  Recent studies of neutral gas-phase reactions characterized by barriers show
that certain complex forming processes involving light atoms are enhanced by
quantum mechanical tunneling at low temperature. Here, we performed kinetic
experiments on the activated C(3P) + H2O reaction, observing a surprising
reactivity increase below 100 K, an effect which is only partially reproduced
when water is replaced by its deuterated analogue. Product measurements of H-
and D-atom formation allowed us to quantify the contribution of complex
stabilization to the total rate while confirming the lower tunneling efficiency
of deuterium. This result, which is validated through statistical calculations
of the intermediate complexes and transition states has important consequences
for simulated interstellar water abundances and suggests that tunneling
mechanisms could be ubiquitous in cold dense clouds.
