We report on the three-body reaction rate of C2–with H2producing
C2H–studied in a cryogenic 16-pole radio
frequency ion trap. The reaction was measured in the temperature range
from 10 to 28 K, where it was found to only take place via three-body
collisions. The experimentally determined termolecular rate coefficient
follows the form ofwithT0= 20 K,
wherea= 8.2(3) × 10–30cm6/s andb= −0.82(12) denotes the temperature
dependence. We additionally performed accurateab initiocalculations of the forces between the interacting partners and
carried out variational transition state theory calculations, including
tunneling through the barrier along the minimum energy path. We show
that, while a simple classical model can generally predict the temperature
dependence, the variational transition state theoretical calculations,
including accurate quantum interactions, can explain the dominance
of three-body effects in the molecular reaction mechanism and can
reproduce the experimentally determined reaction coefficients, linking
them to a temperature-dependent coupling parameter for energy dissipation
within the transition complex.