The laser spectroscopy and dynamics of methylpyrimidines and diacetylene

Ralph Edward Bandy, Purdue University

Abstract

The work presented here is composed of two parts. The first part addresses the effects that a methyl rotor has on the spectroscopy and dynamics of a molecule. This study has been carried out on 2-, 4-, and 5-methylpyrimidine using laser induced fluorescence techniques. By studying the methylpyrimidines we were able to examine the methyl rotor interactions as a function of substitution position. We have seen that the methyl rotor becomes a sensitive probe of the local electronic environment. Furthermore, we have seen that the internal rotor levels can have a profound effect on both the spectroscopy and the intramolecular dynamics of the molecule via internal rotation/vibration coupling. At a primary level this coupling is manifested as a Fermi resonance shifting both the position and intensity of the allowed electronic transitions in both 2-methylpyrimidine and 4-methylpyrimidine. In addition, coupling at a secondary level is observed in all three molecules due to the enhanced density of states brought about by the presence of the low frequency internal rotor energy levels. The second part of this work presents a laser spectroscopy and photochemical study of diacetylene(C$\sb4$H$\sb2$). The spectroscopic study of the S$\sb2 \gets$ S$\sb0$ transition of C$\sb4$H$\sb2$ is carried out using resonant two photon ionization (R2PI) with mass spectrometric detection. The spectroscopy of this molecule is complicated by both Herzberg-Teller and Renner-Teller coupling making definitive assignments of excited state vibrational modes a difficult task. However, by conducting the study in a supersonic jet expansion we were able to determine that the S$\sb2$ excited state lifetime is less than 1 ps due to coupling to a dense manifold of dark background states. The photochemistry (i.e. intermolecular dynamics) of the electronically excited molecule is investigated using laser photoexcitation and vacuum ultraviolet (VUV) ionization. The photochemical studies revealed the nature of the photochemical products for the reaction C$\sb4$H$\sb2$** + C$\sb4$H$\sb2$ for the first time. Primary products observed were C$\sb6$H$\sb2$, C$\sb8$H$\sb2$, and C$\sb8$H$\sb3$. Secondary products observed were C$\sb{10}$H$\sb3$ and C$\sb{12}$H$\sb3$. The implications these results have on current models of planetary atmospheres is discussed.

Degree

Ph.D.

Advisors

Zwier, Purdue University.

Subject Area

Analytical chemistry

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