A comparative
study of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO)
and its 2-methyl-substituted analogue (2-Me-DMPO) has revealed their
contrasting reaction pathways of oxaziridine and lactam (pyrrolidone)
formation. The initial photoexcitation populates the second excited
singlet states (S2) in both the systems with S0–S2 transition moment value of 3 D (oscillator
strength 0.4); this subsequently undergoes (S0/S1) conical intersection through a structure having a CNO-kink and
situated around 35–40 kcal/mol below the vertically excited
geometry of the first excited singlet state (S1). This
conical intersection is found to be responsible for the formation
of the oxaziridine photoproduct in these systems. In DMPO, this oxaziridine
eventually forms the corresponding lactam compound through a [1,2]-H
shift after overcoming a barrier of 35 kcal/mol and following the
imaginary frequency of 1517i cm–1. The reverse thermal process of parent nitrone formation proceeds
through a transition state situated at 60 kcal/mol above the oxaziridine
geometry, and the corresponding imaginary frequency is 1514i cm–1. On the other hand, in 2-Me-DMPO,
the oxaziridine formed is more stable, and lactam formation does not
happen from it in a similar manner.
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