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Ultraviolet photoelectron spectroscopy : a study of unstable molecules

University of British Columbia

The technique of ultraviolet photoelectron spectroscopy (UV PES) has been used to detect unstable molecules in the gas phase. The production of the species under: investigation can be continuously monitored, allowing experimental conditions to be altered to optimize the yield of the desired product. It has therefore proven possible to develop methods of producing high purity gas phase samples of four different groups of unstable molecules; hypochlorites (HOCl, CH₃OCl, C₂H₅OCl, (CH₃)₃COCl), halamines (NH₂Cl, NHCl₂, NCl₃, CH₃NHCl, (CH₃)₂NCl, CH₃NCl₂, NH₂Br, CH₃HBr, (CH₃)₂NBr, CH₃NBr₂, NHF₂, NDF₂) , substituted ketenes (ClHC=C=O, Cl₂C=C=O, BrHC=C=O, Br₂C=C=O, CH₃HC=C=O, (CH₃)₃C=C=O) and formic anhydride, (HCO)₂O. For the latter compound the stable anhydrides (CH₃CO)₂O, (CF₃CO)₂O, (CClF₂CO)₂O, (CCl₃CO)₂O were also investigated for comparison. The Hel photoelectron spectra thus obtained are free from impurities in most cases, this permitting the valence electronic energy levels of these molecules below 21.22eV to be fully studied. The effect of halogen or alkyl substitution upon the functional group has been determined providing information on electronic interactions within the molecule e.g. inductive, resonance and through bond. Several of the molecules studied here have never been observed before in the gas phase eg. CH₃NHBr, CH₃NBr₂, Cl₂C=C=O, Br₂C=C=O and many are sufficiently small that a full assignment of the Hel photo- electron spectrum can be given on the basis of the observed vibrational fine structure (providing information on the ionic vibrational frequencies), comparison with known isoelectronic molecules and the substituent effects observed within the group. In cases where additional support for an assignment was necessary the experimental ionization potentials have been compared to semi-empirical molecular orbital calculations (CNDO/2, HAM 3) within the limits of Koopmans' theorem.

Text

2010-03-15

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http://hdl.handle.net/2429/21880

Chemistry

University of British Columbia

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