Defense Date

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Samy El-Shall

Abstract

In this dissertation, the ion mobility mass spectrometry technique is used to study the possible mechanisms of formation of polyaromatic hydrocarbons (PAHs) and polyaromatic nitrogen-containing hydrocarbons (PANHs) via the ion-molecule reactions of acetylene neutrals with different aromatic cations, in order to infer the possible mechanisms of formation of PAHs and PANHs in different environments such as interstellar media under different ionizing conditions. Furthermore, this technique is used to probe structures, address the thermochemistry, and measure the kinetics of the product ions originated from these reactions. Reactions of benzene radical cations with acetylene produce styrene and naphthalene-type cations at high temperatures. The second order rate constant of this reaction is found to be in the order of 10-14 cm3.s-1 with a barrier of 3.5 kcal.mol-1. Under low temperature regime, benzene radical cation acts as a catalyst to initiate the formation of higher complex hydrocarbons through the associative charge transfer to the acetylene clusters, (C2H2)n+, n= 6-10. Phenylium cation reactions with acetylene is found to be four orders of magnitudes faster than those of benzene radical cation, as predicted theoretically. In these systems the second addition of acetylene molecule is found to follow the Bittner-Howard mechanism. Phenylacetylene and styrene radical cations reactions with acetylene are also studied and the addition of one acetylene molecule to the radical cation is observed. The second order rate constant of the product ions is found to be in the order of 10-13 and 10-14 cm3.s-1 for the phenylacetylene and styrene radical cations respectively. Ion molecule reactions of pyridine cation, benzonitrile cation and pyrimidine radical cation with acetylene are studied. Formation of complex organics with fused nitrogen atom in an aromatic ring is reported. Condensation products of acetylene via the ion-molecule reactions are observed with pyridine cation of up to five acetylene molecules onto the pyridine cation at room temperature. Meanwhile, condensation of only two acetylene molecules is observed for benzonitrile cation and pyrimidine radical cation respectively. In the later case, these condensation reactions are observed with hydrogen abstraction. Ion mobility measurements, collisional induced dissociation (CID) and ab initio calculations are combined to probe the possible structures of the reaction products. Formation of PAHs over nanoparticle surfaces is carried out by studying polymerization of acetylene over Pd nanocatalyst and supported Pd nanocatalyst over MgO and CeO2 prepared by the LVCC technique under different temperature conditions. C8H10 species are formed at 400 oC. However, at 600 oC, production of C16H10 (pyrene isomer) is found to be the major catalytic product. Self polymerization of acetylene at different temperatures is presented as well. Our results of the ion-molecule reactions, associative charge transfer (ACT) and catalytic polymerization over catalysts surfaces’ of acetylene interacting with different aromatic and heterocyclic cations could explain the different possible pathways of formation of complex organics in different ionizing environments in carbon-rich interstellar media in space.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

May 2011

Included in

Chemistry Commons

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