Long live vinylidene! A new view of the H2C=C: -> HC CH rearrangement from ab initio molecular dynamics

We present complete active space self-consistent field (CASSCF) ab initio m olecular dynamics (AIMD) simulations of the preparation of the metastable s pecies vinylidene, and its subsequent, highly exothermic isomerization to a cetylene, via electron removal from vinylidene anion (D2C=C- --> D2C=C: --> DC=CD). After equilibrating vinylidene anion-d(2) at either 600 +/- 300 K (slightly below the isomerization barrier) or 1440 K +/- 720 K (just above the isomerization barrier), we remove an electron to form a vibrationally e xcited singlet vinylidene-d(2) and follow its dynamical evolution for 1.0 p s. Remarkably, we find that none of the vinylidenes equilibrated at 600 K a nd only 20% of the vinylidenes equilibrated at 1440 K isomerized, suggestin g average lifetimes >1 ps for vibrationally excited vinylidene-d(2). Since the anion and neutral vinylidene are structurally similar, and yet extremel y different geometrically from the isomerization transition state (TS), neu tral vinylidene is not. formed near the TS so that it must Live until it ha s sufficient instantaneous kinetic energy in the correct vibrational models ). The origin of the delay is explained via both orbital rearrangement and intramolecular vibrational energy redistribution (IVR) effects. Unique sign atures of the isomerization dynamics are revealed in the anharmonic vibrati onal frequencies extracted from the AIMD, which should be observable by ult rafast vibrational spectroscopy and in fact are consistent with currently a vailable experimental spectra. Most interestingly, of those trajectories th at did isomerize, every one of them violated conventional transition-state theory by recrossing back to vinylidene multiple times, against conventiona l notions that expect highly exothermic reactions to be irreversible. The d ynamical motion responsible for the multiple barrier recrossings involves s trong mode-coupling between the vinylidene CD2 rock and a local acetylene D CC bend mode that has been recently observed experimentally. The multiple b arrier recrossings can be used, via a generalized definition of lifetime, t o reconcile extremely disparate experimental estimates of vinylidene's life time (differing by at least 6 orders of magnitude). Last, a caveat: These r esults are constrained by the approximations inherent in the simulation (cl assical nuclear motion, neglect of rotation-vibration coupling, and restric tion to C-s symmetry); refinement of these predictions may be necessary whe n more exact simulations someday become feasible.