# Hyperpolarised \({}^{1}\)H-\({}^{13}\)C Benchtop NMR Spectroscopy
## 1 Introduction
SABRE catalytically transfers the latent polarisation in _parahydrogen_ (\(p\)-H\({}_{2}\), the NMR-silent singlet spin isomer of H\({}_{2}\)) to a target molecule without chemical alteration of the target. The mechanism by which this occurs, a simplified scheme of which is shown in Figure 1, is through reversible binding of \(p\)-H\({}_{2}\) and the target analyte to a metal complex (commonly iridium-based) in the presence of a weak polarisation transfer field (PTF) in the range of 0-20 mT. Oxidative addition to the metal complex breaks the symmetry of the \(p\)-H\({}_{2}\) molecule, allowing for its stored polarisation to be transferred through the scalar coupling network of the complex to the target molecule that is also bound to the metal. Both the \(p\)-H\({}_{2}\) and the target analyte bind reversibly, leading to a buildup of hyperpolarised analyte in solution over a period of seconds [50]. Figure 1: A schematic representation of the signal amplification by reversible exchange (SABRE)-polarisation transfer process. Hyperpolarisation is catalytically transferred from the nuclear singlet isomer of hydrogen, _parahydrogen_ (\(p\)-H\({}_{2}\)), to the target analyte via the \(J\) coupling network of the active SABRE catalyst. Rapid reversible exchange of the analyte and \(p\)-H\({}_{2}\) on the catalyst leads to a buildup of hyperpolarised analyte in solution over a period of seconds. In general, this process is optimised in a weak magnetic field of 0–20 mT. The active SABRE catalyst is a positively charged Ir(III) di-hydride complex with two molecules of the analyte bound _trans_ to the hydrides and a N-heterocyclic carbene (IMes, shown in the inset) bound _trans_ to a third molecule of the analyte. Many factors influence the effectiveness of the SABRE-polarisation transfer but of significant importance is the PTF [51]. The PTF determines the pathway of the magnetisation transfer through the coupling network [52]. The resonance condition that leads to optimal transfer varies significantly for \({}^{1}\)H SABRE and for transfer to other nuclei [53]. In the SABRE-SHEATH (SABRE in Shield Enables Alignment Transfer to Heteronuclei) approach, the focus is polarisation transfer to heteronuclei, which requires a PTF below the Earth's magnetic field. In this case, a \(\upmu\)-metal shield coupled with an electromagnet is used to reach the microtesla PTF required for direct polarisation transfer to the heteronucleus (e.g., \({}^{15}\)N) [54; 55; 56]. In principle, SABRE-SHEATH can be used for direct polarisation transfer to \({}^{13}\)C; however, due to the coupling network within the active SABRE-polarisation transfer complex, this direct transfer is often highly inefficient [57]. Optimal \({}^{13}\)C hyperpolarisation is typically achieved through an indirect transfer mediated by \({}^{1}\)H or \({}^{15}\)N [58]. In this work, we investigate the feasibility of SABRE-hyperpolarised benchtop \({}^{13}\)C[\({}^{1}\)H] NMR experiments with a focus on the optimisation of the pulse sequences used for hyperpolarised \({}^{13}\)C detection and an analysis of the challenges and opportunities provided by hyperpolarised 2D \({}^{13}\)C-\({}^{1}\)H benchtop NMR spectroscopy.