  Deuterated species are unique and powerful tools in astronomy since they can
probe the physical conditions, chemistry, and ionization level of various
astrophysical media. Recent observations of several deuterated species along
with some of their spin isomeric forms have rekindled the interest for more
accurate studies on deuterium fractionation. This paper presents the first
publicly available chemical network of multiply deuterated species along with
spin chemistry implemented on the latest state-of-the-art gas-grain chemical
code `NAUTILUS'. D/H ratios for all deuterated species observed at different
positions of TMC-1 are compared with the results of our model, which considers
multiply deuterated species along with the spin chemistry of light hydrogen
bearing species H2, H2+, H3+ and their isotopologues. We also show the
differences in the modeled abundances of non-deuterated species after the
inclusion of deuteration and spin chemistry in the model. Finally, we present a
list of potentially observable deuterated species in TMC-1 awaiting detection.

Astrophysics of Galaxies