How Does the Central Atom Substitution Impact the Properties of a Criegee Intermediate? Insights from Multireference Calculations.

Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI)-based multireference calculations have been performed to better understand the ground state properties and the photodissociation mechanism of SiH2 OO, a silicon analogue of the parent Criegee intermediate, CH2 OO. The CASSCF/aug-cc-pV(T+d)Z results suggest that the ground state of SiH2 OO is severely multireference in nature. This explains why SiH2 OO could not be characterized in recently reported coupled cluster calculations. An important implication of this multireference character is the dramatically enhanced reactivity of SiH2 OO, i.e., the calculated barrier for the cyclization of SiH2 OO is only 4.4 kcal/mol, which is nearly 10 kcal/mol lower than that reported for the CH2 OO case. The MRCI/aug-cc-pV(T+d)Z results on the evolution of the low-lying singlet electronic states along the OO bond suggest that SiH2 OO absorbs strongly in the near UV-vis region. These results improve our fundamental understanding of the thermal and photobehavior of XH2 OO (X = C, Si, Ge, and Sn) that serve as precursors for dioxiranes, an important class of oxidants for the synthesis of value-added chemicals, and also find their applications in optodevices.