Singlet to triplet conversion in molecular hydrogen and its role in parahydrogen induced polarization

Detailed experimental and comprehensive theoretical analysis of singlet-triplet conversion in molecular hydrogen dissolved in a solution together with organometallic complexes used in experiments with parahydrogen (the H₂ molecule in its nuclear singlet spin state) is reported. We demonstrate that this conversion, which gives rise to formation of orthohydrogen (the H₂ molecule in its nuclear triplet spin state), is a remarkably efficient process that strongly reduces the resulting NMR (nuclear magnetic resonance) signal enhancement, here of ¹⁵N nuclei polarized at high fields using suitable NMR pulse sequences. We make use of a simple improvement of traditional pulse sequences, utilizing a single pulse on the proton channel that gives rise to an additional strong increase of the signal. Furthermore, analysis of the enhancement as a function of the pulse length allows one to estimate the actual population of the spin states of H₂. We are also able to demonstrate that the spin conversion process in H₂ is strongly affected by the concentration of ¹⁵N nuclei. This observation allows us to explain the dependence of the ¹⁵N signal enhancement on the abundance of ¹⁵N isotopes.