Probing Reaction Dynamics in Complex Gas-Phase Systems Using Guided-Ion-Beam Scattering and Computational Methods
Item
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Title
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Probing Reaction Dynamics in Complex Gas-Phase Systems Using Guided-Ion-Beam Scattering and Computational Methods
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Identifier
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d_2009_2013:68f3b1ba65c9:11648
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identifier
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12186
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Creator
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Fang, Yigang,
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Contributor
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Jianbo Liu
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Date
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2013
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physical chemistry | computational | gas phase reaction | mass spectrometry | micelle | photooxidation | reaction dynamics
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Abstract
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An electrospray ionization (ESI) guided-ion-beam tandem mass spectrometer was developed in our research lab, including vacuum systems, electronics and instrument control and data acquisition (DAQ) systems using LabVIEW programming. Capitalizing on this instrument, two experimental projects have been accomplished as described below.;In project 1, formation and characterization of multiply positively and negatively charged sodium bis(2-ethylhexyl)sulfosuccinate (NaAOT) aggregates were studied in the gas phase. Mass spectra show the compositions of [(NaAOT) nNaz]z+ for positively charged aggregates and [Nan-zAOTn]z- for negatively charged aggregates, with various aggregation numbers (n) and charges (z). Collision-induced dissociation (CID) of mass-selected micellar ions with Xe was performed to probe structures of gas-phase micellar assemblies, identify effects of charge states, solute-surfactant interactions, and determine preferential incorporation sites of different amino acids. Gas-phase NaAOT surfactants are able to assemble into a reverse micelle-like structure in positive ion mode and a direct micelle-like structure in negative ion mode, respectively. Both direct and reverse micelles are able to incorporate amino acids. Driving forces for amino acid solubilization in gas-phase micelles come from hydrophobic and/or electrostatic interactions, which strongly depend on amino acid hydrophobicity and charge states.;In project 2, the reaction of deprotonated cysteine ([Cys-H]- ) with singlet molecular oxygen (O2[a1Delta g]) has been studied, including the measurement of effects of collision energy (Ecol) on reaction cross sections. [Cys-H] - has a carboxylate anionic structure (HSCH2CH(NH 2)CO2-) in the gas phase. Density functional theory (DFT) calculations were employed to examine the properties of various complexes, transition states and products. Three product channels were observed, corresponding to the formation of NH2CH2CO2 - with remaining neutral products, OSCH2CH(NH 2)CO2- via elimination of OH from an intermediate complex, and HSCH2C(NH)CO2- + H2O2 via abstraction of two hydrogen atoms from HSCH2CH(NH2)CO2- by 1O2, respectively. Quasi-classical direct dynamics trajectory simulations were carried out at Ecol = 0.2 eV using B3LYP/4-31G(d) level of theory. Trajectory results were used to guide the construction of the reaction coordinate, discriminate between different mechanisms, and provide additional mechanistic insights. Analysis of trajectories highlights the importance of complex mediation at the early stage of all reactions, and suggests a partially concerted mechanism for the H2O2 elimination reaction.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Chemistry
