Experimental and theoretical chemical dynamics of small hydrocarbon molecules

The experimental and theoretical dynamics of three small unsaturated hydrocarbon radicals: vinyl, allyl and propargyl in their lowest excited states have been studied. The electronic spectrum of the perdeuterated vinyl radical (C2D 3) A2A″ ← X˜ 2A' has been measured between 505 and 450 nm using cavity ring-down spectroscopy. From the best fit of the experimental spectrum, the rotational constants and linewidths were determined for the first four vibrational bands. It was seen that the vibrational structure is very diffuse and the linewidth increased with increasing vibrational excitation. From this, we infer that the excited state lifetime decreases as the vibrational excitation increases. We believe that the short lifetimes in the excited electronic state of the vinyl radical and the diffuse vibrational structure are due to rapid predissociation. CASSCF calculations indicate that the most likely predissociation mechanism is via a conical intersection between the first excited electronic state and the ground electronic state. The geometry of the vinyl radical at the conical intersection is different from the geometry at the global minimum. It has a long C-C bond length (1.6--1.7 A) and a alpha C-C-H bond angle of less than 90°. After crossing to the ground electronic state at the conical intersection, we believe that the vinyl radical dissociates to give vibrationally excited acetylene (HCCH) plus hydrogen (H) atom. The conical intersection for the vinyl radical lies close in energy to the global minimum in the excited electronic state. Other possible predissociation mechanisms are discussed. The visible/near ultraviolet spectra of the allyl and propargyl radicals are similar to the vinyl spectrum. These spectra have diffuse vibrational structure. The diffuseness of the spectra may be attributed to rapid predissociation in the first excited electronic state. Based on the calculations carried out for the vinyl radical we suspect that the predissociation of the allyl and propargyl radicals may occur through a conical intersection. CASSCF calculations on allyl and propargyl radicals indicate that the most likely predissociation mechanism is via a conical intersection between the ground and first excited electronic states. The geometries of the allyl and propargyl radicals at the conical intersection are different from either of the equilibrium structures. The conical intersections for allyl and propargyl radicals are reached by decreasing the C-C-C angle and by twisting the C-C bonds and they lie close in energy to the zero-point vibrational levels in the excited electronic states. Once allyl and propargyl radicals get to the conical intersections, they may cross to the ground electronic state potential energy surfaces and dissociate into excited allene (H2CCCH2) plus hydrogen (H) atom, and excited propargylene (HCCCH) plus hydrogen (H) atom, respectively.