The photoelectron and microwave spectra of the unstable species thioacetaldehyde,CH3CHS,and thioacetone, (CH3)2CS

Combined photoelectron and microwave techniques have been used to study the unstable species thioacetaldehyde, CH₃CHS, and thioacetone, (CH₃)₂CS. These species are produced when their respective cyclic trimers, 1,3,5-trimethyl s-trithiane and hexamethyl s-trithiane, are pyrolysed. The trimer vapours are flowed at approximately 30 μ Hg pressure via a quartz tube heated to between 500 and 600°C into a photoelectron or microwave spectrometer. Under these low pressure conditions the lifetime of CH₃CHS was about 10 seconds. The lifetime of (CH₃)₂CS was much longer, of the order of several minutes.The first (vertical) ionisation potentials of thioacetaldehyde and thioacetone are 8.98 ± 0.02 eV and 8.60 ± 0.05 eV respectively. The photoelectron spectra of the parent trimers have also been studied as well as s-trithiane, the trimer of thioformaldehyde. The microwave rotational spectra show evidence of hindered internal rotation. Preliminary analyses indicate that in thioacetaldehyde the barrier is 1545 ± 20 cal per mole (6470 joule per mole) and in thioacetone it is 1300 ± 50 cal per mole (5440 joule per mole).