Rovibrational Line Lists of Triplet and Singlet Methylene

The methylene molecule (CH₂) is a short-lived radical with lacking data on its spectral line intensities. Although the lifetime of CH₂ is extremely short under Earth's conditions, it exists in a free form in interstellar media. CH₂ is an important intermediate species in chemical reactions associated with the formation and destruction of complex hydrocarbons. We present the first rovibrational line lists of CH₂ in its ground triplet and first excited singlet electronic state. To this end, our previously developed accurate <i>ab initio</i> potential energy surface (PES) was used for the ground electronic triplet state [Egorov <i>et al</i>. J. Comp. Chem. 2024. V. 45. (2). P. 83] while a new PES for the singlet state was constructed in this work using the single-reference coupled cluster approach [CCSD(T)] combined with the extrapolation to the complete basis set (CBS) limit based on the correlation-consistent orbital basis sets with the core-valence electron correlation effects [aug-cc-pCVXZ, X = T, Q, 5, and 6]. In addition, the contributions to the correlation energy from highly excited Slater determinants [CC(<i>n</i>), <i>n</i> = 3-5] were included as well as the scalar relativistic effects and DBOC. The most accurate description of the infrared band origins of singlet CH₂ was thus achieved for the energy range where the impact of the nonadiabatic coupling due to the Renner-Teller effect can be neglected. To obtain the probabilities of the rovibrational transitions, new <i>ab initio</i> DMSs were constructed both for the triplet and singlet CH₂ using the CCSD(T)/aug-cc-pCVQZ approach. Finally, the absorption spectra of triplet and singlet methylene were predicted from the variationally computed line lists.