Structure and Vibrational Analysis of Protonated Ethane C2H7+

The stability and vibrational frequencies of protonated ethane, C2H7+, have been studied by ab initio molecular orbital theory. The IR spectra for C2H7+ recorded by Y. T. Lee’s group were interpreted from a theoretical point of view. Ab initio computations predict the existence of two isomers, a bridged structure, and a tight and loose open form. The loose open form is represented as an H2 molecule attached loosely to a classical open C2H5+ ion. The different behavior observed in the IR spectra can be attributed to the changing ratio of the bridged C2H7+ to the open C2H5+ isomers. The final interpretation of these data is that the spectrum at 150 Torr (1 Torr = 133.322 Pa) arises from the bridged structure which is the most stable form of C2H7+. The additional band in the spectrum observed at 60 Torr originates form the weakly bound complex which dissociates upon absorbing a single IR photon.