Chapter 18: Singlet Spin Order Originating from Para-H₂

Parahydrogen induced singlet spin order provides an excellent tool for many NMR/MRI applications in the natural sciences and medicine. The huge benefit of para-H₂ for NMR experiments is two-fold: it represents a pure spin state that can be very efficiently used in hyperpolarization experiments, and its singlet spin order may make it long-lived because it is immune to many relaxation mechanisms. By the method of Parahydrogen Induced Polarization (PHIP), para-H₂ can be introduced in certain molecules giving rise to hyperpolarized long-lived singlet spin states under favourable experimental conditions. In this chapter, the quantum mechanical properties of H₂ are briefly introduced, the procedure for para-H₂ enrichment is explained and the basic concept of para-H₂ induced polarization (PHIP) is presented. The application of PHIP for the generation of hyperpolarized long-lived states in low and high magnetic fields is summarized. Because singlet spin states are NMR inactive, a special focus must be set on strategies for a controlled conversion of the singlet state into an NMR detectable triplet state. The advantages and disadvantages of several methods for accomplishing this goal are discussed.