Matthew Jacobson, Robert W. Field and Mark S. Child

Physical and Theoretical Chemistry Laboratory, Oxford University, UK

The acetylene-vinylidene isomerization is a prototypical example of a 1,2-hydrogen shift. Over the past several years, I have carried out experimental and theoretical studies of acetylene when it has *nearly* enough vibrational excitation for it to be capable of isomerizing to the quasi-stable vinylidene species. These studies have uncovered indirect but unambiguous evidence for a class of vibrational states, the "local benders", that sample the isomerization pathway and, in a classical sense, spend much more time near the transition state than they do near the linear equilibrium configuration. However, this talk will primarily look forward rather than backward, and examine the experimental and theoretical challenges that face the study of isomerization in acetylene (and similar systems) when the vibrational energy exceeds the transition state energy, and the hydrogen(s) can undergo bond-breaking internal rotation (i.e., the hydrogens can rotate around the CC core). Recent experimental data will be presented which show tantalizing signs of probing the nascent "isomerization states" in acetylene. In addition, possible spectroscopic signatures of bond-breaking internal rotation in acetylene and other molecules will be discussed, including changes in rotational fine structure constants and the breakdown of the polyad approximation.