Resonance Two Photon Ionization Study of Binary Clusters of Styrene with Polar Molecules

Abstract

RESONANCE TWO PHOTON IONIZATION STUDY OF BINARY CLUSTERS OF STYRENE WITH POLAR MOLECULES By Hatem Ahmed Mahmoud, Ph. D. A Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2004 Major Director: M. Samy El-Shall Professor, Department of Chemistry One-color resonance two-photo ionization (R2PI) spectra of mixed clusters of styrene molecule (S) with polar molecules [water (W), methanol (M), ethanol (E), and trifuoroethanol (T)] were measured through the S1←S0 transition of the styrene molecule. The spectra reveal a rapid increase in complexity with the number of polar molecules in the cluster, associated with van der Waal modes and isomeric forms. The spectral shifts of the cluster origins from the S1-S0 transition of the bare styrene molecule reflect the nature of the intermolecular interactions within the binary clusters. The obtained R2PI spectra xiv xv were compared with the spectra of the analogous benzene containing clusters. The styrenewater and the styrene-methanol complexes exhibited very different spectral shifts and structures as compared to the benzene-water and benzene-methanol complexes, respectively. The favorable interactions of polar molecules with the olefin group of styrene may explain the strong inhibition effects of exerted by small concentrations of water and alcohols on the cationic polymerization of styrene. Size-specified intracluster proton transfer reactions were observed for mixed clusters of styrene dimer with water, methanol and ethanol molecules. It was proposed that the polar molecules tend to aggregate around the olefin center, which promotes the transfer of the charge from the propagating chain to the hydrogen-bonded polar molecules subcluster. The minimum number of polar molecules required for a proton transfer to take place exothermically depends on the proton affinity of the polar molecules subcluster. The estimated upper limit value for the proton affinity of styrene dimer radical was evaluated based on the energetic of the proton transfer reaction to be ≤ 220.4 kcal/mol. No intracluster reaction was observed for styrenetrifluoroethanol (STn) system. In order to provide a comparison between the styrene and benzene systems, the benzene-ethanol (BEn) and benzene-trifluoroethanol (BTn) clusters were studied by using the R2PI technique via the transition of the benzene molecule. Both dissociative electron transfer and dissociative proton transfer reactions were observed within the BE 1 0 6 n clusters, where n = 2 and 3, respectively. Proton transfer reactions were observed following dissociative electron transfer reactions within the (BTn) clusters, where n = 4, to generate the protonated clusters (HTn). Chapter