CH₄ and CH₂O dynamics encapsulated inside the C₆₀ fullerene cage: Far and mid-IR study.

We studied the dynamics of two quantum rotors, spherical top CH₄ and asymmetric top CH₂O encapsulated inside C₆₀, using low-temperature far- and mid-infrared spectroscopy. One quantized center-of-mass translational mode was observed for CH₄@C₆₀ at 124cm^-1, similar to confined atoms A@C₆₀ [1]. However, for CH₂O@C₆₀ two peaks were observed, at 167 and 231 cm ¹. The peak at 231 cm^-1 was attributed to the translation motion predominantly along the long axis of CH₂O and the 167 cm^-1 peak to the motion predominantly perpendicular to it. In the mid-IR rovibrational spectrum, the spectral lines were assigned to meta, ortho, and para species of CH₄@C_60. The ν₃ vibrational mode is blue-shifted, and the ν₄ vibrational mode is red-shifted in CH₄@C₆₀ relative to the gas phase. The rovibrational spectra of CH₂O@C₆₀ did not reveal rotational peaks but instead, one strong peak and two broad sidebands were observed. The former was attributed to pure vibration, whereas the latter to unresolved rotational bands. Out of six IR active vibrations, only ν₄ is blue-shifted while the others are red-shifted with respect to the gas phase. The ortho-para spin isomers conversion was seen in the temporal evolution of CH₂O@C₆₀ spectra.

Reference

T. Jafari, A. Shugai, U. Nagel, G. R. Bacanu, M. Aouane, M. J. Ruiz, S. Rols, S. Bloodworth, M. Walkey, G. Hoffman, R. J. Whitby, M. H. Levitt and T. Rõõm, Phys Chem Chem Phys 24, 9943–9952 (2022)