Acetylene-vinylidene Isomerization Dynamics and Influence on Energetics and Collisional Energy Transfer

Acetylene has proven an interesting case study, both for its ubiquity in nature as well as its implementation in industry. It is readily used in a variety of commercial applications and scientific inquiries, and as such, knowledge of its chemical and physical properties is indispensible. Furthermore, its prevalence as an intermediate in combustion reactions makes the study of highly vibrationally excited acetylene a potentially lucrative endeavor, and perhaps holds the promise of a new and exciting field of study. This promise has compelled us—and other researchers—to probe the dynamics of energized acetylene, through its photodissociation with vinylhalides. This photodissociation is accomplished via vacuum UV at 193nm, which results in rovibrationally excited hydrogen halide (HX) and vibrationally excited acetylene; this excited acetylene is subsequently tracked via time resolved Fourier transform infrared emission spectroscopy; the data obtained in this manner is used in conjunction with molecular dynamics in order to explore the underlying mechanisms that dictate vibrationally excited acetylene’s behavior. With the use of Born-Oppenheimer molecular dynamics (BOMD), we are attempting to explore the behavior of highly excited acetylene and perhaps more importantly, its unstable isomer counterpart, vinylidene. Vinylidene is the bi-radical transient of acetylene and exists only on the order of the picosecond. The highly anharmonic potential surface of the isomerization between acetylene and vinylidene, the high barrier height towards the transition state, and the shallow potential well of the transient make an equilibrium between acetylene and vinylidene seemingly unlikely, but in fact such an equilibrium is observed both theoretically and experimentally given the appropriate detection method. It has further been demonstrated that by neglecting the isomerization equilibrium described above, the kinetics fail and only by its inclusion does theory match observation. We present computational results using Born-Oppenheimer-molecular-dynamics (BOMD) to explore the creation of vibrationally excited acetylene/vinylidene via the photolysis of vinylbromide and vinylfluoride. Calculations on the former resulted in a 3-centered (a-a) dissociation of HBr , while the later produced both a 3 (a-a) and 4-centered (a-b) dissociation pathway for HF. Furthermore, because acetylene in a know intermediate in combustion reactions, we will explore various free-radical and noble-gas collision trajectories in order to determine the resulting vibrational energetics of acetylene and its isomer. We will also present the ground-state energies of relevant species at a refined level of theory.