THE INTERNAL ROTATION SPLITTINGS IN $C_{2}H_{3}^{+}$

$^{1}$ M. W. Crofton, M. -F. Jagod, B. D. Rehfuss and T. Oka, to be published. $^{2}$ R. Escribano, P. R. Bunker, and P. C. Gomez, Chem. Phys. Letters 150, 60 (1988). $^{3}$ R. Lindh, B. O. Boos, and W. P. Kraemer, Chen. Phys. Letters 139, 407 (1987).Author Institution: Herzberg Institute of Astrophysics, National Research Council of CanadaSplittings that are believed to result from the ``Internal rotation'' of the three hydrogen atoms around the C-C core in the $C_{2}H_{3}^{+}$ molecule have been $assigned^{1}$ in what is believed to be the antisymmetric CH stretching band. These results show that the splittings are negligible in the vibrational ground state, in agreement with $results^{2}$ based on an ab initio potential $surface,^{3}$ but that they are significant in the vibrationally excited state. By fitting to these splittings we determine the `effective' internal rotation potential in the antisymmetric CH stretching band. The relation of this potential to the ab initio potential is discussed in terms of the way in which the antisymmetric stretching frequency changes with the internal rotation geometry