A delocalized level near the isomerization barrier energy, exhibiting large probability amplitude at the transition state geometry
The Field Lab
The S1 state of acetylene presents the possibility of low-barrier isomerization from its trans geometry minimum to a local energy minimum at a cis geometry. This process has been the focus of many theoretical studies, but it has been difficult to study experimentally because, though the trans minimum has been exhaustively characterized, the transition from the ground electronic state to the cis geometry is electronically forbidden and no transitions to this geometry have previously been observed. In the course of characterizing the S1 surface, several levels were observed which could not be ascribed to S1 trans or other electronic states. S1 cis seems a likely candidate for explaining these "interloper" levels, which were, surprisingly, observed below the calculated barrier to trans–cis isomerization, and must therefore owe their intensity to mixing via tunneling with trans geometry localized states.
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