Enhancing the NMR signals of plant oil components using hyperpolarisation relayed via proton exchange

In this work, the limited sensitivity of magnetic resonance is addressed by using the hyperpolarisation method relayed signal amplification by reversible exchange (SABRE-Relay) to transfer latent magnetism from para-hydrogen, a readily isolated spin isomer of hydrogen gas, to components of key plant oils such as citronellol, geraniol, and nerol. This is achieved via relayed polarisation transfer in which an [Ir(H)2(IMes)(NH2R)3]Cl type complex produces hyperpolarised NH2R free in solution, before labile proton exchange between the hyperpolarisation carrier (NH2R) and the OH-containing plant oil component generates enhanced NMR signals for the latter. Consequently, up to ca. 200-fold 1H (0.65% 1H polarisation) and 800-fold 13C NMR signal enhancements (0.65% 13C polarisation) are recorded for these essential oils in seconds. Remarkably, the resulting NMR signals are not only diagnostic, but prove to propagate over large spin systems via a suitable coupling network. A route to optimise the enhancement process by varying the identity of the carrier NH2R, and its concentration is demonstrated. In order to prove utility, these pilot measurements are extended to study a much wider range of plant-derived molecules including rhodinol, verbenol, (1R)-endo-(+)-fenchyl alcohol, (−)-carveol, and linalool. Further measurements are then described which demonstrate citronellol and geraniol can be detected in an off-the-shelf healthcare product rose geranium oil at concentrations of just a few tens of μM in single scan 1H NMR measurements, which are not visible in comparable thermally polarised NMR experiments. This work therefore presents a significant expansion of the types of molecules amenable to hyperpolarisation using para-hydrogen and illustrates a real-world application in the diagnostic detection of low concentration analytes in mixtures.

Table S1: NMR characterisation data for 1 in dichloromethane-d2 at 298 K.The resonance labels correspond to those shown in Figure S1.S2.
Table S2: NMR characterisation data for 2 in dichloromethane-d2 at 298 K.The resonance labels correspond to those shown in Figure S2.

S1.3: Geraniol
Table S3: NMR characterisation data for 3 in dichloromethane-d2 at 298 K.The resonance labels correspond to those shown in Figure S3.

S2.1: SABRE-Relay hyperpolarisation of 1
A sample of 1 (25 mM) was exposed to pH2 (3 bar) in the presence of NH3 (25 mM) and the precatalyst [IrCl(COD)IMes] for several hours.The sample was then shaken with pH2 at 6.5 mT for 10 seconds before spectral acquistion was performed at 9.4 T. Example 1 H and 13 C NMR spectra are shown in Figure 2 in the main paper, with the associated NMR signal enhancments being given in Table S4.

S2.2: SABRE-Relay hyperpolarisation of 2
A sample of 2 (25 mM) was exposed to pH2 (3 bar) in the presence of NH3 (25 mM) and the precatalyst [IrCl(COD)IMes] for several hours.The sample was then shaken with pH2 at 6.5 mT for 10 seconds before spectral acquistion was performed at 9.4 T. Example 1 H and 13 C NMR spectra are shown in Figures S4 and S5 respecitvely, with the associated NMR signal enhancments being given in Table S5.S5.S5.S6.S6.S6.

Figure S7: Single scan thermally polarised (above) and SABRE-Relay hyperpolarised (middle and lower) 13 C NMR spectra for a sample of [IrCl(COD)(IMes)] (5 mM), NH3 (25 mM), 3 (25 mM) and pH2 (3 bar) in DCM-d2 (0.6 mL). The hyperpolarised spectrum is recorded immediately after shaking the sample for 10 seconds with fresh pH2 at 6.5 mT. The middle spectrum uses a single 90 o pulse for 13 C detection whereas the INEPT sequence used in the lower spectrum transfers magnetisation from the 1 H domain to the 13 C domain via radiofrequency excitation (see section S1). Note that the lower spectrum is not shown on the same vertical scale as the middle and upper. The associated signal enhancements are given in Table
Table S6:

S2.4 SABRE-Relay hyperpolarisation of 4
The structure of 4 is shown in Figure S8 and its NMR characterisation data is shown in Table S7.It was hyperpolarised by shaking a sample containing [IrCl(COD)IMes] (5 mM), NH3 (55 mM), and 4 (25 mM) in dichloromethane-d2 (0.6 mL) with pH2 (3 bar) at 6.5 mT for 10 seconds before spectral acquisition was performed at 9.4 T. The pH2 shaking was performed after the sample has been left to react with H2 (3 bar) overnight to form [Ir(H)2(IMes)(NH3)3]Cl. 1 H and 13 C NMR signal enhancements are shown in Table S7 with an example 13 C NMR spectra shown in Figure S9 ( 1 H NMR spectra are shown in Figure 3 of the main paper).S7.
Table S7: NMR characterisation data and signal enhancements for 4 in dichloromethane-d2 at 298 K.The resonance labels correspond to those shown in Figure S9.

S2.5 SABRE-Relay hyperpolarisation of 5
The structure of 5 is shown in Figure S10 and its NMR characterisation data is shown in Table S8.It was hyperpolarised by shaking a sample containing [IrCl(COD)IMes] (5 mM), NH3 (40 mM), and 4 (25 mM) in dichloromethane-d2 (0.6 mL) with pH2 (3 bar) at 6.5 mT for 10 seconds before spectral acquisition was performed at 9.4 T. The pH2 shaking was performed after the sample has been left to react with H2 (3 bar) overnight to form [Ir(H)2(IMes)(NH3)3]Cl. 1 H and 13 C NMR signal enhancements are also shown in Table S8 with some example NMR spectra shown in Figure S11.S8.S8.

S2.6 SABRE-Relay hyperpolarisation of 6
The structure of 6 is shown in Figure S12 and its NMR characterisation data is shown in Table S9.It was hyperpolarised by shaking a sample containing [IrCl(COD)IMes] (5 mM), NH3 (40 mM), and 6 (25 mM) in dichloromethane-d2 (0.6 mL) with pH2 (3 bar) at 6.5 mT for 10 seconds before spectral acquisition was performed at 9.4 T. The pH2 shaking was performed after the sample has been left to react with H2 (3 bar) overnight to form [Ir(H)2(IMes)(NH3)3]Cl. 1 H NMR signal enhancements are also shown in Table S9 with example spectra shown in FIugre 3c of the main paper.S9.

S2.7 SABRE-Relay hyperpolarisation of 7
The structure of 7 is shown in Figure S13 and its NMR characterisation data is shown in Table S10.It was hyperpolarised by shaking a sample containing [IrCl(COD)IMes] (5 mM), NH3 (30 mM), and 7 (25 mM) in dichloromethane-d2 (0.6 mL) with pH2 (3 bar) at 6.5 mT for 10 seconds before spectral acquisition was performed at 9.4 T. The pH2 shaking was performed after the sample has been left to react with H2 (3 bar) overnight to form [Ir(H)2(IMes)(NH3)3]Cl. 1 H and 13 C NMR signal enhancements are also shown in Table S10 with some example 13 C NMR spectra shown in Figure S14.Note that 1 H NMR spectra are shown in Figure 3d of the main paper.S10.

Table S10: NMR characterisation data and signal enhancements for 7 in dichloromethane-d2 at 298 K. The resonance labels correspond to those shown in Figure S13. Values for one of the diastereomers of 7 are shown in black, with the other being given in red.
Resonance 1 H (ppm) 1 H Enhancement /fold 13 C { 1 H} (ppm)

S2.8 SABRE-Relay hyperpolarisation of 8
The structure of 8 is shown in Figure S15 and its NMR characterisation data is shown in Table S11.It was hyperpolarised by shaking a sample containing [IrCl(COD)IMes] (5 mM), NH3 (35 mM), and 8 (25 mM) in dichloromethane-d2 (0.6 mL) with pH2 (3 bar) at 6.5 mT for 10 seconds before spectral acquisition was performed at 9.4 T. The pH2 shaking was performed after the sample has been left to react with H2 (3 bar) overnight to form [Ir(H)2(IMes)(NH3)3]Cl. 1 H and 13 C NMR signal enhancements are also shown in Table S11 with some example spectra shown in Figure S16.S11.
Table S11: NMR characterisation data and signal enhancements for 8 in dichloromethane-d2 at 298 K.The resonance labels correspond to those shown in Figure S15.& In hyperpolarised spectra the signal for c is hidden under those of i and j.In thermal spectra it overlaps with the cyclooctane signal.

S3.2: Effect of carrier on SABRE-Relay hyperpolarisation of 2
Samples of 2 (25 mM) were exposed to pH2 (3 bar) in the presence of the indicated carrier molecule (25 mM) and the precatalyst [IrCl(COD)IMes] for several hours.The sample was then shaken with pH2 at 6.5 mT for 10 seconds before spectral acquistion was performed at 9.4 T. 1 H NMR signal enhancments for 2 achieved using the different carriers are shown in Table S13.

S3.2: Effect of carrier on SABRE-Relay hyperpolarisation of 3
Samples of 3 (25 mM) were exposed to pH2 (3 bar) in the presence of the indicated carrier molecule (25 mM) and the precatalyst [IrCl(COD)IMes] for several hours.The sample was then shaken with pH2 at 6.5 mT for 10 seconds before spectral acquistion was performed at 9.4 T. 1 H NMR signal enhancments for 3 achieved using the different carriers are shown in Table S14.

S3.4 Effect of NH3 concentration on SABRE-Relay hyperpolarisation of 1
Samples of 1 (30 mM) were exposed to pH2 (3 bar) in the presence of NH3 and the precatalyst [IrCl(COD)IMes] for several hours.The sample was then shaken with pH2 at 6.5 mT for 10 seconds before spectral acquistion was performed at 9.4 T. The amine concentration was then lowered in the sample by bubbling N2 gas through the solution under an inert atmosphere.The sample voluem was returned to 0.6 mL if necessary and degassed agin to remove dissolved oxygen.This lowered the NH3 amount from 60 mM to 30 mM 1 H and 13 C NMR signals enhancements at each NH3 loading were recorded and the results are displayed in Figure S18.

Figure S1 :
Figure S1: Structure of 1.The NMR data for the labelled resonances is given in TableS1.

Figure S2 :
Figure S2: Structure of 2. The NMR data for the labelled resonances is given in TableS2.

Figure S3 :
Figure S3: Structure of 3. The NMR data for the labelled resonances is given in TableS3.

Figure S5 :
Figure S5: Single scan thermally polarised (above) and SABRE-Relay hyperpolarised (middle and lower) 13 C NMR spectra for a sample of [IrCl(COD)(IMes)] (5 mM), NH3 (25 mM), 2 (25 mM) and pH2 (3 bar) in DCM-d2 (0.6 mL).The hyperpolarised spectrum is recorded immediately after shaking the sample for 10 seconds with fresh pH2 at 6.5 mT.The middle spectrum uses a single 90 o pulse for 13 C detection whereas the INEPT sequence used in the lower spectrum transfers magnetisation from the 1 H domain to the 13 C domain via radiofrequency excitation (see Figure S4 and S5. 13 C NMR signal enhancements are calculated from data recorded with a single 90 o detection pulse.S2.3: SABRE-Relay hyperpolarisation of 3 A sample of 3 (25 mM) was exposed to pH2 (3 bar) in the presence of NH3 (25 mM) and the precatalyst [IrCl(COD)IMes] for several hours.The sample was then shaken with pH2 at 6.5 mT for 10 seconds before spectral acquistion was performed at 9.4 T. Example 1 H and 13 C NMR spectra are shown in Figures S6 and S7 respecitvely with the associated NMR signal enhancments being given in Table 1 H and 13 C NMR signal enhancements for 3 measured from a sample containing [IrCl(COD)(IMes)] (5 mM), NH3 (25 mM), 3 (25 mM) and pH2 (3 bar) in DCM-d2 (0.6 mL).Example spectra used to calculate these enhancements is shown in FigureS6 and S7.13 C NMR signal enhancements are calculated from data recorded with a single 90 o detection pulse.

Figure S8 :
Figure S8: Structure of 4. The NMR data and signal enhancements for the labelled resonances are given in TableS7.

Figure S10 :
Figure S10: Structure of 5.The NMR data and signal enhancements for the labelled resonances are given in TableS8.

Figure
Figure S11: a) Example single scan thermally polarised (above) and SABRE-Relay hyperpolarised (lower) 1 H NMR spectra for a sample of [IrCl(COD)(IMes)] (5 mM), NH3 (40 mM), 5 (25 mM) and pH2 (3 bar) in DCM-d2 (0.6 mL).The hyperpolarised spectra are recorded immediately after shaking the sample for 10 seconds with fresh pH2 at 6.5 mT.Note that hyperpolarised and thermal spectra are shown on the same vertical scale.b) example single scan thermally polarised (above), direct 13 C detection using a single 90 o pulse (lower) following fresh pH2 shaking for the same sample in a).Note a 1 H→ 13 C INEPT is shown in Figure 3b of the main paper.The associated signal enhancements are given in TableS8.

Figure S12 :
Figure S12: Structure of 6.The NMR data and signal enhancements for the labelled resonances are given in TableS9.

Figure S13 :
Figure S13: Structure of 7. The NMR data and signal enhancements for the labelled resonances are given in TableS10.

Figure S15 :
Figure S15: Structure of 8.The NMR data and signal enhancements for the labelled resonances are given in TableS11.

Figure
Figure S19: a) 1 H NMR spectrum of rose geranium oil (0.3 mL) in DCM-d2 (0.3 mL) with zoomed in regions shown in b) and c).Spectra are labelled for the major components 1, 3 and 7, with signals for 2 also labelled (which overlap with those of 3) according to the labelling system used in Figure 1c of the main paper, and section S2 of the supporting information.Other resonances for major components such as isomenthone (A), citronellyl formate (B), and geranyl formate (C) are also labelled.