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.