New Theoretical Infrared Line List for the Methyl Radical with Accurate Vibrational Band Origins from High-Level Ab Initio Calculations

In the present work, high-level ab initio calculations
were carried
out for the ground electronic state of the methyl radical (CH3). Dunning’s augmented correlation-consistent orbital
basis sets were employed up to the quintuple-ζ valence quality
with the core–valence electron correlation [aug-cc-pCV5Z] combined
with the single- and double-excitation unrestricted coupled-cluster
approach with a perturbative treatment of triple excitations [RHF-UCCSD(T)].
The explicitly correlated version of the coupled-cluster approach
[RHF-UCCSD(T)-F12x{x = a, b}] was additionally applied with the core–valence
cc-pCVQZ-F12 basis set in order to study convergence with respect
to the basis set size. The contributions beyond the coupled-cluster
level of the theory like Douglas–Kroll–Hess scalar relativistic
Hamiltonian, diabatic Born–Oppenheimer corrections, and high-order
electronic correlations have been included into the ab initio potential
energy surfaces (PESs). It is shown that the theoretical band origins
of CH3 converge gradually to the experimental values when
applying the ab initio PESs using the aug-cc-pCVXZ [X = T, Q, and
5] basis sets. For the first time, all available experimental band
origins of the gaseous CH3 are reproduced within an accuracy
of 0.2 cm–1 using a newly developed PES extrapolated
to the complete basis set limit [CBS(TQ5Z)]. The reached accuracy
is one order of magnitude better than that of the best available calculations.
A new theoretical infrared line list was generated for astrophysical
applications using an ab initio dipole moment surface computed at
the RHF-UCCSD(T)/aug-cc-pCVQZ level of the theory. The manifestation
of a large-amplitude motion in CH3 is also discussed.