Multiphoton ionization studies of pi-hydrogen bonding molecular clusters containing benzene

Aaron Wayne Garrett, Purdue University

Abstract

Resonant two-photon ionization studies of C$\sb6$H$\sb6$-(H$\sb2$O)$\sb{\rm n}$, C$\sb6$H$\sb6$-(CH$\sb3$OH)$\sb{\rm m}$ and C$\sb6$H$\sb6$-(H$\sb2$O)$\sb{\rm n}$-(CH$\sb3$OH)$\sb{\rm m}$ clusters formed by supersonic expansion have been carried out using C$\sb6$H$\sb6$ as the chromophore. A combination of vibronic level probes and Monte Carlo computer simulations indicates that the minimum energy structures of the molecular clusters involve a network of hydrogen bonded solvent molecules which builds off of one face of the benzene ring. The comparison of the spectroscopy of C$\sb6$H$\sb6$-(H$\sb2$O)$\sb{\rm n}$ with C$\sb6$H$\sb6$-(CH$\sb3$OH)$\sb{\rm n}$ shows strong similarities for clusters with n = 1,2, some divergence at n = 3, and a remarkable dissimilarity for larger clusters. We argue that these are consequences of water's ability to donate two protons to hydrogen bonding interactions while methanol can only donate one. Therefore, a water molecule can orient itself to allow strong hydrogen bonding interactions with both the benzene $\pi$-cloud and other water molecules simultaneously. In contrast, a methanol molecule must donate its single proton to either another methanol molecule or to the benzene ring. This basic difference is reflected in cluster geometries which are very different as the number of solvent molecules is increased. Similar data on the C$\sb6$H$\sb6$-(H$\sb2$O)$\sb{\rm n}$-(CH$\sb3$OH)$\sb{\rm m}$ clusters shows that the methanol rich clusters prefer structures which are similar to C$\sb6$H$\sb6$-(CH$\sb3$OH)$\sb{\rm m}$ clusters while water rich clusters prefer C$\sb6$H$\sb6$-(H$\sb2$O)$\sb{\rm n}$ like structures. The C$\sb6$H$\sb6$-(CH$\sb3$OH)$\sb{\rm n}$ clusters were also found to undergo intracluster ion-molecule reactions. The main reaction pathways observed were dissociative electron transfer and dissociative proton transfer between the benzene chromophore and the methanol cluster. The efficiency of these reaction channels along with other minor reaction channels were studied as a function of cluster size and internal energy and are found to be consistent with energetic thresholds for the various product channels.

Degree

Ph.D.

Advisors

Zwier, Purdue University.

Subject Area

Analytical chemistry|Chemistry

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