A computational investigation on the photochemistry of the popular spin‐trap agent N‐tert‐butyl‐α‐phenylnitrone (PBN) and thermal isomerization pathways of its photoproduct oxaziridine

Abstract

Computational investigation on the low‐lying photo‐excited states of N‐tert‐butyl‐α‐phenylnitrone (PBN), a well‐known spin‐trap agent, has revealed its photo‐product (oxaziridine) formation channel. The S0‐S2 vertical excitation in PBN is subsequently followed by a non‐radiative decay pathway through S2/S1 and S0/S1 conical intersections (CIs) with CNO‐kinked structures, situated around 23 kcal/mol and 45 kcal/mol below the vertically excited S2 state, respectively. The reverse photo‐process of PBN formation involves photo‐excitation of oxaziridine to its S2 and S3 photo‐excited states. The forward photo‐isomerization leads to the trans‐oxaziridine with a backside CNO kink (trans‐OXB) while the reverse path studied by us, connects its front‐side CNO‐kinked analogue (trans‐OXF) with the PBN. Our search for the reverse thermal reaction paths from these two oxazirdines has led to their corresponding transition states, one at 35 kcal/mol and the other at 27 kcal/mol above trans‐OXF and trans‐OXB geometries, respectively. They lead to two different isomers (E and Z) of PBN which supports the reported nature of products from the trans‐oxaziridine in this thermal reaction. The inversion path of the chiral nitrogen atom of this N‐tert‐butyl‐oxaziridine (barrier 21 kcal/mol) has also been tracked. This reaction path has been compared with that of the N‐methyl (barrier 30 kcal/mol) and N‐acyl (barrier 10.5 kcal/mol) oxaziridine analogues.