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tokens: of the H 2 ortho to para spin ratio (OPR), the correlations and anticorrelations between D and 15 N found in primitive materials (van Kooten et al. 2017 ). In this paper, we report calculations that re-evaluate those of W12 in light of the revised R15 fractionation chemistry. This study is necessary because, apart from the rate coefficients adopted, there exist significant differences in the physics and chemistry of the models of W12 and R15 . The calculations of W12 considered densities similar to pre-stellar core conditions, about 10–100 times greater than those of R15 , which were based on a standard dark cloud similar to TMC-1 ( n H = 0.2–2× 10 5 cm −3 ). In contrast to W12 , R15 neither included the depletion of gas-phase species by freeze-out on to dust grains, nor the time-dependence in the H 2 OPR – both effects which have been shown to have substantial impact on the relative fractionation in nitriles and amines (here nitrogen hydrides and chemical derivatives thereof) over time ( W12 ). Furthermore, even when reactions ( 1 )–( 3 ) are not operating, there are alternative fractionation processes that could still become important in the physical conditions of the W12 model. For example, W12 identified as playing a role in the fractionation of N 2 and NH 3 , whereas Rodgers Charnley ( 2008a ) speculated that the neutral–neutral pathway could be viable and efficient if there is a barrier to the reaction However, the reaction appears to have no formation barrier, and the recommended reaction rate from the KIDA data base (Wakelam et al. 2013 , http://kida.obs.u-bordeaux1.fr ) of k (T) = 8.8 × 10 −11 (T/300) 0.42 cm 3 molecule −1 s −1 agrees with laboratory measurements down to 56 K (Daranlot et al. 2012 ). W12 assumed a 200 K barrier for reaction ( 6 ) (see Le Teuff, Millar Markwick 2000 ) and R15 found that isotopic exchange through ( 5 ) was indeed plausible, a process not considered in W12 . We have therefore updated the W12 model with respect to the revisions to nitrogen fractionation advocated by R15 , and specific reactions in the nitrogen chemistry from the KIDA data base (Wakelam et al. 2013 ). The paper is structured as follows. Revisions to the 14 N fractionation chemistry and the updates to specific reactions in the nitrogen chemistry are described in Section 2 . The chemical model is summarized in Section 3 . Results from the model calculations are given in Section 4 where they are compared to previous work. Conclusions from this study are in Section 5 . 2 REVISED CHEMICAL NETWORK 2.1 <sup>15</sup>N fractionation Table 1 lists the major reactions potentially leading to 15 N fractionation in cold, dark clouds, both ion–molecule and neutral–neutral reactions, with ZPE differences, Δ E , as determined by R15 . The resulting equilibrium coefficient, K ( T ) = k f / k r , the ratio between the forward and reverse reaction rate (e.g. Terzieva Herbst 2000 ), is listed for each reaction at 10 K, both for the new fractionation