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Issue 13, 2017
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Reactivity of 4Fe+(CO)n=0–2 + O2: oxidation of CO by O2 at an isolated metal atom

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The kinetics of 4Fe+(CO)n=0–2 + O2 are measured under thermal conditions from 300–600 K using a selected-ion flow tube apparatus. Both the bare metal and n = 2 cations are inert to reaction over this temperature range, but 4Fe+(CO) reacts rapidly (k = 3.2 ± 0.8 × 10−10 cm3 s−1 at 300 K, 52% of the collisional rate coefficient) to form FeO+ + CO2. This is an example of the oxidation of CO by O2 occurring entirely on a single non-noble metal atom. The reaction of the bare metal reaction is known to be endothermic, such that this result is expected; however, the n = 2 reaction has highly exothermic product channels available, such that the lack of reaction is surprising in light of the n = 1 reactivity. Stationary points along all three reaction coordinates are calculated using the TPSSh hybrid functional. These surfaces show that the n = 1 reaction is an example of two-state reactivity; the reaction proceeds initially on the sextet surface over a submerged barrier to a structure with an O–O bond distance longer than that in O2, but must cross to the quartet surface in order to proceed over a second submerged barrier to rearrange to form CO2. The n = 2 reaction does not proceed because, on all spin surfaces, the transition state corresponding to O–O separation is at higher energy than the separated reactants. The difference between the n = 1 and n = 2 reactions is not a result of steric effects, but rather because the O2 is more strongly bound to Fe in the entrance well of the n = 1 case, and that energy is available to overcome the rate-limiting barrier to O–O cleavage. Experimental verification of some of these details are provided by guided ion beam tandem mass spectrometry results. The kinetic energy dependence of the n = 1 reaction shows evidence for a curve crossing and yields relevant thermochemistry for competing reaction channels.

Graphical abstract: Reactivity of 4Fe+(CO)n=0–2 + O2: oxidation of CO by O2 at an isolated metal atom

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The article was received on 20 Dec 2016, accepted on 27 Feb 2017 and first published on 27 Feb 2017

Article type: Paper
DOI: 10.1039/C6CP08703E
Citation: Phys. Chem. Chem. Phys., 2017,19, 8768-8777
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    Reactivity of 4Fe+(CO)n=0–2 + O2: oxidation of CO by O2 at an isolated metal atom

    S. G. Ard, O. Martinez, S. A. Brown, J. C. Sawyer, P. B. Armentrout, A. A. Viggiano and N. S. Shuman, Phys. Chem. Chem. Phys., 2017, 19, 8768
    DOI: 10.1039/C6CP08703E

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