Principle Investigator: Tomasz Ratajczyk

Grant OPUS 27 (National Science Centre, Poland), 2024/53/B/ST4/02653

About the project:

It has only recently been discovered that the NMR signal of orthohydrogen (oH2) molecules freshly converted from parahydrogen (pH2) molecules has the unique line shape of a Partially Negative Line (PNL). PNL is controversial because, according to the above presented textbook definition of o-H2 and its NMR signal, PNL should not occur. An attempt has been made to explain the PNL conundrum, but the currently available hypotheses are contradictory. Moreover, our unpublished results make PNL even more perplexing. Our objective is to verify the accuracy of the current hypotheses that explain PNL and to form a basis for a more cohesive understanding of the nature of PNL. The problem with the nature of PNL is visible when one analyzes the basic aspects of oH2 and pH2. In oH2, spins are parallel, resulting in a triplet state, while in p-H2, spins are antiparallel, yielding a singlet state. Hydrogen gas at thermal equilibrium at 298K consists of 75% oH2 and 25% pH2. This mixture can be pH2-enriched by cooling with a catalyst, enabling para-ortho conversion to take place. A pH2-enriched mixture can be heated to 298K, but without the catalyst, the composition remains unchanged. Equilibration will begin when a pH2-enriched mixture is introduced to a system with a catalyst. The population of the three equally separated levels in oH2 yielded from oH2 can deviate from the Boltzmann equilibrium. This is because the pH2-rich mixture is a vessel of high nuclear spin polarization, which can be located on oH2 energy levels during conversion. The 1H NMR signal of freshly converted oH2 can be recorded. This signal is a result of two transitions involving neighboring levels. Regardless of how the energy levels are populated, the transitions can be summed up or canceled, but they can never result in PNL. Two models were presented to explain PNL. In the first model, pH2 is converted into oH2 to overpopulate the central energy level (such a spin system is called the Two Spin Order -TSO). oH2 which is still coordinated exchanges with free oH2 resulting in PNL. PNL can appear as positive-negative or negative-positive, depending on the sign of the spin-spin coupling constants in a weakly coordinated oH2. The second model suggests that H2 molecules are slightly oriented in a magnetic field. Due to this, the dipolar coupling is not averaged to zero causing a shift in the energy levels in oH2, and when oH2 is in the TSO state, the 1H NMR signal appears as PNL. Both models are based on reasonable assumptions. They have some common features – both models require oH2 to be in TSO. However, these models explain PNL via fundamentally different mechanisms. Thus, the problem is that both of these models cannot be correct. Our objective is to verify the accuracy of the current hypotheses that explain PNL. We will determine whether the formation of PNL is due to chemical processes, such as exchange, or the orientation of hydrogen molecules in a magnetic field. To do this, we will investigate the interplay between PNL and the chemical processes and spin physics involved in the occurrence of PNL. We will thoroughly investigate how PNL varies with the structure of the catalyst, why PNL is more efficient with certain catalysts, and how the PNL activity is modulated by ligands that are present which are interacting with PNL catalysts. An essential part of the project will also examine how experimental conditions influence PNL. In this context, we will study how PNL is modulated by the type of solvent and the strength of the magnetic field in which PNL experiments are conducted. This project will help us to understand comprehensively the origin of PNL. This will add to the knowledge in scientific fields that are concerned with ortho and para hydrogen and their interconversion. We believe that this project can facilitate further applications of PNL. In particular, PNL can be a promising informative parameter for the investigation of hydrogen with catalysts. This, for instance, can also include bioanalytical systems, such as hydrogenases. Our findings will significantly contribute to the field of parahydrogen based-hyperpolarization NMR, where, for example, PNL can reveal how the pH2 interacts with the polarization-transfer center and what the extent is of catalyst activation. The knowledge acquired within this project can also contribute to fields concerned with hydrogen gas storage, as the ortho-para conversion is crucial to this technology.