Infrared vibration–rotation selection rules for chemisorbed molecules with free internal rotation: Results for ethylidyne on Pt(111)

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Abstract
We have studied the infrared spectrum of ethylidyne, CCH3, chemisorbed on the Pt(111) surface over the temperature range 82 to 350 K. We observe three infrared active fundamentals: the C–C stretch at 1118 cm−1, the symmetric CH3 bend at 1339 cm−1, and the symmetric C–H stretch at 2884 cm−1. The absence of three other fundamentals in our spectra confirms that the molecule has C3v symmetry on the surface with the C–C axis oriented along the surface normal as had been determined from other studies. Our IR spectra demonstrate the strict validity of the surface dipole selection rules. We also observe a weak band at 2795 cm−1 which we attribute to the first overtone of the asymmetric CH3 bend at 1410 cm−1. The intensity of the overtone is enhanced by a Fermi resonance with the symmetric C–H stretch. At 82 K the symmetric bend has an unusually narrow intrinsic width of only 1.4 cm−1. The narrowness of this band makes it a good choice for investigating the influence of free rotation about the single C–C bond on the vibrational bands. Free internal rotation in ethylidyne on supported platinum at 77 K has been reported in an NMR study. We consider the vibration–rotation selection rules for an adsorbed molecule freely rotating about the surface normal. We show that the observable vibrational bands cannot exhibit rotational fine structure. However, nonsurface dipole allowed modes observable in transmission IR studies of supported metals are much too narrow to be consistent with free rotation of the CH3 group. The NMR and IR results are consistent if the exchange of the hydrogen atoms among three equivalent sites is fast on the NMR time scale but slow on the IR time scale.

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