Application Note 22Hyperpolarisation in NMR usingpara-hydrogenin collaboration with:
Simon Duckett, Callum Gater, Vicky Leadley & Ben Tickner(Centre for Hyperpolarisation in Magnetic Resonance, University of York, York, U.K.)

Introduction
Nuclear magnetic resonance (NMR)
spectroscopy is one of the most powerful andflexible methods of analysis used in science
today. However, in comparison with otheranalytical techniques, NMR signals haveinherently low sensitivity. This is because thestrength of the NMR signal is dependent on theoverall nuclear magnetisation; and whenmagnetised/polarised by an external magnetic
field, only a small proportion of a bulk sample
contributes towards the overall magnetisation.
Which, for 1H, effectively means that only one inevery 10,000 – 100,000 nuclei are observed(depending on the magnetic field strength), withthe ratios even less favourable for other, lownatural abundance, low gyromagnetic ratio
nuclei, such as 13C, 29Si & 15N.
In order to overcome this low sensitivity in NMR,a wide range of approaches can be used,
together known as hyperpolarisation. These use
a range of methods that feature chemical
manipulations, photochemistry and NMR pulse
sequences; to introduce additional polarisation,and hence increase the intensity of the observed

signal, usually by two to four orders ofmagnitude.[Chem. Rev., 2023, 123, 1417-1551]
In this application note, we show somepreliminary data to demonstrate how easily
hyperpolarised NMR spectra for a range ofnuclei can be obtained on the Oxford
Instruments X-Pulse Broadband NMR
Spectrometer, using the SABRE method toperform the hyperpolarisation without the needfor extensive time consuming optimisation.

SABRE
Signal amplification by reversible exchange(SABRE) was pioneered by Simon Duckett andhis team at the University of York, and uses
para-H2 (a spin isomer of H2, which can beeasily obtained by cooling H2 to cryogenictemperatures) to transfer polarisation via an
iridium complex, to a substrate which is thenobserved by NMR (Scheme 1). Compared withmany other methods for hyperpolarisation,
SABRE is relatively straightforward, requiring
only a source of para-H2,
To perform the SABRE method for
hyperpolarisation, para-H2 is added to a samplecontaining an iridium pre-catalyst, target
substrate, and any co-reagents. These samples
are then shaken in a magnetic field to transfer
the polarisation from the para-H2 to thesubstrate, and a one-dimensional NMR
spectrum obtained, the resulting phase of thehyperpolarised signals depend upon the exact
experimental conditions used.[Annu. Rep. NMR Spectrosc.
2018, 93, 145-212]

Hydrogen-1 (Proton)
A 1H NMR spectrum of a ca 60 mmol/ℓ solutionof methyl-4,6-d2-nicotinate following
hyperpolarisation by SABRE is shown in Figure
1. The two aromatic protons are clearly
hyperpolarised, and observed as singlets withnegative phase; while the methyl ester signal
retains a positive phase without significantenhancement. The two aromatic signals (eachScheme 1 Generic mechanism for Signal
Amplification By Reversible Exchange(SABRE)

corresponding with a single proton), are 150×
more intense than the methyl ester signal(corresponding to three protons).
Another feature to note in the spectrum are thesignals at δH −14.0 & −18.2 ppm, these are
hyperpolarised signals from an iridium hydridecomplex, formed as the pre-catalyst isconverted to the active catalyst (pictured inFigure 1); therefore, showing there’s sufficientsignal enhancement to allow for observationof reaction intermediates on the X-Pulse.

Figure 1 Hyperpolarised 1H NMR spectra (single scan) of methyl-4,6-d2-nicotinate in the prescenceof an iridium dihydride intermediate

Fluorine-19

Carbon-13

Like 1H, 19F NMR spectra can display significant
enhancement with hyperpolarisation. In Figure 2
a single scan spectrum of a ca 60 mmol/ℓ
solution of 3-fluoropyridine, following
hyperpolarisation by SABRE is shown. The
signal-to-noise ratio (SNR) of this signal is over
one thousand. While for the same sample
without hyperpolarisation; the signal can still beobserved in a single scan, but has an SNR of
approximatly two (see Figure 2 insert).

Despite how important carbon-13 NMR
spectroscopy is for organic, organometallic, andinorganic chemistry; it’s amongst the lowest
receptivity nucleus commonly studied by NMR.
Therefore, 13C NMR benefits greatly frommethods to increase the signal intensity, such as
hyperpolarisation and isotopic labelling. Both ofthese methods are applied in this example.
A 13C{1H} NMR spectrum was observed in asingle scan following hyperpolarisation by
SABRE of ca 30 mmol/ℓ sodium 1,2-pyruvate[13C2] (Figure 3); with the polarisation transfer
taking place in a mG field (lower than thestrength of the Earths magnetic field), a method
known as SABRE-SHEATH.[Angew. Chem., Int. Ed., 2019, 58,10271-10275]

There are two doublets clearly visible (withopposite phases) in the spectrum, with 63 Hz,
13C-13C J-coupling. These peaks have SNRs ashigh as 1700; while without hyperpolarisation,even with this level of enrichment, at benchtop
frequencies the sample is too dilute to observe
any signal with just a single scan.
Figure 2 Hyperpolarised 19F NMR spectrum (singlescan) of 3-fluoropyridine (Insert: single scan
spectrum without hyperpolarisation)

Figure 3 Hyperpolarised 13C{1H} NMR spectrum (single scan) of sodium 1,2-pyruvate-[13C2]

Silicon-29
Due to both low natural abundance and a low
gyromagnetic ratio, silicon-29 is amongst thelowest receptivity nuclei observed by NMR. Witheven 1 mol/ℓ samples often taking over an hour(>1000 scans) to observe a signal in a standard
one-dimensional NMR spectrum at benchtop
frequencies. Indeed, polarization transfer
sequences (INEPT & DEPT) are routinely used for
29
Si NMR due to the signal enhancement theygive.
To demonstrate the signal enhancement
possible using SABRE, a sample containing ca
30 mmol/ℓ tris(iso-propyl)silanol, the iridium

catalyst, and benzylamine was exposed topara-H2, and a 29Si{1H} NMR spectrum obtained
where a signal is clearly observed in a single
scan (Figure 4). In this two step mechanism,
polarisation is initially transferred from para-H2
to the benzylamine via the iridium complex; thenfrom the benzylamine to the silanol byexchange of labile protons; with the process
therefore known as SABRE-Relay.[Angew. Chem., Int.
Ed., 2020, 59, 2710-2714]
To directly observe this 29Si
signal, for a compound at this concentration, atbenchtop frequency, without hyperpolarisation,
would take many hours or even days.

Figure 4 Hyperpolarised 29Si{1H} NMR spectrum (single scan) of tris(iso-propyl)silanol

Summary
Hyperpolarisation using SABRE has been
demonstrated for a range of nuclei on theOxford Instruments X-Pulse Broadband
Benchtop NMR Spectrometer. These are only
some of the many nuclei which can be observed
on a single X-Pulse.
The X-Pulse uses a 1.4 T (60 MHz) cryogen-freepermanent magnet, and comes with a fully
tuneable user removable probe with an external
deuterium lock as standard. Along with an
optional 25 position autosampler, optionalvariable temperature capability, and an optional
flow cell. Allowing for maximium flexibility toperform a range of experiments, including
many different methods of hyperpolarisation.

If you have any questions about
this application note, pleasecontact our experts:
magres@oxinst.com

visit nmr.oxinst.com© Oxford Instruments Nanotechnology Tools Ltd trading as Oxford Instruments Magnetic
Resonance, 2023. All rights reserved. Do not reproduce without permission.
Part No: MR/253/0523