<p>In this study, the thermodynamics and barrier heights associated with the fragmentation reactions of a set of fifteen 1,4,2-oxaselenazoles into isoselenocyanates (molecules with promising anticancer activity) and carbonyl derivatives, have been studied using the high-level CBS-QB3 quantum chemical protocol. Of the systems studied, attachment of a CF₃-substituent at the C5-position affords the system with the largest gas-phase free energy barrier (190.1 kJ mol^–1), whilst substitution at the C5-position with two –NMe₂ substituents affords a heterocycle with the lowest free energy barrier (67.8 kJ mol^–1). The presence of solvent (acetonitrile) was shown to reduce the free energy barriers in all cases, with the two systems mentioned above having condensed-phase free energy barriers of 180.8 and 42.0 kJ mol^–1, respectively.</p>