  HNC and HCN, typically used as dense gas tracers in molecular clouds, are a
pair of isomers that have great potential as a temperature probe because of
temperature dependent, isomer-specific formation and destruction pathways.
Previous observations of the HNC/HCN abundance ratio show that the ratio
decreases with increasing temperature, something that standard astrochemical
models cannot reproduce. We have undertaken a detailed parameter study on which
environmental characteristics and chemical reactions affect the HNC/HCN ratio
and can thus contribute to the observed dependence. Using existing gas and
gas-grain models updated with new reactions and reaction barriers, we find that
in static models the H + HNC gas-phase reaction regulates the HNC/HCN ratio
under all conditions, except for very early times. We quantitively constrain
the combinations of H abundance and H + HNC reaction barrier that can explain
the observed HNC/HCN temperature dependence and discuss the implications in
light of new quantum chemical calculations. In warm-up models, gas-grain
chemistry contributes significantly to the predicted HNC/HCN ratio and
understanding the dynamics of star formation is therefore key to model the
HNC/HCN system.
