"Theoretical Studies of Various Formation Mechanisms of Large Polycycli" by Lotefa Binta Tuli
 

Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Chemistry

First Advisor's Name

Alexander M. Mebel

First Advisor's Committee Title

Committee chair

Second Advisor's Name

David C. Chatfield

Second Advisor's Committee Title

committee member

Third Advisor's Name

Jeffrey Joens

Third Advisor's Committee Title

committee member

Fourth Advisor's Name

Wenzhi Li

Fourth Advisor's Committee Title

committee member

Fifth Advisor's Name

Christopher Dares

Fifth Advisor's Committee Title

committee member

Keywords

PAH, Fullerens, Mechanisms

Date of Defense

6-26-2023

Abstract

Electronic structure calculations using ab initio molecular orbital and density functional theory methods have been performed to unravel the formation mechanisms of PAHs in the gas phase in a broad range of temperature and pressure conditions. A complete picture of the reaction process is obtained by mapping local minima and saddle points on the potential energy surfaces (PES) of products, intermediates, and smaller reactants participating in the sequential growth of the PAHs. Once the PESs are characterized using quantum chemical calculations, the kinetics of all reactions involved in the growth mechanism are quantitatively evaluated and described by solving the Rice-Ramsperger-Kassel-Marcus (RRKM) Master Equation. In particular, we report G3(MP2,CC)//B3LYP/6-311G(d,p) calculations of the PES for a variety of PAH growth reactions combined with RRKM Master Equation calculations of temperature- and pressure-dependent rate constants. The suggested kinetic schemes and rate constants are proposed as prototypes for the modeling of the growth of PAHs via the hydrogen abstraction – vinylacetylene addition (HAVA), hydrogen abstraction – acetylene addition (HACA), and phenyl addition – dehydrocyclization (PAC) mechanisms involving an addition of the C4H4 or C2H2 molecules or the phenyl radical, C6H5, to a PAH radical or molecule. Also, our ab initio investigation shows that the buckyball, C60, can be synthesized in the gas phase through dehydrogenation/cyclization mechanism involving corannulene, C20H10, addition to the nano bowl radical, C40H9, under the temperature-pressure conditions mimicking those in circumstellar envelopes of carbon rich stars. These reaction sequences reveal a reaction class in which a PAH radical/molecule undergoes ring expansion while simultaneously forming an out-of-plane carbon backbone central to 3D nanostructures such as bucky bowls and buckyballs. The fundamental reaction mechanisms studied in the project are critical in facilitating an intimate understanding of the origin and evolution of the molecular universe and, in particular, of carbon in our galaxy.

Identifier

FIDC011207

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