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Thermal stability of hypergolic ionic liquids: a mechanistic and spectroscopic approach

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Author(s):
Rafael Boffo de Souza
Total Authors: 1
Document type: Master's Dissertation
Press: São Paulo.
Institution: Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ)
Defense date:
Examining board members:
Rômulo Augusto Ando; Danilo Manzani; Alvaro Takeo Omori
Advisor: Rômulo Augusto Ando
Abstract

This work focused on investigating the main reaction mechanisms that occur during the thermal decomposition processes of a class of energetic ionic liquids (EIL\'s), known as hypergolic ionic liquids, in order to understand the relationship between the variation of the chemical structure of nitrogenous cations and the factors that govern the modulation of the thermal stability of these compounds. For this, the work thus developed was divided into two parts and was based on a systematic investigation of ionic liquids (IL\'s) formed by the combination between the dicyanamide anion and different cationic structures. In Part I of this study, the investigated system was composed of IL\'s containing a series of nitrogenous cations, with distinct central structures and all having butyl and methyl alkyl substituents. The effect of the cation structure on the thermal stability was obtained by TGA/DTG analysis, so that the main decomposition paths were determined by Py - GC/MS, through the identification and quantification of the gaseous products generated during the pyrolytic process. In this way, it was possible to designate that the main mass loss event is associated with the dealkylation reaction of -CH3 group of the cationic structures, following an SN2 mechanism. Allied to theoretical calculations (Hartree-Fock), the results of the energetics analysis for the reaction pathways at the H3C – N+(LG) sites showed that the thermal stability behavior has its origin strictly in the energy barrier of the reaction, ΔE#(ζ), which is accompanied by the same variation of the Tpeak\'s. Using the Activation Strain and Intersection of Harmonic Potential Curves Models to analyze the energy profile of the reactions, it was identified that the energy required to rearrange the reactants until the formation of the transition state (TS) is the portion responsible for governing the variation of the energy barrier for the different cations, in such a way that the strength of the reactive bonds of the type N+ – C reactive bonds is the parameter that modulates the chemical reactivity within the same substituent group. With the aim of probing the electronic structure of the different cations, spectroscopic measurements of 13C chemical shifts, obtained by NMR, showed that the increase in chemical reactivity/decrease in thermal stability, associated with the reduction in the strength of reactive N+ – C bonds, has its origin in the attenuation of the electron density in these regions of the molecular structure.In Part II of this work, based on the previous conclusions obtained previously, it was shown that it is possible to correlate spectroscopic measurements of 13C displacement with chemical reactivity/thermal stability properties for a larger number of energetic cationic structures that decompose via the SN2 reaction. The investigation of the electronic structure of 1-alkyl-3-methylimidazolium cations by means of NMR and vibrational infrared spectroscopy showed that the reduction of thermal stability for these compounds is directly related to the decrease of electronic density in the imidazole ring. With this, the electron withdrawing power associated with the alkyl functionalization directly linked to the cationic structure, presented the following order of hierarchy between the following substituents: -benzil > -alil > -octil ≈ -hexil > -butil > -etil > -metil. (AU)

FAPESP's process: 20/06307-4 - Vibrational spectroscopy and physico-chemical properties of hypergolic ionic liquids
Grantee:Rafael Boffo de Souza
Support Opportunities: Scholarships in Brazil - Master