Anjan Chattopadhyay
BITS-PILANI, India
Title: A computational investigation of the photochemical oxaziridine conversion process of some experimentally analyzed small-chain conjugated nitrones
Biography
Biography: Anjan Chattopadhyay
Abstract
In recent times, we have computationally explored the photochemistry of several conjugated nitrone systems which include the chemopreventive retinyl nitrones, fluorescent naphthyl nitrones and small open-chain nitrones with phenyl substitutions at the C-terminal positions. In the latter category, we have studied the α-styryl N-methylnitrone and the 3,3-diphenylethylene N-methylnitrone, both synthesized and characterized by our group. Results based on high-level computational investigations on the photo-excited states of these two conjugated N-methylnitrone systems have justified their experimentally observed photochemical features. The UV peaks gradually developed near 260 nm has been theoretically predicted to appear from the oxaziridine. Analysis of the IR peaks has also indicated this terminal heterocyclic species as the photoproduct. Photo-excitation of the planar α-styryl N-methylnitrone is found to populate the first excited singlet state through allowed S0-S1 transition of roughly 7 Debye transition moment value. This is subsequently followed by a reaction path leading towards the lowest-energy conical intersection (S0/S1) with a terminal CNO-kinked geometry (27-30 kcal/mol below the relaxed S1 state). Following the gradient difference vectors of this conical intersection, an oxaziridine structure was located around 14 kcal/ mol above the ground state. In contrast, the photo-excitation of the non-planar 3,3-diphenylethylene N-methylnitrone leads to two strongly allowed singlet-singlet transitions (S0-S1 and S0-S2). The initially photo-excited S2 state relaxes to the S1 state which is further followed by oxaziridine formation through the terminally twisted conical intersection. However, the S0–S1 transition in this nitrone is found to follow another route by transfer of huge amount of non-bonding electron cloud of oxygen to the π* orbital, and thus forming a stable excited state geometry with an elongated N-O bond which gets involved in a sloped conical intersection with the ground state; this can be related to the experimentally observed slow decay of the longer wavelength UV peak of this nitrone.