Perturbation activated transitions in the high resolution infrared spectrum of C2H6:: rotational constants and torsional splitting in the ground state

The l-type perturbation mechanisms that activate vibration-rotation transitions with Delta k= +/- 2, Delta(l modulo 3) = -/+ 1 in the perpendicular bands of molecules with threefold axial symmetry have been investigated in the case of ethane-like molecules. It is shown that in this class of molecules such transitions, combined with the usual ones with Delta k = Delta(l modulo 3)= +/- 1, give information not only on the values of the K-structure rotational parameters, but also on the torsional splitting of the lower combining state. Transitions of this type have been examined in two perturbed regions of the infrared spectrum of C2H6, in the v(8) fundamental and in the v(4)+v(10) combination. Assuming that the four torsional components in the ground state of C2H6 follow the energy pattern generated when tunneling occurs from one minimum of the torsional potential barrier to the two adjacent minima only, a torsional splitting of 0.00566 +/- 0.00015 cm(-1) in each rotational level has been evaluated. The values of the ground state rotational constants A '' = 2.669693 +/- 0.000020 cm(-1) and 10(5) . D-k '' = 0.885 +/- 0.050 cm(-1) have also been determined.