@jehin2009; @aleon2010; @mumma2011]]{}. Observational surveys have been made of galactic star-forming regions [@adande2012] and dark clouds [@hily-blant2013] and also point to a range of values, as well as variations by source and the molecular species observed. Some variation of ${}^{14}$N/${}^{15}$N is clearly due to the details of local nuclear synthesis, with an increasing excess of ${}^{15}$N towards the galactic centre [@wilson1999; @adande2012]. Another proposed explanation for the observed ratios of $\mathrm{C^{14}N}/\mathrm{C^{15}N}$, $\mathrm{HC^{14}N}/\mathrm{HC^{15}N}$ and $\mathrm{{}^{14}NH_3}/\mathrm{{}^{15}NH_3}$ is low-temperature isotopic-exchange chemistry. Models considering this mechanism [[e.g., @rodgers2008; @wirstrom2012; @le_gal2013]]{} have been successful in introducing strongly non-elemental abundance ratios under specific conditions. The critical exchange reactions, e.g., $$\rm
  HC^{14}NH^+ + {}^{15}N \leftrightharpoons HC^{15}NH^+ + {}^{14}N + energy,$$ rely on the lower zero-point energy of the heavier molecular species and are exothermic by $\leq30$K [@terzieva2000], limiting their significance to cold environments. This is in contrast to similar reactions affecting the H/D abundance in molecules which may proceed at temperatures as high as 170K.

An alternative route to the mass fractionation of molecules is isotope selective photodissociation. For the case of $\mathrm{N_2}$ dissociation, self-shielding has been shown to drive the $\mathrm{HC^{14}N}/\mathrm{HC^{15}N}$ ratio in the atmosphere of Titan [@liang_etal2007]. Additionally, there is strong observational evidence that the 