EXPERIMENTAL AND THEORETICAL-STUDIES TOWARD A CHARACTERIZATION OF CONCEIVABLE INTERMEDIATES INVOLVED IN THE GAS-PHASE OXIDATION OF METHANE BY BARE FEO+ - GENERATION OF 4 DISTINGUISHABLE [FE,C,H4,O]+ ISOMERS

Sections of the potential energy surface of [Fe,C,H-4,O]+ 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), CH3O(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), CH3O(H)Fe+ (4), and CH2=FeOH2+ (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 H2O and FeCH2+, and 3 is accessible by SO2 loss from the Fe+ complex of CH3SO2OH. 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.