Reactive intermediates are useful in organic synthesis, industrial chemistry and many biological processes. Knowledge of their structure and energetic behaviour enables the mechanisms of reactions to be further understood. The initial aim of this project was to determine the gaseous molecular structures of various short-lived ketenes using a new combined gas electron diffraction / mass spectrometry (GED/MS) apparatus, as well as a full analysis using computational methods. The proof-of-concept was to be focussed on ketene (ethenone) which can be generated cleanly from pyrolysis of diketene (4- methylideneoxetan-2-one). Due to the COVID-19 pandemic we were unable to get the required apparatus delivered to the University of Canterbury, therefore the project was refocused to a fully computational investigation of ketene and substituted ketenes.

Computational modelling of the structures and reaction pathways for the generation of ketene and substituted ketenes has been undertaken using the Gaussian 09 and NWChem suite of programs, utilizing the New Zealand e-Science Infrastructure (NeSI). The parent diketene can break down to give either ketene, or allene and CO₂. Both of these pathways have been studied in detail and this work demonstrates that the reaction pathway for the formation of ketene has a lower activation barrier than the alternative pathway (formation of allene and CO2) using CBS-QB3 calculations. The predicted entropy, enthalpy and free energy of pyrolysis dissociation of diketene at the CCSD(T)/CBS level however indicates that the ketene formation pathway is spontaneous only at elevated reaction temperature.

The second reactive species discussed is methyleneketene. Pyrolysis of diazotetranoic acid [3- diazofuran-2, 4(3H, 5H)-dione] could give either methyleneketene and CO₂ or carbon sub-oxide and formaldehyde with liberation of nitrogen gas in both cases. Both pathways have been studied computationally. The methyleneketene and CO₂ formation pathway was found to be exergonic with a lower energy barrier than the carbon sub-oxide and formaldehyde formation pathway by CBS-QB3 level of theory. The thermochemical parameters have also been studied using CCSD(T)/CBS calculations.

Finally a series of substituted ketenes obtained from the decomposition of derivatives of Meldrum’s acid (MA) are discussed. Methyleneketene, methyl-ketene, and dimethyl-ketene can all be obtained from the pyrolysis of Meldrum’s acid (MA) derivatives, and all decomposition pathways have been studied theoretically using CBS-QB3 calculations. All were found to be thermodynamically feasible at 298.15 K with the exception of methylene-Meldrum's acid (MeMA). All reactions are endothermic, requiring energies in the range of 190 to 250 kJ/mol to cross the energy barrier towards the formation of substituted ketene.