Excited state relaxation mechanisms of paracetamol and acetanilide.

The photochemical pathways of acetanilide and paracetamol were investigated using the XMS-CASPT2 quantum chemical method and the cc-pVDZ (correlation consistent polarized valence double- $\zeta$ ) basis set. In both compounds, the bright state is the second excited state, designated as a ${}^1({\mathrm{\pi \pi }}^{*}$ L_a) state. Through a detailed exploration of the potential energy profile and the conical intersection structure between the ${}^1({\mathrm{\pi \pi }}^{*}$ L_a) and ground states, we gained a better understanding of how cleavage might occur in both molecules upon photoexcitation. Other potential relaxation mechanisms, including crossings with the dark ${}^1\left(n{\pi }^{*}\right)$ and ${}^1({\mathrm{\pi \pi }}^{*}$ L_a) states, are also discussed in detail.