Analysis of the SABRE (Signal Amplification by Reversible Exchange) Effect at High Magnetic Fields

A detailed study of the Signal Amplification By Reversible Exchange (SABRE) effect at high magnetic fields is performed. SABRE is formed by spin order transfer from parahydrogen to a substrate in a transient organometallic complex. Typically, such a transfer is efficient at low magnetic fields; at high fields it requires radio-frequency (RF) excitation of spins in the SABRE complex. However, recently it has been shown (Barskiy et al. in J. Am. Chem. Soc. 136:3322–3325, 2014) that high-field SABRE is also feasible due to “spontaneous” spin order transfer (i.e., transfer in the absence of RF excitation) although the transfer efficiency is low. Here, we studied the SABRE field dependence for protons in the field range 1.0–16.4 T and found an increase of polarization with the field; further optimization of proton polarization can be achieved by varying the viscosity of the solvent. As previously, polarization transfer is attributed to cross-relaxation; this conclusion is supported by additional experiments. For spin-½ hetero-nuclei, such as 15N and 31P, spontaneous spin order transfer is also feasible; however, in contrast to protons, it is based on a coherent mechanism. Consequently, higher transfer efficiency is achieved; moreover the 15N and 31P spectral patterns are remarkably different from that for protons: multiplet (anti-phase) polarization is seen for hetero-nuclei. Our study is of importance for enhancing weak nuclear magnetic resonance (NMR) signals by exploiting non-thermally polarized spins. Although the efficiency of high-field SABRE is lower than that of low-field SABRE; the high-field SABRE experiment is easy to implement for improving the sensitivity of NMR methods.