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NMR Coupling Constant - Effect of Frequency

Supposing I run a sample at 400 MHz and then again at 200 MHz.

Would the coupling constant change ? I think it would be larger at 400 MHz but I dont fully see why.

(edited 10 years ago)

Reply 1

Big-Daddy

No. The coupling constant will not change. It is the same as the chemical shift; the constant of proportionality introduced by your external magnetic field simply cancels out.

A proper mathematical justification may be more complicated though. I assume you want this, but unfortunately I cannot provide serious derivations over the Internet. The books I recommend are OCP 32, "Nuclear Magnetic Resonance" by PJ Hore, and OCP 92, "NMR: The Toolkit" also by PJ Hore, in that order.

Reply 2

BJack

Original post by Big-Daddy

No. The coupling constant will not change. It is the same as the chemical shift; the constant of proportionality introduced by your external magnetic field simply cancels out.

The coupling constant doesn't change because it's a quantum mechanical interaction that doesn't depend on the external field strength.... This is why you want to run with a high field, as this means the coupling constant is relatively small and you stay in the weak coupling regime. (i.e. simple spectra — don't need to worry about roofing, overlapping multiplets and all that.)

Reply 3

saurabhrai

Coupling constant won't change as it doesn't depends on external magnetic field. Also bit clarification about chemical shift, it the frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm. The scale is made more manageable by expressing it in ppm and is indepedent of the spectrometer frequency.