Abstract
The spatial and electronic structure and absorption spectra of some representatives of spiropyrans (SP) are calculated in order to study the mechanism of monomolecular chemical reactions. The calculated energies of the triplet states of SP are close to the experimental activation energies of the thermal opening of the pyran ring. The corresponding lowlying electron transitions are localized on the pyran ring, as indicated by spin density distribution in the triplet state. It is concluded that cleavage of SP occurs via the lowest triplet state of SP. For colored open forms of SP having a planar structure, the quantum chemical calculation predicts a transition between the zwitterionic and quinoid states, depending on the character of intermolecular interactions; this transition is accompanied by a significant change in the absorption spectrum.
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