Ab Initio Rotational and Vibrational Spectroscopy of CH Radicals at the Coupled Cluster Level.

Combustion and pyrolysis processes of allene and propyne are known to involve radicals with the structural formula CH, the most stable of which is the classic resonance-stabilized allyl radical. In addition to allyl, four other isomers of CH are possible: the propene derivatives -1-propenyl, -1-propenyl, and 2-propenyl, as well as the cyclopropane derivative cyclopropyl. Among these 5 species, the allyl radical has been extensively studied both theoretically and spectroscopically; however, little is known about the spectroscopy of the cyclopropyl radical, and virtually no experimental spectroscopic data are available for the remaining three. Here, we carry out an ab initio investigation of the CH radicals at the coupled cluster level of theory with a focus on computing the spectroscopic parameters relevant for pure rotational and rotationally resolved vibrational spectroscopy. The rotational constants, dipole moments, spin-spin tensors, and Fermi contact parameters are evaluated at the CCSD(T)/cc-pwCVQZ level of theory, while vibrational properties such as vibration-rotation interaction parameters and centrifugal distortion constants are calculated at the frozen-core CCSD(T)/ANO1 level. Vibrational analysis was carried out using second-order vibrational perturbation theory with an explicit treatment of resonances. Finally, the inversion tunneling potential in cyclopropyl as well as the methyl internal rotation potentials in the 1- and 2-propenyl isomers are computed and relevant spectroscopic parameters for modeling their resultant tunneling splittings are derived. Results from the calculations compare favorably with the extensive body of spectroscopic literature on the allyl radical, giving confidence that the computed parameters for the remaining isomers will be useful for guiding the interpretation of future high-resolution experimental spectroscopy of these radicals.