Photochemistry of chlorobenzene interacting with the surface of amorphous ice

Author

Terry Evans Vaughn

Date of Award

12-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Fred Grimm

Committee Members

Charles Feigerle

Abstract

The photochemistry of chlorobenzene adsorbed to an amorphous ice surface has been investigated at ultra-high vacuum (UHV). Ultra-thin ice films (< 50 Å) were grown on the surface of a Ni (111) crystal in a vacuum chamber evacuated at a pressure on the order of 10^-10 torr. These thin ice films were determined to be continuous and void of large cracks and fissures which might expose the underlying nickel substrate. Various exposures of chlorobenzene were dosed to the amorphous ice surface at both submonolayer and multilayer coverages. Broad band UV irradiation of this system produced benzene as the primary photolysis product as determined by thermal desorption spectroscopy (TDS). The chlorine, once photodissociated from the phenyl ring, was determined to diffuse through the ice layers to the nickel surface. A thermal desorption spectrum taken after irradiation indicated no chlorine species to desorb from the surface of the ice except for the parent chlorobenzene. The chlorobenzene adsorbate showed only physisorption to the ice surface with no evidence for any stronger interaction. Deuteration studies indicated that the source of hydrogen to form the benzene during irradiation was primarily from an adjacent phenyl ring species and not from the ice surface. This implies stronger lateral interaction between the adsorbates and appreciable mobility of chlorobenzene on the ice surface. Thermal desorption profiles of multilayer chlorobenzene adsorbed to ice also corroborates the apparent weak interaction at the interfacial region. Interestingly, it was found that multilayer chlorobenzene had a higher desorption temperature than did monolayer chlorobenzene from the ice surface. This indicates that the chlorobenzene-chlorobenzene interaction is potentially larger than the chlorobenzene-H₂O interaction. A possible explanation for the observed desorption profiles is that island aggregation of chlorobenzene is occurring on the ice surface. These islands could contain multilayer coverages of chlorobenzene at the center and taper to monolayer coverages at the edges. Desorption for the monolayer coverage at the edges of the islands with a steady reduction in island size is thought to explain the observed temperature profiles of monolayer and multilayer coverages. Island aggregation would also facilitate the experimentally observed abstraction of hydrogens from adjacent phenyl compounds. Theoretical calculations at the ab initio level, including Hartree-Fock and higher order corrections, were performed to investigate the interactions at the interfacial region of the adsorbate and ice surface. One to one molecular complexes of chlorobenzene and H₂O were explored as precursors to the larger macroscopic interfacial system. Binding energies of benzene, chlorobenzene, and fluorobenzene with H₂O were calculated. Benzene and fluorobenzene were theoretically investigated for the purposes of comparing the effects of ring substitution on the binding interactions. It was found that benzene possessed a higher binding energy than did the substituted benzene compounds. Interestingly, the chlorobenzene and fluorobenzene complexes were shown to have very similar binding energies. Global minimums for all three hydrogen bonded complexes were found with H₂O acting as the proton donor and the phenyl ring acting as the electron donation source. Chlorobenzene was found to be participating in a possible double hydrogen bonded complex with H₂O. One of the H₂O hydrogens was preferentially directed toward the para-carbon on the phenyl ring in what appears to be a o-type hydrogen bond with the py orbital of the para-carbon. The other hydrogen is directed over the center of the ring in a more traditional rr-type hydrogen bond. Vibrational frequencies were also calculated to complement the theoretical investigation of the hydrogen bonding interaction of the complexes. Calculations on the possible interactions of atomic chlorine with ice were also carried out to explore chlorine radical complexation with H₂O monomers within the ice with relevancy to possible mechanisms for chlorine diffusion. Binding energies and preferred complex geometries for the chlorine anion and chlorine radical complex formation with H₂O were determined for one to one complexes. H₂OCI has been investigated previously and is used here for comparative purposes. The structure and binding energies reported previously are in excellent agreement with our work. H₂OCI has not been previously investigated either experimentally or theoretically. The H₂OCI radical complex was found to have a binding energy of 1.8 kcal/mol. The interactions between a chlorine anion and a chlorine radical with H₂O were found to be distinctly different in mechanism and orientation. The chlorine radical is bound to the H₂O monomer through a lone pair on the oxygen in a tetrahedral arrangement whereas the chlorine anion displayed a more traditional hydrogen bonded interaction. The geometry of the ionized form of the radical complex, H₂OCI⁺, was also theoretically determined. Calculated vibrational frequencies were compared to recently reported experimental values of the cation complex. The complex has been recently proposed to act as an intermediate in heterogeneous reactions occurring on stratospheric ice particulates over Antarctica.

Recommended Citation

Vaughn, Terry Evans, "Photochemistry of chlorobenzene interacting with the surface of amorphous ice. " PhD diss., University of Tennessee, 1998.
https://trace.tennessee.edu/utk_graddiss/9392