Selected reactions of photo-excited carbonyl compounds in the liquid phase

Abstract

The work presented in this thesis falls into two parts; firstly, a study or the cis-trans isomerisation or methyl tiglate (trans 2-carbometho:xy-2-butene) and secondly, a study or the quenching of the Norrish Type II reaction by cyclohexene. Both sets of experiments were carried out by semi-micro photolytic techniques developed for the photolysis of small quantities of de-oxygenated liquids. A full description and comparison or the methods developed is presented.
In the first part of the work methyl tiglate was isomerised to its cis isomer, methyl angelate by the absorption of 254 run light. The reaction was studied as a function or temperature, ester concentrated and wavelength. The quantum yeild of tiglate to angelate isomerisation was measured as 0.090 * 0.003 and was found to be independent or initial ester concentration. Temperature was shown to have a small effect. The reaction was not affected by change or solvent and did not occur at 313 nm. The quantum yield of the reverse reaction was measured as 0.032 and was also independent of concentration. A minor side reaction to the 8,y-ester was investigated and had a quantum yield of 0.005. Methyl angelate was shown to be the precursor for this reaction and incorporation or deuterium in a suitable experiment showed the participation of an enol intermediate. A six-membered transition state is proposed.
The isomerisation was found to be partially quenched by the addition of oxygen and paramagnetic salts. Photosensitisation was investigated with a number of compounds using 313 and 366 run exciting light. Acetone, acetophenone, benzophenone and anthraquinone were found to sensitise the reaction. Sensitisation was accompanied by oxetane formation in the case of the latter two compounds.
The results are discussed in relation to more detailed investigations of the isonerisation of ethylenic compounds and a potential energy versus angle of twist diagram is proposed for the lowest electronic states of the esters. There appears to be an efficient mechanism for internal conversion from the excited singlet state of both isomers as has been suggested for certain other n,w* systems. A mechanism for photosensitised isomerisation is proposed involving a Schenck type of intermediate, based on studies of the effect of ester concentration on the benzophenone sensitised isomerisation.
The results presented in the second part of the work show how quenching of the photoelimination reaction in a ketone by an olefin of very similar triplet state energy can be used to obtain values of the quantum yields for the production of, and reaction from, singlet and triplet states of the ketone. It was shown, however, that quenching by compound formation observed in benzene is not the dominant reaction in cyclohexane. It is suggested that this is due to slight changes in the relative triplet state energies of donor and acceptor.