  Polarization transfers are crucial building blocks in magnetic resonance
experiments, i.e., they can be used to polarize insensitive nuclei and
correlate nuclear spins in multidimensional NMR spectroscopy. The polarization
can be transferred either across different nuclear spin species or from
electron spins to the relatively low-polarized nuclear spins. The former route
occurring in solid-state NMR (ssNMR) can be performed via cross polarization
(CP), while the latter route is known as dynamic nuclear polarization (DNP).
Despite having different operating conditions, we opinionate that both
mechanisms are theoretically similar processes in ideal conditions, i.e., the
electron is merely another spin-1/2 particle with a much higher gyromagnetic
ratio. Here, we show that the CP and DNP processes can be described using a
unified theory based on average Hamiltonian theory (AHT) combined with
fictitious operators. The intuitive and unified approach has allowed new
insights on the cross effect (CE) DNP mechanism, leading to better design of
DNP polarizing agents and extending the applications beyond just
hyperpolarization. We explore the possibility of exploiting theoretically
predicted DNP transients for electron-nucleus distance measurements--like
routine dipolar-recoupling experiments in solid-state NMR.
