Para-Hydrogen Induced Polarization (PHIP) experiments were performed in
coupled multispin systems at variable magnetic fields. We studied the magnetic
field dependence of PHIP in styrene, which is the product of hydrogenation of
phenylacetylene. At low magnetic fields where the spins are coupled strongly
by scalar interaction efficient polarization transfer among the interacting
protons takes place. The experimentally observed spectra are in good agreement
with the simulation, which takes into account eight coupled spins. We also
demonstrate effects of nuclear spin level anti-crossings on the PHIP pattern.
It is shown that rapid passage through the level anti-crossing enables highly
efficient polarization transfer between specific spin orders. In addition, we
studied PHIP transfer to 13C and 19F hetero-nuclei. It is shown that hetero-
nuclei can be efficiently polarized in a wide field range; in particular, for
polarizing them it is not necessary to go to ultra-low fields, which provide
their strong coupling to protons. The resulting polarization is of the
multiplet type and gives strong enhancements of the individual NMR lines. In
general, variation of the magnetic field gives the opportunity for
manipulating PHIP patterns and transferring polarization to target spins of
choice