Experimental and theoretical studies toward a characterization of conceivable intermediates involved in the gas-phase oxidation of methane by bare FeO+. Generation of four distinguishable [Fe,C,H4,O]+ isomers

Abstract

Sections of the potential energy surface of [Fe,C,H4,0]+ ions are probed experimentally by using collisional activation mass spectrometry as well as theoretically by ab initio MO studies. Evidence is presented for the existence of several, clearly distinguishable isomers, some of which are of relevance in the context of methane activation by FeO+ in the gas phase. While this process initially gives rise to the formation of an encounter complex, CH4-FeO+ (1), both spontaneous and post-collisional isomerization occurs to other intermediates formed from 1. The data point to the intermediacy of CH3-Fe-OH+ (3), CH30(H)Fe+ ( 4), and CH2=Fe-OH2+ ( 7); these species account for the formation of FeOH+, Fe+, and FeCH2+, respectively. The isomers CH3-Fe-OH+ (3), CH30(H)Fe+ ( 4), and CH2=Fe-OFi2+ ( 7) can be generated in a clean fashion using appropriate ion/molecule reactions in the gas phase. 4 results by termolecular association of CH3OH and Fe+ as well as decarbonylation of HCOOCH3, 7 is formed in a termolecular reaction from H20 and FeCH2+, and 3 is accessible by S02 loss from the Fe+ complex of CH3S020H. Mixtures of 3 and 7 are obtained by the Fe+-mediated losses of C2H4 from n-C3H7OH and decarbonylation of HCOCH2OH and CH3COOH, pointing to a relatively complex potential energy surface of these systems. No experimental evidence has yet been obtained for the existence of the insertion products H3CO-Fe-H+ ( 8) and H-Fe-CH2OH+ (9) as stable gas-phase species. The former, generated initially by decarbonylation of HCOOCH3, undergoes facile isomerization to H3CO(H)Fe+ (4), and the latter, when generated by decarbonylation of HCOCH2OH, isomerizes to a 1:4 mixture of H3C-Fe-OH+ (3) and H2C=Fe-OH2+ (7). Some of the experimentally probed isomers were further characterized by means of ab initio MO calculations using the MP2/ECP-DZ level of theory, and the computed structural features were found to correlate with the experimental results. The order of stabilities are discussed and compared, where possible, with existing or estimated thermochemical data.

Theoretical Study on the Mechanism of the Reaction of CH4 + MgO

“…The structure of HOMgCH 3 is similar to that of HOFeCH 3 + in the dissociation of CH 4 on FeO + reaction. 23 From the atomic natural charges in Table 2, the electronic features support the view that HOMgCH 3 can be described as either CH 3 -interacting with MgOH + or OHinteracting with MgCH 3 + , since both the CH 3 and OH moieties are close to -1 valence and both MgOH and MgCH 3 moieties are close to +1 valence. These interactions are so large that the HOMgCH 3 molecule is especially stable.…”

“…4 To obtain further information about this process, many theoretical studies of the interaction of methane with metal oxide heve been performed using an oxometal cluster model 9,16,[18][19][20][21] and gaseous metal oxide molecules. [22][23][24][25][26][27] Of particular interest is the direct oxidation of methane by metal oxide fragments, [22][23][24][25][26][27] because this process can be justifiably regarded as providing models for the more complicated processes of heterogeneous or enzymatic methane monooxygenase catalysis. It is obvious that a better understanding of the details of the activation processes would be desirable in order to improve the catalysts.…”

Gas-phase activation of methane by ligated transition-metal cations

“…MoO 3 ϩ exhibits a KIE of the same magnitude, thus pointing to the operation of a similar mechanism (60). In the case of OFeOH ϩ (59) and FeO ϩ (52, 53), even larger KIEs are observed that, at least in the latter case, can be ascribed to the formation of an intermediate insertion species in conjunction with spin cross-over (81)(82)(83)(84)(85).…”

Darstellung, Reaktivität gegenüber Kohlenwasserstoffen und elektronische Struktur von heteronuclearen Vanadium‐Phosphor‐Sauerstoff‐Clusterionen

“…Diese Erkenntnis ist nicht zuletzt Gasphasenstudien geschuldet, bei denen Metalloxide unterschiedlicher Stöchio-metrie unter wohldefinierten Bedingungen untersucht wurden; hierzu gehören [MgO]C + , [8] [FeO]C + , [9] [PbO]C + , [10] [ [19] In Anbetracht dieser drastischen Unterschiede schien es angebracht, Cluster mit systematisch variiertem Vanadium/PhosphorVerhältnis zu studieren. Das enorme Interesse an solchen Systemen hängt auch mit ihrer engen Beziehung zu den Vanadiumphosphoroxid(VPO)-Katalysatoren zusammen, die in der heterogenen Katalyse sehr selektive Oxidations-und Bindungsaktivierungsprozesse ermöglichen.…”