Large amplitude motion in 9-methylanthracene: High-resolution spectroscopy and Ab Initio theoretical calculation^†

CH₃ internal rotation is one of the typical large amplitude motions in polyatomic molecules, the spectral analysis and theoretical calculations of which, were developed by Li-Hong Xu and Jon Hougen. We observed a Doppler-free high-resolution and high-precision spectrum of 9-methylanthracene (9MA) by using the collimated supersonic jet and optical frequency comb techniques. The potential energy curve of CH₃ internal rotation is expressed by a six-fold symmetric sinusoidal function. It was previously shown that the barrier height (V₆) of 9MA-d₁₂ was considerably smaller than that of 9MA-h₁₂ [M. Baba, et al., J. Phys. Chem. A 113, 2366 (2009)]. We performed ab initio theoretical calculations of the multicomponent molecular orbital method. The barrier reduction by deuterium substitution was partly attributed to the difference between the wave functions of H and D atomic nuclei.