NMR Signal Enhancement by using Parahydrogen Induced Polarization (PHIP) and Appropriate Pulse Sequences

Introduction Parahydrogen induced polarization (PHIP) has turned out to be a versatile technique to obtain hyperpolarized molecules exhibiting strong NMR signals via a chemical approach. PHIP makes use of breaking the high initial symmetry of parahydrogen during homogeneously catalyzed hydrogenations of unsaturated substrates and, therefore, of the creation of nonequivalent protons in the products. Consequently, the population of the energy levels of their spin states deviates from the Boltzmann distribution characteristic for systems in thermal equilibrium. This leads to absorption and emission signals in the NMR spectra if recorded in situ and a theoretical signal increase of up to 10, which is in practice limited by relaxation processes in the product. Transfer of polarization to hetero-nuclei can be implemented randomly in weak magnetic fields or selectively via special pulse sequences. Optimization of polarization transfer to e.g. C is crucial for applications like metabolic imaging where the highest possible C polarization is required to obtain high SNR images [1].