Investigation on the Unexplored Photochemistry of 5,5-Dimethyl-1-pyrroline 1-Oxide (DMPO).

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 1517 i 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 1514 i 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.