$1.5\times10^{-5}$, @linsky2003atomic), deuterated molecules can be found in the interstellar medium (ISM) with enormously large amounts and are routinely used as proxies for the cold environments typical of the early stages of the star formation process. It is well known that deuterium fractionation processes, *i.e.*, the deuterium enrichment in a given molecular species, can occur at very high rate under some specific physical conditions including, above all, a low gas temperature (5–20K) and strong depletion [@caselli2012our]. Inheriting such conditions from their pre-stellar core phase, Class 0 protostars are the astronomical objects which exhibit the largest molecular deuteration found so far [@ceccarelli2014deuterium]. In the last few decades, several deuterium-bearing species (including doubly- and triply-deuterated forms) have been detected in these sources, e.g., water [@butner2007discovery], formaldehyde [@ceccarelli1998detection], hydrogen sulfide [@vastel2003first], methanol [@parise2004first], and ammonia [@roueff2005interstellar; @vdTak2002nd3].

The Class 0 protostar IRAS 16293-2422, located in the nearby $\rho$-Ophiuchi star-forming region (at a distance of $\sim$120pc), is considered a specimen for super-deuteration phenomena [@ceccarelli2007ps0]. Indeed, besides an exceptionally rich chemistry in its inner and warmer regions [@van1995molecular; @jorgensen2012detection; @fayolle2017protostellar], this source harbors a large number of interstellar deuterated species in the outer and cold envelope [@loinard2001doubly]. Among them, the family of neutral nitrogen hydrides 