Publication Date

8-2017

Date of Final Oral Examination (Defense)

6-12-2017

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Chemistry

Department

Chemistry

Supervisory Committee Chair

Michael P. Callahan, Ph.D.

Supervisory Committee Member

Dale D. Russell, Ph.D.

Supervisory Committee Member

Don Warner, Ph.D.

Abstract

Organic compounds in meteorites were likely transformed by a variety of processes on the asteroid parent body including aqueous, thermal, and radiolytic alteration. Previous studies have identified a suite of purine nitrogen heterocycles in carbonaceous chondrites (a class of meteorites) and determined that their likely origin was due to cyanide chemistry. The thesis research described here consisted of two parts: investigating thermal effects on aqueous ammonium cyanide reactions and the production/survivability of organics (Chapter 2) and investigating gamma radiation effects on purine nitrogen heterocycles to understand how prolonged radiation exposure influenced the distribution and abundance of nitrogen heterocycles measured in meteorites today (Chapter 3).

A temperature study of 1 M ammonium cyanide was conducted from room temperature to 200 °C (temperatures similar to the aqueous alteration of some carbonaceous chondrites) using a high-pressure reaction vessel. The resulting liquid supernatant and water insoluble cyanide (hetero)polymer were isolated and analyzed by attenuated total reflectance Fourier transform infrared spectroscopy and thermochemolysis gas chromatography-mass spectrometry using tetramethylammonium hydroxide or high-performance liquid chromatography with UV detection (HPLC-UV). A strong correlation was observed between thermochemolysis products of cyanide polymers and the initial reaction temperature - production of aromatic compounds increases and nitrogen containing compounds decrease with respect to increasing reaction temperature. The data presented in this thesis suggests that the polymer may be a less efficient source of nitrogen heterocycles when produced at high temperatures. HPLC-UV analysis showed that the supernatant of heated NH4CN reactions is a complex mixture containing many unknown UV-absorbing chromophores. In addition, multiple nitrogen heterocycles were tentatively identified in the supernatant of heated NH4CN reactions including the high temperature 200 °C reactions.

Finally, HPLC was used to determine decomposition of seven purine nitrogen heterocycles exposed to gamma radiation from a cobalt-60 source in an effort to extrapolate abundances during the early formation of the Solar System. Generally speaking, purine nitrogen heterocycles in the solid state are very stable to high doses of γ-radiation; however, guanine experienced a 53% decomposition over the course of ~1 MGy of radiation dose. The radiolysis study correlates to similar conditions in asteroids after their initial aqueous alteration period (i.e. dry alteration) and suggests that meteorite abundances for some compounds (such as adenine) may have remained relatively unchanged over time, but other compounds (like guanine) may have had “original” abundances greater than those currently observed in carbonaceous chondrites.

DOI

https://doi.org/10.18122/B2MT40

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