# Hyperpolarised \({}^{1}\)H-\({}^{13}\)C Benchtop NMR Spectroscopy
## 3 Results
### Optimal Detection of SABRE-Enhanced \({}^{13}\)C Benchtop NMR Spectra
(**b**) \({}^{13}\)C[\({}^{1}\)H] NMR spectra acquired using PA (top) and INEPT transfer from \({}^{1}\)H (bottom). A refocusing delay of (3\(J_{CH}\))\({}^{-1}\) with \(J_{CH}\) = 10 Hz was included prior to signal acquisition in both cases. A second possible explanation for the lower signals observed following the INEPT transfer is that the efficiency of hyperpolarisation transfer to \({}^{1}\)H is not optimised under these experimental conditions for molecules containing \({}^{13}\)C. Both of the fully-coupled, SABRE-enhanced \({}^{13}\)C NMR spectra in Figure 3a contained peaks that had anti-phase character with respect to a relatively long range \({}^{1}\)H-\({}^{13}\)C coupling on the order of \(J_{CH}=10\) Hz. This anti-phase character is consistent with previous observations in the literature involving indirect SABRE-polarisation transfer to \({}^{13}\)C via \({}^{1}\)H [66]. In the case of the PA experiment, the anti-phase character indicated that the SABRE process had enhanced two-spin-order terms involving non-directly-bonded \({}^{1}\)H-\({}^{13}\)C pairs within the analyte [49]. In the case of the INEPT transfer experiment, we found that the optimal \({}^{13}\)C NMR signal was observed using a constant of \(J_{CH}=10\) Hz for the transfer step. Interestingly, the \({}^{13}\)C NMR signal observed for a larger one-bond coupling constant of \(J_{CH}=140\) Hz was reduced. This is in contrast to INEPT experiments carried out on samples at thermal equilibrium, where the most efficient transfer of polarisation is achieved between directly bonded \({}^{1}\)H and \({}^{13}\)C nuclei. These results indicate that protons directly bonded to \({}^{13}\)C are less efficiently hyperpolarised via the SABRE process under our experimental conditions. The lower efficiency of \({}^{1}\)H hyperpolarisation for protons directly bonded to \({}^{13}\)C can be understood by considering the resonance condition that facilitates spontaneous polarisation transfer in SABRE. Efficient polarisation transfer requires that the difference in chemical shift between the source of the polarisation (the \(p\)-H\({}_{2}\)-derived hydrides in the polarisation transfer complex) and the target nuclei of the analyte bound to the complex be approximately equal to the dominant \(J\) coupling constant within the coupling network. Typically, this is \(J_{HH}\)-8 Hz between the pair of hydrides. This gives rise to an optimal PTF for transfer to aromatic \({}^{1}\)H resonances in the analyte of \(\sim\)6.5 mT and of a few \(\mu\)T for direct transfer to other nuclei such as \({}^{15}\)N and \({}^{13}\)C. However, if the protons in the target analyte are directly bonded to \({}^{13}\)C, the dominant coupling will be the one-bond \({}^{13}\)C-\({}^{1}\)H coupling on the order of 140 Hz. This will significantly shift the resonance condition to a PTF on the order of \(\sim\)0.1 T. Therefore, polarisation transfer to protons directly bonded to \({}^{13}\)C is inefficient in the PTF of 6.1 mT used in these  
Figure 4: (a) \({}^{13}\)C and (b) \({}^{1}\)H SABRE-hyperpolarisation lifetimes measured using a single-shot pulse sequence (Figure 2e) at 1 T (43 MHz) for 4-methylpyridine in methanol-\(d_{4}\) with 5.2 mM SABRE catalyst. Values are the average over five experiments and error bars are the standard error across the repeat measurements. experiments. This effect has been observed previously in the case of SABRE hyperpolarisation of \({}^{13}\)C in acetonitrile, where no polarisation transfer to the methyl carbon is observed [66]. In addition, [PERSON] and coworkers have performed \({}^{13}\)C SABRE experiments over a wide range of PTF values [52]. In their experiments, significant \({}^{13}\)C SABRE hyperpolarisation was observed in a relatively strong PTF field of 90 mT. Therefore, it is likely that the efficiency of the INEPT approach could be greatly increased by carrying out the SABRE-polarisation transfer in a much stronger PTF and using a one-bond / coupling constant for the transfer.