Ultrafast molecular dynamics in complexed trans-stilbene

Author: Heikal, Ahmed Abdou

Year: 1996

Degree: Dissertation (Ph.D.)

Advisor: Zewail, Ahmed H.

Committee Members: Zewail, Ahmed H.; Culick, Fred E. C.; Corngold, Noel Robert; Marcus, Rudolph A.; McKoy, Basil Vincent; Goddard, William A., III

Option: Applied Physics

DOI: 10.7907/p3gz-kj57

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

Ultrafast molecular dynamics in large molecular systems have been studied using either pump/probe time-of-flight mass spectrometry, or laser-induced fluorescence with time-correlated single photon counting technique. Intramolecular vibrational energy redistribution (IVR) and photoisomerization reaction in alkane-solvated and substituted trans-stilbenes are presented. Absent, restricted, and dissipative IVR were identified in stilbene-hexane[...] van der Waals complex. The dramatic increase of the density of states upon hexane solvation shifts the threshold for an efficient IVR to much lower excess-energy (~300 [...] for the dissipative regime) in comparison with the bare molecule (~1170 [...]). The effects of symmetry, density of states, and number of atoms on IVR were studied by using 4-methoxystilbene, 4,4'-dimethoxystilbene, and 2-phenylindene as model systems. The solvation and structural effects on the isomerization reaction in trans-stilbene is even more dramatic. For the first time, the lowering of the isomerization barrier (~700±100 [...]) in trans-stilbene upon one-hexane complexation is observed experimentally upon and compared with that in the bare molecule (~1250±100 [...]). Furthermore, the isomerization rate above the barrier in stilbene-(hexane)[...] complex is slightly slower than that in bare trans-stilbene. As the excess-vibrational energy exceeds the calculated binding energy of the (1:1) complex, the vibrational predissociation channel become open and was accounted for by using a simple kinetic model. Further decrease in the isomerization rate in stilbene-hexane[...] clusters was observed as the number of solvent molecules [...] increases. As the cluster size increases, the rate becomes less dependent on the excess energy in the [...]-state and the separation of the barrier must await theoretical calculations. The solvent polarity (in stilbene-hexanenitrile[...] complexes) and isotope effects (in stilbene-hexane-[...] complex) on the overall isomerization reaction were also examined. Similar studies were carried out on stilbene-(ethane)[...] clusters which has a lower binding energy than stilbene-(hexane)[...] clusters which, in turn, make the fragmentation of the cluster more severe. On the other hand, interesting observations were made on the reaction dynamics in a designed set of substituted stilbenes. 4-Methoxystilbene, 4,4'-dimethoxystilbene, 4,4'-dihydroxystilbene, trans-[beta]-1-cyclohexene-styrene and 2-phenylindene were used as model systems to study the role of electron conjugation and the torsional motion around [...]-[...] single bond in the isomerization reaction in trans-stilbene.

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