A method of transferring hyperpolarization among scalar-coupled nuclear spins
is proposed, which is based on spin mixing at energy Level Anti-Crossing (LAC)
regions. To fulfill LAC conditions a resonant RF-field was applied with
properly set frequency and amplitude. In this situation LACs occur between the
nuclear spin levels in the rotating doubly tilted reference frame. The
validity of the approach is demonstrated by taking as an example the transfer
of para-hydrogen induced polarization in a symmetric molecule, whose coupled
spin network can be modeled as a four-spin AA′MM′-system with two pairs of
‘isochronous’ spins. For this spin system LAC positions have been identified;
rules for the sign of spin polarization have been established. The dependence
of the polarization transfer efficiency on the RF-field parameters and on the
time profile of switching off the RF-field has been studied in detail;
experimental results are in excellent agreement with the theory developed. In
general, exploiting LACs in the rotating doubly tilted frame is a powerful
tool for manipulating hyperpolarization in multispin systems
