Spectroscopic characterization and photochemistry of nitrogen-containing molecules relevant to Titan's atmosphere

Deepali N Mehta-Hurt, Purdue University

Abstract

Titan's atmospheric chemistry has been the source of intrigue since the planetary body's discovery. Though there is a growing understanding of the atmospheric chemistry of small molecules on Titan, much less is known about larger molecule formation, and particularly about nitrile chemistry. This dissertation characterizes nitrile/isonitrile intermediates that are postulated to be important in Titan's atmosphere, adding to the necessary foundation for understanding Titan's atmospheric chemistry. The vibronic spectroscopy of para-diisocyanobenzene (pDIB, C≡N-Ph-N≡C) has been characterized as a first step towards photochemical studies that can test the transformation of the isonitrile group to other nitrogen-based functionalities. pDIB was found to have a excitation spectrum dominated by Herzberg-Teller vibronic coupling to a nearby second electronic excited state, and the S0-S1 origin was found to be weak, due to the near cancellation of the transition dipole moment upon electronic excitation. The phenylcyanomethyl radical (PCM, Ph-ĊH-C≡N), a doubly resonance-stabilized radical (RSR), was also spectroscopically characterized for the first time. The ionization potential, excited state lifetime, and rotational band contour of PCM at the D0-D1 electronic origin were obtained. The spectroscopy of (E)- and (Z)-phenylvinylnitrile ((E)- and (Z)-PVN, Ph-CH=CH-C≡N), structural isomers of the simplest polycyclic aromatic nitrogen heterocycle (PANH) quinoline, was also characterized. Both (E)- and (Z)-PVN were found to be planar in the ground and first excited electronic state. (E)-PVN was found to participate in extensive Duschinsky mixing, whereas Duschinsky mixing was not as prominent in (Z)-PVN. The spectral signatures for (E)- and (Z)-PVN, were additionally used to carry out ultraviolet hole-filling (UVHF) experiments to test for the formation of quinoline through photoisomerization. Lastly, the microwave spectroscopy of 4-pentenenitrile (CH2=CHCH2CH2C≡N), 4-pentynenitrile (N≡C-CH2- CH2-C≡CH), and glutaronitrile (N≡C-CH2-CH2-CH2-C≡N) was studied. Microwave transitions of each molecule were assigned to particular conformations, providing high resolution microwave data that can be used in future microwave searches for their presence in Titan and elsewhere in interstellar space.

Degree

Ph.D.

Advisors

Zwier, Purdue University.

Subject Area

Atmospheric Chemistry

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