A Diels-Alder reaction with 125 MHz benchtop NMR

The Q Magnetics 125 MHz benchtop NMR spectrometer is a powerful and convenient tool for monitoring chemical reactions. No reconfiguration is required to periodically transfer samples by flow from an external reaction vessel, or to track reactions carried out directly in the polymer sample tube. In this post, we demonstrate monitoring of a Diels-Alder reaction. This particular reaction has recently been studied as a route to producing phthalic anhydride from lignocellulosic biomass, a promising renewable feedstock [1].

125 MHz NMR reaction monitoring in non-deuterated DMSO

An equimolar solution of cyclohexadiene and maleic anhydride was prepared in non-deuterated DMSO, then immediately injected by syringe into the spectrometer’s sample tube. Spectra were recorded every 5 minutes for 3 hours. No shimming or other spectrometer adjustments were made after the sample was injected, or during the 3-hour period. No solvent-suppression techniques were used or needed. The figure above shows integrations of the vinylic proton signals from the reactants and the adduct product plotted versus time. These signals are sufficient to track the progress of the reaction and quantitatively characterize the second-order kinetics. All other expected signals are also present (DMSO and its satellites at 3.09, 2.54, 1.98; cyclohexadiene at 2.09; adduct at 3.29, 3.03, 1.12-1.68).

When the spectrometer’s polymer sample tube is filled using the front-panel fittings, there is no change of the sample geometry. This means there is normally no need for shimming before starting scans, as would be required with a spectrometer that uses NMR tubes. Data collection can start immediately after a sample is injected or immediately after it is transferred by flow from an external vessel. No special insert is required for sample delivery by flow. If a precipitation accident occurs and the sample tube becomes blocked, it can be quickly replaced without disturbing the magnet, and experiments can resume within a few minutes.

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References

1. Eyas Mahmoud, Donald A. Watson and Raul F. Lobo, Green Chem. 16, 167 (2014).

There's a lot going on in nicotinamide at 125 MHz

Nicotinamide appears on the World Health Organization’s model list of essential medicines as humanity’s go-to treatment for pellagra, a disease caused by severe deficiency of niacin, or vitamin B3. In the early 19th century, pellagra was a major public health problem in the southern United States. Nicotinamide is also sold as an acne treatment, and there is evidence that it reduces the risk of some kinds of skin cancer. The material used here came from a 500 mg capsule sold over-the-counter under the Nature’s Way brand. The capsule’s content was dissolved in 10 mL of water, then filtered, dried, and redissolved in deuterated solvents.

The proton NMR spectrum of nicotinamide has several interesting features, as revealed by the new Q Magnetics 125 MHz benchtop spectrometer. In the figure below, I show the aromatic region under three conditions: in D2O at neutral pH, in D2O with 2 drops of concentrated HCl added per mL, and in DMSO. The spectra are referenced to TMS in D2O and to DSS (set to 0.1 ppm) in DMSO.

125 MHz proton NMR spectra of nicotinamide in D2O, acidified D2O, and DMSO

The D2O spectrum at neutral pH is easiest to assign. Two of the multiplets are clearly broader than the others. These must be from the two protons adjacent to the fast-relaxing nitrogen [1]. The broad multiplet with the larger splitting must be H6, since it is adjacent to another proton, while the other broad multiplet must be H2. The remaining sharp multiplet with one large J-coupling must be H4, and the sharp multiplet with two large J-couplings must be H5. This assignment agrees with several in the literature [2,3].

The aromatic region changes dramatically when the D2O solvent is acidified by adding two drops of concentrated HCl per mL, so that the ring nitrogen becomes protonated (or rather deuterated, since there are many more exchangeable deuterons in the sample than exchangeable protons). The chemical shifts of the two ortho protons H2 and H6 increase by about 0.4 ppm, while H4 increases by 0.8 ppm and H5 by 0.7 ppm. As a result, the H4 and H6 multiplets are now overlapping. The smaller shift of the ortho protons is characteristic of pyridines [4]. By measuring the chemical shift of any of the ring protons versus pH, it is possible to determine pKa of the ring nitrogen, with the result pKa = 3.54. This experiment is simple enough that it can be included in a laboratory course [5].

When the spectrum is acquired in d6-DMSO, the H4 and H5 multiplets take on an odd asymmetric appearance. In a recent paper on methods development for qNMR, Achanta, Chen, and Pauli [3] show that this is due to overlap with two very broad peaks at 7.6 ppm and 8.2 ppm, assigned to the amide protons H7 and H8, and visible in DMSO because the solvent has no exchangeable protons.

Curiously, the ortho protons H2 and H6 that show broadening in D2O, seem to be sharper in both the DMSO and the acidified D2O spectra. This would seem to imply that the broadening in D2O is not due solely to fast quadrupolar relaxation of 14N.

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References

1. J.P. Kintzinger and J.M. Lehn Mol. Phys. 14, 133 (1968).

2. BMRDB bmse000281, doi:10.13018/BMSE000281.

3. Prabhakar S. Achanta, Shao-Nong Chen, and Guido F. Pauli, Magn. Reson. Chem. 59, 746 (2021).

4. Mosher, Sharma, and Chakrabarty, J. Mag. Res. 7, 247 (1972).

5. Zivkovic et al., Magnetochemistry 3, 29 (2017).

The path to 125 MHz benchtop NMR

Welcome to the new Q Magnetics web site and blog. In this first post, I’ll try to put our new company and technology in context with the help of some NMR history.

Liquid-phase NMR spectroscopy began in 1951 with the 32 MHz spectrum of ethanol shown below [1]. It is not quite recognizable to us today as an NMR spectrum because the multiplet structure is not resolved. The team at Stanford struggled with field stability, and reported that their water-cooled electromagnet had to be “…stabilized by voltage, current, and proton controlled regulators.” Proton multiplets due to spin-spin scalar coupling were first resolved at CERN in 1955 [2,3], not with an electromagnet, but with a more-easily stabilized permanent magnet based on Alnico V, the highest energy-product hard ferromagnetic material of the day.

(a) 1951 NMR spectrum of ethanol at 32 MHz. (b) 1955 30 MHz spectrum with multiplets resolved.

In the earliest days of NMR spectroscopy, electromagnets and permanent magnets vied for supremacy. At the ETH in Zurich, Hans Primas pursued both technologies [4], achieving results that ultimately led to the first Bruker spectrometers. However, by the late 1950s, electromagnets had won the contest. Despite challenges with stability, their need for water-cooling, and for large amounts of electrical power, electromagnets had a feature that would ensure their success: they could achieve higher magnetic fields.

Readers familiar with the basics of NMR will know why higher fields are nearly always desirable. The equilibrium polarization of nuclear spins placed in a magnetic field is proportional to the applied field strength, and this initial polarization is the precious resource that NMR experiments depend on. The Larmor frequency at which spins precess is also proportional to field. These two effects together mean that, other things being equal, the signal voltage is proportional to the second power of field strength. Higher field also means higher dispersion in the spectrum. Multiplet splitting, measured in Hz, is fixed by the strength of spin-spin interactions, but the chemical shifts of NMR spread out the multiplets in proportion to field strength.

Record high fields for the four different magnet technologies used in NMR spectroscopy: electromagnets, floor-standing permanent magnets, benchtop permanent magnets, and superconducting magnets. Data from [5,6,7,8].

Every NMR magnet technology has been pushed to higher and higher fields as it has developed. This repeating history of NMR spectroscopy is illustrated by the record fields plotted above for four different magnet technologies. I have tried to include every record field for each magnet technology that has been demonstrated in an NMR spectrometer capable of resolving proton multiplets.

Electromagnets reached 100 MHz proton Larmor frequency in 1960. A leading example was the Varian HR-100, with a magnet mass of over 1300 kg, requiring a 30 ampere three-phase power source. Electromagnets were the dominant technology for research applications of NMR spectroscopy until the early 1970s, when they were displaced by superconducting magnets. Today, electromagnets have almost disappeared from NMR labs, except for a few niche applications that require variable fields.

The field strength of floor-standing permanent magnets rose much more slowly than electromagnets. I believe all of the record-field floor-standing permanent magnets shown on the plot above were based on Alnico alloys. The maximum field of 90 MHz was reached in 1975, with the introduction of the 900 kg Varian EM-390 magnet. By then, permanent magnets were competing with superconducting magnets, offering much lower cost of ownership, but also lower field. Many floor-standing 60 MHz and 90 MHz magnets are still in use today, making them the longest-surviving magnet technology for NMR.

The slow and spectacular rise of the field strength of superconducting NMR magnets is well-know to modern chemists, and is a story better told elsewhere. Like the state-of-the-art 100 MHz electromagnets from the 1960s, these are also expensive and demanding machines to own, though for completely different reasons.

In the mid-2000s, several companies started developing modern permanent-magnet NMR spectrometers, taking advantage of three new technologies: rare-earth ferromagnets (SmCo and NdFeB) with much higher energy products than Alnico, integrated RF digital acquisition and signal processing, and fast embedded programmable processors. First-to-market in 2010 was the 45 MHz picoSpin spectrometer. At 4.8 kg and the size of a shoe-box, I believe it is still the smallest complete NMR spectrometer able to resolve proton multiplets. The new ‘benchtop’ permanent-magnet spectrometers are all much more compact than older Alnico-based designs, but they are diverse instruments with masses ranging up to 170 kg, some stretching the benchtop designation. They target correspondingly diverse markets and applications.

With the introduction of our 125 MHz benchtop spectrometer, Q Magnetics is leading a fourth repeat-of-history in NMR magnet development. Because the underlying materials science is quite mature, we anticipate that field increase for this new class of magnets is already complete, and will now plateau in the same way that electromagnets and floor-standing permanent magnets did previously.

As I am writing this first post, we are about a week from our product launch and from going live with our web site. It’s an exciting time at Q Magnetics! We hope you will look around our site to share the excitement, and return to our blog and site from time to time in the coming months and years to learn more about Q Magnetics spectrometers and their applications.

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References

1. J.T. Arnold, S.S. Dharmatti, and M.E. Packard, J. Chem. Phys. 16, 507 (1951).

2. J.T. Arnold, Phys. Rev. 102, 136 (1956).

3. Weston A. Anderson, Phys. Rev. 102, 151 (1956).

4. Richard R. Ernst, Angew. Chem. Int. Ed. 49, 8310 (2010).

5. Gareth A. Morris, J. Mag. Res. 306, 12 (2019).

6. “Introducing: The low-cost Varian EM-390,” Chem. and Eng. News, May 19, 1975.

7. Ian D. Campbell, Biomedical Spect. and Imaging 2, 245 (2013).

8. “Bruker installs world’s first 1.2 GHz NMR,” Chem. and Eng. News, May 15, 2020.