Question

Question: What effect does increasing the operating frequency of a¹HNMR spectrum have on each value: (a) the chemical shift in$\delta$ ;(b) the frequency of an absorption in Hz; (c) the magnitude of a coupling constant J in Hz?

Short answer

Answer

(a)The chemical shift does not change on increasing the operating frequency.

(b)The frequency of absorption must be higher if the operating frequency is higher.

(c)The magnitude of the coupling constant does not change on increasing the operating frequency.

Step-by-step solution

 1H NMR

¹H NMR can also be termed proton NMR. The signal intensity is plotted on the y- axis and the chemical shift value on the x-axis in an NMR spectrum.

Chemical shift 

The horizontal part of the NMR spectrum shows the chemical shift. The formula that is employed to calculate chemical shift can be given

$\mathit{c}\mathit{h}\mathit{e}\mathit{m}\mathit{i}\mathit{c}\mathit{a}\mathit{l}\mathbf{}\mathit{s}\mathit{h}\mathit{i}\mathit{f}\mathit{t}\mathbf{}\mathbf{(}\mathit{i}\mathit{n}\mathbf{}\mathit{p}\mathit{p}\mathit{m}\mathbf{}\mathit{i}\mathit{n}\mathbf{}\mathit{\delta }\mathbf{}\mathit{s}\mathit{c}\mathit{a}\mathit{l}\mathit{e}\mathbf{)}\mathbf{=}\frac{\mathbf{o}\mathbf{b}\mathbf{s}\mathbf{e}\mathbf{r}\mathbf{v}\mathbf{e}\mathbf{d}\mathbf{}\mathbf{c}\mathbf{h}\mathbf{e}\mathbf{m}\mathbf{i}\mathbf{c}\mathbf{a}\mathbf{l}\mathbf{}\mathbf{s}\mathbf{h}\mathbf{i}\mathbf{f}\mathbf{t}\mathbf{}\mathbf{(}\mathbf{i}\mathbf{n}\mathbf{}{\mathbf{H}}_{\mathbf{Z}}\mathbf{)}\mathbf{}\mathbf{D}\mathbf{O}\mathbf{W}\mathbf{N}\mathbf{F}\mathbf{I}\mathbf{E}\mathbf{L}\mathbf{D}\mathbf{}\mathbf{F}\mathbf{R}\mathbf{O}\mathbf{M}\mathbf{}\mathbf{T}\mathbf{M}\mathbf{S}}{\mathbf{v}\mathbf{}\mathbf{o}\mathbf{f}\mathbf{}\mathbf{N}\mathbf{M}\mathbf{R}\mathbf{}\mathbf{s}\mathbf{p}\mathbf{e}\mathbf{c}\mathbf{t}\mathbf{r}\mathbf{o}\mathbf{m}\mathbf{e}\mathbf{t}\mathbf{e}\mathbf{r}\mathbf{}\mathbf{(}\mathbf{i}\mathbf{n}\mathbf{}\mathbf{M}{\mathbf{H}}_{\mathbf{Z}}\mathbf{)}\mathbf{}}$

Coupling constant

The separation between any two peaks situated near to each other is denoted as the coupling constant. The information about the adjacent protons can be obtained using the coupling constant.

Effect of operating frequency on several factors

(a) Chemical shift provides absorption as a fraction of NMR operating frequency. So it does not depend on the spectrometer that is employed to record the spectrum. The value of chemical shift does not get altered on increasing the operating frequency.

(b) The energy variation between two nuclear spin states is larger when the operating frequency is stronger. Hence a higher frequency of absorption is required on increasing the operating frequency.

(c) The magnitude of the coupling constant depends upon how the hydrogen atoms are arranged. It is independent of the operating frequency. Hence, the magnitude of the coupling constant is unaffected by the operating frequency.