[macman104]

I have a question that asks for the Hz in 1ppm for a 500MHz machine when run for 1H and 13C.  It says "hint they are not the same".

I'm very confused.  I was always told that the Hz = ppm * machine_frequency, which would give the same value regardless whether it was 1H or 13C NMR.  Am I operating under an incorrect assumption?  I would have assumed the answer was 500Hz/ppm on a 500MHz machine.

[StarvinMarvin]

Of course they're not the same for 1H and 13C. Can you tell what are the resonance frequencies for protons and carbons on 500 MHz machine?

[gfunk]

Haha, my friend was grilled for this in his honours thesis defense.  He was doing solid-state caesium NMR, and referred to the spectrometer as "the 600".  One of the professors asked him, "You mentioned the 600MHz spectrometer, but that's the resonant frequency for protons.  What is the resonant frequency for the nucleus of interest in your research?"

What's constant between the nuclei is the field strength of the magnet in Tesla.  From there, you want to find the Larmor frequency?  I don't remember all of my NMR theory.  Anyhow, a quick way to find out is to look at an NMR printout from the spectrometer.  It should give you the frequency lock.

[StarvinMarvin]

There's even a quicker way for protons and carbons: the frequency for 13C is usually 1/4 of the proton frequency on the particular machine. So, in case of 500 MHz machine, carbons are recorded at 125 MHz.

[macman104]

Thanks for the info.  I knew of Larmor frequency, but the significance of it wasn't really covered at all in my undergrad classes.