What you should know about signal dispersion in benchtop NMR

Resolution and signal dispersion in NMR are related terms that are frequently –and wrongly– used interchangeably. Signal dispersion refers to how squeezed or spread the signals are over the chemical shift axis, while resolution refers to the ability to distinguish two separate peaks and is directly related to the line width of the peaks, which is dependent upon its T₂ (spin-spin relaxation).¹

Signal dispersion plays a pivotal role especially in ¹H NMR due to the small chemical shift range. Like the signal-to-noise ratio, this is directly dependent on the magnetic field strength of the NMR spectrometer: the stronger the magnetic field, the higher the signal dispersion, thus the larger the space between signals in the spectrum, assuming all other variables remain the same. For example, at 60 MHz, 1 ppm spans 60 Hz while at 100 MHz, 1 ppm spans 100 Hz, and on a 400 MHz NMR spectrometer, 1 ppm spans 400 Hz, etc. - you get the idea. With J couplings in Hertz being constant at different field strengths, in the unifying ppm scale, a multiplet simply spans a wider region in the spectrum at lower field strengths. Thus, overlap with neighboring signals is more likely to be observed. This is easier to visualize in the following image showing the predicted ¹H NMR spectra of 2,2-difluoroethyl p-toluenesulfonate at 60 MHz, 100 MHz, and 400 MHz in the Hertz and ppm scale, respectively:

Figure 1: Simulated spectra of 2,2-difluoroethyl p-toluenesulfonate at different field strengths in the Hertz (left) and ppm scale (right). Signals H4 and H5 display the signal dispersion most clearly in spectral overview.

Why should you care about the distance between two signals? Three reasons:

1) Baseline separation of two signals allows individual – and more accurate – under-curve area integration

2) More complex molecular structures or mixtures of analytes lead to multiple signals, the more you can differentiate, the more information you get out of the data.

3) Second order effects get more prominent with lower signal dispersion which adds more complexity to spectra.¹ Here is a direct ¹H NMR spectral comparison of bromobutane at 60 MHz and 100 MHz. The increased signal dispersion at 100 MHz leads to complete baseline separation of the methyl group H1 and the methylene group H2. The higher dispersion even almost allows to fully separate methylene protons H2 from methylene group H3.

Figure 2: Stacked spectra of 1-bromobutane acquired on a Nanalysis 60 (top) and 100 (bottom). Separation of signals H2 and H3 as well as a lower degree of second order effects are directly related to the signal dispersion.

While narrower line widths offer precision, signal dispersion reduces spectral complexity, facilitating accurate spectral analysis. For a more detailed analysis of signal dispersion and resolution check our paper or infographic.

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References:

[1] J. Chem. Educ. 2021, 98, 1227 – 1232. - Incorporating Benchtop NMR Spectrometers in the Undergraduate Lab: Understanding Resolution and Circumventing Second-Order Effects