Formation routes, structures, and spectroscopic parameters of butyl isocyanides: the case of <i>t</i> -C4H9NC isomer in TMC-1

ABSTRACT 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.