The low-barrier methyl internal rotation in the rotational spectrum of 3-methylphenylacetylene

The rotational spectrum of 3-methylphenylacetylene has been recorded in the 2–8 GHz region using a chirped-pulse broadband microwave spectrometer. Torsion-rotation transition splittings are observed from a tunneling motion along the methyl internal rotation axis. The XIAM program was used to characterize the splitting, yielding an internal rotation barrier, , of cm−1. While this barrier is considered low, fits of A-state only transitions yield a quality, rigid-rotor fit, and are compared to the combined A/E fits. Computationally predicted barriers are estimated between 14.4 and 28.9 cm−1. Journal of Molecular Structure, Volume 1213, 2020 Sérgio R. Domingos, Cristóbal Pérez, Mark D. Marshall, Helen O. Leung, Melanie Schnell