Results from chemical models. In each row, left panel: temporal evolution during the quasi-static evolution of the core (step 1), and right panel: evolution of the inner core as a function of the central density during the collapse (step 2). The bulk ¹⁴N:¹⁵N isotopic ratio is = 330. (a) Gas-phase abundances, relative to n_H, of the main isotopologs of relevant species. (b) ¹⁴N/¹⁵N abundance ratios when adopting the TH00 set of fractionation reactions. (c) Same as (b) but with the fractionation reactions from R15.