  The time irreversible evolution of the two-nucleus spin systems interacting
with a magnetic field is analyzed in the framework of the subdynamics theory.
The spin systems are determined by the H(1) and C(13) nuclei. The investigation
is based on the complex spectral representation of the Liouvillian which gives
a rigorous description of irreversibility. The evolution of the density matrix
is analyzed for the short and long time region. Quantum Zeno effect is
discussed.
