chemical shift of TMS is defined as delta= 0 ppm'. Ok so have I understood this correctly? TMS produces a single peak because all the carbon atoms are in the same environment. But why is it defined as delta= 0 ppm? Is it because frequency at which energy is absorbed is lower than for a lot of other compounds/molecules? Or has it just been randomly chosen as 0 for no particular reason?
Original post by tammie123
'Each molecule of TMS has 12 equivalent protons and these give rise to a single, sharp NMR signal which can be identified easily. The chemical shift of TMS is defined as delta= 0 ppm'. Ok so have I understood this correctly? TMS produces a single peak because all the carbon atoms are in the same environment. But why is it defined as delta= 0 ppm? Is it because frequency at which energy is absorbed is lower than for a lot of other compounds/molecules? Or has it just been randomly chosen as 0 for no particular reason?
Yes, you get a single peak in 1H and 13C NMR because all the protons are in the same environment and all the carbon atoms are in the same environment.
You're also right that the shift is defined as 0 ppm for convenience, as the vast majority of organic spectra will only have carbon and proton environments with larger delta than in TMS.
Original post by tammie123
'Each molecule of TMS has 12 equivalent protons and these give rise to a single, sharp NMR signal which can be identified easily. The chemical shift of TMS is defined as delta= 0 ppm'. Ok so have I understood this correctly? TMS produces a single peak because all the carbon atoms are in the same environment. But why is it defined as delta= 0 ppm? Is it because frequency at which energy is absorbed is lower than for a lot of other compounds/molecules? Or has it just been randomly chosen as 0 for no particular reason?
It hasn't been randomly chosen, it's been defined as such. This is similar to the atomic mass unit being defined as 1/12 the mass of 12C
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