The "non-reaction" of ground-state triplet carbon atoms with water revisited.
We introduce a novel experimental setup for the generation of carbon atoms by means of pulsed laser ablation with a pulse rate optimized to avoid warming of the matrix. The combination of this technique with annealing of the matrix, recooling, and spectral recording (e.g. IR) allowed us to differentiate between the reactivity of triplet and singlet carbon atoms towards water under matrix-isolation conditions. Our experimental procedure assures the relaxation of all unreacted carbon atoms to their triplet ground state in the 10 K matrix before spectral recording. In agreement with CCSD(T)/cc-pVTZ+ZPVE computational data and earlier lower level results, we find that triplet carbon atoms indeed do not react with water, despite their high initial energy. Intersystem crossing of the triplet to singlet states of hydroxy carbene are less important, as the barriers for rearrangement of the initial complex of triplet carbon atoms and water to covalently bound species are too high, and dissociation is more likely. We found no evidence for a direct O--H bond-insertion path for triplet carbon atoms. Self-condensation reactions of triplet carbon atoms are clearly favored and yield carbon clusters that show reactivity of their own. The proposed formation of aldoses in extraterrestrial environments can therefore only derive from "hot" carbon atoms or through photoreactions.