Compilation of Molecules with Millimeter-Wave Spectroscopy of Rare, Doubly, and Multiply Substituted Isotopologues

This document provides supplementary tables to the ALMA2040 Colzi et al. white paper “From local chemistry to galactic evolution: unlocking the power of interstellar isotopes with ALMA2040” . It compiles diatomic, small inorganic, carbon-chain, and complex organic molecules for which rare, doubly, and multiply isotopically substituted species have millimeter- and/or submillimeter-wave spectroscopic data available in major molecular spectroscopy catalogues.

The tables list the parent molecule and the corresponding isotopologues for which spectroscopic information is available, including substitutions involving carbon, nitrogen, oxygen, sulfur, silicon, and deuterium. Particular emphasis is placed on multiply substituted isotopologues, which are especially relevant for exploiting isotopic information with current and future high-sensitivity, wide-bandwidth facilities such as ALMA and the planned ALMA2040 capabilities.

Table 1 covers diatomic molecules, small inorganic species, molecular ions, and selected organic molecules. Table 2 focuses on carbon-chain species and complex organic molecules, including several molecules of particular relevance to astrochemical studies. The tables also highlight cases in which an isotopically substituted species becomes detectable at millimeter wavelengths despite the parent molecule having no permanent dipole moment because of molecular symmetry. Molecular oxygen is noted separately because its rotational spectrum can be observed through magnetic-dipole transitions.

The compilation is based primarily on the Cologne Database for Molecular Spectroscopy (CDMS; Endres et al. 2016), the Jet Propulsion Laboratory (JPL) Molecular Spectroscopy Catalog (Pickett et al. 1998), and, for Table 2, the Lille Spectroscopic Database has also been used (Motiyenko et al. 2025). The tables are intended as a reference resource supporting the discussion in the ALMA2040 white paper of the isotopologue space accessible to millimeter/submillimeter observations and the potential of future ALMA capabilities for comprehensive isotopic inventories of interstellar molecules.

This document is provided as supplementary material to the associated ALMA2040 white paper and should be read in conjunction with that publication. The compilation reflects the spectroscopic catalogues consulted at the time of preparation and is not intended to constitute an exhaustive or permanent inventory of all experimentally characterized isotopologues.