title: Reactivity of4Fe+(CO)n=0-2+ O2: oxidation of CO by O2at an isolated metal atom.
abstract: The kinetics of4Fe+(CO)n=0-2+ O2are 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, but4Fe+(CO) reacts rapidly (k = 3.2 ± 0.8 × 10-10cm3s-1at 300 K, 52% of the collisional rate coefficient) to form FeO++ CO2. This is an example of the oxidation of CO by O2occurring 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 O2is 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.
authors.affiliation: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, NM 87117, USA. rvborgmailbox@us.af.mil.
authors.forename: Shaun G
authors.initials: SG
authors.lastname: Ard
authors.affiliation: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, NM 87117, USA. rvborgmailbox@us.af.mil.
authors.forename: Oscar
authors.initials: O
authors.lastname: Martinez
authors.affiliation: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, NM 87117, USA. rvborgmailbox@us.af.mil.
authors.forename: Steven A
authors.initials: SA
authors.lastname: Brown
authors.affiliation: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, NM 87117, USA. rvborgmailbox@us.af.mil.
authors.forename: Jordan C
authors.initials: JC
authors.lastname: Sawyer
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
authors.forename: P B
authors.identifier: http://orcid.org/0000-0003-2953-6039
authors.initials: PB
authors.lastname: Armentrout
authors.affiliation: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, NM 87117, USA. rvborgmailbox@us.af.mil.
authors.forename: Albert A
authors.initials: AA
authors.lastname: Viggiano
authors.affiliation: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, NM 87117, USA. rvborgmailbox@us.af.mil.
authors.forename: Nicholas S
authors.identifier: http://orcid.org/0000-0002-0274-2644
authors.initials: NS
authors.lastname: Shuman