Isomer -specific spectroscopy and conformational isomerization energetics of flexible benzene derivatives

My thesis research investigated the spectroscopy and dynamics of molecules involved in key steps leading toward soot formation in combustion environments. A critical feature of models of combustion is something shared by many areas of science; namely, the need to assess and characterize the role played by structural isomers in chemistry. In this thesis structural isomers of benzene derivatives, some of which lead directly to polyaromatic hydrocarbon formation, were studied. A supersonic jet was used to cool these molecules into their zero-point levels. Then, various laser-based spectroscopies were used to provide the infrared and ultraviolet spectral signatures of individual structural isomers free from interference from one another. Unique infrared and ultraviolet spectral signatures of ortho-, meta-, and para-ethynyl styrenes (oES, mES, and pES), para- and meta-divinylbenzene (pDVB and mDVB), 3-benzyl-1, 5-hexadiyne (BHD), 5-phenyl-1-pentyne (PP), and 4-phenyl-1-butyne (PB) were obtained. These studies showed that mES, mDVB, BHD, PP, and PB all have multiple conformations present in the expansion. Assignments of individual conformations were made for BHD, PP, and PB based on the observed and calculated frequencies in the alkyl and acetylenic CH stretch regions, S0-S1 electronic origin shifts, and rotational band contour analysis. High resolution fluorescence excitation studies were used to assign the two conformations of m ES. Armed with this foundation of spectroscopic data on individual conformational isomers, detailed studies of the conformational isomerization dynamics were carried out on mES and mDVB. These studies used the method of stimulated emission pumping-population-transfer spectroscopy (SEP-PT) as a means to place narrow bounds on the energy thresholds for isomerization between individual reactant-product isomer pairs. This method utilizes selective excitation of a single conformation by means of SEP in the early portions of the gas-phase expansion, followed by collisional re-cooling of the vibrationally excited population into the conformational minima for subsequent conformation-specific detection. These studies were used to determine the thresholds for isomerization, the relative energies of the different conformations in the ground state, and the pathways for isomerization. In addition, single vibronic level fluorescence studies were used to fit the vinyl torsional levels observed in m DVB to obtain the barrier to isomerization in a second manner