  Development of the methods to exploit nuclear hyperpolarization and search
for molecules whose nuclear spins can be efficiently hyperpolarized is an
active area in nuclear magnetic resonance. Of particular interest are those
molecules that have long nuclear relaxation times, making them to be suitable
candidates as contrast agents in magnetic resonance imaging. In this work, we
present a detailed study of SABRE SHEATH (Signal Amplification By Reversible
Exchange in Shield Enabled Alignment Transfer to Heteronuclei) experiments of
15N,15N' azobenzene. In SABRE SHEATH experiments nuclear spins of the target
are hyperpolarized by transfer of spin polarization from parahydrogen at
ultralow fields during a reversible chemical process. The studied system is
complicated, and we are concerned only about a subset of the data, presenting
details for the molecules that experience fast chemical exchange at the
catalytic complex and thus are involved in polarizing the free azobenzene.
Azobenzene exists in two isomers trans- and cis-. We show that all nuclear
spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable
magnetic fields. Enhancement factors (relative to 9.4 T) reach several
thousands of times for 15N spins and a few tens of times for the 1H spins.
There are two approaches to observe either hyperpolarized magnetization of
15N/1H spins or hyperpolarized singlet order of the 15N spin pair. We compare
these approaches and present the field dependencies of SABRE experiments for
them. No hyperpolarization of trans 15N,15N' azobenzene was observed. The
results presented here will be useful for further experiments where
hyperpolarized cis-15N,15N' azobenzene is switched by light to trans 15N,15N'
azobenzene for storing the produced hyperpolarization in the long-lived spin
state of the 15N pair of trans-15N,15N' azobenzene.
