Laser-ion beam photodissociation studies of ionic transition metal cluster fragments and model peptides

Abstract

Ionic cluster fragments of the type M[x](CO)[y]+ are studied by laser-ion beam photodissociation. The systems studied include Mn[x](CO)[y]+ (x =1-2; y=0-10, derived from Mn2(CO)10), Fe[x](CO)[y]+ (x =1-3; y=0-12, derived from Fe3(CO)12), and Co[x](CO)[y]+ (x=2-4; y=0-12, derived from Co4(CO)12). Relative photodissociation cross-sections for each ion and upper limits to bond dissociation energies for M-M and M-CO are reported. In addition, limitations in the laser-ion beam photodissociation technique for providing accurate estimates of bond dissociation measurements are discussed. The use of a biased activation cell for performing photodissociation experiments on a triple sector ion-beam mass spectrometer is described. The objective of this work is to eliminate or reduce background signals from metastable ion and/or residual collision-induced dissociation and thereby increase sensitivities for laser-ion beam photodissociation. Biasing the activation cell results in separation of a significant fraction (>85%) of the metastable noise from the photodissociation signals, thereby increasing signal-to-noise ratios and shortening data collection times in the photodissociation experiment. In addition, the rates of photodissociating ions can be obtained from analysis of the data. The use of pulsed valves for performing fast-atom bombardment (FAB) ionization on a sector ion-beam mass spectrometer is described. The objective of this work is to improve the sensitivity of tandem mass spectrometry. Improvements in signal-to-noise ratios for collision-induced dissociation and photodissociation as well as total ion yields by pulsed ionization are compared with cw fast-atom bombardment. Visible (458-514.5 nm) and uv (333-385 nm) photodissociation of the [M+H]+ ions of dinitrophenyl (DNP) derivatized amino acids and peptides is reported. The DNP substituent serves as the chromophore for photoexcitation, however, this energy is rapidly randomized to all degrees-of-freedom of the molecule. Thus, dissociation occurs via all energetically accessible reaction channels. The site selective photoexcitation results are rationalized with particular emphasis on "energy selective" dissociation channels of large ionic systems.