Electronic structure and spectroscopic properties of astrochemically relevant C_nHN isomers

The C_nHN isomer families, which include cyano (-CN) and isocyanide (-NC) functional groups, constitute an important class of nitrogen-bearing carbon chains detected in the interstellar medium (ISM). Here, we present a quantum-chemical characterization of selected low-energy neutral C_nHN (n = 2-5) isomers using a combination of grammar-driven isomer generation with SmilX and high-level ab initio methods, including explicitly correlated CCSD(T)-F12 and CCSD(T)-F12/RI approaches. A total of 21 lowest-energy singlet and triplet species were characterized in terms of their relative energies, equilibrium geometries, charge distributions, dipole moments, vertical ionization potentials, vibrational frequencies, and rotational constants. Except for the C₄HN family, whose global minimum corresponds to a cyclic ring-chain structure, the lowest-energy isomers are linear or quasi-linear species containing terminal -CN groups. A previously uncharacterized C₅HN isomer (5-5), consisting of a three-membered HC₃ ring connected to a -CCN substituent, was also identified. Dipole moments generally increase with carbon-chain length within structurally related series, whereas vertical ionization potentials are governed primarily by the local bonding environment and terminal connectivity rather than chain length alone. Anharmonic vibrational treatments generally improve agreement with available experimental frequencies, although quasi-linear species exhibit larger deviations associated with large-amplitude motions. The calculated rotational constants agree with the available experimental data with deviations of 0.02-2.15% for the directly comparable constants, supporting the reliability of the computational protocol for spectroscopic predictions. The resulting dataset provides reference spectroscopic and electronic properties for undetected C_nHN isomers and may assist future laboratory and astronomical studies of nitrogen-containing carbon chains in the ISM.