What to expect: Chemical Shifts & Coupling Constants in Low-field NMR Spectroscopy

One of the questions that we always get at tradeshows and conferences is how our instrument compares to high-field data. There are significant inherent differences between low-field and high-field instruments, but the most important from a chemistry point of view are sensitivity (S/N) and resonance dispersion (signal separation).

1. Sensitivity. The signal to noise ratio (S/N) is proportional to the formula in Figure 1. For the same sample acquired at 60 vs. 400 MHz the theoretical SNR ratio is 17.2, and vs. 600 MHz is 31.6.

FIGURE 1. n: # NUCLEI IN RESONANCE; γ_e : GYROMAGNETIC RATIO OF EXCITED NUCLEI; T: TEMPERATURE OF THE SAMPLE (K); γ_d : GYROMAGNETIC RATIO OF DETECTED NUCLEI; B₀ : APPLIED FIELD STRENGTH (T); NS : NUMBER OF SCANS

2. Resonance dispersion. After acquiring the data, the FID is transformed from the time domain to the frequency domain by a Fourier transformation. The frequency (in Hz) is proportional to the strength of the external field. Since spectrometers with different magnetics field strengths are commonly used, it was decided to express the chemical shifts using an independent unit (ppm), as shown in Figure 2. As there are more Hz/ppm at 400 MHz, and the couplings remain constant, the signals appear narrower and are better resolved than they are at 60 MHz despite the fact that each signal contains the same structural information.

FIGURE 2. PPM SCALE CALCULATION

For this blog entry I have decided to show you a couple of spectra acquired at 60, 400 and 600 MHz in order to illustrate the effect of sensitivity and signal dispersion. Figure 3 shows the proton NMR spectra of 4-hydroxypropiophenone at 250 mM concentration acquired at different field strengths. Between the spectra at 400 and 600 MHz is not possible to see a significant difference in S/N using the naked eye, but we can clearly see a difference comparing them with the spectrum at 60 MHz.

FIGURE 3. SPECTRA ACQUIRED AT 60, 400 AND 600 MHz

To illustrate the difference in signal dispersion I have zoomed-in very specific regions in figure 3 (see Figure 4). As you can see all the resonances are centered at the same chemical shift, but the signals at 400 and 600 MHz are significantly narrower. The difference in signal dispersion comparing the 400 and 600 MHz spectra is not significant because the field is only 1.5 times stronger. However, the field strengths at 400 and 600 MHz are almost 7 times and 10 times stronger, respectively. What’s important to highlight here is that despite the significant difference in signal dispersion, the spectra acquired at different field strengths contain the same chemical and structural information. Pay close attention to the aromatic doublet in Figure 4, it’s the same splitting pattern in all the fields!

FIGURE 4. SPECTRA ACQUIRED AT 60, 400 AND 600 MHz

Even though it’s very nice to see the sharp spectra generated using high-field instruments, you will realize that a lot of times the spectra can be analyzed using a low-field instrument, with a fraction of the cost and time. Contact us if you want to know more about differences in high-field and low-field or if you have any other benchtop NMR inquiries!