Monitoring reactive organic intermediates and short-lived organometallic species via NMR spectroscopy

Abstract

Short-lived organic and organometallic species produced photochemically using a UV laser or UV lamp were studied by using NMR spectroscopy. The compounds studied included photochromic compounds, cyclic ketenimines, indole compounds and s-alkane complexes. A new method of laser alignment was tested in this project for directing laser light into an NMR tube. A photochromic molecule was used as the reacting compound. This photochromic molecules switches rapidly between a merocyanine form in the presence of strong UV light, back to a spirooxizine form without UV irradiation. By observing the composition of the solution via NMR spectroscopy, the laser can be aligned by maximizing the amount of merocyanine form. However, this molecule only reverts rapidly when the light induced changes occur around room temperature. Thus, the application of this method is limited to around room temperature. Phenyl azide and 3,5-dimethoxyphenyl azide were photolyzed in different solvents with the aim of observing a didehydroazepine reactive intermediate. Unfortunately, no intermediate was directly observed. The photolysis of 4,6-dimethoxy-1,2-diphenyl indole, was also studied in this thesis. This indole sample can be photo reacted with singlet O2 in solution and it is expected to form short lived peroxide and oxetane intermediates. The indole compound was photolyzed in different solvents at low temperature in the presence of a sensitizer and oxygen, but no intermediates were observed. An organometallic compound, (hexaethylbenzene)tungstentricarbonyl has been shown previously to generate short lived s-alkane complexes when photolyzed in alkane solvents. This tungsten compound was previously found to have a selective binding of methyl groups when forming tungsten alkane complexes. In this work, the tungsten compound was photolyzed in methylcyclopentane at low temperature. By analysing the 2D 1H NMR spectra, the product composition was found to be a mixture of two tungsten(methylcyclopentane) complexes with the tungsten bound either to the methylene groups which are the furthest from the methyl group ( methylene groups on the C-3) or bound to the methyl group. The binding ratio to the C-3 methylene groups and methyl group was 5.8:1.