Abstract Iminomethylium, HCNH+, is a ubiquitous molecular cation in the interstellar medium (ISM). Although isotopologues of HCNH+ precursors are known to exist, no isotopically substituted derivative of HCNH+ has been identified in space, primarily due to the absence of hyperfine-resolved spectral data. In this work, we present the first hyperfine-resolved spectroscopic data for DCNH+, H13CNH+, HC15NH+, and HCND+. In addition, we provide hyperfine-resolved collisional rate coefficients with H2 and Einstein coefficients that enable non-LTE predictions of line intensities in cold molecular clouds. Radiative-transfer calculations tailored to TMC-1 indicate that HCND+ is the most detectable isotopologue, with the j = 2–1 rotational line expected at ~7–40 mK (while the j = 1–0 line is blocked by atmospheric O2), whereas H13CNH+ and HC15NH+ yield mK-level emission for the j = 1–0 and 2–1 lines. On the other hand, DCNH+ lines are intrinsically weak owing to its nearly-zero dipole moment. The frequencies, hyperfine patterns, and radiative and collisional coefficients reported here remove the main barrier to targeted searches of HCNH+ isotopologues and enable robust non-LTE determinations of isotopic ratios in the cold ISM.
Bop et al. (Tue,) studied this question.