Computer simulation of the isomerization mechanism and spectral characteristics of spiro[1,3,4]oxadiazines

Abstract

Structural and spectral characteristics of spiro[1,3,4]oxadiazines were calculated by density functional theory method (B3LYP/6-311++G(d,p)). It is shown that a fundamental difference of spirooxadiazines from known spirocyclic compounds is that the ring-opened form is more energetically favorable (by 4.9 kcal mol^–1) compared to the ring-closed one. The closeness between the total energies of closed and open forms of spirooxadiazines suggests all isomers to coexist in solution. The intramolecular rearrangements of compounds under study associated with bond cleavage—formation are accompanied by overcoming of lower-energy barriers than those in the case of analogous spiropyrans and spiroxazines. The calculated electronic spectra (TD DFT) and overlapping absorption band maxima of the closed and open spirooxadiazine forms suggest a low probability of photoinitiated ring opening—closure in this compound.