Knowledge Bank

The goal of the present work is to indicate what sorts of rotation-vibration spectra might be expected for protonated acetylene when the three protons in the molecule are allowed to interchange roles in a cyclic mannner by migrating around an elliptical orbit enclosing the two carbon atoms, as suggested by ab initio $calculations1,2$. [FIGURE] On the one hand, approximate rotation-vibration-tunneling energy levels can be obtained by mapping the protonated acetylene problem onto an internal rotation problem already discussed in the $literature3$ in terms of numerical calculations in a Principal-Axis-Method treatment. On the other hand, semi-quantitative rotation-vibration-tunneling energy levels can be obtained for the high barrier limit by applying to protonated acetylene an Internal-Axis-Method-like formalism originally set up to treat the water dimer. The agreement betwen the numerical PAM calculations and the algebraic IAM results serves as a check on both. Unfortunately, the development of this formalism, with its tunneling splittings, statistical weights, selection rules, etc., has not yet led to an assignment of the spectrum of $C2H3+$ reported last year at this meeting by Crofton and $Oka4$.