Coherent polarization transfer effects in a coupled spin network have been
studied over a wide field range. The transfer mechanism is based on exciting
zero-quantum coherences between the nuclear spin states by means of non-
adiabatic field jump from high to low magnetic field. Subsequent evolution of
these coherences enables conversion of spin order in the system, which is
monitored after field jump back to high field. Such processes are most
efficient when the spin system passes through an avoided level crossing during
the field variation. The polarization transfer effects have been demonstrated
for N-acetyl histidine, which has five scalar coupled protons; the initial
spin order has been prepared by applying RF-pulses at high magnetic field. The
observed oscillatory transfer kinetics is taken as a clear indication of a
coherent mechanism; level crossing effects have also been demonstrated. The
experimental data are in very good agreement with the theoretical model of
coherent polarization transfer. The method suggested is also valid for other
types of initial polarization in the spin system, most notably, for spin
hyperpolarization