Why 100 MHz Benchtop NMR?
While low-field NMR has extremely favourable accessibility and affordable characteristics, the most common question we get asked about our family of benchtop NMR spectrometers is with respect to any trade-offs that come from moving to lower-field. For either structural elucidation or quantification (qNMR) applications, there are two main obstacles imposed at low-field: (i) resolution/ dispersion; and (ii) what I will call spectral complexity. While resolution and peak overlap can contribute to spectral complexity, this it is primarily due to second order effects.
Resolution and dispersion are related, but subtly different. Resolution refers to the ability to fully separate two peaks. Dispersion, on the other hand, refers to the spread of an individual peak over the chemical shift axis (the x-axis). To illustrate: at 60 MHz, a 1 Hz wide peak would cover 0.02 ppm, whereas at 100 MHz, a 1 Hz wide peak would cover only 0.01 ppm. This means that as the field strength increases, each peak appears narrower. Correspondingly, resonances are more likely to be resolved even if they have similar chemical shifts.
To illustrate the dispersion/resolution phenomena, the low-field benchmark compound of the ibuprofen proton NMR is shown in figure 1 acquired at 60 and 100 MHz. In the 60 MHz spectrum, the methine peak at 1.77 ppm, overlaps with both of its neighbours, so although the peak multiplicity allows the user to infer the structure from the spectrum, it is not totally clear – the septet is not fully observed and there is also error in the integration. In the 100 MHz spectrum, however, there is a clear separation between the methine peak and its methyl neighbours. The fine structure of the septet can be observed in the expected 1:6:15:20:15:6:1 Pascal’s triangle ratio.
Figure 1: 100 and 60 MHz stacked plot of ibuprofen in DMSO-d6. Asterisk denotes 13C satellites and residual solvent.
The second benefit of increasing field strength is the reduction of spectral complexity. To illustrate this effect the 1H NMR spectrum of dyfonate in chloroform is shown in figure 2 at 60 and 100 MHz. Dyfonate is an agrochemical with a complex (and interesting!) 1H NMR spectrum, owing to the observation of both standard vicinal proton-proton bonding (3JH-H), as well as 2JP-H and 3JP-H heteronuclear coupling due to the presence of the phosphorus-31 (100% natural abundance, I = ½).
Figure 2: 100 and 60 MHz stacked plot of dyfonate in CDCl3.
To highlight the difference, it is easy to look at the resonances between 1.0-2.5 ppm, which can be seen as a zoom-in inset in figure 2. At 60 MHz, this presents as two multiplets from 1.5-2.4 ppm and 0.9-1.5 ppm, integrating to 2 and 6 protons respectively, but with no real indication as to the molecular structure. In the 100 MHz spectra, however, we can start to see some multiplicities which make sense visually. For the ethyl group directly bonded to phosphorus, one would expect to see the methyl group as a doublet of triplets, and the methylene as a doublet of quartets. While still second order at 100 MHz (and also 600 MHz for that matter), one can clearly make out each individual reference of the CH3 of the ethyl group with a triplet at 1.16 pm. Unfortunately, the second ethyl CH3 triplet is overlapping with the ethoxy CH3 at 1.35 ppm, which itself appears as a triplet (it appears that we do not observe any 4JP-H coupling in the methyl). The doublet of quartets for the ethyl CH2, does overlap at approximately 2.0ppm, and we see only 6 lines out of the expected 8, but in a recognizable pattern.
While not fully first order, the chemical shift and coupling constants of these can be extracted directly from the 100 MHz spectrum, without requiring collection of higher-field data. This added information of the 100 MHz is extremely interesting, because it means that we may be able to use software tools, such as quantum mechanical modelling software like NMR Solutions, to extract all required chemical shift and coupling constant information. By moving to higher field, we can begin to address more industries including agrochemicals, flavour and fragrance, personal care and cosmetics.
For further reading on field strength and NMR spectra, I suggest you have a look at this recently published paper in ACS Journal of Chemical Education, or this blog entry.
For more information on our 60 or 100 MHz model, please contact sales@nanalysis.com or fill out this form.