Femtochemistry and reactive intermediates : application to atmospheric and organic chemistry

Citation

Herek, Jennifer Lynn (1996) Femtochemistry and reactive intermediates : application to atmospheric and organic chemistry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qj6m-w225. https://resolver.caltech.edu/CaltechETD:etd-04082008-084916

Abstract

The logical sequence of primary steps in a reaction mechanism is governed by the reactive intermediates which are postulated to ensure smooth transformations between stable species. Historically, such constructs were merely hypothetical as their (assumed) extreme reactivity and/or unstable nature precluded their observation. The introduction of femtosecond lasers to chemistry has allowed insight and understanding to many fundamental processes in reaction dynamics. This thesis presents applications of femtosecond spectroscopy to atmospheric and organic chemistry, specifically probing the reactive intermediates mediating various chemical transformations. Direct observation of these fleeting species can validate previously proposed mechanisms as well as offer new perspectives to their reaction dynamics. Studies of diradicals, the molecular species hypothesized to be archetypal of chemical bond transformations in many reactions in organic chemistry, have been made using femtosecond laser techniques combined with mass spectrometry in a molecular beam. Evolution of the diradical intermediate is monitored in real time throughout the course of the chemical reaction. These studies offer the first direct evidence of singlet 1,3- and 1,4-diradicals. Hydrogen-atom transfer processes are fundamental to many reactions in organic chemistry. The dynamics of the motion may be localized, with little nuclear motion aside from the H-atom transfer between two "anchors," or may be global, involving a multidimensional potential. Using methyl salicylate as a prototype system, direct studies of the nuclear motion in this important transformation are made with femtosecond resolution. Many reactive intermediates have been linked to stratospheric ozone depletion in recent years. In particular, catalytic cycles involving halogens are considered critical to establishing the overall mechanism for ozone depletion. Direct studies of the photochemically-activated dissociation of key species in atmospheric chemistry can offer insight to the global dynamics by providing important rate constants and branching ratios. Femtosecond reaction dynamics of OClO in a supersonic molecular beam are reported. The observations reveal the nuclear motions and couplings between potential energy surfaces relevant to the dissociation process. Comparisons with results of ab initio calculations are made.

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