Deuterium labelling and thermochemical studies of dissociative ionization of methyl substituted monosilacyclobutanes. Ring expansion in the molecular ion of 1,1,3-trimethyl-1-silacyclobutane

Dissociative ionization of 1,1‐dimethyl‐1‐silacyclobutane, 1,1‐bis(trideuteromethyl)‐1‐silacyclobutane, 1,1,3‐trimethyl‐1‐silacyclobutane, 1,1‐bis(trideuteromethyl)‐3‐methyl‐1‐silacyclobutane and 1,1,2‐trimethyl‐1‐sila‐cyclobutane have been studied. Low energy electron impact fragmentation of 1,1‐bis(trideuteromethyl)‐3‐methyl‐1‐silacyclobutane results mainly in the loss of ethene with the involvement of the C‐methyl group from the rearranged molecular ion. No fragment ions indicating rearrangement of the molecular ion have been detected in mass spectra of 1,1‐bis(trideuteromethyl)‐1‐silacyclobutane. The ionization energies for 1,1,3‐trimethyl‐1‐silacyclobutane, 1,1,2‐trimethyl‐1‐silacyclobutane and 1,1‐dimethyl‐1‐silacyclopentane, and also the appearance energies for the [M − 28]+˙ and [M − 42]+˙ ions, have been measured by photoioniza‐tion mass spectrometry. The heats of formation of these ions and of 1,1,3‐trimethyl‐1‐silacyclobutane and 1,1‐dimethyl‐1‐silacyclopentene molecules have been calculated as have the enthalpies of the transformation processes.