Knowledge Bank

The bending fundamentals of the ~{A}$1Au$ state of C$2$H$2$ ($\nu 4$, torsion and $\nu 6$, in-plane cis bend) are nearly degenerate, and interact strongly by $a$-and $b$-axis Coriolis coupling}\underline{\textbf{98}},~2742, 1993.}. Rotational analysis of the overtone polyads with v$4$+v$6$= 2 - 5 shows that, in addition, they suffer from extremely strong Darling-Dennison resonance, with the coupling parameter $K4466$ equal to $-$51cm${}^{-1}$.Theresultingvibrationallevelpatternsresemblethoseresultingfromthepresenceofanangularmomentum,thoughwithverylargesplittings.Itisshownthat,althoughC${}_2$H${}_2$ in its ~{A} state is an asymmetric top, an angular momentum-like pattern must arise when two nearly degenerate vibrations are strongly coupled. Several vibrational bands are found at higher energy which cannot be accounted for in the manifold of the trans-bent ~{A} state. In view of the possibility that they represent levels from the cis well of the ~{A} state tunnelling through the cis-trans isomerization barrier, the rotational selection rules have been considered, with the aim of determining the vibrational symmetries (and possible vibrational assignments) of these ``interloper'' levels.