Level anti-crossings in ParaHydrogen Induced Polarization experiments with Cs-symmetric molecules

[Display omitted] ► The hydrogenation of symmetric molecules with parahydrogen is studied. ► Theoretical considerations for the behavior of a 4-spins model are presented. ► The level anti-crossing arises as the responsible for the in-phase signal pattern. ► The experiments performed on a suitable molecule confirm the theoretical spectrum. ► The dependence of the signal on the magnetic field is also experimentally verified. Hyperpolarization by means of ParaHydrogen Induced Polarization (PHIP) has found increasing applications since its discovery. However, in the last decade only a few experiments have been reported describing the hydrogenation of symmetric molecules. A general AA′BB′ system is studied here. Calculations of the spin dynamics with the density matrix formalism support the experimental findings, providing profound understanding of the experiments in Cs-symmetric molecules. Level anti-crossings between states related to the triplet and the singlet state of one pair of the protons are identified as being responsible for hyperpolarization transfer in a PHIP experiment, when the former p-H2 protons occupy the sites AA′. The hydrogenation of acetylene dicarboxylic acid dimethylester with parahydrogen is used to illustrate the case. The theoretical treatment applied to this particular reaction explains the signal enhancements in both groups of protons in the spectrum when the sample is placed in the proper magnetic field strength, including the phase inversion of the signal of the methyl group. The treatment described here can be extended to every molecule which can be approximated as an AA′BB′ system.