Infrared Spectra of the 10-μm Bands of 1,2-Difluoroethane and 1,1,2-Trifluoroethane: Vibrationally Mediated Torsional Tunneling in 1,1,2-Trifluoroethane

✍ Scribed by Stephen C. Stone; C.Cameron Miller; Laura A. Philips; A.M. Andrews; G.T. Fraser; B.H. Pate; Li-Hong Xu

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 601 KB
• Volume: 174
• Category: Article
• ISSN: 0022-2852
• DOI: 10.1006/jmsp.1995.0004

✦ Synopsis

The 3-MHz-resolution infrared spectra of the 10-mm bands of the gauche conformer of 1,2difluoroethane (HFC152) and the C 1 -symmetry conformer of 1,1,2-trifluoroethane (HFC143) have been measured using a molecular-beam electric-resonance optothermal spectrometer with a tunable microwave-sideband CO 2 laser source. For 1,2-difluoroethane, two bands have been studied, the n 17 B-symmetry C-F stretch at 1077.3 cm 01 and the n 13 B-symmetry CH 2 rock at 896.6 cm 01 . Both bands are well fit to a asymmetric-rotor Hamiltonian to better than 0.5 MHz.

The n 13 band is effectively unperturbed, while the n 17 band is weakly perturbed, as shown by the large change in centrifugal distortion constants from the ground state values. Two bands have also been studied for 1,1,2-trifluoroethane, the n 11 symmetric CF 2 stretch at 1077.2 cm 01 and the n 13 C-C stretch at 905.1 cm 01 . One of the two bands, n 11 , is unperturbed and fit to near the experimental precision. The n 13 vibration, on the other hand, is weakly perturbed by an interaction with a nearby state. This perturbation leads to a doubling or splitting of the lines, due to a perturbation-induced lifting of the degeneracy of the symmetric and antisymmetric tunneling states associated with tunneling between the two equivalent C 1 forms. For the J, K a states studied, the splittings are as large as 37 MHz. Combining this observation with published low-resolution far-infrared measurements of torsional sequence-band and hot-band frequencies and calculations from an empirical torsional potential allows us to identify the perturbing state as n 17 / 6n 18 . Here, n 17 is the CF 2 twist and n 18 is the torsion. The matrix element responsible for this interaction exchanges eight vibrational quanta! ᭧ 1995 Academic Press, Inc.