STUDIES OF ION DISSOCIATION KINETICS AND MECHANISMS BY SURFACE-INDUCED DISSOCIATION AND INFRARED MULTI-PHOTON DISSOCIATION/SOFT-LANDING

摘要

This dissertation presents dissociation mechanism and dissociation kinetics studies of gas-phase ions using mass spectrometry (MS). Dissociation of a gas-phase ion is related to its fundamental properties such as composition and structure. However, the detailed processes, internal energy deposition during ion activation as well as the mechanism of dissociation, are not fully known. In the present work, ion structural studies from which mechanisms can be inferred were performed using infrared multiphoton dissociation (IRMPD) spectroscopy, soft-landing, IR spectroscopy, and quantum chemical calculations. Kinetics studies involved instrument modification to add surface-induced dissociation (SID) capability and peak shape analysis. Structural studies were performed to determine dissociation mechanisms. The b₂⁺ ion from AGG is an oxazolone structure as indicated by the IRMPD spectrum and quantum chemical calculations. Protonated 4-ethoxymethylene-2-phenyl-2-oxazolin-5- one is also an oxazolone-type structure, while protonated cyclo-AG is a diketopiperazine structure. Soft-landing experiments were carried out to corroborate IRMPD results. Soft-landed protonated cyclo-AG and protonated 4-ethoxymethylene-2-phenyl-2- oxazolin-5-one underwent neutralization and retained their structures. The soft-landed b₂⁺ ion of AGG showed evidence of ring opening and conversion into a linear structure. The modified matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometers with SID capability were used to study fast dissociation kinetics (sub-microsecond dissociation). Silicon nanoparticle assisted laser desorption/ionization (SPALDI) allows the study of small molecule dissociation kinetics for ions without the matrix interference observed in MALDI. Well characterized systems, such as, N(CH₃)₄⁺, N(CD₃)₄⁺, and substituted benzylpyridinium ions were used to confirm reliability of the peak shape analysis. Obtained dissociation rates, of submicrosecond order, are consistent with the known dissociation theories. Dissociation of fullerenes, C₆₀ and C₇₀, was also investigated with the SID method using a fluorocarbon self-assembled monolayer (FSAM) surface. Fullerene ions produced C(2n)⁺ fragments ion in the kinetic energy range of 150-300 eV. At higher than 400 eV, mass spectra showed additional small fragment ions composed of odd numbers of C units. Energy resolved MS/MS curves support parallel dissociation at high SID energies while peak shape analysis explains sequential dissociation at about 150 eV range. Instrument modification of a MALDI-TOF mass spectrometer with SID capability allowed successful studies of fast unimolecular dissociation kinetics of small ions and fullerenes.