2025MNRAS.544.3472S

Cyanides and isocyanides are key prebiotic molecules in the interstellar medium, although the latter have been rarely detected. The observed variations in the isocyanide-to-cyanide ratio among different sources indicate incomplete understanding of their formation chemistry. We report a comprehensive computational study of the conformational landscape of butyl isocyanides (n-, i-, s-, and t-C$_{4}$H$_{9}$NC), including structural and spectroscopic characterization of the low-energy conformers. Equilibrium geometries and rotational constants were obtained using the Pisa Composite Schemes combined with second-order vibrational perturbation theory, yielding mean deviations from experimental data of $\sim$0.1 per cent, equivalent to $\sim$1 mÅ in bond lengths and $0.1^{\circ }$ in bond angles. Simulated rotational spectra in the 20-200 GHz range show that the most intense transitions of all isomers occur mainly between 75 and 160 GHz at 100-300 K, shifting to 50-110 GHz at 10 K. Formation and destruction pathways of t-C$_{4}$H$_{9}$NC and its precursors were analysed under cold interstellar conditions, and global rate coefficients were derived for chemical modelling. The predicted abundance of t-C$_{4}$H$_{9}$NC is very low ($\sim$1.45 $\times$ 10$^{-21}$), consistent with its current non-detection in astronomical surveys.