Knowledge Bank

The study of the enigmatic infrared spectrum of protonated acetylene $C2H3+$ which has occupied us for the last three years has come to a plateau and the results are summarized. About 80 R-branch and P-branch lines have been assigned. The general asymmetric rotor pattern of a parallel band with intensity alternation for the $K=1$ K.doublets shows that the spectrum is due to $C2H3+$ with the non-classical bridged structure. The absence of other C-H stretching vibrations also supports this conclusion. Combination differences for the ground state fit well to the asymmetric rotor pattern with the standard deviation of $0.0084cm-1$ The following rotational constants have been obtained: $B=1.1422(5)cm-1C=1.0467(4)cm-1$. These rotational constants agree well with those predicted from ab initio $calculations.1,2,3$ The anomaly of the spectrum is in high J.K levels and in the excited state. Attempts have been made to find out whether or not this anomaly is caused by proton tunnelling via the non-classical structure. This tunnelling changes the two proton problem of the rigid model to a three proton problem4 and thus changes the spin statistical ratio of 3:1 to 4:2. These problems and our study of the $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ species will be discussed.