Graduate Thesis Or Dissertation
 

Polarization spectroscopy and photodissociation studies of nitroaromatic compounds in the gas phase

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  • The central purpose of this dissertation was to explore and expand the use of an uniform electric field for orientation and for subsequent spectroscopic investigation of asymmetric polar molecules in the gas phase. The systems of study were nitrobenzene (NB) and the nitrotoluene (NT) isomers. We were able to quantitatively determine the direction of the electronic transition dipole relative to the permanent dipole for these molecules, thus providing quantitative information on the symmetry of the second and third singlet excited electronic states. Transition to the second singlet excited state (S2) was shown to have a dipole predominantly perpendicular to the permanent dipole, consistent with a localized excitation of the -NO2 moiety. The transition dipole to the S3 state for the meta and para isomers was almost completely parallel to the permanent dipole, opposite to that observed for the S2 transition. The success of these experiments has demonstrated the ability of the Brute Force Orientation technique to quantitatively characterize the transition dipole properties of large molecules. The importance of this technique lies in the fact that most large molecules undergo rapid internal conversion and slow dissociation after absorption, therefore information on the symmetry properties of these systems is otherwise unattainable. In addition to the determination of the transition dipole direction, we were able to characterize many details of the dissociation process by analyzing the internal energy distribution of the Nitric Oxide (NO) photofragment. The Resonantly Enhanced Multi Photon Ionization spectrum of NO has revealed that the methyl group causes significant perturbation in the dissociation process, while it seems to have minimal effect on the transition dipole direction among NB and NT. All NT isomers showed significantly more vibrational excitation than previously reported for NB. Additionally, the meta and para isomers were observed to have a vibrational inversion behavior for v” ≥ 3, with the higher vibrational levels having larger populations. The higher vibrational levels were also found to have higher degree of rotational excitation. A bimodal behavior was also observed in the rotational distribution of several vibrational bands. A preference was observed for the π lobe being in the plane of rotation of the fragment. After a thorough comparison with relevant literature reports, and based on our experimental results, we present a model for the dissociation of NT isomers. We believe the bimodal rotational distribution and vibrational inversion indicate at least two dissociation channels in which isomerization of the nitro group and an NO2 photofragment play essential roles.
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