Acetylene at the threshold of isomerization

This article reviews recent research on acetylene which is intended as a contribution to the understanding of intramolecular vibrational energy flow when it is poorly described by either statistical (i.e., RRKM) or purely separable (i.e., harmonic oscillator/normal mode) models. The experimental spectra that inform this investigation are similar to 7 cm(-1) resolution dispersed fluorescence spectra of the acetylene S-1-->S-0 system. Above 10 000 cm(-1) of vibrational energy, these spectra are extremely congested and cannot be analyzed using conventional spectroscopic assignment procedures. Instead, a numerical pattern recognition procedure is utilized to disentangle spectroscopic patterns that are associated with approximately conserved polyad quantum numbers. This pattern recognition analysis makes possible detailed modeling of the short-time (similar to 1 ps) but large-amplitude vibrational dynamics of acetylene at high energy (15 000 cm(-1)), which is demonstrated here to be dominated by regularity even for the low-frequency bending motions (22 quanta of bend excitation). That is, a few stable motions dominate the large-amplitude bending dynamics, including local bend (one hydrogen bending), which is closely related to the acetylene-vinylidene isomerization coordinate, and a new type of vibrational motion that we call counter-rotation, in which the two hydrogens undergo circular motions on opposite ends of the CC core.