Theory of Low Temperature Gas-Phase Reactions

Cyclopropenyl cation – the simplest Huckel's aromatic molecule – and its cyclic methyl derivatives in Titan's upper atmosphere

“…This is in line with what discussed above, i.e., that the cross beam measurements (Sonnenfroh and Farrar, 1986) of center of mass translational energy distributions for C 3 H 3 + reflect the product branching ratio of cyclic to linear increases with the decrease of reagent relative collision energy from 2.0 to 0.5 eV. The dominance of long-range attractive potential (Clary, 1987;Klippenstein and Georgievskii, 2008) in the entrance channel and the multiple transition state models (Lopez et al, 1996) in the product exit channel thus provide clear evidence of the formation of the cyclopropenyl cation, as the major product in the conditions of Titan's upper atmosphere and interstellar clouds at low temperatures (10-200 K). The process of isomerization between linear and cyclic isomer of C 3 H 3 + does not occur at temperatures relevant to the chemistry of Titan's atmosphere, because of a very large barrier of about 50 kcal/mol relative to the linear isomer (Fig.1).…”

“…In these ion-molecule reactions, in the entrance channel the transition states connecting collision complexes to the covalently bound global minimum all lie well below the reactants (Sonnenfroh and Farrar, 1986;Lopez et al, 1996;Almlof et al, 1984). Thus, simple capture rates (Clary, 1987;Klippenstein and Georgievskii, 2008) in the limit of long range attractive forces can provide an accurate description of the kinetics of the formation of ion-molecule complexes at higher energies around room temperature as well as at the lower collision energies prevailing in planetary atmospheres and interstellar clouds. In treating the unimolecular decomposition of intermediate collision complexes to the products (Wardlaw and Marcus, 1988;Gilbert and Smith, ed.…”

“…The saddle point in the entrance of the potential energy surface connecting the corner-protonated cyclopropyl cation (C 3 H 7 + ) intermediate (Almlof et al, 1984;Hariharan et al, 1974) to the covalently bound short-lived sec-C 3 H 7 + structure tends to lie (Sonnenfroh and Farrar, 1986) well below the energy of the reactants, because of long-range increased attractions in these ionmolecule complexes. In this respect, simple capture theory (Clary, 1987;Klippenstein and Georgievskii, 2008) provides an accurate description of the long-range part of this attractive potential.…”

“…Titan's upper atmosphere is chemically complex. The reactions involving neutrals, ion-molecule reactions and the chemistry of negative ions initiate this complexity (Vuitton et al, 2009a(Vuitton et al, , 2009bWaite et al, 2009;Coates et al, 2007;Sittler et al, 2009;Ali et al, 2010;Klippenstein and Georgievskii, 2008;Balucani, 2009;Larsson et al, 2012) and start the organic inventory in the second largest moon of our solar system. No other planetary environments in our solar system are known to exceed such chemical complexity.…”

“…Temperatures, densities, and time scales are the three most important parameters that govern the gas phase reactions involving carbocations and carbanions in Titan's cold upper atmosphere. The organic chemistry is also driven by fast neutral reactions (Klippenstein and Georgievskii, 2008;Canosa et al, 2008) such as radical-molecule reactions. Since the translational energy of molecular species at temperatures of Titan's thermosphere and ionosphere is typically of the order of 1 kJ mol -1 , only exothermic reactions with practically no entrance barriers can take place.…”