Isotope effects in the vibrational spectrum of perfluoroethane

• The IR spectrum of perfluoroethane with natural isotope composition in liquid N2 at T = 77 K is recorded.
• The frequencies and intensities of 12 vibrational bands in the fundamental and overtone regions are found.
• The characteristics of vibrational bands of 13C12CF6 are determined.
• The «anomalous change» of intensity upon isotopic substitution in the molecules with the center of inversion is explained.

IR spectrum of solution of perfluoroethane with the natural isotope composition in liquid N2 at T = 77 K is recorded. Relative intensities for the *ν1, *ν5, and *ν10 bands of the asymmetric isotopologue 13C12CF6 have been determined. The ν5(A2u) and ν10(Eu) bands of the C2F6 molecule were found to behave quite differently upon isotopic substitution. The ν10 band splits into two bands close in intensity, one of which falls within the spectral region of the main isotopologue. This fact explains the low (less than 1%) intensity of the *ν10 band. At the same time the isotopic substitution does not change intensity of the ν5 band, and the *ν5 band intensity corresponds to the natural isotope abundance (2%). This interpretation has been confirmed by a calculation performed for the C2F6 and 13C12CF6 molecules using the GF matrix formalism.