Electron impact and multiphoton ionization and fragmentation of dicarbonyl cyclopentadienyl methyl iron and pentamethylcyclopentadienyl dicarbonyl methyl iron

Abstract

Electron impact (EI) and multiphoton ionization (MPI) mass spectra of the title compounds CpFe(CO) 2CH 3 (1) and ∗CpFe(CO) 2CH 3 (2) are reported. Under EI molecular ions and large fragment ions are formed from both compounds. Direct analysis of metastable ions showed that successive detachments of ligands from 1 lead to Fe + ions while loss of Fe atoms is more important for ionic fragments of 2. Ionization energies of 7.56 eV (1) and 7.25 eV (2) were determined by extrapolating ion abundances versus energy in semi-log plots. In the same way, appearance energies of major fragment ions were obtained for both compounds. Derived data include ion dissociation energies and enthalpies of ion formation. Typically, the dissociation energies required were below 1 eV for splitting off CO ligands and above 2 eV for CH 3 detachment. Upon MPI of both compounds at the excimer wavelengths 351, 308 and 248 nm Fe + ions are formed almost exclusively, probably by breakdown of the neutral parent compounds yielding bare metal atoms that are subsequently ionized. At 193 nm, molecular ions appear and large fragment ions are generated with low abundance, depending on laser intensity. High laser intensities lead to complete breakdown and Fe + ion formation. At low intensities loss of Fe atoms is observed only in the case of 2. A dependence on pressure was observed for abundances of molecular ions and some fragment ions of 2. The abundances of C 10H 13 1 and of Fe + ions are independent of pressure.